EXTRACTION STUDIES OF PALLADIUM WITH DITHIZONE.

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University Miaorilms

International 300 N. Zeeb Road Ann Arbor, Ml 48106

1319466

SIMONZADEH, NINUS

EXTRACTION STUDIES OF PALLADIUM WITH DITHIZONE

The University of Arizona M.S. 1982

University Microfilms

International 300 N. Zeeb Road. Ann Arbor, MI 48106

EXTRACTION STUDIES OF PALLADIUM

WITH DITHIZONE

by

Ninus Simonzadeh

A Thesis Submitted to the Faculty of the

DEPARTMENT OF CHEMISTRY

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 8 2

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfill­ment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowl­edgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: tfl/'ykAA —

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

H. FJlEISER Professor of Chemistry

D G?e

ACKNOWLEDGMENTS

The author wishes to thank Dr. Henry Freiser for

his assistance in the preparation of this thesis.

iii

TABLE OF CONTENTS

Page

LIST OF'ILLUSTRATIONS vi

LIST OF TABLES vii

ABSTRACT viii

INTRODUCTION 1

Oxidation States 8 Kinetic Effects 9 The (+2) State: Pt(II) and Pd(II) 11 Pt(IV) and Pd(IV) Complexes 12 Complex Formation of Palladium with

Chelating Agents 12 The Structure of Dithizone 17 The Primary (2:1) Metal Dithizonates 19 Secondary Metal Dithizonates 21

STATEMENT OF PROBLEM 23

EXPERIMENTAL 24

Materials 24 Apparatus 24 Procedure 25

Purification of Dithizone 25 Standardization of Pd Solution by

EDTA-Indirect Titration Method, and by the Iodide Spectrophotometric Method ... 27

Preparation of the Primary and the Secondary Palladium Dithizonates 28

Distribution Studies 28

RESULTS AND DISCUSSION 30

Preliminary Experiments 30 Effect of V0 (Organic Phase Volume)

on the Precision of Measurements 30

iv

V

TABLE OF CONTENTS — Continued

Page

Effect of Shaking Time on Extraction of Palladium 32

Effect of Varying Concentrations of Sulfuric Acid on Extraction of Pd 35

Effect of Various Masking Ligands, under Specified Conditions, on the Extraction of Palladium 37

The Effect of Varying pH on LogD1

in the Presence of Thiosulphate 38 Effect of Varying Thiosulphate

Concentration on LogD' 41 Effect of Varying Reagent Concentration

on the Extraction of Palladium in the Presence of Thiosulphate 46

Effect of Varying Concentrations of Chloride on the Rate of Extraction of Pd via a High-Speed Stirring Technique 47

SUMMARY 57

REFERENCES 59

LIST OF ILLUSTRATIONS

Figure Page

1. Electrode potentials for PGM in acidic and basic solutions 6

2. High-speed stirring-extraction apparatus .... 26

3. Determination of stoichiometry for the primary Pd-dithizonate in 0.2M H„S04, at 637 nm in CHClg 7 31

4. Equilibration time as a function of amount of ligand present in solution 33

5. Effect of sulfuric acid on extraction of palladium 36

6. Effect of pH on extraction of palladium .... 40

7. Effect of varying thiosulphate concentration on extraction of palladium at pH 7.40 43

8. Effect of varying thiosulphate concentration on extraction of palladium at pH 9.86 45

9. Effect of varying ligand concentration on extraction of palladium 49

10. Effect of chloride concentration on the observed rate constant 53

11. Determination of elementary rate constants (k3 and k4) for the formation of primary palladium dithizonate 56

vi

LIST OF TABLES

Table Page

1. Some representative compounds and ions of the PGM 3

2. Some halo-complexes of the platinum metals ... 4

3. Chelating agents for the extraction of PGM ... 15

4. Effect of pH on the distribution of palladium 39

5. Effect of thiosulphate concentration on the distribution of palladium at pH 7.40 ... . 42

6. Effect of thiosulphate concentration on the distribution of palladium at pH 9.86 ... . 44

7. Effect of varying ligand concentration on the distribution of palladium at pH 9.86 ... . 48

8. Observed rate constants (M^sec-"*") at half lives for the formation of (2:1) Pd-dithizonate at various chloride concentrations 51

vii

ABSTRACT

A study was conducted on the extraction behavior of

palladium with dithizone in an aqueous-chloroform system.

Palladium was found to form an incredibly stable (2:1)

chelate with dithizone, extractable from up to 9M sulfuric

acid solution.

Thiosulphate was found to be a suitable sequestering

(masking) agent which suppressed the extraction of palladium.

The dependence of the extraction on thiosulphate concentra­

tion, pH, and ligand concentration was investigated.

Some rate studies with the use of chloride ion as

the masking agent were also carried out using a high-speed

stirring technique.

viii

INTRODUCTION

The metals which comprise the platinum group are

palladium, platinum, iridium, osmium, ruthenium and rhodium.

The 4d and 5d orbitals in these series are continuously

filled, beginning with Y and La with one or two electrons

present in a 5s or 6s orbital. Metal atoms in the same

column of the periodic table (Ru, Os, Rh and Ir, Pd, Pt)

exhibit marked similarity in chemical properties due to simi­

larity in outer electron configuration and atomic radii.

The chemistry of the noble metals in aqueous solution

is complicated by a wide range of oxidation states, which is

by far greater than that shown by other transition metals.

The highest possible, as well as the most common oxidation

states increase with increasing atomic numbers in the same

column of the periodic table due to the increasing distance

of the outer electrons from the nucleus. The oxidation states

decrease along a row from left to right due to the gradual

filling of the d-orbitals.

Ruthenium and osmium sometimes appear in the highest

oxidation states (VII) and (VIII). These are stabilized by

ligands which form ionic bonds with the metal ion; oxygen

donors and oxyhalides are examples of such ligands. Oxidation

state (VI), associated mainly with Os and Ru, appears much less

1

2

frequently in Pt, Ir and Rh and is characteristic of oxygen

compounds. Ligands such as water, ammonia, amines, halogens

and sulfur donors usually stabilize metals such as Ru, Os,

and Ir in their (IV) and (III) oxidation states, Pt in its (IV)

state and Pt and Pd in their (II) state. Lower oxidation

states (I, 0) are associated with few elements and are ob­

served mainly in carbonyl compounds.

Shown in Table 1 are representative compounds and ions

of oxidation states 0 to +8 of the PGM, and in Table 2, common

halo-ions of these metals.

Aside from a wide range of oxidation states, the

chemistry of the noble metals is further complicated by the

wide variation in kinetics found among such metals. Ligand

exchange reactions of the type:

MX + L -> MX , + X m m-1

are relatively slow for the noble metals. For many transition

metals reactions of this type are extremely rapid. For exam­

ple, the equilibrium for the reaction

Cu(H20)^+ + 4C1" "• CuCl^" + 6H20

is reached almost instantly.

The combination of kinetic factors, wide range of

oxidation states, and the variety of complexes formed, offers

the analytical chemist a variety of approaches by which he

Table 1. Some representative compounds and ions of the PGM .

Oxidation State Pd Pt Ir Rh Ru Os

0 Pd(CNR)2 Pt<ra3>4 Ir(NH3)5 Rh2(CO)g Ru(CO)c D Os(CO)5

+1 Rh2(CO)4Cl2 Os(NH3)6Br

+2 PdO,PdCLg PtO Ir(NH3)4Cl2 [Rh(dipy)2Cl]+ Os(CN)g"

+3 Ir2°3NH2° [RhCl2en]2 [rU(nh3)5ci]2+ Os(NH3)6Br,

+4 Ir02 Ru°2 OSgOClg

+5 RuF5

+6 PtF„ 6 IrF6 RhF

6 RuF6 0sF6

+7 RU°- OsOFc D

+8 Pt(CO)Fg RU°4 0s04

a. See Inorganic Chemistry, A Guide to Advanced Study, p. 712, by R. B. Heslop.

o«

Table 2. Some halo-complexes of the ct

platinum metals.

•

Oxidation State Pd Pt Ir Rh Ru Os

4-+2 PdCl~; 4

PtClT 4

IrBrc 6

p<ii° PtI4

+3 lrF6~ . tRh(H20)nC16--„ln~3 RuFe~ OxCl3"

IrCl^" RhClj!" 6

6

IrBre"

+4 PdC16~ Ptcie~

[Ir(H20)nCl 6-nln~2 RhF6~ RuF6~ OsCl^~

6

• PdF6~ PtBrs" IrF6~ RhCl^" 6

RuCl^" 6

+5 ptF- IrP6 RhF" OsF"

a. See Inorganic Chemistry, A Guide to Advanced Study, p. 714, by R. B. Heslop.

5

can carry out the separation of the noble metals from the

transition base metals, and in turn from each other.

Extraction techniques which are presently used for

the separation of the noble metals from other base metals or

from each other are, apparently, not highly selective. Os

and Ru, however, form stable tetroxides, and may be selec­

tively extracted into chloroform or carbon tetrachloride^

from acidic solutions. Other more selective methods for the

extraction of the platinum metals involve the use of chelating

agents.

The chemistry of many of the separational schemes is

not entirely obvious. An attempt is, therefore, made here to

briefly review the solution chemistry of the PGM. Thermody­

namic differences among the base metal ions is the basis upon

which wet chemical separations of these metals is achieved.

The noble metals, on the other hand, possess drastically

different characteristics, and separations are carried out

utilizing differences in kinetic, as well as thermodynamic

behavior among these metals.

"Nobility" of the PGM is governed by a number of

kinetic and thermodynamic factors:

1. The unfavorable potential of the reaction (see Figure

1) for electrochemical potentials):

M° •* Mn+ + ne

6

- 0 . 6 2 -PdC14 -1-288 Pd02 -1.2

Palladium Pd o -0.987 -Pd 2+

PdCl ?_

PdO,

-0 .01 Pd(OH),

Plat in urn

IN°BAIE Pt(°H)2 -0.1 to -0.4 Pt(OH) - 2

Pt' -1.2 -Pt 2+

PtC

-0.73 PtCl" -0.68

, 2 -x6

+0.95 -PtS

Rhodium

IN acid -0.44

RhClg—ca"1-2—RhClg"

Rh zO^Rh+^0^Rh2+_^Rh3+^li4_RhC)2+_zl^_Rho:

i -0 . 8 !

IN base

Rh "°-04 ' Rh203 Rho2 RhO^

IN acid solution

>-1. 1T..2+ >-1.1 i ir 1

Iridium Ir (<~1'Io^ lr3+ =^1 irO_ >"1'3 IrO= * r

-0.93

{ ' "°'77 IrCla" ~1-017 irci" 6 6 T _ 4 -0.99t „ 3-I r B r g — I r B r g

IN basic solution

Ir -0-1 lr203 Iro2_^°J_lrO*

Figure 1. Electrode potentials for PGM in acidic and basic solutions (data obtained from Oxidation Potentials, 2nd Ed., by Windell M. Latimer).

Rutherium Ru ~°-45 ru++—<L-l_RuC1_ 1,3 RuClcQH ~1,75 RuO,,—^-RuO,—^-RuO,

-0.4 -0.6 -1.5"

-1.25

IN acidic solution

Os —0s2+ ——OsCl? —OsCl? —OsO.(S)

-0.71

6

-0.97

-0.85

Osmium IN basic solution

9 9 Os 5 ®s2^3 0s0r

0.15

-0.1 « -0.3 7T/*\ OsO. HOsO, 4

- 0 . 2

Figure 1. — Continued

8

2. Subsequent formation of an inert layer of oxide or

chloride on the surface of the metal after dissolu­

tion.

3. The slow rate of oxidation, as in (1).

Factors (2) and (3) are dominant in the case of secondary PGM

(Ru, Os, Ir, Rh), whose dissolution in chloride medium

(4-6M HC1) takes place at a much slower rate than Pd or Pt.

Oxidation States

A summary was given in earlier pages on the possible

oxidation states that these metals can attain, and the types

of ligands that would stabilize such oxidation states.

Separation of PGM is to a great extent dependent on mainte­

nance of certain oxidation states for these metals, as evi­

denced by the example shown below. Kinetic effects play an

important role in the separation of the noble metals. Ir(IH)

and Pt(IV) behave differently in aqueous media, and use is

made of this difference in chemical properties for the purpose

of separating these two. The reduction potentials of the

following half reactions:

Pt(IV) + 2d -»• Pt(II) E = +0.77

Ir(IV) + e "• Ir(III) E = 0.87

are fairly close, and it would not seem feasible to selectively

reduce Ir(IV) to Ir(III) for the purpose of separating these

9

two metals from each other. In practice, however, the equi­

librium between the two oxidation states of iridium is estab­

lished almost instantly, whereas for platinum the reduction

is slow, and hence a clean separation of these two can be made

possible.

Kinetic Effects

As stated earlier, ligand exchange reactions of the

type

MX + L t MX , + X m m-1

are relatively slow for the PGM. On the other hand, many

transition metals undergo very fast ligand exchange reactions.

The rate of ligand exchange for each ion is nearly equal to

the rate of water exchange for that ion (approximately slower

by an order of magnitude). Among the noble metals, equiva­

lent ligand exchange equilibrium would take hours for Pd(II)

and years forPt(IV). Generally kinetics for substitution

reactions for the PGM vary as shown below:

Os(IV) Pt(IV) Ir(IV) Inert

Pt(II) Pd(IV) Ru(IV) Moderately inert

Ru(III) Rh(III) Ir(III) Os(III) Fairly rapid

Pd(II) Extremely rapid

For a noble metal, in an oxidation state which is not

10

amenable to the fast ligand exchange kinetics (i.e., Ir(IV)),

the presence of small amounts of another, or the same metal

in an oxidation state, amenable to rapid ligand exchange, may

enhance the rate of ligand substitution for the first. The

following example demonstrates the preceeding argument for

Pt(IV) and Pt(II):

1. Pt(IV)(NH3)4Cl2++Cl" sloW'Pt(IV)(NH3)4^l-Cl+Cl"

2. Pt(II)(NH3)4++Cl ->• Pt(NHg)4$l+(fast pre-equilibrium)

3. Pt(IV)(NH3)4Cl^+ + Pt(II)(NH3)4$l+ +

H3N\ ̂ 3 H3V * 3+

ci —^.p<"— ci - - — ci -»•

NH3 h3n nh

3

Pt(IV)(NH3)4^lCl2+ + Pt(NH3)4Cl+; followed by:

4. Pt(II)(NH3)4Cl+ + Cl~ -»• Pt(NH3)4^l+ + CI" .

Reactions (1) and (2) demonstrate the inertness and lability

of ligand substitution reactions for Pt(IV) and Pt(II), respec­

tively. The presence of a small quantity of Pt(II)(NHg)4$l+

can, apparently, enhance the ligand substitution kinetics for

Pt(IV), as shown in (3).

A brief review of the coordination chemistry of

platinum and palladium in their (+2) and (+4) oxidation

states follows.

Platinum and palladium form complexes in a range of

oxidation states from (0) to (+5). Principal oxidation

states for these in aqueous media are (+2) and (+4) and (+1)

where there exists metal-metal bonding. In the (0) state

it-complexes with carbonyls and tertiary phosphines, also

clusters of the type Ptg and Pt^ are formed.

The (+2) State: Pt(II) and Pd(II)

O Complexes formed have a d configuration. They are

usually square-planar and diamagnetic. Pt(II) and Pd(II)

have a tendency to form stable complexes with "soft" bases

containing sulfur, phosphorus nitrogen, arsenic, and the soft­

est in the halogen series (iodide, bromide and chloride, and

with N-donor ligands as in aliphatic amines and NOg. The

high stability of these complexes is in main, a consequence

of large orbital overlap between the empty outer shell

orbitals of the heavy atoms with filled dir orbitals (dxz,

dyz, dxy) of the metal ion.

Pd(II) and Pt(ll), generally form complexes of low

stability with oxygen-containing ligands and with fluoride.

There exists, however, various y-OH dimers and trimers of

2 unusual stability.

12

Pt(IV) and Pd(IV) Complexes

These are invariably octahedral, and have the same

arrangement of ligands in the equatorial positions as with

the square-planar complexes that Pd(II) and Pt(H) form.

Complexes of the type M(IV)(X) (NHQ)l~n are common, where n o b-n

m:l-6; and X = Br,- CI, NOg, SCN. Other nitrogen donors which

are found in such complexes include hydroxylamine, ethylene-

diamine and hydrazine. The hexa-chloro or hexa-bromo com­

plexes are the more stable. The Pt(IV) complexes, apparently,

are a great deal more stable and kinetically inert than those

of Pd(IV).

Complex Formation of Palladium with Chelating Agents

In the following few pages a summary is presented on

the complex formation of palladium with various chelating

agents, some of which are used in extraction of this metal.

Palladium forms stable complexes with NTA (nitrile-

3 triacetic acid; log$^ = 19.5, and EDTA (ethylene diamine

3 tetraacetic acid; log$^ = 18.5, in aqueous solution, where

the amino nitrogen and the carboxyl oxygen in these ligands

are used in bonding with the metal. EDTA forms a 1:1 chelate

with Pd(H) and can occupy two or four of the coordination

sites of the planar Pd(II) complex.

A variety of commercial reagents of the hydroxyoxime

13

type have a large capacity for palladium. These are divided

into two groups, a and 3 hydroxyoximes. In B-hydroxyoximes

the hydroxy1 group is phenolic. Both a and $ oximes have

high distribution coefficients for Pd at moderately high

2 acidity (D * 10 ; 0.5M HC1). Examples of a and B hydroxy­

oximes include LIX 63 for a-oxime and LIX 65N and LIX 70 for

8-hydroxyoximes. These reagents provide fairly good selec-

4 tivity against Pt (separation factor of >100).

Dimethylglyoxime (DMG), normally used for extraction

2+ of Ni from an ammoniacal solution is used to extract Pd

5 from acid solution. The chelate thus formed dissolves in

alkali media, probably due to the formation of hydroxocomplex.

DMG can extract Pd(II) from 0.2 to 0.3M HCL or 1.0M HgSO^

solution. This reagent can be used to separate Pd(II) from

the rest of the PGM.

8-Hydroxyquinoline (oxine) and its derivatives are

also used to extract Pd(II). Palladium can be extracted

quantitatively from pH 0 to 10 into chloroform with a solution

g of 0.01M oxine in CHClg. The kinetics of extraction are

fairly slow, however. The complex absorbs at 425 nm in chloro­

form.

B-Diketones form six-membered ring chelates with

Pd(H) and are used to extract this metal. Acetyl acetone

(acac) can extract Pd quantitatively in a pH range of 1.5 -

14

7 8 10. Thenoyltrifluoroacetone (TTA) extracts palladium quan­

titatively from pH 4.5 to 8.8 into n-butanol, methyl propyl

ketone and benzene.

Thiooxine (8-Mercaptoquinoline), being a sulfur donor,

forms a stable complex with Pd(II), and can extract this

metal quantitatively from a highly acidic solution (6B 4C1)

g into chloroform. Dithizone forms an incredibly stable

chelate with palladium and can extract this metal quantita­

tively from acid solution (~9M HgSO^) into chloroform. The

only metal ions that can be co-extracted from highly acidic

solutions are Pt(II), Hg(II), and Au(IH).

Thiourea forms a stable, water-soluble complex with

palladium. The complex is formed from an acid solution. It

can be used for the spectrophotometric determination10 of

this metal. Thiourea is used to precipitate Pd in a weakly

alkaline solution for gravimetric determination of this metal.

Thioureaa complex of palladium is decomposed by the action

of sulfuric acid and with subsequent formation of the sulfide

which can be used for the purpose of gravimetric determina­

tion.

Diethyldithiophosphoric acid forms a yellow precipi­

tate with Pd(II) in neutral or acidic media. The reagent can

be used for the gravimetric determination of Pd(II). The

precipitate once formed is insoluble in concentrated sulfuric

Q 12 Table 3. Chelating agents for the extraction of PGM. '

Metal Reagent Aqueous Phase Solvent

Pd(II) PAR 8N H2S04 CC14, CHClg

PAN pH 2-5, w or w/o EDTA CHClg

Salicylaldixime pH 3 4-methyl-2-pentanone CHClg or CgHg

Diethyldithinocarbamate pH 11 + EDTA CC14, CHClg or CgHg

a-nitroso, B-maphthol pH 1-2, w or w/o EDTA CgH , toluene or isopentanol

Pt(H) dithizone 1:3 HCl or 1-10.5N H2SO4 with SnClg

CgHg, CC14, CHClg

diethyldithiocarbamate acid solution C6H6

Pt(IV) TTA 5-9N HCl 2:1 mixture of n-butan< and acetophenone

1,1 diantipyrinylbutane 4-6N HCl 1,2 dichloroethane

Os(IV) catechol pH 3 CHClg

1,1-diantipyrinyl butane 4-6N HCl 1,2 dichloroethane

Ru a-nitroso, B-naphthol solution containing ascorbic acid

CC1. 4

Table 3. — Continued

Metal Reagent Aqueous Phase Solvent

Ru(III) Oxine pH 4-6.4 n-propanol, CHC1 or

C6H6

Ru(VII) uetraphenyl arsonium chloride

0.0IN NaOH CHClg

Ir(IV) Tetraphenylphosphonium bromide

- 0.1N HC1 CHClg

Rh(H) 2,Mercapto-4,5-dimethyl thiazole

3-9N HC1 CHClg

Rh(IH) TTA acetic acid/sodium acetate buffer

C6H6

diethyldithiocarbamate pH 8 4-methyl 2-pentanone

a. An elaborate discussion on the spectrophotometric methods for PGM is found in the Analytical Chemistry of the Noble Metals, Vol. 24, Pergamon Press, by F. E. Beamish.

17

or hydrochloric acid. The chelate can also be extracted

from acid solution into organic solvents (eOQC = 3.1 x 104,

e340 = 3.6 x 103).

Other sulfur donor-ligands used to extract Pd(II)

include, thio-TTA, and 5-sulfothiooxine. These can extract

palladium in a relatively lower pH range, and form chelates

with higher absorptivities than the corresponding ligands,

coordinating through oxygen and/or nitrogen.

The following table (Table 3)"^ lists many of the

chelating agents used for the extraction of platinum metals;

many of these, particularly oximes, sulfur-containing ligands

and azo compounds, are used to extract palladium and platinum,

and only a small number are used to extract other noble

metals.

The Structure of Dithizone

It would seem from the visible absorption spectrum of

dithizone in various organic diluents that it exists in two

tautomeric forms. Two distinct bands are seen in the visible

spectrum of the reagent; the one in the lower wavelength

region is ascribed to the enol form, and the one in the long

wavelength region, the keto form. The assignment of the

~445 nm (in CHClg) band of dithizone to the enol form could

be supported by the fact that the formazans (i) have a charac­

teristic absorption at ~420 nm; also the anion of the reagent

18

(HDz~) has a maximum absorption at 470 nm. Upon chelation,

however, as with many other sulfur or oxygen-containing lig-

ands, there is a shift toward the longer wavelength region of

the thiol band of the reagent anion. Evidently most metal

dithizonates have their maximum absorption in 440-550 nm

region. Palladium chelate (2:1) has two absorption maxima

(445, 637 nm in CHClg), with a shoulder band at 565 nm.

These are probably due to charge transfer as with nickel

13 dithizonate. In the palladium chelate there is a batho-

chromic shift even beyond the maximum at 605 nm of the thione

band of the reagent alone.

The whole idea of keto-enol tantomerism of dithizone

in solution could be supported by the fact that the peak

ratio of the two absorption bands of dithizone in solution

is not a constant and changes with the type of solvent used;

i.e., R = 2.59 in CHClg and 1.09 in n-hexane. If dithizone

in solution existed in only one,form, and the two peaks

observed in the visible spectrum originated from a single

structure, then, depending on the solvent type, the peak

heights would either increase or decrease, but the peak

ratio should remain constant. The very fact that there exist

two distinct bands with different peak ratios depending upon

the solvent used, would suggest the existence of two differ­

ent species absorbing at different wavelengths.

19

Recently, however, other workers have concluded

14 based on spectroscopic investigations, i.e., NMR and

15 Raman , that structure (II) is the only form of dithizone

in solution.

•From the preceeding arguments, it would seem apparent

that the structure of dithizone in solution is yet uncertain

and a complete understanding of the behavior of dithizone in

organic solvents would have to await accumulation of more

data.

Nik H - - s • < /* X I yV «. \y\/

" • < d x = a n

The Primary (2:1) Metal Dithizonates

Primary dithizonates vis-a-vis secondary ones are

the most important from an analytical viewpoint. They are

formed by metal-sulfur, metal-nitrogen bond formation.

M-S, M-N bonding is evidenced by a large body of X-ray crys-

16 17 tallographic data obtained up to the present. ' It has

20

been realized through three-dimensional X-ray determinations

that the structures shown on the following page are those of

the primary dithizonates with some divalent metal ions.

Wherein (III), Zn(II) and Hg(II) are tetrahedrally coordina-

Q

ted through nitrogen and sulfur atoms, and in (IV) the d

metal ions [Ni(II), Pt(II), Pd(II)] are bonded to N and S

atoms in a square planar configuration. The nickel complex

is planar due to conjugation, the phenyl rings nearest to the

metal ion and sulfur atoms, however, are twisted out of plane

by some 64° due to steric repulsion between them and the

neighboring sulfur atoms. Depending on the oxidation state

of the central metal ion, metal chelates of different stoi-

chiometry can be formed; for example, Co(III) and Bi(III) form

CO(HDz)g and (Bi(HDz)g, and with mixed complexes such as

Bi(HDz)2Cl-2H20, and for Au(III),

It is known that metal ions in oxidation states higher

than (+3), i.e., Pd(IV) and Pt(IV), do not form extractable

dithizonates. However, in light of the "hardness" of these

metal ions, with inherent inertness toward ligand substitution

as a consequence of all inner d-orbitals being, at least,

singly occupied and hence not available to accommodate ligand's

lone pairs of electrons, their reluctance toward forming

stable dithizonates is understood.

21

« "X rvV\ vv \,J hi-

rV \ ' n ) )K VS/VJ H / V f c

2+ pd^+ pt2+ III (M = Hg(II) or Zn(II)) IV (M = Ni ' Fa ' Ft )

Secondary Metal Dithizonates

These are almost invariably formed only with certain

metals when there is an excess of metal present. Their

low solubility in organic phase suggests that they are poly-

18 19 meric species. ' Due to the observed (1:1) stoichiometry

of the many of the secondary or "enol dithizonates," it was

assumed that the imino-proton was dissociated upon chelate

formation. This, however, would seem unlikely since many of

these so-called "enol-dithizonates" are formed in acidic

solutions. For example, Pd-chelate (1:1) can be formed from

2M HgSO^, and it is observed that dithizone itself does not

20 lose its imino proton below pH 14.

21 22 As postulated by many workers, ' Cu(II), when

present in excess of the reagent (dithizone) formed the enol

22

or the secondary (1:1) complex with the removal of the imino

18 proton upon chelation. Freiser and Freiser, however, dis-

2*3 puted this argument, and subsequently Brand and Freiser

proved, clearly, primarily by photoelectron spectroscopic

measurements, that the so-called "enol Cu(II)-dithizonate"

is, in fact, a Cu(I) species. This posed a challenge to the

accepted view, that in the case of the copper chelate the

imino proton is replaced by the metal ion. The implications

here are that there would be no need for the removal of the

18 N-attached hydrogen, and a polymeric species of 1:1 stoi-

chiometry could result. The coordination requirements of

copper in such a 1:1 complex in the organic phase may be

satisfied via intermolecular bonding.

An alternative explanation for the observed (1:1)

stoichiometry for secondary dithizonates other than copper,

would involve the formation of the mixed complexes of the

type XM(II)HD (X = Cl~, N0~, etc.) where there would be no z o

need for the dissociation of the imino proton upon the for­

mation of the secondary dithizonates. Mixed Hg(II)

24 complexes such as ClHg(II) HDz are known.

Despite the preceeding discussion, however, it is

concluded that the true structure of the "enol complexes" of

metal dithizonates will be revealed only by a complete X-ray

structural determination.

STATEMENT OF PROBLEM

In this study, dithizone, a sulfur donor ligand, was

used to extract palladium. The effect of pH, reagent concen­

tration and masking agent concentration on the extraction

behavior of palladium was studied.

The aim of this work was to gain some insight into

some of the fundamental properties of the palladium-chelate

chemistry in aqueous-organic solvent system. Such fundamen­

tal studies are essential in order to better understand

existing analytical methodology and to develop new methods

in the field of solvent extraction chemistry.

23

EXPERIMENTAL

Materials

A.R. grade reagents were used for all experiments.

Reagent grade dithizone, obtained from Eastman Kodak, was

further purified (see section entitled Procedure for Purifi­

cation of Dithizone).

Stock solutions of PdClg (Alfa Inorganics) were pre­

pared by dissolving weighed amounts of the salt in de-

ionized water with small amounts of HC1 added to facilitate

dissolution. Solution was standardized as described in

Procedure.

Chloroform, purchased from Fisher Scientific, was

used without further distillation. A small amount of ethanol

present in chloroform was found to suppress the rate of decom­

position of dithizone in chloroform. All other reagents were

used without further purification.

Apparatus

A Perkin Elmer model 552, UV-vis spectrophotometer

with a Perkin Elmer, model 261 chart recorder, and 1-cm

matching quartz cells were used for all absorbance measure­

ments and obtaining of spectra. pH of solutions was measured

with an Orion Research model 701 digital pll meter. An

24

V

25

Eberbach shaker, with a shaking speed of 280 oscillations per

minute was used for the purpose of mixing small volumes of

3 organic and aqueous phase placed in 44 cm extraction vials

(28 mm o.d., 108 mm in height, obtained from VWR Scientific).

The extraction kinetics apparatus (Figure 2) consisted of a

500 ml Morton flask, equipped with a high-speed vacuum

4 stirrer (0-2 x 10 rpm), purchased from Cole Palmer Instru­

ment Company (#4660, 4666). Distilled water was further

de-ionized in a crystal deeminizer (Crystal Research Labora­

tories) .

Procedure

Purification of Dithizone

_3 A stock solution of dithizone (5 x 10 M) was prepared

by dissolving 0.128 g of dithizone in 100 ml CHCl^ solution.

15 to 25 ml of this solution were then pipetted into a

separatory funnel and equilibrated with several portions of

NH^OH solution (~0.5M). The aqueous portions containing

dithizone (anionic form) were then combined and placed back

into a separatory funnel; the total aqueous phase volume was

about 50 ml. Approximately 50 ml of A.R. chloroform with

10 ml of 6M sulfuric acid were pipetted into the separatory

funnel. Upon shaking for about ten seconds, the dithizone

was back extracted into chloroform. The organic phase was

separated and traces of water removed by adding a small

26

c

Figure 2. High-speed stirring-extraction apparatus. — (A) High-speed motor; (B) Stir shaft; (C) Nitrogen inlet; (D) Sample outlet; and (E) Morton flask.

amount (~1 g) of anhydrous sodium sulfate to the chloroform

phase. The solution was then made up to volume (100 ml) with

CHClg. Concentration and purity of this stock solution were

then determined as follows. Absorbance of a diluted solution

of dithizone was measured, and using a molar extinction coef-

27 -1 -1 ficient of 40,500, M cm , the concentration in the orig­

inal stock solution determined. The ratio of absorbance

values at 605 nm and 445 nm serves as a measure of purity of

a dithizone solution in chloroform. A ratio of 2.59 repre-

20 sents a solution of >99% purity.

Standardization of Pd(II) Solution by EDTA-Indirect Titration Method, and by the Iodide Spectrophotometric Method

25 In the EDTA method, a small excess of disodium salt

_3 solution of EDTA (1.07 x 10 M) was added to a specified

volume of Pd** (3-4 mol of 1 x 10~^M). The pH of the solution

was adjusted to 10 + 0.10 with a HC0~/C0g buffer. Four drops

of the EBT (erichrome black T) indicator (0.2%) were added

to this solution. The solution was then titrated with a

-4 standard solution of Zn(NOg>2 (9.55 x 10 M) and the appear­

ance of a tinge of pink that persisted indefinitely marked

the end-point.

26 In the iodide method, various amounts (1-5 ml) of

_3 the Pd stock solution (1 x 10 M) were equilibrated with

- 2+ excess iodide (Nal), (I /Pd - 12,000) to ensure formation

of the orange-iodo complex (Pdl~). All solutions had a pH

28

of 3.6. Absorbance measurements were made at 406 nm, e =

10,500; and at 319 nm, e = 15,800 M~^cm~^.

Preparation of the Primary and the Secondary Palladium Dithizonates

The primary (2:1) palladium chelate, in the absence

of a masking agent (i.e., SgOg) was prepared by adding an

excess of a purified dithizone solution in chloroform

(HDzQ/Pd >_ 2.2) to an acidified solution of palladium

(pH < 1). Sulfuric and/or hydrochloric acids were used for

acidification. The two phases were then shaken for an

appropriate period of time using a mechanical shaker. Acidity

of the solutions was adjusted using hydrochloric or sulfuric

acids, or with various buffers according to the choice of the

masking agent used. For example, with the use of thiosul-

phate as the masking agent the pH of the solution was kept in

the region of 7 to 10.5

The secondary palladium chelate was prepared from

moderately acidic solutions (i.e., 1.2M HC1 or 0.5M HgSO^).

In this case an excess of palladium was used. Palladium to

dithizone ratio was varied from 10 to about 70 and'changes

in the absorption spectra of the chelate, noted.

Distribution Studies

For all equilibrium studies, mixtures were prepared

by adding 15 ml of chloroform phase and 10 ml of the aqueous

29

phase to the extraction vials, and shaking for an appropriate

period of time. The two phases were then separated, and

subsequent measurements were performed on each phase.

Studies on the kinetic effect of chloride on the rate

of extraction of palladium were made as follows: 100 ml of

the organic phase (dithizone in CHClg) and 100 ml of the

aqueous phase, containing palladium and the masking agent

(chloride) were carefully added to the Morton flask, minimiz­

ing agitation of the solutions. The two phases were then

4 stirred, at high speed (~10 rpm) (Figure 2). 5 to 10 ml

samples were removed, by purging the reaction vessel with

nitrogen gas, at 15-second intervals. The organic phase was

washed with ammonium hydroxide (0.5M) to remove excess dithi­

zone, followed by measuring the absorbance of the organic

phase at 637 nm.

RESULTS AND DISCUSSION

Preliminary Experiments

Some preliminary experiments involving primary and

secondary palladium chelates were performed. From a plot

of absorbance the ratio of the amount of dithizone to the

amount of palladium present (Figure 3), a stoichiometry of

2:1 was deduced for the primary chelate formed from acid

solution (0.2M HgSO^) in the presence of 0.3M chloride.

Evidently high chloride concentration did not affect the

stoichiometry of the chelate formed. A value of (3.41 + .20)

4 -1 -1 x 10 M cm was calculated for the molar absorptivity of the

2:1 chelate in chloroform (637 nm).

The secondary palladium chelate was prepared from a

1.2M HC1 solution by having a Pd/HDz mole ratio of 70 to 1.

Increasing the concentration of Pd beyond this ratio did not

cause a change in the absorbance of the chelate formed. A

3 -1 -1 molar absorptivity of (25.2 + 0.2) x 10 M cm was calcu­

lated for the secondary palladium chelate on the basis of

assumed 1:1^® stoichiometry (482 nm in CHClg).

Effect of V (Organic Phase Volume) on the Precision of Measurements

Some initial distribution studies were conducted using

30

31

.60

• • m • • •

•

•

.20

.10 •

JZL 1 .0 2.0 5 .0

HDz/Pd

4.0

Figure 3. Determination of stoichiometry for the primary Pd-dithizonate in 0.2M H^SO^, at nm :i"n ^HCl^,

32

equal volumes (10 ml) of the organic and the aqueous phase.

In all cases the absorbance of the chelate formed in the

organic phase was measured. It was found that due to the

time lag from the point of removal of the aqueous phase, fol­

lowed by the addition of anhydrous sodium sulphate for drying,

to the point when the absorbance measurement was made, that

some chloroform would evaporate and hence, reproducibility of

the measurements would suffer a bit. It was then decided to

increase the volume of the chloroform phase to 15 ml, and all

subsequent absorbance measurements were found to be reproduc­

ible to within + 0.003 abs. units.

Effect of Shaking Time on Extraction of Palladium

Varying the ratio of the ligand to metal concentration

seemed to affect the equilibration time, notably (Figure 4).

In all subsequent experiments involving the use of masking

agents such as thiosulphate, sulphite, etc., a 24-hour shaking

time was found to be sufficient to reach equilibrium. However,

a minimum 36-hour shaking time was allotted to ensure complete

equilibration.

The distribution of a particular metal ion, in its

simplest form, between the aqueous and the organic phase,

is expressed as:

(1) Mn+ + nHL t ML , N + nH+

' o n(o)

33

5.5 •

4.5 T3 ft

N

S

3.5

• 2.5

• m

"O 7T0 12 V? 22 27 Equilibration Time (hrs)

Figure 4. Equilibration time as a function of amount of ligand present in solution. --[Pd2+] = 1.978 x 10~^M

34

or [HL]

(2) D = K — ex [H ]n

where D is the distribution constant of a metal ion of charge

n, defined as

(3) d = Tm L aq.

K is the extraction, or the equilibrium constant of reac-

tion (1); [HL]o is the concentration of the free ligand in

the organic phase.

In a more complicated situation where the distribution

of a metal ion is not as apparently obvious as (1) would sug­

gest , and where a masking agent is present at some concentra­

tion in the aqueous phase, where it forms a water-soluble

complex with the metal ion, then a new constant can be defined

with an additional term included in the expression.

Let

(4) D' = f[(HL)o, (H+), (X)],

where (X) is the concentration of the masking agent in the

aqueous phase. Expressed more concisely,

(5) D' = K^x[HL]°[H] P [ x ] S

where K' is the conditional extraction constant of a metal ex

35

with a particular reagent. A change in each one of the vari­

ables in (5) should then accompany a change in D1.

The very high stability of palladium dithizonate

prompted experiments to be carried out to observe, via spec-

trophotometric (absorbance) measurements, changes in the

distribution of palladium between two phases. Experiments

were performed where the reagent concentration in organic

phase, hydrogen ion concentration, and the concentration of

the masking agent were varied, separately, in order to

measure a change in the amount of palladium extracted.

In pages to follow, the effect that each of the

variables mentioned has on the distribution of palladium

between aqueous phase and chloroform, is studied.

Effect of Varying Concentrations of Sulfuric Acid on Extraction of Pd

It is evident from the results shown in Figure 5 that

increasing acidity of the solution from 0.20M to 9M sulfuric

acid had virtually no effect on percent extraction of the

palladium chelate. The aqueous phase consisted of 1.98 x

_ 5 10 MPd(II) with a modest amount of chloride present (8.8 x

-5 10 M) as PdCL2 and some HC1 initially added to the palladium

stock solution. The organic phase contained dithizone in

total amounts varying from a mole ratio of HL/Pd = 5.4 to 3.0.

In all cases, percent extraction remained greater than 99.

a*

36

.50 1 .5 2 .5 5 .5 4 .5 5 .5 6 .5 7 .5 8.5 9 .5 Sulfuric Acid Concentration

Figure 5. Effect of sulfuric acid on extraction of palladium

37

The nature of the experiments was such that no change in D'

(distribution constant), when all values of D' were above

99, could be measured accurately.

Effect of Various Masking Ligands, under Specified Conditions,

on the Extraction of Palladium

Throughout the course of this study, experiments were

conducted to obtain conditional extraction constants in the

presence of varying masking agent concentrations. Of the ones

tested, only thiosulphate had an observable effect on the

extraction behavior of palladium. As for the others, chloride,

bromide, iodide, sulphate, sulphite and EDTA, none had any

observable effect on extraction of palladium. Palladium was,

in all cases, greater than 99% extracted.

Pdl~ with a logB^ of 24.5, y = 1.0, would be the most

stable complex in aqueous phase yis-a-vis the complexes formed

with other masking agents tested except thiosulphate. In a

solution of 0.293M I~(NaI) and 0.30M H^HgSO^), Equation (5)

was used to calculate a minimum conditional extraction con­

stant (logK = 11.39) for palladium-dithizonate in aqueous-6X

chloroform system. Of course, with the other masking agents

tested, Cl~, Br-, etc., the value for the conditional extrac­

tion constant would be smaller.

Thiosulphate, among all the masking ligands tested,

was the only one that had a significant effect on the extrac­

38

tion behavior of palladium. In the presence of thiosulphate,

changes in the distribution constant (D') as a result of

varying the concentration of any of the variables, for example

that of the reagent, above stoichiometric (2:1) levels, could

be monitored spectrophotometrically. The effect of thiosul­

phate as a masking agent is discussed in this section.

The Effect of Varying pH on LogD', in the Presence of Thiosulphate

It was found that thiosulphate could only be used in

a range of (7< pH < 10.5); below this range thiosulphate was

found to decompose into elemental sulphur and possibly ions

29 = = with sulphur in a higher oxidation state (i.e. , SO^, SO^

^4^6' e*-c-)- Above a pH of about 10.5, distribution values

were somewhat low, possibly due to formation of hydroxo

complexes of palladium.

pH of solutions was varied from 6 to 10.4 and a plot

of log D' versus pH was obtained (Figure 6). See also Table

4. It is evident from Figure 6 that below pH of ~7.2, low

values of logD' are obtained. The presence of traces of free

sulphur was noted. A line of slope zero was obtained in the

range (8 < pH < 10.5), which would indicate that the distribu­

tion of palladium is independent of hydrogen ion concentration

in this pH range. This, however, can be explained by the

following mechanism:

Table 4. Effect of pH on the distribution of palladium. — In the presence of 7.0 x 10~2m thiosulphate.

Abs. LogD' PH

0.090 -0.602 6.25

0.103 -0.527 6.72

0.240 0.058 7.40

0.269 0.172 8.80

0.270 0.176 9.17

0.269 0.172 9.31

0.271 0.180 9.40

0.268 0.168 9.97

0.265 0.156 10.20

40

.20 0 G3GP m m

m

o.o

- .20 . . Q fcUD O J

- .40 . .

•

- .60 . . 0

6.5 7 .5 8.5 pH

9.5 10

Figure 6. Effect of pH on extraction of palladium. — In the presence of thiosulphate (7 x 10 M).

41

(1) [Pd(0H)2S203r + 2HDZO t 2PdDz2(o) + 2H20 + S^

where no protons appear on either side of the equation.

Effect of Varying Thiosulphate Concentration on LogD'

By keeping a constant pH, and varying the thiosul­

phate concentration, plots of logD'-21og[HDz] versus

-log[S20~] at pH 7.40 + .02 and pH 9.86 + .02 were obtained

(see Figures 7 and 8, and, correspondingly, Tables 5 and 6).

At pH 7.40 a slope of ~2 indicated that two thiosul­

phate anions were involved in the reaction as postulated in

the following mechanism:

(2) Pd(S203)= + 2HDz0 t PdDZ2(o) + 2H+ + 28^

This is reasonable, since the pH is low enough that

any hydroxo complex is replaced by the more stable thiosul­

phate complex of palladium. At pH 9.86, however, a slope of

1.15 (Figure 8) indicates that only one thiosulphate anion

is involved in the reaction; and by adhering to the coordina­

tion requirements of Pd(II), the following reaction is likely

to take place:

(1) [Pd(0H) 2S 20 3 ]= + 2HDZQ Z Pd(Dz)2(Q) + 2H20 + SgOg

Since not a large excess of dithizone was used in

these experiments, varying the concentration of thiosulphate

Table 5. Effect of thiosulphate concentration on the distribution of palladium at pH 7.40.a — [Pd2+] = 1.978 x 10~^M; ionic strength = 0.5M.

VHDz Used CHDzq Log(HDz) LogD' (ml) 10+4(M) (free ligand) "log(S2°3) logD'-21og(HDz)Q

0.191 -0.132 4 1.076 -4.757 0.981 9.382

0.227 7.72xl0~3 4 1.046 -4.836 1.141 9.680

0.240 5.80xl0~2 4 1.060 -4.848 1.157 9.753

0.272 0.184 4 1.07 -4.900 1.282 9.984

0.294 0.275 4 1.06 -4.957 1.388 10.189

0.300 0.301 4 1.06 -4.971 1.418 10.289

Slope: 2.041

Y-intercept: 7.373

Correlation Coefficient: 0.9984 _2

Standard Deviation of Slope: 5.476 x 10 _2

Standard Deviation of Intercpet: 2.036 x 10

a. See Figure 7.

43

10.4

a

10.z a

~°10.0 N a

bC 0 I—I N 1

o 9.8 buO O •J

9.6

9.4

o

1.0 1 .1 1 .2 1.3 1 * 4 1 .5

-Log(thiosulphate)

Figure 7. Effect of varying thiosulphate concentration on extraction of palladium at pH 7.40.

Table 6. Effect of thiosulphate concentration on the distribution of palladium at pH 9.86.a — [Pd2+] = 1.978 x 10~5M; ionic strength = 0.5M.

Abs. LogD 1

CHDz X VHDz Used log{HDz] o o Jo

10+^M (ml) (free ligand) -log[S203] LogD'-21og(HDz)o

0.220 -1.93 X o 1 to

9.895 5.00 -4.782 8.55 x 10-1 9.545

0.245 +7.74 X 10"2 9.533 5.00 -4.844 1.0314 9.765

0.274 1.92 X 10"1 9.918 5.00 -4.854 1.156 9.901

0.282 2.25 X 10_1 9.697 5.00 -4.886 1.253 9.998

0.311 3.50 X 10"1 9.609 5.00 -4.945 1.455 10.240

0.328 4.30 X 10"1 9.740 5.00 -4.963 1.554 10.356

Slope: 1.149

Y-intercept: 8.569

Correlation Coefficient: 0.9998

Standard Deviation of Slope: 1.46 x 10-2

_3 Standard Deviation of Intercept: 8.528 x 10

a. See Figure 8.

45

io.4

m

10.2 ID

10.0 •

m

9.8

•

9.6

m

.90 1 .1 1 .3 -Log(thiosulphate)

1.5

Figure 8. Effect of varying thiosulphate concentration on extraction of palladium at pH 9.86.

46

would affect the concentration of free dithizone in the

organic phase and hence a correction needed to be made when

studying the effect of thiosulphate on the distribution of

palladium, while keeping all other concentrations constant.

Since

D' = K^x[HL]n[H+]p[X]s

then

(6) logD' = logK' + nlogHDz + plogH + slog(X) CA O

Since the value of logD' was found to be independent of pH

in a range of (8 < pH < 10.5) then the above equation would

simply reduce to

(7) logD' = logK' + nlogHDz = slog(X) ©X o

S is the number of thiosulphate anions involved in the reac­

tion of palladium with dithizone. Since some measurements

were made at high pH of 9.86 (HC0~/C0~ buffered solutions),

a correction for the concentration of dithizonate anion in

the aqueous phase had to be made.

Effect of Varying Reagent Concentration on the Extraction of Palladium in

the Presence of Thiosulphate

Figure 9 and the corresponding table (Table 7) show

the effect of varying dithizone concentration on the distribu­

tion of palladium between two phases. All solutions were

buffered with a HC0~/C0g buffer, pH 9.86 + 0.02. A constant

thiosulphate concentration of 0.0698M was maintained. For

all experiments. As shown in Figure 9, a slope of ~0.5 is

obtained from a plot of logD' versus log[HDz]Q. Log D' and

log[HDz]Q values included in Table 7 are the averages in a

set of six experiments performed under identical conditions

— 5 (i.e., constant metal concentration (1.978 x 10 M, constant

pH 9.86 + .02 and constant thiosulphate concentrations,

6.98 x 10"2M).

The value of 0.5 for the slope suggests that a sub-

stoichiometric species of 2:1 stoichiometry (two metal ions

for each dithizonate anion) is formed. However, this is

highly unlikely since in all cases a significant excess of

dithizone was used, which would tend to form the normal 2:1

chelate (HL/Pd = 2). The absorption spectrum of the metal-

chelate formed in the presence of thiosulphate is identical

with the absorption of the normal 2:1 chelate of palladium-

dithizonate formed under acidic conditions (0.2M HgSO^) in

the absence of any masking agent. It is not possible at the

present time to offer an explanation for the results obtained.

Effect of Varying Concentrations of Chloride on the Rate of Extraction

of Pd via a High-Speed Stirring Technique

In the rate study experiments, the concentration of

chloride was varied, while maintaining all other concentra-

2+ tions constant. In all cases an initial Pd concentration

48

Table 7. Effect of varying ligand concentration on the distribution of palladium at pH 9.86.a

Abs. LogD' CHDZ o

VHDz Used o

(ml) LogHDZQ

0.385 0.7725 6.098 x : 10~4 5 -3 .828

0.379 0.727 8.50 X 10"4 3 -3 .915

0.363 0.620 8.86 X 10"4 2 -4 .099

0.328 0.4295 8.50 X 10-5 10 -4 .5115

0.349 0.5385 8.50 X •10-5 15 -4 .275

0.300 0.301 8.28 X 10-5 7 -4 .7615

0.274 0.192 8.50 X 10-5 5 -4 .966

Slope: 0.477

Y-intercept: 2.582

Correlation Coefficient: 0.9992 _3

Standard Deviation of Slope: 8.507 x 10 - 2

Standard Deviation of Intercept: 1.014 x 10

a. See Figure 9.

49

.80

.70

. 60 . .

.50 . . Q bfi O J

.40

• m

•

m

•

30 •

20

-4 .5 -4 .0 -3 .5 3 .0 Log(HDz)o

Figure 9. Effect of varying ligand concentration on extraction of palladium.

50

-5 of 1.87 x 10 M and an initial dithizone concentration of

-5 9.35 x 10 M were used. Acidity was maintained at 0.12M in

HgSO^. 100 ml of organic and of aqueous phase were used in

all experiments. The apparatus used for the rate studies is

30 shown in Figure 2.

It is evident from Table 8 that the chloride ion

concentration has a significant effect on the rate of extrac­

tion of palladium. A second-order rate equation was used for

the purpose of studying the rate of transfer of palladium

into chloroform.

Assuming:

d[Pd]

(1> - -dF^ = kobs. tPd][HDz]

d[Pd] CHDz -2[Pdo]

(2> + -air2 - kobs. icPd - <Pd>oi —^

where and Cp{j are total concentrations of the ligand and

metal, respectively.

Integration yields:

K_p CPd<CHDz " atM]0)

(3) C - 2C C TC - I PdI T - ̂ obst " HDz Pd HDz ( Pd L 6 ; oDS

o

Substitution of appropriate parameters into (3)

yields:

51

Table 8. Kinetic parameters including observed second-order rate constants and half lives for the formation of (2:1) Pd-dithizonate at various chloride concentrations.

_ _rj -i -I k _(a) (a) , [CI ]M kQxlO (M sec ) ^ x 10 ' Kf4[Cl ] tl/2

0.03 5.36 9.13 0.703 30

0.07 3.94 10.40 1.641 59

0.10 3.22 10.77 2.344 70

0.13 2.98 12.06 3.047 72

0.20 2.55 14.79 4.688 95

a. From a plot of k /a, versus Kf.[CI ], k. = slope = ° 7 -1 -1

(1.34 + .096) x 10 M sec and k, = intercept = 7 -1 -1

(8.05 + 0.29) x 10 M sec . See also Figures 10 and

11.

52

Sr log CPd(eCHDz ' 2A) _ k

2CPd WeCPd " A) obs ' HDz o

where e and A are the molar absorptivity and the absorbance

chloride consisted of eight absorbance measurements at 15-sec

intervals.

to 0.20 M, the predominant species present in solution were

PdClg and PdCl^, with almost a negligible concentration of

other chloro-complexes. From a plot of the kQ (observed rate

constant) against chloride concentration (Figure 10), it is

observed that kQ approaches a limiting minimum value with

increasing chloride concentration. The fact that the value

of kQ varies with chloride concentration suggests that kQ is

a function of one or more elementary rate constants; the

following mechanism for the chloride concentration range of

0.02 - 0.20 M can be postulated, where the formation of the

3 n ion pair (PdCl 0Dz) " is the rate determining step.

The introduction of appropriate parameters and rearranging

yields:

of the palladium chelate in chloroform, respectively.

Each typical run at a specified concentration of

Since the chloride ion concentration varied from 0.02

(5) k3tPdC13KHDz] + k4[PdCl4][HDz]

(6) Cnftl]= [pd]T[HDz]{k3a3 + k4a4}

53

70

60

m

•

• •

_EL 2 . 0 7.0 12 17 22

[Chloride] x 10 M

Figure 10. Effect of chloride concentration on the observed rate constant.

54

where cx [PdCl~]

(7) % = ~ [Pd]T

and [PdCl ]

(8 ) a 4 = 4

[Pd]T

and where

(9) [Pd]T * [PdCl~] + [PdCl~],

or from (1)

<10) ko = k3a3 + k4a4

(11) {kjCPdCl"] + k4K(4[PdCl"][Cl ]}

where

(12) Kf4 = [PdC14]

f4 [PdCl-JLCl-J

Subsequent rearrangement yields:

(13) kQ = a3 {k3 + k4Kf4[Cl"]}

or

(!4) ^ = {k3 + k4Kf4[Cl"]}

From

and

[PdCl~] (15) . 2+ J o = Bo

[Pd2+][C1 ]3 3

[PdCl=] (16 ) z = 3 4

[Pd2+][C1 ]4 4

We can arrive at:

55

^3 (18 ) a 3 - d

33 + B4[C1"]

So therefore,

k k {& + 34[C1-]}

(195 ^ - S3 k3 + VW" 3

Using log3g =10.17

log34 = 11.54

and log Kf4 = 1.37

A plot of kQ/ag against K^4[C1~] yielded a straight

line with slope = k4 and intercept = kg. See Table 8 and

Figure 11.

V

160

150

140

co

130

M

56

m

120 m

110

100

0

G3

CD 1 . 0 2 . 0 3.0 4.0

Kf4[Cl]

• • »

s.o

Figure 11. Determination of elementary rate constants (k3 and k4) for the formation of primary palladium dithizonate

SUMMARY

Palladium was found to form a strong (2:1) chelate

with dithizone, extractable into chloroform from up to 9M

3 —1 -1 sulfuric acid solution, £g37 = (34.1 + 0.2) x 10 M cm

in chloroform. The effect of various masking agents on the

extraction of Pd was studied. Of the ones tested, chloride,

bromide, iodide, sulfate, sulfite, thiosulphate and EDTA,

only thiosulphate was found to have an observable effect on

the distribution of palladium between two phases, as all

measurements were made spectrophotometrically.

It was found that thiosulphate formed a 2:1 complex

with palladium at pH 7.40 and a 1:1 complex in a pH range of

(8-10.5). The distribution of palladium between aqueous

phase and chloroform, in the presence of thiosulphate, was

found to be independent of pH in the range of (8-10.5). A

8 3+0 93 conditional extraction constant of 10 ' - ' was calculated

for the primary (2:1) palladium-dithizonate in the presence

of thiosulphate.

A kinetic study indicated that chloride has a signifi­

cant effect on the rate of extraction of palladium dithizonate

into chloroform. The rate of extraction can be decreased by

increasing the chloride concentration in the aqueous phase.

57

58

7 -1 Second-order rate constants on the order of 10 M

sec-"*" were calculated for a postulated mechanism:

* ̂ = k3[PdCl-][Hdz] + k4[PdCl=][Hdz] .

32 These are seemingly larger than expected for palladium,

which is supposed to be relatively inert toward ligand dis­

placement. No adequate explanation could be given for this,

at the present time.

Future work on palladium, in the context of this

study, may involve finding other suitable ligands as masking

agents of the sulfur-donor type, and possibly an X-ray struc­

tural determination on the primary and secondary palladium

dithizonates.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

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