Dr. Jehad M DiabFaculty of pharmacyDamascus University

Gravimetric Analysisالتحلیل الوزني

Pharma.analytical chemistry II

Gravimetric Analysis

Gravimetry is the Quantitative measurement of theanalyte by weighing a pure solid form of theprecipitate.

Gravimetric Analysis is one of the most accurateand precise methods of macro-quantitative analysis.

Often required for high precision

Obtaining pure solids from solutions containingan unknown amount of a metal ion is done byprecipitation. Dr.Jehad Diab

Electrogravimetric methods. analyte isprecipitated on cathode as metal or on anodeas metal oxide Dr.Jehad Diab

طرائق الترسیب

طرائق التطایر

طرائق التحلیل الوزني الكھربائي

and wash

and calculateDry and ignite

precipitating

Precipitation steps in gravimetry

Dr.Jehad Diab

مراحل التحلیل الوزني

Dr.Jehad Diab

خواص الرواسب

Mechanisms of precipitation1.Saturation: the amount of salt dissolved ismaximum under certain condition of pressure andtemperature

2.Supersaturation: the amount of salt is larger than atsaturation step, turbid solution

3.nucleation: when a small number of ions ,atoms,molecules initially unite either spontaneous orinduced resulting in very small aggregates of asolid during precipitation.

4.Particle growth: the three dimensional growth ofparticle nucleus converted into a larger crystal

Dr.Jehad Diab آلیات الترسیب

اإلشباع

فوق اإلشباع

التنوي

النمو الجزیئ

Dr.Jehad Diab

Dr.Jehad Diab

Particle size and filterabilityVon Weimarn described an equation that can control

the particle size of ppt; named Von Weimarn ratio• RSS= Q-S/S (Rss = Relative supersaturation, Q is

concentration of the solute at any instant. S is itsequilibrium solubility)

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Proportional to RSS

Inversely Proportional to RSS

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pH control of precipitation

Ca2+ + C2O42- CaC2O4 (s)

H2C2O4 2 H+ + C2O42-

Feeder Reaction:

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Weak basic medium

10-6 - 10-4 mm

10-1 – 10 mm

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Primary adsorbed layer

AgNO3 + NaCl →AgCl↓ + NaNO3

reagent analyte

(Secondaryadsorbedlayer)

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Dr.Jehad Diab

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coagulationpeptization

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adsorbed

adsorbed

The result is coagulation of colloidal precipitate

of adsorbed

Dr.Jehad Diab

Colloids particles must collide with one another tocoalesce. However ,the negatively charged ionicatmospheres of particles repel one another. the particles,therefore ,must have enough kinetics energy to overcomeelectrostatic repulsion before the can coalesce. Heatpromotes coalescence by increasing the kinetic energy .

And coagulation of colloidal precipitate is resulted in.

And coagulation of colloidal precipitate is resulted in.Dr.Jehad Diab

increasing electrolyte concentration (HNO3 for AgCl) decreasesthe volume of the ionic atmosphere and allows particles tocome closer together before electrostatic repulsion becomesignificant. For this reason .most gravimetric precipitations aredone in the presence of an electrolyte.

High Electrolyte Concentrationto Aid Precipitation

Excess charge on colloid creates ionicatmosphere around particle

D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686

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Washing a colloid with water to remove excess counterion or trapped impurities can result in peptization.

معالجة الببتزة او التشبغر

Use a volatile electrolyte

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Digestion and aging

,to reduce impurities present and largerparticles obtained

Or more

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التھضیم والتعتیق

Fig. 10.1. Ostwald ripening.

During digestion at elevated temperature:

Small particles tend to dissolve and reprecipitate on larger ones.

Individual particles agglomerate.

Adsorbed impurities tend to go into solution.

During digestion at elevated temperature:

Small particles tend to dissolve and reprecipitate on larger ones.

Individual particles agglomerate.

Adsorbed impurities tend to go into solution.

©Gary Christian,Analytical Chemistry,

6th Ed. (Wiley)التھضیم أو نضوج استوالد

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الرواسب البلوریة

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(inclusion)

Sources of Coprecipitation

(trapped impurities)

adsorption

(interferences)

Post precipitation الترسیب التالي: Sometimes a precipitate standingin contact with the mother liquor becomes contaminated by theprecipitation of an impurity on top of the desired precipitate.

Dr.Jehad Diab

الترسیب المشترك

اإلمتزاز السطحياإلحتباس

اإلحتواء

adsorbed

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اإلمتزاز السطحي

()l

silver and nitrate ions, soAgNO3 is coprecipitated with the AgCl.

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Coprecipitation error: negative or positive errorsEx: (a). in Cl analysis, colloidal AgCl + AgNO3 → (+) error(b). in Ba2+ → BaSO4 analysisif Ba(NO3)2 (larger FW than BaSO4) → (+) errorif BaCl2 ( FW: BaCl2 < BaSO4) → (-) error

Dealing with surface adsorption

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Dealing with surface adsorption

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Occlusion

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اإلحتباس

Occlusion

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Occlusion

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dDetermination of NH4+ as NH4MgPO4 , K+ will cause a mixed

crystal containing KMgPO4

(inclusion اإلحتواء)

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Dr.Jehad Diab

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Homogeneous precipitation. The best precipitates are obtained withhomogeneous precipitation, in which theprecipitating reagent is gradually generated in thesample solution, through a slow chemical reaction.

In this way there is never a large excess ofreagent, so that nucleation is slow, giving allexisting nuclei plenty of time to grow.

The precipitating reagent is usually formed bythe slow hydrolysis of an organic compound atelevated temperature, but even synthesis ispossible, as in the generation of dimethylglyoxime

Dr.Jehad Diab

الترسیب المتجانس

Methods have been worked out to generate avariety of precipitants, such as hydroxide, sulfide,sulfate, phosphate, oxalate, 8-hydroxyquinoline,and chromate.

Both the temperature and the pH must becontrolled, because both usually affect the rates ofthe hydrolysis reaction. Moreover, as we saw in thepH is often crucial in the formation of theprecipitate.

Homogeneous precipitation

Dr.Jehad Diab

Solid formed by homogeneous precipitation are generally purer and moreeasily filtered than precipitate generated by direct addition of a reagent tothe analyte solution.

3

Preparation of NH4OH as precipitant by hydrolysis of urea

Al(OH)3,Fe(OH)3,Sn(OH)4,Bi(OH)3,Th(OH)4

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∆

thioacetamide CH2·CS.NH2 + H20 → CH2·CO.NH2 + H2S Cd, Cu, Mo, Sb

urea (NH2)2CO + 3H20 → CO2 + 2NH4+ + 20H- AI, Bi, Ga, Fe, Sn, Th

Sulfamic acid NH2S03H + H20 → NH4+ + H + + SO4

2+ Ba, Ca, Pb, Sr

Trimethyle phosphate (CH30)3PO + 3H20 → 3CH30H + 3H+ + PO43- Zr

Dimethyl oxalate CH30.CO.CO.OCH3 + 2H20 → 2CH30H + 2H + + C2042- Ca, Mg, Zn

8-acetoxyquinoline + H20 → CH3C02H + 8-hydroxyquinoline AI, Mg, U, Zn

2Cr3+ + BrO3- + 5H20 → Br- + l0H+ + 2CrO42- Pb

Biacetyl plus hydroxylamine CH3·CO.CO.CH3 + NH20H → dimethylglyoxime + 2H20 Ni

Table. Some common reactions for homogeneous precipitationreagent generating reaction used to precipitate

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Dr.Jehad Diab

تجفیف الراسب

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250 oC 600 oC

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Dr.Jehad Diab

الحسابات في التحلیل الوزني

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Dr.Jehad Diab

Dr.Jehad Diab

Gravimetric calculations

f.wt analyte(g/mol) a (mol analyte)

gf = ------------------------- × ------------------------

f.wt ppt(g/mol) b(mol ppt)

gf = g analyte / g ppt

g analyte = g ppt × gf

% analyte =( g analyte / g sample) × 100

% analyte = (g ppt × GF) / g sample) ×100% analyte = (g ppt × GF) / g sample) ×100

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Cl2 → AgCl(s)

GF=1/2 × Cl2 /AgCl = Cl2 / 2AgCl=35.5*2/2(108+35.5)= 0.25

AlCl3 → AgCl(s)

GF=1/3 × AlCl3 / AgCl = AlCl3 / 3AgCl I → Hg5(IO6)2

GF=2I/ Hg5(IO6)2

Determine GF:

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Problems: Calculate the mass of analyte interm of grams to each gram of ppt for the

following analytes:

Analyte ppt

P(31g) → Ag3po4 (711g )

K2HPO4(136g) → Ag3po4 (711g )

Bi2S3 (514 g) → BaSO4 (233 g)

g analyte = g ppt × gf

g p =g Ag3PO4 ×GF= 1 ×31 / 711=0.044 g p/1g ppt

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Problem : Determine the gravimetricfactors in term of symbols for thedetermination of:

AnalyteAnalyte pptppt G.F.G.F.CaCaOO CaCaCOCO33 CaOCaO/CaCO/CaCO33

FeFeSS BaBaSSOO44 FeSFeS/BaSO/BaSO44

UUOO22(NO(NO33))22 UU33OO88 33UOUO22(NO(NO33))22/U/U33OO88

CrCr22OO33 AgAg22CrCrOO44 CrCr22OO33//22AgAg22CrOCrO44

Dr.Jehad Diab

Problem : Determine the gravimetric factors interm of symbols for the determination of:

(a) Aluminum as its hydroxyquinolate,AI(C9H60N)3 (Al AI(C9H60N)3)

(b) Phosphorus as phosphomolybdic anhydride,P2Mo24078 (P P2Mo24078 )

(c) Potassium as its chloroplatinate

K → K2PtCl6(d) Sulfur as barium sulfate,( S → BaS04)

(e) Nickel as nickel dimethylglyoxime,

( Ni → Ni(C4H702N2)2Dr.Jehad Diab

Problem : Determine the gravimetric factors for thedetermination of:

1. In -> In2O3 GF

2. HgO -> Hg5(IO6)2 GF

3. K3PO4 -> K2PtCl6 GF

4. K3PO4 -> Mg2P2O7 GF

5. AgIO3 -> Ba(IO3)2 GF

6. AgNO3 -> AgIO3 GF

7. (NH4)2SO4 -> BaSO4 GF

8. Mn3O4 -> MnO2 GF

9. Cu2HgI4 -> Cu GF

Cr -> Cr203 GFDr.Jehad Diab

10.

Example: in an organic sample (0.352g) phosphorouswas dissolved and converted to Mg2P2O7 precipitate(0.223 g). Calculate the percentage %P in the original

sample.

Dr.Jehad Diab

2P(gfw 31 g) → Mg2P2O7 (gfw 222.6)GF=2× 31 /222.6 =0.2783% analyte =(g ppt × GF / g sample) × 100

%P =( 0.223 × 0.2783 / 0.352) ×100 = 17.1 %

Example: When an sample of impure potassiumchloride (0.4500g) was dissolved in water andtreated with an excess of silver nitrate, 0.8402 g ofsilver chloride was precipitated. Calculate thepercentage KCl in the original sample.

Answer:KCl ( gfw =74.50) => AgCl ( gfw =143.50)GF= 74.50/143.50=0.519%KCl = (mass of AgCl *GF/mass of KCl) *100=

=( 0.8402*0.519/0.4500 )*100 = 96.90%

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Dr.Jehad Diab

Mainly Al and Mg

Dr.Jehad Diab

Dr.Jehad Diab

Organic reagents

Table: some of inorganic precipitating agentsprecipitants analyte and (formed precipitate, weighed)*

NH3(aq) Be (BeO),Al (Al2O3),Cr(Cr2O3), Fe(Fe2O3)

(NH4OH) Sn (SnO2),Zr (ZrO2)

H2S Zn (ZnS→ZnO),As (As2S3 →As2O3 Or

As2O5),Bi (Bi2S3

(NH4)2HPO4 Mg(Mg2P2o7),Zn(Zn2P2O7),Cd(Cd2P2O7)

H2SO4 Sr,Cd,Pb,Ba (all as sulphate)

H2PtCl6 K (K2PtCl6)

HCl Ag (AgCl), Hg(Hg2Cl2)

AgNO3 C l(AgCl), Br (AgBr), I (AgI)Dr.Jehad Diab

Table: some of inorganic precipitating agentsprecipitants analyte ( formed precipitate, weighed)* BaCl2 SO4

2- (BaSO4)( NH4)2S Hg (HgS) HNO3 Sn4+ (SnO2) H5IO6 Hg (Hg5(IO6)2 NaCl, Pb(NO3)2 F (PbClF) MgCl2,NH4Cl PO4

3- (Mg2P2O7)Ca2+ H2C2O4 CaCO3 or CaO

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-------------------------------------------------------------------------------------------------------* After drying or ignitionFe3+ + OH-→Fe(OH)3 → Fe2O3

weighed formppt

Mainly Mg and AL

بعض عوامل الترسیب : جدولالعضویة

Cs+

Mn+ +nHR→ MRn + nH+

M2+ +2HR →MR2 + 2H+

2 , Ag+ , Cu+

M+ +NaR →MR + Na+

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interfere

Mn+ +nNH4R -> MRn + nNH4+

M2+ +H2R →MR + 2H+

An- + nRCl -> RnA + nClDr.Jehad Diab

Ca2+→ CaC2O4 → CaO

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Dr.Jehad Diab

Dr.Jehad Diab

القیاس الوزني الكھربائي

•used for electroplating, extraction and purification ofof the analyte

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كمون التفكك

current-voltage relation for electrolysis .

Cu2+ +2e => Cu

Ag(CN)2- + e => Ag +2CN-

Pb2+ +2H2O => PbO2 + 4H+ +2e

Dr.Jehad Diab

Dr.Jehad Diabفاراداي ویقدر بالكولوم1= 96500

CU2+Determine the number of grams of Cu2+ thatcould be deposited on cathode as Cu,if currentof 6 amperes is applied for 5 minutes

Dr.Jehad Diab

Dr.Jehad Diab

Dr.Jehad Diab

RevisionDiminishing importance of gravimetry, as a resultof the development of much faster (though usuallyless precise) instrumental methods.

Consequently, gravimetry is used only when itssuperior precision is really needed. In that case,great care must be exercised to avoidcoprecipitation of other sample components,occlusion of solvent in the precipitate, andadsorption of excess reagent, otherwise the extraeffort is negated by an impure or otherwise poorlydefined precipitate

Dr.Jehad Diab

We have emphasized the factors that facilitate theformation of coarse, pure, easily filterableprecipitates, explained why precipitates arepreferably generated homogeneously, and why theyare usually washed with electrolyte solutions ratherthan with water.

Ammonium salts are often used for this purpose,because they will readily volatilize upon subsequentheating of the precipitate.

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The End

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