Precipitation titration

LL ee cc tt uu rr ee 7 7

Associate profAssociate prof . L.V. Vronska. L.V. VronskaAssociate profAssociate prof . M.M. Mykhalkiv

OutlineOutline

1. Essence and classification of methods precipitation titration

2. Titration Curves

3. Argentometry

4. Thiocyanatometry

5. Mercurometry

6. Sulphatometry

7. Hexacianoferratometry

A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration - a precipitation titration.

1. Essence and classification of 1. Essence and classification of methods precipitation titrationmethods precipitation titration

Requirements to reactions and defined Requirements to reactions and defined substances: substances:

The defined substance should be dissolved in The defined substance should be dissolved in water and give an ion which would be active in water and give an ion which would be active in sedimentation reaction. sedimentation reaction.

The received precipitate should be practically The received precipitate should be practically insoluble (insoluble (KKspsp<10<10-8 -8 - 10 - 10, , S<10S<10-5 -5 mol/Lmol/L).).

Results of titration should not be deformed by Results of titration should not be deformed by the adsorption phenomena (coprecipitation). the adsorption phenomena (coprecipitation).

Precipitate should form enough quickly.Precipitate should form enough quickly. There should be a possibility of fixing of an There should be a possibility of fixing of an

equivalence point. equivalence point.

Classification of methods precipitation Classification of methods precipitation titrationtitration (on(on titrant)titrant)::

1. Argentometry

2. Thiocyanatometry

3. Mercurometry

4. Sulphatometry

5. Hexacianoferratometry

2. Titration Curves2. Titration Curves

Curve plotting of titration is based on Curve plotting of titration is based on a rule of a rule of solubility productsolubility product..

And accordingly And accordingly

nBmABAnm

nm BAKsp ][][

Precipitation titration curve for 50.0 mL of0.0500 M Cl– with 0.100 M Ag+. (a) pClversus volume of titrant; (b) pAg versus

volume of titrant.

The factors which define valueThe factors which define value of of inflection pointsinflection points of titration on curves of of titration on curves of

precipitation titrationprecipitation titration

Concentration of titrant solutions and a Concentration of titrant solutions and a defined ion (than more concentration, the defined ion (than more concentration, the titration inflection point is more)titration inflection point is more)

Solubility of a precipitate (than solubility Solubility of a precipitate (than solubility less, the titration inflection point is more)less, the titration inflection point is more)

Influence of precipitate solubility on Influence of precipitate solubility on titration inflection pointtitration inflection point

Determinate the end-point Determinate the end-point by potenthiometric wayby potenthiometric way

Influence of different factors on Influence of different factors on inflection point ininflection point in precipitation precipitation

titrationtitration

TemperatureTemperature (than more temperature, the (than more temperature, the solubility of a precipitate will be more and the solubility of a precipitate will be more and the inflection point is less)inflection point is less)

Ionic strengthIonic strength of a solution (than more ionic of a solution (than more ionic strength of a solution, the solubility of a strength of a solution, the solubility of a precipitate will be more and the inflection precipitate will be more and the inflection point is less)point is less)

3. Argentometry3. Argentometry

This is a precipitation titration in which Ag+ is the titrant.XX-- + Ag + Ag++ = AgX = AgX

where: Xwhere: X-- = Cl = Cl--, Br, Br--, I, I--, CN, CN--, SCN, SCN--, etc.., etc.. Titrant:Titrant: AgNO AgNO33 – secondary standard solution – secondary standard solution StandardizationStandardization оn primary standard solution of оn primary standard solution of

Sodium chlorideSodium chloride

AgNOAgNO33 + NaCl = + NaCl = AgClAgCl + NaNO+ NaNO33

IndicatorIndicator by standardization by standardization - 5 % Potassium chromate - 5 % Potassium chromate KK22CrOCrO4 4 (to appearance (to appearance reddish-browneddish-brown precipitate of precipitate of Silver chromate):Silver chromate):

2AgNO2AgNO33 ++ K K22CrOCrO44 == AgAg22CrOCrO44+ 2KNO+ 2KNO33..

ArgentometryArgentometry::

without indicator:without indicator:

- Gay-Lussac method (method of even - Gay-Lussac method (method of even turbidity)turbidity)

- method to point enlightenment- method to point enlightenment with inicator:with inicator:

- Mohr method- Mohr method

- Fajans – Fisher – Khodacov method- Fajans – Fisher – Khodacov method

- Volhard method- Volhard method

Gay-Lussac method (method of Gay-Lussac method (method of even turbidity)even turbidity)

If solution NaBr titrate by solution AgNOIf solution NaBr titrate by solution AgNO33 (or on the (or on the

contrary) there is a reaction:contrary) there is a reaction:

BrBr-- + Ag + Ag++ = AgBr = AgBr↓↓ For fixation of equivalence point it is necessary to select For fixation of equivalence point it is necessary to select

two identical portions of a solution before the titration two identical portions of a solution before the titration end. To one of them add a drop of AgNOend. To one of them add a drop of AgNO33 solution, on solution, on

another - a drop of NaBr solution at the same another - a drop of NaBr solution at the same concentration. Titration finish when will be identical concentration. Titration finish when will be identical intensity of turbidity in both portions of solution.intensity of turbidity in both portions of solution.

Method to enlightenment pointMethod to enlightenment point The method of titration to an enlightenment point can be The method of titration to an enlightenment point can be

applicable when insoluble compounds is in colloidal state. For applicable when insoluble compounds is in colloidal state. For example, definition of Іexample, definition of І-- ions by silver nitrate, AgІ forms, it is ions by silver nitrate, AgІ forms, it is adsorbing Іadsorbing І-- and receive negative charges (colloidal solution of and receive negative charges (colloidal solution of AgІ forms).AgІ forms).

As more and more ІAs more and more І-- ions react with Ag ions react with Ag+ + ions, particles AgІ ions, particles AgІ gradually lose adsorbed by them Іgradually lose adsorbed by them І-- ions, and their charge ions, and their charge decreases. In the end of titration occur coagulation of particles decreases. In the end of titration occur coagulation of particles and their sedimentation. The solution thus is absolutely clarified. and their sedimentation. The solution thus is absolutely clarified.

Mohr methodMohr method Titrant:Titrant: AgNO AgNO3 3 – secondary standard solution– secondary standard solution StanardizationStanardization on primary standard solution of on primary standard solution of

sodium chloride NaCl (by sodium chloride NaCl (by a measured volume of primary standard solution)::

AgNOAgNO33 + NaCl = + NaCl = AgClAgCl + NaNO + NaNO33

IndicatorIndicator - 5 % potassium chromate - 5 % potassium chromate KK22CrOCrO44 (to (to formation formation precipitate of reddish-brown Ag2CrO4):):

2AgNO2AgNO33 + K + K22CrOCrO44 = = AgAg22CrOCrO44+ 2KNO+ 2KNO33

Determinate substanceDeterminate substance: chloride Cl: chloride Cl--, bromide Br, bromide Br--.. MediumMedium: рН~ 6,5-10,3.: рН~ 6,5-10,3. UsageUsage: quantitative definition of sodium chloride, : quantitative definition of sodium chloride,

potassium chloride, sodium bromide, potassium potassium chloride, sodium bromide, potassium bromide, etc.bromide, etc.

Restrictions of usage of Mohr method:Restrictions of usage of Mohr method: It is impossible to use titation in acidic solutions:It is impossible to use titation in acidic solutions:

22CrOCrO442-2- + 2H + 2H++ = = CrCr22OO77

2-2- + H + H22OO It is impossible to use titration in the presence of It is impossible to use titration in the presence of

ammoniaammonia, etc. ions, molecules which can be ligands on , etc. ions, molecules which can be ligands on relation to Silver ionsrelation to Silver ions

It is impossible to use titration in the presence of many It is impossible to use titration in the presence of many cations (cations (BaBa2+2+, Pb, Pb2+2+,, etc.) which form the painted etc.) which form the painted precipitates with chromate ions precipitates with chromate ions CrOCrO44

2-2-

It is impossible to use titration in the presence of It is impossible to use titration in the presence of reducers which reduce chromate ions reducers which reduce chromate ions CrOCrO44

2-2- to to CrCr3+3+ ions ions

It is impossible to use titration in the presence of many It is impossible to use titration in the presence of many anions (POanions (PO44

3-3-, AsO, AsO443-3-, AsO, AsO33

3-3-, S, S2-2- etc.) which with etc.) which with Silver ions give the painted precipitatesSilver ions give the painted precipitates

Fajans – Fisher – Khodacov methodFajans – Fisher – Khodacov method Titrant:Titrant: AgNO AgNO3 3 – secondary standard solution– secondary standard solution

StanardizationStanardization on primary standard solution of on primary standard solution of sodium chloride NaCl (by a measured volume of sodium chloride NaCl (by a measured volume of primary standard solution):primary standard solution):

MediumMedium: рН~ : рН~ 6,5-10,36,5-10,3 (by definition of (by definition of cloridesclorides)) andand рН~ рН~ 2,0-10,32,0-10,3 (by definition of (by definition of bromidesbromides andand iodidesiodides))..

IndicatorsIndicators of methodof method: :

- - dichlorofluorosceindichlorofluoroscein (by definition of (by definition of cloridesclorides))

- - eosineeosine (by definition of (by definition of bromidesbromides andand iodidesiodides))

The mechanism of indicators The mechanism of indicators actionaction

AgNOAgNO33 + NaCl = + NaCl = AgClAgCl + NaNO + NaNO33

HIndHInd H H++ + + IndInd--

Adsorption indicator whose color when adsorbed to the precipitate is different from that when it is in solution

ColourColour

A number adsorptionA number adsorption abilitiesabilities of anion at рН~7 on a of anion at рН~7 on a precipitate surface of Silver chlorideprecipitate surface of Silver chloride

II-- > CN > CN-- > SCN > SCN-- > Br > Br- - > > eosineeosine > Сl > Сl-- > > dichlorofluoroscein > NO > NO33

-- > ClO > ClO44--

IndicatorIndicator SolutionSolution Surface of Surface of precipitateprecipitate

dichlorofluoroscein greenish yellow pink

eosineeosine yellowish-redyellowish-red redish - violetredish - violet

Conditions of titration:Conditions of titration: Acidity of solutions Acidity of solutions Concentration of reacting solutions Concentration of reacting solutions The account adsorption abilities of indicators and The account adsorption abilities of indicators and

ions which present in a solutionions which present in a solution Titration near equivalence pointTitration near equivalence point is necessary to is necessary to

spend slowlyspend slowly Titration with adsorption indicators spend in a Titration with adsorption indicators spend in a

diffused lightdiffused light

Use:Use: Quantitative definition of chlorides, bromides, Quantitative definition of chlorides, bromides,

iodides, thiocyanides, cyanides. iodides, thiocyanides, cyanides.

Volhard methodVolhard method TitrantTitrant:: AgNOAgNO33,, ammonium or potassiumammonium or potassium

thiocyanidethiocyanide NHNH44SCNSCN, , KSCNKSCN - - secondary standard secondary standard solutionsolution

StardadizationStardadization AgNOAgNO33 on primary standard solutionon primary standard solution NaCl,NaCl, NHNH44SCN, KSCNSCN, KSCN on standardization solutionon standardization solution AgNOAgNO33: :

AgNOAgNO33 + NH + NH44SCN = SCN = AgAgSCNSCN + NH + NH44NONO33

IndicatorIndicator byby standardizationstandardization of ammonium or of ammonium or potassium thiocyanidepotassium thiocyanide withwith ironiron (ІІІ) (ІІІ) salts salts (NH(NH44Fe(SOFe(SO44))2212H12H22O O in presence of nitric acidin presence of nitric acid) ) to to the formation of the reddish colored Fe(SCN)the formation of the reddish colored Fe(SCN)2+2+ complexcomplex::

FeFe3+3+ + SCN + SCN-- = = [Fe(SCN)][Fe(SCN)]2+2+

MediumMedium:: in presence of nitric acidin presence of nitric acid Indicators of methodIndicators of method: : ironiron (ІІІ) (ІІІ) salts salts

NHNH44Fe(SOFe(SO44))2212H12H22O O in presence of nitric acidin presence of nitric acid

Determinate substanceDeterminate substance:: halogenide halogenidess, , thiocyanides, cyanidesthiocyanides, cyanides, , sulphidessulphides, , carbonatescarbonates, , chromateschromates, , oxalatesoxalates, , arsenatesarsenates etcetc..

Until equivalence point (e.p.)Until equivalence point (e.p.)

HalHal-- + Ag+ Ag++ (excess) = AgHal (excess) = AgHalAgAg++(rest) + (rest) + SCNSCN-- = AgSCN= AgSCN

After e. p.After e. p.

FeFe3+3+ + + SCN-SCN- = = [Fe(SCN)][Fe(SCN)]2+2+

(reddish)(reddish)

!!!!!! At definition of iodides the indicator is added in the end of At definition of iodides the indicator is added in the end of titration to avoid parallel:titration to avoid parallel:

2Fe2Fe3+3+ + 2I + 2I-- = 2Fe = 2Fe2+2+ + I + I22

Advantages of Volhard methodAdvantages of Volhard method

Titration possibility:Titration possibility: - In very acidic solutions- In very acidic solutions - In the presence of many cations which interfere by definition in - In the presence of many cations which interfere by definition in

Mohr methodMohr method

4. Thiocyanatometry4. Thiocyanatometry This is a precipitation titration in which SCNSCN- is the titrant.

TitrantTitrant:: ammonium or potassium thiocyanideammonium or potassium thiocyanide NHNH44SCN, KSCN - SCN, KSCN - secondary standard solutionsecondary standard solution

Stardadization:Stardadization: on primary standard solution ofon primary standard solution of AgNOAgNO33: :

AgNOAgNO33 + NH + NH44SCN = AgSCNSCN = AgSCN + NH + NH44NONO33

IndicatorIndicator byby standardizationstandardization of ammonium or of ammonium or potassium thiocyanidepotassium thiocyanide withwith ironiron (ІІІ) (ІІІ) salts:salts:

FeFe3+3+ + SCN + SCN-- = = [Fe(SCN)][Fe(SCN)]2+2+

MediumMedium:: in presence of nitric acidin presence of nitric acid IndicatorIndicator:: ironiron (ІІІ) (ІІІ) salts salts NHNH44Fe(SOFe(SO44))2212H12H22O O in in

presence of nitric acidpresence of nitric acid

Determinate substanceDeterminate substance:: drugsdrugs, which contain , which contain SilverSilver (Albumosesilber, colloid silver - Kollargol, silver (Albumosesilber, colloid silver - Kollargol, silver nitrate). nitrate).

!!! !!! At the analysis of drugs which contain nonionic At the analysis of drugs which contain nonionic silver, preliminary it is heated with sulphatic and nitric silver, preliminary it is heated with sulphatic and nitric acids (receive ionic compound). acids (receive ionic compound).

!!!!!! At definition of At definition of iodides iodides the indicator is added in the indicator is added in the the endend of titration to avoid parallel: of titration to avoid parallel:

2Fe2Fe3+3+ + 2I + 2I-- = 2Fe = 2Fe2+2+ + I + I22

Advantages of ThiocyanatometryAdvantages of Thiocyanatometry

Titration possibility:Titration possibility: - In very acidic solutions- In very acidic solutions - In the presence of many cations which - In the presence of many cations which

interfere by definition in Mohr methodinterfere by definition in Mohr method

5. Mercurometry5. Mercurometry MercurometryMercurometry – – this is a precipitation titration

in which HgHg222+2+

is the titrant

2Cl2Cl-- + Hg + Hg222+2+ = Hg = Hg22ClCl22 KKspsp = 1,3 = 1,31010-18-18

2I2I-- + Hg + Hg222+2+ = Hg = Hg22II22 KKspsp = 4,5 = 4,5 1010-29-29

TitrantTitrant:: Hg Hg22(NO(NO33))22 -- secondary standard secondary standard solutionsolution

Stardadization:Stardadization: on primary standard on primary standard solutionsolution of of NaCl:NaCl:

HgHg22((NONO33))22 + 2 + 2NaClNaCl = = HgHg22ClCl22 + 2 + 2NaNONaNO33

Indicators:Indicators: 1) solution of iron (ІІІ) thiocyanide (from 1) solution of iron (ІІІ) thiocyanide (from reddish to colourless)reddish to colourless)

2Fe(SCN)2Fe(SCN)2+2+ + Hg + Hg222+2+ = Hg = Hg22(SCN)(SCN)22 + 2Fe + 2Fe3+3+

!!! For the account of titrant volume which is used on !!! For the account of titrant volume which is used on indicator titration, do “blind test”indicator titration, do “blind test”

2) 1-2 % spirit diphenylcarbazone (to blue 2) 1-2 % spirit diphenylcarbazone (to blue colour)colour)

!!! Indicator is added before end of titration!!! Indicator is added before end of titration

Determinate substanceDeterminate substance:: chlorides and iodides. chlorides and iodides. MediumMedium:: very acidic (may be to 5 mol/L H very acidic (may be to 5 mol/L H+ + ions).ions).

Advantages of mercurometry:Advantages of mercurometry: Possibility usage titration in very acidic solutionsPossibility usage titration in very acidic solutions Titrant is cheaperTitrant is cheaper Mercury (I) Salts are less soluble, therefore accuracy Mercury (I) Salts are less soluble, therefore accuracy

and sensitivity of titration more, and inflection point on and sensitivity of titration more, and inflection point on a curve is much morea curve is much more

Possibility of titration with the diphenylcarbazone Possibility of titration with the diphenylcarbazone indicator in the muddy and painted solutionsindicator in the muddy and painted solutions

Probably reversive titration of Mercury (І) saltsProbably reversive titration of Mercury (І) salts

Lack:Lack: Mercury (І) salts are Mercury (І) salts are very toxicvery toxic..

6. Sulphatometry6. Sulphatometry SulphatometrySulphatometry – – this is a precipitation titration in which SOSO44

2-2- is

the titrantSometimesSometimes bariummetrybariummetry –– this is a precipitation titration in which

Ba2+2+ is the titrant

ReactionReaction of methodof method::

BaBa2+2+ + + SOSO442-2- = BaSO = BaSO44↓↓

determinate determinate titranttitrantsubstancesubstance

TitrantTitrant:: secondary standard solutionsecondary standard solution ofof H H22SOSO44, Ba(NO, Ba(NO33))22, BaCl, BaCl22.. Stardadization:Stardadization: solution ofsolution of H H22SOSO44 on primary standard solution on primary standard solution

ofof Na Na22BB44OO77 oror Na Na22COCO33 (crystal.)(crystal.) withwith methyl orange; Ba(NO methyl orange; Ba(NO33))22 oror BaClBaCl22 on secondary standard solution ofon secondary standard solution of H H22SOSO44 withwith nitrchromazo ornitrchromazo or orthanilic А. orthanilic А.

IndicatorsIndicators:: use use metallochrome indicator - metallochrome indicator - nitrchromazo or nitrchromazo or (orthanilic С), orthanilic А. (orthanilic С), orthanilic А.

BaInd BaInd + H + H22SOSO44 = BaSO = BaSO44 + + HH22IndInd

violet violet white roze white roze precipitate solution precipitate solution

Determinate substance:Determinate substance: Direct titration:Direct titration:- by sulphatic acid – content of Barium- by sulphatic acid – content of Barium- by barium chloride or barium nitrate – content of sulphatesby barium chloride or barium nitrate – content of sulphates

!!!!!! For completeness of sedimentation of barium ions use as solvent For completeness of sedimentation of barium ions use as solvent of of 50 % ethanol50 % ethanol

!!!!!! Sometimes, as the indicator use sodium rodizonate Sometimes, as the indicator use sodium rodizonate

7. Hexacyanoferratometry7. HexacyanoferratometryHexacyanoferratometryHexacyanoferratometry - this is a precipitation

titration in which KK44[Fe(CN)[Fe(CN)66]] is the titrant TitrantTitrant:: secondary standard solution secondary standard solution

KK44[Fe(CN)[Fe(CN)66]]

StardadizationStardadization:: on standard solution of KMnO on standard solution of KMnO44 in sulphatic acidin sulphatic acid

5K5K44[Fe(CN)[Fe(CN)66] + KMnO] + KMnO44 + H + H22SOSO44 = 5K = 5K33[Fe(CN)[Fe(CN)66] + MnSO] + MnSO44 + + 3K3K22SOSO44 + 4H + 4H22OO

Indicator by standardizationIndicator by standardization - Methyl violet - Methyl violet (from yellow-dreen to red-brown).(from yellow-dreen to red-brown).

IndicatorsIndicators:: diphenylaminediphenylamine at the presence of a small at the presence of a small

amount of potassium hexacyanoferrate (ІІІ) for amount of potassium hexacyanoferrate (ІІІ) for definition Zndefinition Zn2+2+, Mn, Mn2+2+

3,3-dimetilnaftizin3,3-dimetilnaftizin for definition Cu for definition Cu2+2+, Zn, Zn2+2+, , CdCd2+2+, Pb, Pb2+2+, Fe, Fe2+2+, Ni, Ni2+2+ etc. etc.

alizarin red Salizarin red S for definition Zn for definition Zn2+2+, Pb, Pb2+2+, Th(IV) , Th(IV) diphenylcarbazonediphenylcarbazone for definition Cd for definition Cd2+2+

Determinate substanceDeterminate substance:: metal cations (direct metal cations (direct titration), displacement titration – phosphate titration), displacement titration – phosphate ions.ions.

Thanks for your attention!