Titrasi Presipitasi

Outline

1. Essence and classification of methodsprecipitation titration

2. Titration Curves

3. Argentometry3. Argentometry

4. Thiocyanatometry

5. Mercurometry

6. Sulphatometry

7. Hexacianoferratometry

Precipitation Titratons

Precipitation titrimetry, which is based onreactions that yield ionic compounds of limitedsolubility, is one of the oldest analyticaltechniques. The slow rate of formation of mostprecipitates, however, limits the number ofprecipitating agents that can be used in titrationsprecipitating agents that can be used in titrationsto a handful. The most widely used and importantprecipitating reagent, silver nitrate, which is usedfor the determination of the halogens, thehalogen-like anions. Titrations with silver nitrateare sometimes called argentometric titrations.

Requirements

• The analyte should be dissolved in water and

give an ion which would be active in

precipitation reaction.

• The precipitate should be practically insoluble

(K <10-8 to 10-10, S<10-5 mol/L).(Ksp<10-8 to 10-10, S<10-5 mol/L).

• There is no coprecipitation.

• Precipitate should form quickly enough.

• There should be a way for determining the

equivalence point.

Classification of precipitation titrationmethods (based on titrant):

1. Argentometry

2. Thiocyanatometry

3. Mercurometry3. Mercurometry

4. Sulphatometry

5. Hexacianoferratometry

2. Titration Curves

Curve plotting of titration is based on a ruleof solubility product.

And accordingly

nBmABAnm

nm BAKsp ][][

The Shapes of Titration Curves

Titration curves for precipitation reactions arederived in a completely analogous way to themethods described for titrations involving strongacids and strong bases. P-functions are derived forthe preequivalence-point region, the post-equivalence point region, and the equivalenceequivalence point region, and the equivalencepoint for a typical precipitation titraton.

Most indicators for argentometric titrationsrespond to changes in the concentration of silverions. As a consequence, titraton curves forprecipitation reactions usually consist of a plot ofpAg versus volume of AgNO3.

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 value ofinflection points of titration on curves of

precipitation titration

• Concentration of titrant solutions and theanalyte (the higher the concentration, thesharper the slope of the inflection point.)sharper the slope of the inflection point.)

• Solubility of a precipitate (The smaller thesolubility, the sharper the slope of theinflection point)

Influence of precipitate solubility ontitration inflection point

Influence of other factors on inflectionpoint of the precipitation titration

• Temperature (the higher the temperature,the higher the solubility of a precipitateand the less sharp slope inflection point)and the less sharp slope inflection point)

• Ionic strength of the solution (the higherthe ionic strength of a solution, the higherthe solubility of a precipitate and the lesssharp slope inflection point)

Titration of 50 mL of 0.05 M NaCl by0.1000M AgNO3 continued

pAg = –log 10[Ag+].Precipitation AgCl

Volume AgNO3

Sharp increase inFree Ag+

Successful titrations of ions by SiverNitrate

pAg = –log 10[Ag+].

Precipitation AgI

Precipitation AgBr

Volume AgNO3

A Sharp increase inFree Ag+ signals the endpoint in each case

Precipitation AgCl

Successful titrations of Mixtures by SiverNitrate

pAg = –log 10[Ag+].

Precipitation AgI first

Volume AgNO3

A Sharp increase inFree Ag+ signals the endpoint in each case

Precipitation AgCl second

Sigmoidal titration curve Linear segment titration curve.Spectrophotometric titrationcurve of transferrin with ferricnitriloacetate.

Titration curve for the titration of 50.00 ml of 0.1000M AgNO3 with0.1000M KSCN.

Factors influencing the sharpness of end points

1) Reagent concentration[T], [A] eT

2) Completeness of reactionSolubility e.p. change jump

I– Ksp=8.3 ×10–17I Ksp=8.3 ×10Br– Ksp=5.0 ×10–13

Cl– Ksp=1.8 ×10–10

IO3– Ksp=3.0×10–8

BrO3– Ksp=5.7×10–5

Titration of a mixture by potentiometric detection

Two precipitable ions(ex. Cl–, I–)

Titrant (ex. Ag+)

Ex. 0.0502M KI + 0.0500M KClEx. 0.0502M KI + 0.0500M KCl0.0845 M AgNO3Ksp AgI << Ksp AgCl

8.3×10 –17 1.8 ×10–10

Coprecipitation[Ag+][Cl–] / [Ag+][I–]= [Cl–] / [I–] = 8.3×10 –10/1.8×10–17

= 1/ 4.6 ×10–7

Titration curves for 50.0 ml of a solution 0.0800 M in Cl and 0.0500 M inI or Br.

End Point for Argentometric TitrationsThree types of end points are: (1) chemical, (2)potentiometric, (3) amperometric. Potentiometricend points are obtained by measuring the potential.To obtain an amperometric end point, the currentgenerated between a pair of silver microelectrodes ismeasured and plotted as a function of reagentmeasured and plotted as a function of reagentvolume. The chemical end point consists of a colorchange or the appearance or disappearance ofturbidity. The requirements are (1) the color changeshould occur over a limited range in the p-function,and (2) the color change should take place within thesteep portion of the titration curve.

3. Argentometry

This is a precipitation titration in which Ag+ is the titrant.

X- + Ag+ = AgX

where: X- = Cl-, Br-, I-, CN-, SCN-, etc..

• Titrant: AgNO3 – secondary standard solution• Titrant: AgNO3 – secondary standard solution

• Standardization оn primary standard solution ofSodium chloride

AgNOAgNO33 ++ NaClNaCl == AgCl + NaNO+ NaNO33

• Indicator for standardization - 5 % Potassium chromateK2CrO4 (to appearance reddish-brown precipitate ofSilver chromate):

2AgNO2AgNO33 ++ K2CrO4 == Ag2CrO4+ 2KNO+ 2KNO33..

Argentometry:

• without indicator:

Gay-Lussac method (method of eventurbidity)

method to point enlightenment

• with indicator:

Mohr method

Fajans – Fisher – Khodacov method

Volhard method

Gay-Lussac method (method ofeven turbidity)

If solution NaBr titrate by solution AgNO3 (or on thecontrary) there is a reaction:

Br- + Ag+ = AgBr↓

• For determination of equivalence point it is• For determination of equivalence point it isnecessary to select two identical portions of asolution before the titration end. To one of themadd a drop of AgNO3 solution, on another - a dropof NaBr solution at the same concentration.Titration will finish when there will be identicalintensity of turbidity in both portions of solution.

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

be applicable when insoluble compounds is in colloidalstate. For example, determination of І- ions by silvernitrate, AgІ forms, it is adsorbing І- and receivenegative charges (colloidal solution of AgІ forms).

As more and more І- ions react with Ag+ ions, particles As more and more І- ions react with Ag+ ions, particlesAgІ gradually lose adsorbed by them І- ions, and theircharge decreases. In the end of titration occurcoagulation of particles and their sedimentation. Thesolution thus is absolutely clarified.

Formation of a Colored PrecipitateThe Mohr Method

Sodium chromate can serve as an indicator for theargentometric determination of chloride, bromide,and cyanide ions by reacting with silver ion to form abrick-red silver chromate (Ag2CrO4) precipitate in theequivalence-point region. The reactions involved inthe determination of chloride and bromide (X-) are

titration reaction: Ag+ + X- AgX(s) [white]indicator reaction: 2Ag+ + CrO4

2- Ag2CrO4(s)[red]

The solubility of silver chromate is several timesgrater than that of silver chloride or silver bromide.

Mohr method• Titrant: AgNO3 – secondary standard solution• Stanardization on primary standard solution of

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

AgNOAgNO33 ++ NaClNaCl == AgCl + NaNO+ NaNO33

• Indicator - 5 % potassium chromate K2CrO4 (toformation precipitate of reddish-brownformation precipitate of reddish-brownAg2CrO4):

2AgNO3 + K2CrO4 = Ag2CrO4+ 2KNO3

• Determinate substance: chloride Cl-, bromideBr-.

• Medium: рН~ 6,5-10,3.• Usage: quantitative definition of sodium

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

Restrictions of usage of Mohr method:

• It is impossible to use titation in acidic solutions:2CrO4

2- + 2H+ = Cr2O72- + H2O

• It is impossible to use titration in the presence ofammonia, etc. ions, molecules which can beligands on relation to Silver ions

• It is impossible to use titration in the presence ofmany cations (Ba2+, Pb2+, etc.) which form themany cations (Ba , Pb , etc.) which form thepainted precipitates with chromate ions CrO4

2-

• It is impossible to use titration in the presence ofreducers which reduce chromate ions CrO4

2- toCr3+ ions

• It is impossible to use titration in the presence ofmany anions (PO4

3-, AsO43-, AsO3

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

Adsorption Indicators: The Fajans MethodAn adsorption indicator is an organic compound thattends to be adsorbed onto the surface of the solid ina precipitation titration. Ideally, the adsorption occursnear the equivalence point and results not only in acolor change but also in a transfer of color from thesolution to the solid (or the reverse).

Fluorescein is a typical adsorption indicator useful forFluorescein is a typical adsorption indicator useful forthe titration of chloride ion with silver nitrate. Inaqueous solution, fluorescein partially dissociatesinto hydronium ions and negatively chargedfluoresceinate ion that are yellow-green. Thefluoresceinate ion forms an intensely red silver salt.Titrations involving adsorption indicators are rapid,accurate, and reliable.

Fajans – Fisher – Khodacov method

• Titrant: AgNO3 – secondary standardsolution

• Standardization on primary standardsolution of sodium chloride NaCl (by ameasured volume of primary standardsolution):solution):

• Medium: рН~ 6,5-10,3 (for determination ofclorides) and рН~ 2,0-10,3 (fordetermination of bromides and iodides).

• Indicators of method:– dichlorofluoroscein (for determination of clorides)

– eosine (for determination bromides and iodides)

The mechanism of indicators action

AgNO3 + NaCl = AgCl + NaNO3

HInd H+ + Ind-

Adsorption indicator whose color when adsorbedto the precipitate is different from that when it is insolution

ColourIndicatorIndicator SolutionSolution Surface ofSurface of

precipitateprecipitate

Dichloro-fluoroscein

greenish yellow pink

eosineeosine yellowishyellowish--redred redishredish -- violetviolet

• A number adsorption abilities of anion at рН~7on a precipitate surface of Silver chloride

• I- > CN- > SCN- > Br- > eosine > Сl- >dichlorofluoroscein > NO3

- > ClO4-

eosineeosine yellowishyellowish--redred redishredish -- violetviolet

Conditions of titration:• Acidity of solutions

• Concentration of reacting solutions

• The account adsorption abilities of indicatorsand ions which present in a solution

• Titration near equivalence point is necessaryto spend slowlyto spend slowly

• Titration with adsorption indicators spend in adiffused light

Use:• Quantitative definition of chlorides, bromides,

iodides, thiocyanides, cyanides.

The Volhard Method (Colored Complex)

In the Volhard method, silver ions are titrated with astandard solution of thiocyanate ion:

Ag+ + SCN- AgSCN(s)

Iron (III) serves as the indicator. The solution turnsred with the first slight excess of thiocyanate ion:

Fe3+ + SCN- Fe(SCN)2+

red

The titration must be carried out in acidic solution toprevent precipitation of iron(III) as the hydratedoxide.

KFe SCN

Fe SCNf

( ).105 10

3-

2+

3+

…continued…

The most important application of theVolhard method is for the indirectdetermination of halide ions. A measuredexcess of standard silver nitrate solution isexcess of standard silver nitrate solution isadded to the sample, and the excess silverion is determined by back-titration with astandard thiocyanate solution.

Volhard method• Titrant: AgNO3, ammonium or potassium

thiocyanide NH4SCN, KSCN - secondarystandard solution

• Stardadization AgNO3 on primary standardsolution NaCl, NH4SCN, KSCN onstandardization solution AgNO3:AgNO3 + NH4SCN = AgSCN + NH4NO3AgNO3 + NH4SCN = AgSCN + NH4NO3

• Indicator by standardization of ammoniumor potassium thiocyanide with iron (ІІІ) salts(NH4Fe(SO4)212H2O in presence of nitricacid) to the formation of the reddish coloredFe(SCN)2+ complex:

Fe3+ + SCN- = [Fe(SCN)]2+

• Medium: in presence of nitric acid

• Indicators of method: iron (ІІІ) saltsNH4Fe(SO4)212H2O in presence of nitricacid

• Determinate substance: halogenides,thiocyanides, cyanides, sulphides,carbonates, chromates, oxalates, arsenatesetc.

Until equivalence point (e.p.)

Hal- + Ag+ (excess) = AgHal

Ag+(rest) + SCN- = AgSCN

After e. p.

Fe3+ + SCN- = [Fe(SCN)]2+

(reddish)

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

2Fe3+ + 2I- = 2Fe2+ + I2Advantages of Volhard method

Titration possibility:

• In very acidic solutions

• In the presence of many cations which interfere bydefinition in Mohr method

End point detection in argentometric titration

Detection techniques in precipitation titrations : Indicator Potentiometry Light scattering / turbidimetry of nephelometry

Titration Mohr Volhard Fajans

Titration Ag+ + Cl– AgCl Ag+ + Cl– AgCl Ag+ + Cl– AgClreaction white Back titration :

Ag+ + SCN– AgSCNwhite

white

End point 2Ag+ + CrO42– Ag2CrO4 SCN – + Fe3+ FeSCN2+ Electric double

layerreaction red soluble red with adsorptionInd.

pH 7~10.5 Kf= 1.05×103

Dichlorofluorescein

Use Cl –, Br –, CN – Cl –, Br –, I – Cl –, Br –, I –, SCN –

No use I –, SCN –

Table 13-3, p.362

4. Thiocyanatometry• This is a precipitation titration in which SCN- is the

titrant.

• Titrant: ammonium or potassium thiocyanideNH4SCN, KSCN - secondary standard solution

• Stardadization: on primary standardsolution of AgNO3:

AgNO + NH SCN = AgSCN + NH NOAgNO3 + NH4SCN = AgSCN + NH4NO3

• Indicator by standardization of ammoniumor potassium thiocyanide with iron (ІІІ) salts:

Fe3+ + SCN- = [Fe(SCN)]2+

• Medium: in presence of nitric acid• Indicator: iron (ІІІ) salts NH4Fe(SO4)212H2O in

presence of nitric acid

Determine substance: drugs, which contain Silver(Albumosesilber, colloid silver - Kollargol, silvernitrate).

!!! At the analysis of drugs which containnonionic silver, preliminary it is heated withsulphuric and nitric acids (receive ioniccompound).compound).

!!! At determination of iodides the indicator is addedin the end of titration to avoid parallel:

2Fe3+ + 2I- = 2Fe2+ + I2

Advantages of Thiocyanatometry

Titration possibility:

• In very acidic solutions

• In the presence of many cations whichinterfere by determination in Mohrmethodmethod

5. Mercurometry• Mercurometry – this is a precipitation

titration in which Hg22+ is the titrant

2Cl- + Hg22+ = Hg2Cl2 Ksp = 1,310-18

2I- + Hg22+ = Hg2I2 Ksp = 4,5 10-29

• Titrant: Hg2(NO3)2 - secondary standardsolution

• Stardadization: on primary standardsolution of NaCl:

Hg2(NO3)2 + 2NaCl = Hg2Cl2 + 2NaNO3

• Indicators: 1) solution of iron (ІІІ) thiocyanide (fromreddish to colourless)

2Fe(SCN)2+ + Hg22+ = Hg2(SCN)2 + 2Fe3+

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

2) 1-2 % spirit diphenylcarbazone (to blue2) 1-2 % spirit diphenylcarbazone (to bluecolor)

!!! Indicator is added before end of titration

• Determinate substance: chlorides and iodides.

• Medium: very acidic (may be to 5 mol/L H+ ions).

Advantages of mercurometry:

• Possibility usage titration in very acidic solutions

• Titrant is cheaper

• Mercury (I) Salts are less soluble, thereforeaccuracy and sensitivity of titration more, andinflection point on a curve is much moreinflection point on a curve is much more

• Possibility of titration with the diphenylcarbazoneindicator in the muddy and painted solutions

• Probably reversive titration of Mercury (І) salts

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

6. Sulphatometry• Sulphatometry – this is a precipitation titration in which

SO42- is the titrant

Sometimes bariummetry – this is a precipitation titration inwhich Ba2+ is the titrant

Reaction of method:

Ba2+ + SO42- = BaSO4↓

determinate titrantdeterminate titrantsubstance

• Titrant: secondary standard solution of H2SO4, Ba(NO3)2,BaCl2.

• Stardadization: solution of H2SO4 on primary standardsolution of Na2B4O7 or Na2CO3 (crystal.) with methylorange; Ba(NO3)2 or BaCl2 on secondary standard solutionof H2SO4 with nitrchromazo or orthanilic А.

• Indicators: use metallochrome indicator - nitrchromazoor (orthanilic С), orthanilic А.

BaInd + H2SO4 = BaSO4 + H2Indviolet white roze

precipitate solutionDeterminate substance:• Direct titration:- by sulphatic acid – content of Barium- by barium chloride or barium nitrate – content of sulphates- by barium chloride or barium nitrate – content of sulphates

!!! For completeness of sedimentation of barium ions use assolvent of 50 % ethanol

!!! Sometimes, as the indicator use sodium rodizonate

SulphatometryEx. Ba2+ + SO4

2– BaSO4

(white)T

V(titrant, ml)

End pointTurbidimetry : The intensityof light scattered byparticles of precipitate ismeasuredmeasured

Detection of end point:1) Turbidimetry :

stabilizer(glycerol-alcohol mixture)2) Indicator :

rhodizonate + Ba2+ Red ppt

7. Hexacyanoferratometry

Hexacyanoferratometry - this is a precipitationtitration in which K4[Fe(CN)6] is the titrant

• Titrant: secondary standard solutionK4[Fe(CN)6]

• Stardadization: on standard solution of• Stardadization: on standard solution ofKMnO4 in sulphatic acid

5K4[Fe(CN)6] + KMnO4 + H2SO4 = 5K3[Fe(CN)6] + MnSO4 +3K2SO4 + 4H2O

• Indicator by standardization - Methylviolet (from yellow-dreen to red-brown).

Indicators:diphenylamine at the presence of a small

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

3,3-dimetilnaftizin for definition Cu2+, Zn2+,Cd2+, Pb2+, Fe2+, Ni2+ etc.alizarin red S for definition Zn2+, Pb2+,alizarin red S for definition Zn2+, Pb2+,

Th(IV)diphenylcarbazone for definition Cd2+

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

sample KCl + K2SO4 mass= 2,44gvol= 250mL

Aliquot 1 vol= 50mL

Mohr titration 41,4mL 0,05818M AgNO3Aliquot 2 vol= 50mL

add NaB(Ph)4 Vol= 40mL 0,1082M

ppt redissolve --> Mohr titration 50mLppt redissolve --> Mohr titration 50mLAliquot 1 --> mmol Cl=(V*M)AgNO3 titration 1=

2,40632mmol (1)Aliquot 2 --> mmol K+ = (mmol Cl + 2*mmol SO4)

= (V*M)AgNO3 titr2 = 2,90784mmol (2)mmol SO4= 0,5*(mmol(2)-mmol(1))=

0,25076mmol%KCl= 36,71%

=D17/D18*F24*(39+35,5)/1000*1/D16*100

%K2SO4= 8,94%

13-28

sample= 1,998g Cl- + CLO4-

vol= 250mL

Aliquot 1 50mL

titration AgNO3 13,97mL 0,08551M

Aliquot 2 50mL

reduction of Cl)4---> Cl-

titration AgNO3= 40,12mL

from Aliquot 1 mol Cl-=mol AgNO3= (V*M)AgNO3 titration 1= 1,194575mmol (1)from Aliquot 1 mol Cl-=mol AgNO3= (V*M)AgNO3 titration 1= 1,194575mmol (1)

from aliquot 2= mol Cl- + mol ClO4-= (V*M)AgNO3 titration 2= 3,430661mmol (2)

mmol ClO4-= (2) -(1) 2,236087mmol

%Cl= 10,61%

=B19/B20*G26*35,5/1000*1/B18*100

%ClO4= 55,68%

=B19/B22*G28*(35,5+4*16)/1000*1/B18*100