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Precipitation titrimetry

Gábor Galbács

Precipitation titrimetry

This is one of the oldest methods of analytical chemistry. It is avolumetric titration method where the reaction between thetitrant and sample solutions yield precipitate (low solubility,

ll i i d )usually ionic compounds).

But is it possible at all to develop a titrimetric method based onprecipitation reactions?

the formation of precipitate can be stoichiometric

the reaction can be quantitative

? precipitate formation is usually slow

? evolving of the reaction is apparent, but the end-pointis obscured, hazy…

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Argentometry

Argentometry, where the titrant is a standard AgNO3 solution isthe most common precipitation titrimetric method, because

il i it t ll hi hl i l bl• silver precipitates are usually highly insoluble• many species form stoichiometric precipitates with Ag+

(e.g. Cl-, Br-, I-, F-, CN-, SCN-, CrO42-, PO4

3-, AsO43-, etc.)

• these precipitates are formed quickly

Examples:

10 13 17 13Ksp, AgCl=1.8×10-10 Ksp, AgBr=5.2 × 10-13 Ksp,AgI=8.3× 10-17 Ksp,AgSCN= 4.9×10-13

Argentometry - standardization

Titrant is a standardized AgNO3 solution. It is not aprimary standard, because silver ions are easily reduced toelemental Ag e g they are also light sensitive Theelemental Ag, e.g. they are also light sensitive. Thetitrant needs to be stored in a dark (brown) container andstandardization is also needed.

The standardization is done by a standard alkali-halidesolution (e.g. NaCl, KCl)

Ag+ + NO - + Na+ + Cl- AgCl + NO - + Na+Ag+ + NO3 + Na+ + Cl AgCl + NO3 + Na+

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Argentometric titration curves

In argentometry, the titration curves usually plot pAg against theadded titrant volume. Note, that an increasing pAg meansstrongly decreasing [Ag+]. During the titration, the precipitateequilibrium determines pAgequilibrium determines pAg.

Example: 50 mL 0.1 M KSCN sample0.1 M AgNO3 titrantKsp,AgSCN= 4.9×10-13

Before the equivalence point:A i t ll d b th i i SCN t tipAg is controlled by the remaining SCN- concentration

At the equivalence point:pAg can be directly calculated from Ksp

After the equivalence point:pAg is controlled by the excess Ag+

Titration of KSCN with AgNO3

Before the equivalence point

0 mL titrant added (0%):

pAg can not be calculated as Ag+ is not present in samplepAg can not be calculated, as Ag+ is not present in sample…

Ksp= [Ag+]×[SCN-]

10 mL titrant added (20%):

pAg= -lg (Ksp / cSCN-, remaining) because solubility is neglectable…

cSCN-, remaining= ((cKSCN, total·VKSCN) - (cAgNO3·VAgNO3, added)) / Vtotal

pAg = -lg (4.9×10-13 / 0.0666 M) = 11.13

25 mL titrant added (50%):

pAg = - lg (4.9×10-13 / 0.0333 M) = 10.83

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Region 2: At the equivalence point

50 mL titrant was added (100%):

Titration of KSCN with AgNO3

there is only the precipitate present

pAg = 6.15

)Klg(pAg sp−=

Titration of KSCN with AgNO3

After the equivalence point

75 mL titrant added (150%):

[Ag+] is controlled by the excess titrant as the solubility of the [Ag+] is controlled by the excess titrant, as the solubility of the precipitate is relatively neglectable

pAg= -lg (cAg+, excess)

cAg+, excess= ((cAgNO3·VAgNO3, added)-(cKSCN, total·VKSCN)) / Vtotal

pAg= -lg ( 0.02 M ) = 1.69

100 mL titrant added (200%):

pAg = -lg ( 0.033 M ) = 1.47

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Argentometric titration curves

Effect of Ksp on titration curves

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Measuring halides side by side

Measuring halides side by side

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Effect of concentration on titration curves

0.05 M

0.005 M

End point detection methods

As was alluded to before, end point detection is crucial for thesuccess of precipitation titrations. The following end-pointdetection methods are (or has been) used in the practice:

the Gay-Lussac method

the Mohr method

the Volhard method

the Fajans method

the instrumental method

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End-point detection methods: Gay-LussacThis approach actually uses no indicator. Instead,the end-point is detected via the point offlocculation. If the titration is performed slowlyand carefully, it can be observed that at the end-point no further precipitate forms as a result of

Before EP

point, no further precipitate forms as a result ofthe next droplet of titrant, but the precipitatesuddenly flocculates, that is fluffy largeagglomerates are formed (the sample solutionsort of clears up).

The cause for this flocculation is that precipitateparticles tend to adsorb their own ions,especially the one which is in excess (the halide

i b f th EP d il ti ft thAfter EP

anions before the EP and silver cations after theEP), thereby acquiring electric charge. Becauseof this, they repel each other. At exactly the EP,there is no excess of ions, so no charge, thus theparticles attach to each other (flocculate).

This method is not very reliable. The more insoluble is the precipitate, the better is the accuracy.

End-point detection methods: Mohr

This approach uses chromate ions(CrO4

2-) as the indicator. The ideabehind the method is that Ag2CrO4 is ared precipitate, the solubility of whichi h t b tt th th t f this somewhat better than that of thesilver halide precipitates. Thus, if acalculated amount of chromate ions isadded to the sample and the titrationcommences, as soon as we pass the EP,the excess of silver ions will initiate theprecipitation of red Ag2CrO4.

The reliability of this methoddepends largely on theconcentration of the indicatoranion and the pH, as the latterinfluences the solubility of theindicator precipitate. Also becauseof the carcinogenic nature ofCr(VI), this method is seldomlyused.

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End-point detection methods: Volhard

This method is based on back-titration and uses a soluble,intensively colored substance (Fe(III)-thiocyanate complex)as indicator. Silver ions in a known amount, but in excess areadded to the sample. All halide ions are precipitated and theremaining free silver ions are then back-titrated using a KSCNsolution in the presence of Fe(III) ions. Fe(III) ions form a reddishcomplex with thiocyanate ions. This color will show as soon as theEP is passed during titration.

This method is reliable, but not very accurate (positive andnegative errors can both be present due to the interaction ofthe two silver precipitates). Accurate measurements can bedone, but preventive actions – via the addition of furtherreagents – are needed.

End-point detection methods: Fajans

This method is based on the use ofadsorption indicators. Fajans’s idea stemsfrom the fact that at the end-point, thes face cha ge of sil e halide p ecipitate

Before EP

surface charge of silver halide precipitateparticles will change polarity.

If indicator anions are added to the sample(e.g. eosin, p-etoxi-chrysoidine, fluorescein),that form a colorful precipitate with silverions, then these anions can only be adsorbedon the surface of the silver halide precipitatef om the end point on On the s face

After EPfrom the end-point on. On the surface,concentration conditions allow the formationof the corresponding precipitate, and hencethe color of their appearance allows the end-point detection. For example, fluorescein andeosin gives pink color.

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End-point detection methods: Fajans

Dye adsorption makes the silver halide susceptible tophotochemical reduction (silver deposition), therefore it is best toperform the titration in dark.

This method is reliable and accurate, but the accuracydepends again on the pH and experiencedness of theanalyst.

End-point detection methods: instrumental

Sample solution

Reference electrode

mV meter

electrode

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Precipitation titrations: some applications

Checklist for precipitation titrations

Remember:

• the discussed formulas of precipitate equilibriap p q

• effect of concentration and Ksp on the titration curve

• methods of standardization for the AgNO3 titrant

• end-point detection methods

potential applications• potential applications