1. Precipitation Titrimetry
Dr. Mohammad Khanfar

2. Precipitation Titrimetry
Precipitation titrimetry, which is based on reactions that yield ionic compounds of limited solubility.
Because of the slow rate of formation of most precipitates, however, there are only a few precipitating agents that can be used in titrimetry.
By far the most widely used and most important precipitating reagent is silver nitrate, which is used for the determination of the halides, the halide-like anions (SCN-, CN-, CNO-), mercaptans, fatty acids, and several divalent and trivalent inorganic anions.
Titrimetric methods based on silver nitrate are sometimes called argentometric methods.

3. Precipitation Titrimetry
The four parameters that may be considered for a feasible argentometric analysis:
1) Precipitate formed must be insoluble,
2) Precipitation process should be fast and rapid,
3) Co-precipitation effects must be minimal, and
4) Detection of equivalence point must be apparently visible.

4. Precipitation Titrimetry, theory
When silver nitrate solution is run into a solution of sodium chloride a precipitate of silver chloride is formed:
The end point is the point at which all the chloride has been precipitated as silver chloride.
However, detection of complete precipitation by observing the point at which addition of reagent causes no further precipitation is too tedious in practice and use is usually made:
1) A chemical reaction to give a colored precipitate or,
2) colored solution at the end point

5. Precipitation Titrimetry, theory
For instance, in the former example potassium chromate (K2CrO4) is added to the solution. As the silver nitrate solution is added, the chloride is precipitated as silver chloride, but when all the chloride has been precipitated, the next drop of silver nitrate causes precipitation of red chromate, indicating that the end point has been reached ( as Ag2CrO4).
Kf: is the formation constant of the precipitate.
The equilibrium constant for such a reaction in its reversed direction is known as solubility product or constant (Ksp)
Ksp = [Cl-] [Ag+] / [AgCl] Kf
Ksp

6. Precipitation Titrimetry, theory
The higher the formation constant of the insoluble product the more complete and spontaneous the reaction would be. i.e. the lower the Ksp of certain product the more complete the reaction would be.
Titration curve
A titration curve for this method usually consists of a plot of pAg versus the volume of silver nitrate added.
Calculate the pAg for titration curve of mL of 0.05 M NaCI with 0.10 M AgN03 (for AgCI, Ksp = 1.82 X ).
At equivalence point.
M1V1 = M2V2
Ksp = [Cl-] [Ag+]
[Cl-] = [Ag+]

7. Precipitation Titrimetry
After addition of 10 ml (preequivalence)
Ksp = [Cl-] [Ag+] = 1.82 X 10-10
Postequivalence-Point
10 ml of silver nitrate after the equivalence point
[Ag+] = [AgNO3] excess

8. Precipitation Titrimetry
The Effect of Reaction Completeness (Kf) on Titration Curves
The higher the Kf value (i.e. the lower
the solubility product, Ksp) the sharper
the end point.

9. Precipitation Titrimetry
Indicators for Argentometric Titrations
The end point produced by a chemical indicator usually consists of a color change or, occasionally, the appearance or disappearance of turbidity in the solution being titrated.
Three methods to determine the end points:
1) The Mohr Method, Chromate Ion
2) The Fajans Method, Adsorption Indicators
3) The Volhard Method, Iron(III) Ion

10. Precipitation Titrimetry
Chromate Ion; The Mohr Method
Sodium chromate can serve as an indicator for the argentometric determination of chloride, Bromide, and cyanide ions by reacting with excess silver ion to form a brick-red silver chromate (Ag2CrO4) precipitate in the equivalence-point region.
The silver ion concentration at chemical equivalence in the titration of chloride with silver ions is given by

11. Precipitation Titrimetry
The chromate concentration required at the equivalence point to initiate red color is:

12. Precipitation Titrimetry
This method is associated with positive systematic error for two reasons:
1) chromate ion concentration of 6.6 X 10-3 M imparts such an intense yellow color to the solution that formation of the red silver chromate is not readily detected, and lower concentrations of chromate ion are generally used for this reaction. As a consequence, excess silver nitrate is required before precipitation begins.
2) An additional excess of the reagent must also be added to produce enough silver chromate to be seen.
A correction for this error can readily be made either by blank titration or standardization of silver nitrate solution against primary-standard-grade sodium chloride
The Mohr titration must be carried out at a pH of 7 to 10 (why?)

13. Precipitation Titrimetry
Adsorption Indicators: The Fajans Method
An adsorption indicator is an organic compound that tends to be adsorbed onto the surface of the solid in a precipitation titration. Ideally, the adsorption occurs near the equivalence point and results not only in a color change but also in a transfer of color from the solution to the solid (or the reverse).
Fluorescein is a typical adsorption indicator that is useful for the titration of chloride ion with silver nitrate.
Another adsorption indicator is the eosine, structurally similar to fluorescein.

14. Precipitation Titrimetry
In aqueous solution, fluorescein partially dissociates into hydronium ions and negatively charged fluoresceinate ions that are yellow-green. The fluoresceinate ion forms an intensely red silver salt.

15. Precipitation Titrimetry
How it works?
The fluoresceinate anion tends to be adsorbed on the surface of precipitated silver chloride, but not so strongly as chloride.
In the first stages of a titration, chloride ions in solution are adsorbed preferentially on the surface of the precipitate. As the titration proceeds, the chloride ion concentration decreases until it approaches a small value at the equivalence point.
After the equivalence point a slight excess of the silver ion adsorbed at the surface of the precipitate, and the attraction between adsorbed silver ion and fluoresceinate anion causes the latter to be adsorbed onto the surface of the precipitate.
This adsorbed layer is pink, and the appearance of this pink color on the precipitate is taken to be the end point.
Because the color change takes place on the surface of the precipitate, it is sharper if the surface area is large.

16. Precipitation Titrimetry
Iron(III) Ion; The Volhard Method
In the Volhard method, silver ions are titrated with a standard solution of thiocyanate (titrant) ion:
lron(lII) serves as the indicator. The solution turns red with the first slight excess of thiocyanate ion:
FeSCN2+ is soluble product not precipitate
The titration must be carried out in acidic solution to prevent precipitation of iron(III) as the hydrated oxide.

17. Precipitation Titrimetry
The most important application of the Volhard method is the indirect determination of halide ions.
A measured excess of standard silver nitrate solution is added to the sample, and the excess silver is determined by back titration with a standard thiocyanate solution.
The strong acidic environment required for the Volhard procedure represents a distinct advantage over other titrimetric methods of halide analysis

18. Precipitation Titrimetry
Silver chloride is more soluble than silver thiocyanate (i.e. the Kf of AgSCN is higher than AgCl) . As a consequence, in chloride determinations by the Volhard method, the reaction
occurs to a significant extent near the end of the back-titration of the excess silver ion. This reaction causes the end point to fade and results in an overconsumption of thiocyanate ion, which in turn leads to low values for the chloride analysis. This error can be circumvented by filtering the silver chloride before undertaking the back-titration.
Filtration is not required in the determination of other halides because they all form silver salts that are less soluble than silver thiocyanate.

19. Some applications of precipitation titrimetry
Chloral hydrate (sedative and hypnotic drug)
The analysis of chloral hydrate depends on acid-base titration with sodium hydroxide based on the this reaction:
As the chloroform generated, it undergoes chemical reaction with the alkali to a certain degree; therefore, addition of alkali followed by back titration does not afford the correct assay.
Thus, we have : 1 2

20. Some applications of precipitation titrimetry
Therefore, the sample is dissolved in excess NaOH. After standing for 2 min (reaction 1), the excess alkali is back titrated with H2SO4 using phenolphthaline as indicator. Because reaction (2) has occurred to some extend, the neutral solution is titrated with 0.1M AgNO3 using potassium chromate as indicator, which gives the amount of NaOH taken up in this reaction. The amount of AgNO3 taken by this step plus the amount of H2SO4 by the previous back-titration when subtracted from the volume of NaOH originally added, gives the amount of NaOH required by the sample in the reaction 1.

21. Some applications of precipitation titrimetry
Example: 4 g of chloral hydrate ( g/mol) was dissolved in 10 ml of DW and then 50ml of 0.5 M NaOH. After 2 minutes, the solution was titrated with 32ml of 0.1M H2SO4, employing phenolphthalein solution as indicator till a color change from pink to colorless was achieved. The neutralized liquid was titrated with 8ml of 0.1 N AgNO3 using potassium chromate solution as indicator till precipitation of red chromate was obtained. Calculate the w/w% of chloral hydrate.
Extra 2 points on the midterm exam.

22. Some applications of precipitation titrimetry
A 20-tablet sample of soluble saccharin was treated with mL of 0.08 M AgN0 3 . The reaction is
After removal of the solid, titration of the filtrate and washings required 2.81 mL of 0.05 M KSCN. Calculate the average number of milligrams of saccharin ( g/mol) in each tablet.