ARGENTOMETRIC PRECIPITATION METHODS
In general, titrations governed by precipitation reactions do not really constitute an appreciable number in volumetric determinations in comparison to either redox or acid-base reactions. The interaction between silver-nitrate and sodium chloride in solutions result into the precipitation of silver chloride as shown below :
NaCl + AgNO3 → AgCl ↓ + NaNO3
In actual practice, however, such titrations are more or less restricted to those involving precipitation of Ag+ with anions, for instance : halogens (Cl–, Br–, I–) and thiocyanate (SCN–). Generally, it is quite difficult and tedious to locate the exact point at which further addition of reagent affords no more precipitation. There-fore, the choice and wisdom of a chemical reaction is preferably sought so as to result in either a coloured solution or a coloured precipitate at the end point. A typical instance may be cited by application of potassium chromate (K2CrO4) solution in the above case whereby any extra drop of silver nitrate, after all the chloride has been precipitated, immediately causes precipitation of red chromate showing that the end point has been duly achieved.
It is, however, interesting to observe here that such reactions do offer limited usage because of the following two facts, namely :
(i) Co-precipitation effects do not give a real composition of the precipitate, and
(ii) Choice of appropriate indicator is very much limited.
Besides, the foregoing facts another vital aspect to be taken into consideration is the solubility product that plays a major role in such titration. Hence, the equilibrium constant of the reaction giving the precipitate of AgCl may be expressed as :
From the above expression the solubility product constant Ksp may be designated as :
assuming the activity of solid AgCl being constant.
Following are the four cardinal parameters that may be considered for a feasible argentometric analysis, namely :
(i) Precipitate formed must be insoluble,
(ii) Precipitation process should be fast and rapid,
(iii) Co-precipitation effects must be minimal, and
(iv) Detection of equivalence point must be apparently visible.