Argentometric Precipitation Methods: Theory

THEORY

In the precipitation reaction involving chloride and silver nitrate, the addition of even a small quantity of the latter shall effect precipitation of AgCl provided that Ksp has been exceeded significantly. At this juncture, the concentrations of both Ag⁺ and Cl^– are related by the solubility-product equilibrium constant  thus, we have :

Chromate ion concentration required to initiate the precipitation of Ag₂CrO₄ commences at the equivalence point and may be calculated with the solubility products for AgCl and Ag₂CrO₄ :

In actual practice, the concentration of chromate produces an intense yellow colour to such an extent that the end point is masked. Therefore, normally concentrations of 5 × 10 ^–3 M are employed in analytical procedures. It suggests that [Ag⁺] shall be > 1.3 × 10 ^–5 M at the end-point thereby introducing a positive determinate error. However, it has been proved experimentally that even with concentrations as low as 2 × 10 ^–3 M, the extent of error caused is negligibly small.

Adsorption-coprecipitation phenomenon using fluorescein, dichlorofluorescein and tetrabromofluo-rescein (eosin) essentially impart the fluoresceinate ion that is absorbed on the AgCl particles. At the equiva-lence point, the AgCl particles change from white to pink due to the coprecipitation of silver fluoresceinate. In short, the adsorption indicator method is quite rapid and capable of providing very accurate results for the estimation of Cl^– with AgNO₃.

Furthermore, Br^–, I^– and SCN^– ions can also be titrated with AgNO₃ employing eosin as an adsorption indicator.