Argentometric methods
1. Introduction
Argentimetric titrations are a type of complexometric titration which is used to accurately
measure the concentration of specific anions (e.g. chloride ions) through their complexation
with silver cations (Ag+). For this reason, argentometric titrations are regarded as precipitation
titrations with one of the resulting precipitates from the reaction of the titration being an
insoluble silver salt.
Through a technique known as ‘Volhard’s Method’ (named after the German chemist Jacob
Volhard), the concentration of thiocyanate ions (SCN-) in aqueous solution was determined
quantitatively by titrating a standardised solution of silver nitrate against a solution of
thiocyanate of unknown molarity using ammonium iron (III) sulfate as the indicator. The
addition of the SCN- initially produces a brown precipitate of silver thiocyanate (AgSCN) as
shown by the following equation.
Ag+(aq) + SCN-(aq) → AgSCN(s)
Once all of the Ag+ is used up through this reaction, a slight excess of the thiocyanate reacts
with the aqueous Fe3+ present to form the complex [FeSCN]2+, otherwise known as
thiocyanatoiron.
Fe3+(aq) + SCN-(aq) → [FeSCN]2+(s)2
The resulting brown complex induces a relatively observable colour change of white (due to
the silver chloride precipitate) to brown (due to the [FeSCN]2+ complex)