Volhard Method

What is the Volhard Method?

The volhard method is a procedure for titrating silver(I) with standard potassium thiocyanate (KSCN).

The titration is carried out in acidic solution. When the silver(I) has been precipitated as white silver thiocyanate, the first excess of titrant and the iron(III) indicator react and form a soluble red complex. The method is widely used for silver and chloride, because the titration can be done in an acidic solution.

Volhard Method Procedure

The titration of silver with NH₄SCN with ferric alum as an indicator is an example of the class of titration involving the formation of a coloured substance in the solution. During titration AgSCN is formed while at the end point excess NH₄SCN reacts with Fe(III) to form deep red [FeSCN]²⁺. The amount of thiocyanate which will give a visible colour is very small. Thus, the end point error is very small but the solution should be shaken vigorously at the end point as silver ions are absorbed on the precipitate and are then desorbed.

In Volhards method, one can easily determine chloride ions in acidic solution, otherwise in basic media Fe³⁺ will hydrolyse. An excess of AgNO₃ is added to chloride solution and part of it is unreacted. Ag solution is back titrated with ferric alum as indicator, but the method has a source of error. AgSCN is less soluble than AgCl, So

AgCl + SCN^– ⇾ AgSCN + Cl^–

This will consume more NH₄SCN and chloride content will appear lower. This error can be eliminated by filtering off AgCl precipitate before back titration or if a little nitrobenzene is added, it will adhere to AgCl and protect it from reaction with thiocyanate however, nitrobenzene slows down the reaction.

This can be avoided if Fe(NO₃)₃ and a small measured amount of NH₄SCN are added to the chloride solution at the start with HNO₃ and the mixture titrated with AgNO₃ until the red colour disappears.

Volhard Method in Precipitation Titration

The Volhard method is designed for the direct titration of silver in dilute nitric acid with standard thiocyanate solution. Added ferric ion forms the clear, red colored ion [FeSCN]²⁺, at the precipitation endpoint.

Here, chloride can be determined indirectly by adding a known excess of standard silver solution, then back titrating with the Volhard method. This is a less desirable approach. However, since the ethacrynate back titrant tends to react slightly with the AgCl to form the even more stable precipitate AgSCN.

As with the Mohr method, a blank or a titer is advised for both the direct and indirect approaches. In the Volhard method a standard solution of thiocyanate ion (SCN) is titrated against Ag⁺ using Fe³⁺ as the indicator.

Volhard Method Calculation

In the Volhard method silver ions are titrated with a standard solution of thiocyanate ions. The volhard method is an indirect or back titration method in which an excess of a standard solution of silver nitrate is added to a chloride containing sample solution.

The excess silver is then back titrated using a standardized solution of potassium or ammonium thiocyanate with ferric ion as an indicator. The amount of silver that is precipitated with chloride in the sample solution is calculated by subtracting the excess silver from the original silver content.

Ag⁺ + Cl^– ⇾ AgCl

Ag⁺ + SCN^– ⇾ AgSCN

SCN^– + Fe³⁺ ⇾ [FeSCN]²⁺