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 forming 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.
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+
Frequently Asked Questions on Volhard Method
Why is Nitric acid used in the Volhard method?
The Volhard method is a back-titration developed to tackle this problem. For this process, using excess uniform silver nitrate, chlorides are precipitated to the solution. The silver nitrate that is used is created by the nitric acid reaction of silver foil.
What are the methods of determination of chlorides?
The chloride ion concentration is calculated by subtracting the titration findings of the silver ion moles which reacted with thiocyanate from the total moles of silver nitrate added to the solution. This approach is used when the solution’s pH is acidic, after the sample is prepared.
What is the Volhard method of analysis?
Volhard process for assessing chlorine, bromine, and iodine in the form of halides by precipitating them with excess silver nitrate and using a thiocyanate solution to titrate excess.
Which indicator is used in the Volhard method?
The expression ‘excess’ is used because the moles of added silver nitrate are considered to surpass the moles of sodium chloride present in the sample so that all the present chloride ions react. The indicator Fe3+ (ferric ion) is then added, with the potassium thiocyanate solution titrated to the water.
What is called the Argentometric method?
Argentometry is a form of titration which involves the silver(I) ion in analytical chemistry. This is usually used to determine the chloride content present in a sample. A solution of silver nitrate of known concentration is titrated against the sample solution.