Estimation of Sodium Chloride: An Introduction
Analytical chemistry is the branch of chemistry that deals with the identification and analysis of the quantity or quality of chemicals or elements in a sample under consideration. Argentometry is a quantitative analytical method. It is a titration involving silver (I) ions. Argentometry is mainly used for the estimation of chloride ions in a sample. Because chloride ions can easily react with a silver(I) and form silver chloride. Silver chloride is insoluble and remains a white colour precipitate. Volhard method is a method in argentometry that helps in the estimation of sodium chloride in a sample.
Volhard Process
The Volhard process is a method used in argentometry for the estimation of chloride ion in a sample. The Volhard process is named after the scientist Jacob Volhard and is an example of back titration. In this method, excess silver nitrate is added to the sample containing chloride ions. Silver nitrate and sodium chloride are inorganic compounds. The precipitated silver chloride is filtered off then the remaining sodium nitrate is titrated against ammonium thiocyanate. Ferric ammonium sulphate is added as an indicator. Endpoint is the appearance of blood red colour due to the formation of a complex.
Theory of Volhard Method
Volhard's method is famous for the estimation of sodium chloride in a sample. Excess silver nitrate is added to the sample which reacts with the chloride ion of sodium chloride. The reaction of sodium chloride and silver nitrate is an example of a precipitation reaction where a white coloured silver chloride is precipitated.
The reaction of sodium chloride and silver nitrate is as follows
NaCl + AgNO3 →AgCl + NaNO3
The precipitated AgCl is filtered off. The reaction is carried out under acidic conditions. Addition of AgNO3 should be followed by addition of concentrated HNO3. Excess AgNO3 is added to ensure complete precipitation of AgCl.
A concentrated solution of KMnO4 is added to oxidize any organic matter that is not disposed of by HNO3. Following boiling cooling and dilution the excess AgNO3 is back-titrated against KSCN. And add ferric ammonium sulphate as an indicator. Ferric ammonium sulphate reacts with excess thiocyanate and forms a red blood-coloured complex.
SCN- can react with AgCl ppt. To prevent this diethyl ether or nitrobenzene is added to the solution.
Applicability
This method is useful for the estimation of salt-processed meat products at a level ≥ 0.08%.
Procedure of Volhard Method
Excess silver nitrate followed by concentrated HNO3 is added to the analyte. The precipitated AgCl is filtered off. Concentrated KMnO4 is added to oxidize the organic component which could not disposed of by con. HNO3. The mixture is boiled, cooled and diluted. It is then titrated against potassium thiocyanate. Ferric ammonium sulfate is added as the indicator. Nitrobenzene or diethyl ether is added to prevent the reaction of AgCl remaining in the solution with SCN-. The endpoint is determined by the appearance of red blood colour.
Result Calculation In Volhard Method
Volhard method is the back titration for the estimation of chloride ions. Here excess standard sodium nitrate solution is added to the sample. The excess silver is then back titrated against potassium thiocyanate. The amount of silver that is precipitated with chloride ions is calculated by subtracting the excess silver from the original silver content.
Ag+ + Cl– → AgCl
Ag+ + SCN– → AgSCN
SCN– + Fe3+ → [FeSCN]2+
Assay of Sodium Chloride
Assay in chemistry means the process of determining the composition or quality of a substance. For the assay of sodium chloride Volhard, the process is used.
The Volhard method of assay of sodium chloride is described below Weigh accurately 0.1 g of sample and dissolve it in 50 mL water. To this add 50 mL of 0.1 M silver nitrate, 5mL of 2M nitric acid and 2mL of concentrated KMnO4 . Shake well and titrate with 0.1 M ammonium thiocyanate taken in the burette. 2 ml of ferric ammonium sulphate solution is added as an indicator. Titrate until the colour become reddish brown.
Mohr method is also used for the assay of sodium chloride.
Interesting Facts
The reaction could not be carried out in a medium containing HCl. Because chloride ions will volatilize to HCl. The vapor pressure of HCl is higher than HNO3.
Geoforry in 1729 gives the first true description of a titration.
Key Features
An assay of sodium chloride is done by the Volhard titration method
Volhard titration is a back titration with excess silver nitrate against thiocyanate
Endpoint is indicated by a red blood colour by adding ferrous ammonium sulfate indicator
The amount of silver precipitated is calculated by subtracting the amount of excess silver from the original amount of silver.
FAQs on Estimation of Sodium Chloride
1. What are the advantages of the Volhard method?
It is capable of direct silver analysis and indirect chloride analysis. colour change is very visible.
2. What is the role of nitric acid in the assay of sodium chloride?
Nitric acid helps to maintain a high electrolyte concentration and this addition is critical to ensure complete precipitation of the chlorides.
3. Differentiate Mohr’s method and Volhard's method?
Mohr’s method is the reaction between silver ions with chloride ions in the presence of a chromate indicator. Volhard reaction is the reaction between excess silver ion and halide.
