Sodium Chloride determination
Evaluation of salt concentration (sodium chloride) present in foodstuffs is very important mainly for the reason of preservation and taste of the food products.
Total chloride in the food is usually determined and can be presented as sodium chloride content.
Insignificant components of foods may also provide chloride ions but those food products to which salt is added are essentially analysed for sodium chloride content.
Salt content of the food may be analytically determined in one of the following ways:
1. Chemical techniques
Titrimetric methods are the most widely used for salt determination and are usually based on the following methods:
– Titration Mohr method
The Mohr method uses chromate ions as an indicator in the titration of chloride ions with a silver nitrate standard solution. Following the precipitation of the whole amount of chloride (usually as white silver chloride) the first excess of titrant results in the production of a silver chromate precipitate, that indicates the end point.
When the stoichiometry and moles consumed at the end point are identified, the amount of chloride in the experimental sample can be determined.
Following the experiment in this report (to determine sodium chloride content in the sample), the salt is initially required to be extracted from the food sample (Tomato ketchup) by means of accurate ashing at 500 – 550°C (alkali chlorides are virtually volatile at higher temperature) with subsequent dissolution of the ash.
As mentioned earlier Mohr procedure involves direct titration with 0.1M silver nitrate in which the chloride content is measured in the absence of acid.
– Titration Volhard method
Chloride ion can be determined by Volhard procedure during which the food sample is boiled in diluted nitric acid. The method involves the addition of excess silver nitrate and back titration with potassium thiocyanate.
Adding an excess of silver nitrate solution to a solution containing chloride ions, results in the precipitation of silver chloride.
The concentration of chloride can then be analysed by back-titrating of the excess (unreacted) silver ions with thiocyanate solution to create a silver thiocyanate precipitate.
Fe3+ (ferric ion) is usually used as an indicator for the titration because as soon as all the silver ions have reacted, the minimum excess of thiocyanate will react with Fe3+ to produce a bright red complex.