Evaluation of sodium chloride (salt) content of the food products is important to ensure that the requirements of food quality and food safety are met. Sodium chloride content is usually determined by the essay of total chloride concentration in the foods. Food products to which salt is added are essentially analysed for sodium chloride content.
There are various methods to measure the salt content of the foods, among which the following prevail:
1. Chemical techniques
Titrimetric methods are the most widely used techniques for the salt determination of foods. They include titration Mohr and titration Volhard 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.
The reactions are summarised as below:
Before end point: Ag++ Cl– –> AgCl
At end point: 2Ag+ + CrO42- –> Ag2CrO4
The salt is initially required to be extracted from the food sample by means of accurate ashing at 500 – 550°C with subsequent dissolution of the ash. Mohr procedure involves direct titration with 0.1M silver nitrate in which the chloride content is measured in the absence of acid.
Titration (Mohr) method is a well known, simple and accurate assay for chloride determination. Chloride ions however, are liable to be lost during the experiment at the stage of ashing as they are sensitive to the volatilisation.
– Titration Volhard method
Chloride ions 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.
(ferric ion) is usually used as an indicator for the titration. This is because as soon as all the silver ions have reacted, the minimum excess of thiocyanate will react with Fe
to produce a bright red complex.
2. Flame – photometry (flame emission spectroscopy)
Flame photometric methods are commonly used for analysis of metal salts, particularly Na, K, Li, Ca, and Ba. The samples are prepared as aqueous solutions. The metal salts are analysed quantitatively by determining the flame emission of the aqueous solutions. As a result of the high temperature of the flame, the alkali metals will absorb energy from the heat and will develop to an excited state in their atomic forms. When these individual atoms become cool, they will return to their original unexcited condition and re-emit their absorbing energy by means of radiation at certain wavelengths; some of which are in the visible zone.
In other words, the alkali metal (in solution) is aspirated into a high temperature flame in an aerosol form which in turn results in the radiation of a separate frequency that can be isolated by an optical filter. The ignition which is in low concentration, is related to the number of atoms returning to the ground state. This is in turn in correlation with the number of atoms ignited, i.e. the concentration of the sample.
Flame photometry is a straightforward and almost cheap method with high throughput of the samples. The method can offer applications for chemical, biological and environmental evaluations. It is limited to easily ionised metals due to the low temperature of the natural gas and air flame in comparison with other ignition methods involving sparks, gas plasmas, etc. As the temperature isn’t considerable enough to ignite transition metals, the method is especially used for identification of alkali and alkali earth metals.
Low temperature in the flame photometry may lead to some disadvantages. These include interference and stability of the flame; aspiration rates; fuel and oxidant flow rates; solution viscosity and concomitants in the samples. Therefore it is very important to determine the emission of the standard and the unknown solution under relatively identical conditions.
Unexpected results of the salt determination are probably due to the presence of the compounds (other than salt as sodium chloride) which contain sodium and/or chloride. For instance, Sodium present in the additives such as sodium nitrite phosphate, sodium nitrite and sodium bicarbonate which can significantly affect the analysed sodium content. Chloride may also be present in compounds like potassium chloride which can result in over estimation of the total chloride content of the analysed food.