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:
Flame – photometry (flame emission spectroscopy)
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 degrees Celcius (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.
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 ferric ion to produce a bright red complex.
Flame photometric methods are usually used for analysis of metal salts, particularly Na, K, Li, Ca, and Ba. in samples that are easily prepared as aqueous solutions.
The alkali metals, when increased to a sufficiently high temperature, will absorb energy from the source of heat and be raised to an excited state in their atomic form.
When these individual atoms 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.
So if an alkali metal in solution is aspirated into a high temperature flame in an aerosol form it will radiate (after excitation by the flame) a separate frequency that can be isolated by an optical filter.
This ignition is related (in low concentrations only) to the number of atoms returning to the ground state, which is in turn in relation to the number of atoms ignited, i.e. the concentration of the sample.
Flame photometry is a straightforward, almost cheap method with high throughput of sample that can offer applications for chemical, biological and environmental evaluations.
This method 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.
However the low temperatures may result in some disadvantages usually in association with intervention and the durability of the flame and aspiration system. Other factors affecting these consist of fuel and oxidant flow rates, aspiration, solution consistency and concomitants in the samples.
Therefore it is crucial to determine the emission of the standard and experimental solutions under the same conditions as much as possible.