Introduction
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.
The reactions are summarized below:
Before end point: Ag
+ + Cl
- --> AgCl
At end point: 2Ag
+ + CrO
42- --> Ag2CrO4
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.
2. Flame – photometry (flame emission spectroscopy)
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.
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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.
Discussion & Conclusion
In general unexpected or imperfect results of salt determination are probably due to the fact that compounds of sodium and chloride except for the salt (as sodium chloride), may be present in the food.
For instance, Sodium present in potential additives such as sodium nitrite phosphate (stabilizer), sodium nitrite (preservative) and sodium bicarbonate (raising agent) may considerably affect the analysed sodium content. Chloride may also be present in compounds like potassium chloride which is similarly important in evaluation for the following reasons:
If sodium content is to be achieved as the basis for salt determination (Flame photometry), then the presence of sodium from other compounds (e.g. additives) can result in over-estimation of the equivalent salt content.
If Chloride content is to be calculated in order to achieve equivalent salt content; Following this estimation, sodium content is determined assuming that the sodium is only extracted from the salt (as NaCl), whereas the sodium may be present as part of other sources that hasn’t been taken into account, so the actual level of sodium is underestimated.
In the same way, if estimated chloride has been derived from other sources (e.g. potassium chloride), then the equivalent salt content and the sodium content will be overestimated if determined from the chloride concentration.
Although titration (Mohr) method seems to be a well known, simple, direct and perhaps accurate method for chloride determination, the methods in which salt content is calculated from initially determined sodium concentration (spectrometry) are believed to be more precise and reliable than chloride-based analysis.
Chloride ions are liable to be lost during the experiment at the stage of ashing as it is sensitive to volatilisation.
However it should be remembered that low temperature in the flame photometry may lead to some certain disadvantages in connection with interference and stability of flame and aspiration conditions. Aspiration rates, fuel and oxidant flow rates and purity, solution viscosity and concomitants in experimental samples may contribute to these consequences. Therefore it is very important to determine the emission of the standard and unknown solution under relatively identical conditions. [1159]
