Group 7: Halogens
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I am currently doing a project on determining the salt content in crisps. I have been using a volumetric method, a Mohr titration. I have ground up crisps and added distilled water in order to dissolve the NaCl and made it up to a standard solution and filtered. I have titrated these solutions with silver nitrate with potassium dichromate as an indicator, as the silver will react with the chloride ions in the salt preferentially and then will react with the dichromate ions, when there is no chloride ions left, turning the solution brick red. I want to now try to determine the salt content of the crisps gravimetrically by producing a silver chloride salt, but I'm not quite sure where to start.
270110
Igloo writes ...
The first thing to mention is that for the analysis to be successful via a gravimetric method, you must have access to a weighing balance of high precision, e.g. weighing to at least 0.01g or, better still, to 0.001g.
Make up your salt solution just as you did for the silver nitrate titration. Take a known volume (via a pipette), transfer to a beaker and add an excess of silver nitrate solution, i.e. enough to ensure that all the chloride ions have been precipitated as silver chloride. The white cloudy solution should be kept away from sunlight since AgCl decomposes in UV light. Filter the mixture, wash the precipitate by adding distilled water through the residue in the filter paper, and then dry the residue completely (using an oven at about 120oC). Finally, the precipitate can be weighed.
The mass of AgCl obtained is then multiplied by 35.5/(108+35.5) to find the mass of chloride ions within it. You can then deduce from this the mass of salt originally present in the volume of solution used, and then, by scaling up, the mass of salt in the original mass of crisps used.
One or two practical details to note:
You will need to calculate what volume of silver nitrate represents an excess (your titration results should help here) and then add at least this quantity. Don’t add too much – silver nitrate is expensive.
Filtration under reduced pressure is to be preferred – perhaps using a Buchner flask. Silver chloride tends to be colloidal and slow to filter. You must, however, use a good quality filter paper which holds back all the AgCl.
Thorough drying is essential. Even small amounts of residual water/moisture will ruin your results. You may decide to run a small volume of ethanol through the solid in the filter paper, after the washing with water. This helps to speed up the dryness process. Why?
You will need to think about how you are going to remove the solid from the filter paper (after? or before? drying).
By the way, you might decide that to get more accurate results you should use larger quantities all round. This is to ensure that you obtain a mass of AgCl ‘worth weighing’. Inaccuracies tend to be more significant as the mass being weighed becomes smaller. Can you see why?
Yes, there are many practical problems to overcome in order to achieve good results, and when you weigh, make sure that you are doing so in a draught-proof environment.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 27 January 2010
