Practical investigations
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I have chosen to use a silver nitrate titration in my individual investigation to find the salt content of food. However, I'm not sure how this whole reaction works!
Igloo writes
Silver nitrate titrations are not very commonly carried out, owing to the high cost of the silver nitrate and to the difficulties associated with detecting the end-point.
You will have to check that other ingredients in the food do not interfere with the titration - or make it impossible to detect the end-point.
However, I’ll give you a brief account of the principles on which this technique is based.
The solution containing chloride ions is measured out into a conical flask, a few drops of (yellow) potassium chromate are added and the standard silver nitrate solution is slowly run in from a burette. As the titration proceeds an off-white precipitate of silver chloride is formed, so the mixture becomes increasingly murkier and murkier!
Ag+(aq) + Cl-(aq) -> AgCl(s)
At the end-point, when all the chloride ions have been destroyed, any extra silver ions added form a red precipitate with the chromate ions of the indicator, and so the sign that this point has been reached is to detect a colour change of the precipitate from white to red.
2Ag+(aq) + CrO42-(aq) -> Ag2CrO4(s)
This is not at all easy, so you will probably need several practice runs to perfect your technique.
Before you start try to ensure that the concentration of chloride ions in your solution of foodstuff is similar to that of the silver ions in the silver nitrate solution, so that “sensible” titration readings are obtained, and that precious silver nitrate solution is not wasted. Remember to make up the silver nitrate solution in de-ionised/distilled water rather than tap water (which contains Cl-), and ask your supervisor if there is a “silver residues” bottle for all your silver waste materials.
Do realize that this titration detects the presence of chloride ions, as opposed to sodium chloride specifically, so for example, if there is any potassium chloride present, this will be detected as if it were sodium chloride. The presence of bromide and iodide ions will also interfere with this method.
Always carry out a risk assessment and check with your teacher before starting any practical work.
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: 22 January 2006
