Group 7: Halogens
Read our general notes on Risk Assessment
Are there any ways to analyse flourine concentrations in water in a class lab?
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Igloo replies ... The short answer is no!
Fluoride ions are difficult to detect since they are relatively unreactive – they cannot be easily oxidised for example – and there are no straightforward class lab tests available.
However, one feature of the fluoride ion is that it doesn’t precipitate out with silver ions (as chloride, bromide and iodide ions do), yet its lead salt is fairly insoluble. So, one rather cumbersome approach might be to add dilute nitric acid, followed by a sample of aqueous silver nitrate to the water sample. No precipitate should form, and this therefore eliminates the possibility of chloride, bromide and iodide ions being present.
Next, an aqueous solution of lead(II) nitrate solution can be added, and this should form a white precipitate of lead(II) fluoride, PbF2, if fluoride ions are present. However, this will only happen if the fluoride ions are in a sufficiently high concentration. If the concentration of fluoride ions is low, no precipitate will probably form, since the solubility product of lead(II) fluoride will not have been reached.
There are commercial test papers available for the detection of fluoride ions, but the chemical processes involved are highly complex, and in any case are not appropriate for use in a school or college lab.
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: 27 March 2008
