chapter  18
13 Pages

Sample preservation prior to analysis

Aqueous solution of some anions eg chloride and sulphate are quite stable (ie do not undergo decomposition) when stored up to one month at ambient temperatures. However, it is necessary to consider what happens if the sample also contains a further anion which upon storage decomposes to produce additional amounts of the anion of interest. For example, if a sample which is required to analyse for sulphate also contains sulphide, polysulphides, sulphite, thiosulphate, dithionate, polythionates then it is certain that the true initial sulphate content of the sample at the moment of sampling will not be obtained even if the sulphate analysis is carried out within 1h of the sample being taken. A similar situation would exist for a sample in which it is required to determine chloride but which also contains anions such as chlorite, chlorate and hypochlorite and possibly perchlorate.