Aqueous samples are susceptible to rapid chemical and physical reactions between the sampling time and analysis. Since the time between sampling and analysis could be greater than 24 hours, the following preservation techniques are recommended to avoid sample changes resulting in errors: all samples except metals must be refrigerated. Refrigeration of samples to 4°C is common in fieldwork, and helps stabilize samples by reducing biological and chemical activity (EPA 1979).
In addition to refrigeration, specific techniques are required for certain parameters (see section 10.9). The preservation technique for metals is the addition of nitric acid (diluted 1:1) to adjust the pH to less than 2, which will stabilize the sample up to 6 months; for cyanide, the addition of 6N caustic will adjust the pH to greater than 12, and refrigeration to 4°C, which will stabilize the sample for up to 14 days. Little other preservation can be performed on solid samples.
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