For nonpolar solvents, such tests are simply made by comparing the readings over saturated salt solutions before and after the addition of a drop of the test solvent. More polar solvents may dissolve significantly in the saturated salt solution and, hence, alter its true water activity. In these cases, we prefer to use pairs of solid salt hydrates, again with and without the addition of test solvent. The water activity would only be altered here if the organic solvent reacted chemically with the solid salt (e.g., forming an alternative solvate); this appears to be rare.
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