The method of changing parameter in photometry has been introduced to allow the determination of a substance in the presence of a strongly interfering unknown matrix. The theory of this method is now discussed in detail. The discussion is illustrated by the case of the determination of calcium in a strongly interfering matrix of HCl and LaCl<sub>3</sub>, using an atomic absorption flame photometer. The derivations are, however, general enough to apply to any photometric method, and even to some nonphotometric methods having the same formal dependence on a changing parameter. It is shown that a changing parameter can be sensitive enough to prove the existence of interference, while not being sensitive enough to evaluate such an interference. It is also shown that even a very considerable interference can be difficult to detect and to evaluate.

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