The IR spectrum of a sample in acidic and alkaline solutions cannot be retrieved adequately when only the spectrum of pure water is subtracted. After such an operation, some water bands remain in the spectrum, which also has a distorted baseline. An analysis of a series of IR spectra of HCl and NaOH solutions showed that they could be represented by two pairs of eigenspectra, one pair for the acidic solutions and the other for the basic solutions. The fraction of each eigenspectrum of a sample in an acidic or alkaline solution is determined with the 2100 and 3300 cm-1 water bands. After subtraction, no baseline adjustment is necessary. The effectiveness of the method used to subtract the water bands is illustrated with solutions of malic acid at low and high pH.

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