ABSTRACT
This chapter shows that the application of acid-base equilibria for the difference spectrophotometric determination of drug. The application of difference spectrophotometry is very advantageous both in qualitative and quantitative analysis. However, the real great importance of difference spectrophotometry is in quantitative analysis as a consequence of its potential to increase the selectivity of the measurement. The use of difference spectrophotometry is of no importance when color reactions are applied, since in the visible range matrix-based background absorptions do not usually interfere. Although the reaction between primary aromatic amines and formaldehyde leading to Schiff's bases does not cause strong bathochromic and hyperchromic shifts, even this small shift is sufficient to serve as the basis of highly selective difference spectrophotometric method. As a result of the complex formation at pH 7 and 6, respectively, an intense bathochromic shift of the spectrum of the diphenol takes place.
