Effect of pH on absorbance of azo dye formed by reaction between nitrite and sulfanilamide/N-(1-naphthyl)ethylenediamine in residual nitrite methods for foods.

Current methods used for determining residual nitrite concentration in foods involve forming an azo dye that is measured spectrophotometrically. Conventional procedures do not specify control of pH for the final colored solution. Because many indicator dyes are pH-dependent, absorbance of the azo dye may vary not only because of nitrite concentration differences, but also because of pH variation. Thus, erroneous results could arise from a standard curve developed at one pH and used for a test sample at a different pH. This would cause an inaccurate concentration conversion from the standard curve. In this study, absorbances at lambda max wavelengths were recorded at 28 pH values (range 1.0-4.5) for each of 5 nitrite concentrations. A mathematical equation was fitted to these data and its plot gives a 3-dimensional response surface showing the relationships between pH, nitrite concentration, and absorbance. A modification for the spectrophotometric measurement of nitrite is proposed.