Interference of ascorbic and isoascorbic acids in the spectrophotometric determination of nitrite by the diazotisation-coupling technique

Abstract

The problem of the interference of ascorbic and isoascorbic acids in the spectrophotometric determination of nitrite by the diazotisation-coupling technique is important because these substances are frequently added to cured meats containing nitrite. The chemistry of the reaction of ascorbic and isoascorbic acids with nitrite is discussed. The effect of pH on the reaction at room temperature and on digestion at 80 °C was investigated. Two composite diazotisation-coupling reagents [sulphanilamide and N-(1-naphthyl)ethylenediamine; 4-nitroaniline and N-(1-naphthyl)ethylenediamine] were used in this investigation. At room temperature 1 mg of ascorbic or isoascorbic acid (per 50 ml) interfered only slightly or moderately at pH 10 and 12 but interfered very significantly at pH 8 and below. When solutions at pH 8, 10 and 12 were heated at 80 °C for 1 h, satisfactory recovery was obtained in the presence of 2.5 mg of ascorbic or isoascorbic acid (per 50 ml). A study of the stability of pure nitrite solutions indicated that solutions at pH 6.5, 8, 10 and 12 show no decomposition after standing at room temperature for 24 h. The solutions at pH 8, 10 and 12 showed no significant decomposition on heating at 80 °C for 2 h. A tentative method is proposed for the determination of nitrite in cured meats containing ascorbic or isoascorbic acid, whereby a slurry of 5 g of the sample in 0.01 M sodium hydroxide is digested at 80 °C for 90 min, zinc sulphate is added as a clarifying agent, the volume is made up to 250 ml, the solution is filtered and nitrite in an aliquot of the filtrate is determined.