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 diazotisationcoupling 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 diazotisationcoupling reagents [sulphanilamide and N-( 1-naphthy1)ethylenediamine;4-nitroaniline and N-(1-naphthy1)ethylenediaminel 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.

Thermal and photochemical degradation of myoglobin pigments in relation to colour stability of sliced dry-cured Parma ham and sliced dry-cured ham produced with nitrite salt

“…The factor used to convert percentage N to percentage crude protein was 6.25. -Nitrite content (ppm) determined by the Norwitz-Keliher method [11], in which the NEDD concentration was doubled as described by Binstok et al [12]. -Nitrate content (ppm) determined by a method based on that of Miranda et al [13] where vanadium(III) reduces nitrate to nitrite.…”

“…-Nitrate content (ppm) determined by a method based on that of Miranda et al [13] where vanadium(III) reduces nitrate to nitrite. The digestion, clarification and filtration of the meat samples were performed as described by Norwitz and Keliher [11]. The filtrate (25 ml) was mixed with 12.5 ml NEDD (1 w/v in H 2 O), 12.5 ml SULF (2% w/v in 5% HCl), 25 ml of a saturated solution of VCl 3 in 1 M HCl and water to make 100 ml.…”

Rapid and Selective Electrochemical Determination of Nitrite in Cured Meat in the Presence of Ascorbic Acid

“…Although the AOAC method is very sensitive, low recoveries are reported when used for nitrite determination in meat due to the ascorbic acid interference [3]. Norwitz and Keliher [5] have proposed modifying the AOAC method (referred to here as the modified AOAC method) using an alkaline medium (0.01 M NaOH) for the digestion step and introducing a new step based on addition of a clarifying agent (ZnSO 4 ) for protein precipitation. In this way the centrifugation step can be replaced with a simple filtration step.…”

“…For spectrophotometric determinations the Norwitz & Keliher method [5] was followed. 5 g of homogenised meat were added to approximately 70 mL 0.01 M NaOH and blended.…”

“…1a). Different methods have been used to reduce the ascorbic acid interference including the digestion of aqueous slurry of the sample at 80 C for 2 h (AOAC Official Method) or the digestion of a slightly alkaline slurry at 80 C for 1.5 h followed by clarification with zinc sulphate [5]. Binstok et al [6] proposed an improved procedure of the Norwitz & Keliher method based on the use of sand during the digestion step and a larger quantity of NED during the diazotisation step.…”