Rate Constant and Activation Energy for Formation of a Nitrosoascorbic Acid Intermediate Compound

The rate of decomposition of nitrite as a result of its reaction with ascorbic acid was determined from spectrophotometric measurements at various pH values and temperatures. The reaction proceeded initially as second order. The product was nitrosoascorbic acid, and it was formed with an increasing rate constant with decreasing pH. The rate was proportional to the concentration of nitrite from pH 4.2 to 5.38. As the reaction proceeded further at lower pH and higher temperature, it deviated from the second order reaction plot, giving kinetic evidence that the formation of 2,3-dinitrosoascorbic acid occurred more easily at low pH and high temperature. The activation energy for formation of nitrosoascorbic acid decreased with increasing pH from 10.90 Kcal/M at pH 4.35 to 6.46 Kcal/M at pH 5.49. Apparently there are two different reactions in the activation energy; i.e., the reaction of nitrous acid with the undissociated form of ascorbic acid (high activation energy) and the reaction of nitrous acid with once dissociated ascorbic acid (low activation energy). The significance of the reaction of nitrite with ascorbic acid at the pH values encountered in meats cured with nitrite and ascorbate is emphasized in terms of low activation energy and heating which increase the rate constant.