Redox vs neutralization titrations for determination of ascorbic acid’s concentration in food supplements

Abstract

Introduction. Ascorbic acid is required for the optimal activity of several important biosynthetic enzymes and it is therefore essential for various metabolic pathways in the body. The recommended dosage for men is 90 mg per day and for women 75 mg per day. During pregnancy, it takes about 85 mg per day while breastfeeding 120 mg per day. Tobacco destroys vitamin C in the body, because of which smokers should consume up to 200 mg per day. However, there are several categories of the population, which cannot provide optimal amounts of all necessary nutrients through the food. In these situations, the use of supplements can help. European regulation provides that any supplement of ascorbic acid may be one of five compounds: L-Ascorbic Acid, Sodium-L-Ascorbate, Potassium-L-Ascorbate, Calcium-LAscorbate, and L-Ascorbyl-6-Palmitate. According to the legislation, in the Republic of Moldova, the state quality control of food supplements is not mandatory, being based on the quality control of producer. In this context, it becomes appropriate to prove the content of ascorbic acid in food supplements. Aim of the study. Evaluation of redox and neutralization methods of quantitative determination of ascorbic acid in food supplements. Materials and methods. Electronic databases: Medline, Cochrane, Embase and Springer were accessed using “vitamin C analysis”, “ascorbic acid assay” and “vitamin C quantitative determination”. Also, the search was conducted by using printed pharmaceutical and chemical journals. 108 bibliographic sources were eligible for our study. Results. For the determination of ascorbic acid, a wide range of techniques and methods is available, each with its own advantages and disadvantages. In most of the articles (65%), alkalimetric method was used in order to determine the content of ascorbic acid in food supplements. It is an acidic compound due to the facile ionization of hydroxyl group on carbon 3 (pKa = 4.17) while the hydroxyl group on carbon 2 is much more resistant to ionization (pKa = 11.79). Also, most frequently (35%) the iodometric method was applied. As the iodine is added during the titration, the ascorbic acid is oxidised to dehydroascorbic acid, while the iodine is reduced to iodide ions. Conclusions. Both alkalimetric and iodometric methods were applied successfully for the determination of ascorbic acid in food supplements. The iodometry was more accurate than alkalimetry in determination of ascorbic acid from samples that contain additional acids, which do not interfere with the oxidation of ascorbic acid by iodine.