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Vitamin C, also known as ascorbic acid or L-ascorbic acid, is an essential nutrient involved in the repair of tissues and the enzymatic production of certain neurotransmitters. It is required for the functioning of several enzymes and immune performance, and is also an important antioxidant. This nutrient is found in many foods and is often used as a dietary supplement.

USP general chapter <580> describes a titration technique to determine the assay of Vitamin C as ascorbic acid, sodium ascorbate, and calcium ascorbate dehydrate, or their mixture in finished dosage forms as capsules, tablets, and oral suspensions.

This Application Note demonstrates the Vitamin C determination in water-soluble vitamin tablets. The methodology can also be applied for oil-soluble vitamin or mineral tablets, as well as oil- and water-soluble vitamin or mineral capsules.

The method is demonstrated for water-soluble vitamin tablets.

Several tablets are accurately weighed and then ground into a fine powder. A portion is transferred into a volumetric flask, to which metaphosphoric and acetic acid are added. After dissolution, the volumetric flask is filled up to the mark with carbon dioxide-free water.

905 Titrando with tiamo. Example setup for the determination of vitamin C.
Figure 1. 905 Titrando with tiamo. Example setup for the determination of vitamin C.

This bivoltametric analysis is carried out on a 905 Titrando system equipped with a magnetic stirrer and a double Pt sheet electrode for indication.

To a reasonable amount of prepared sample, metaphosphoric acid, acetic acid, and carbon dioxide-free water are added. The vitamin C content is then titrated against dichlorophenol-indophenol until the first equivalence point.

A blank analysis is performed in the same way.

The analysis demonstrates acceptable and reproducible results and well-defined titration curves. For the tested water-soluble vitamin tablet, a vitamin C content of 97.7% (n = 6, SD(rel) = 0.23%) is obtained, which is within the given USP criteria of 90–150%. An example titration curve is displayed in Figure 2.

Figure 2. Example titration curve for vitamin C determination.

After sample preparation, the determination of vitamin C in vitamin capsules or tablets can efficiently be carried out by using a Metrohm autotitrator. Fast and precise determination according to USP <580> is possible.

This method is also suitable for samples such as:

  • Oil- and water-soluble vitamins capsules
  • Oil- and water-soluble vitamins oral solution
  • Oil- and water-soluble vitamins tablets
  • Oil- and water-soluble vitamins with minerals capsules
  • Oil- and water-soluble vitamins with minerals oral solution
  • Oil- and water-soluble vitamins with minerals tablets
  • Water-soluble vitamins capsules
  • Water-soluble vitamins tablets
  • Water-soluble vitamins with minerals capsules
  • Water-soluble vitamins with minerals tablets

Metrohm AG

9100 Herisau