ISO 6557-2:1984

Overview

ISO 6557-2:1984 describes routine laboratory procedures for the determination of ascorbic acid (expressed as dehydroascorbic acid equivalent) in fruits, vegetables and derived products. The standard specifies two complementary approaches: a titrimetric method (Method A) using 2,6-dichlorophenolindophenol dye and a spectrometric method (Method B) after extraction with xylene. Both methods are intended for routine quality control and monitoring of agricultural and food products.

Key Topics

• Scope and principle

  • Method A: direct titration of extracted ascorbic acid with 2,6-dichlorophenolindophenol until a stable salmon-pink end point.
  • Method B: reduction of dye by ascorbic acid, extraction of excess dye into xylene and spectrometric measurement (500 nm) of the xylene layer.

• Reagents and extraction

  • Use either a 2% oxalic acid extraction solution or a metaphosphoric acid/glacial acetic acid mixture as described in the standard.
  • The dyestuff is 2,6-dichlorophenolindophenol prepared and stored under refrigerated, light-protected conditions; periodic renewal is recommended.

• Apparatus and measurement

  • Typical laboratory equipment: analytical balance, burettes/microburettes, mixer, centrifuge, and spectrometer capable of 500 nm readings for Method B.

• Quality controls and interferences

  • The standard highlights known interferences (iron, copper, tin, hydrosulfides, sulfites, sulphur dioxide and reducing agents).
  • Procedures for detection and correction are provided: methylene blue and carmine indigo checks, hydroquinone treatment for xylene-extractable pigments, and formaldehyde treatment to correct for reducing substances in some products.

• Repeatability and reporting

  • Method B repeatability: difference between two determinations on the same sample by the same analyst should not exceed 3% of the mean.
  • Test reports must state the method used, results, any non-standard steps taken and incidents likely to influence results.

Applications

• Routine quality control of raw fruits and vegetables, juices and derived products for nutritional labeling and process control.
• Investigations of product stability, storage effects and heat damage where ascorbic acid retention is a quality marker.
• Suitable for strongly coloured solutions where spectrometric extraction (Method B) improves accuracy.

Related Standards

• ISO technical committee guidance (ISO/TC 34) covers agricultural food product testing and may include related parts for ascorbic acid determination.
• Users should consult other ISO methods for sample handling, storage conditions, or expanded analytical protocols when full traceability and accreditation are required.

This summary emphasizes practical value: straightforward titration for uncomplicated samples and an extraction-spectrometric route for coloured or complex matrices, enabling reproducible ascorbic acid analysis in food quality workflows.