Test No. 108: Complex Formation Ability in Water

This Test Guideline describes the polarographic method. The ability of a substance to form complexes with metals can be assessed by means of polarographic techniques which allow the determination of stability constants for some complexes.

The polarographic method can be applied to substances with a water solubility greater than 10-5 M. This method is based on the fact that the reduction potentials of metal ions are shifted, usually to more negative values, as a result of complex formation. A minimum of four known concentrations of the chemical being tested should be investigated with a known concentration of metal ions. The chemical being tested should normally be present in at least a 25-fold excess over the metal ion concentrations. The current should be measured at applied potentials in the range - 0.2 V to -1.0 V. In order to detect complexes which form slowly, it is necessary to allow the solutions to stand under a nitrogen atmosphere for a minimum period of 24 hours. If the results are not significant, it is necessary to use methods based on different physicochemical principles, such as spectrophotometry or nuclear magnetic resonance spectrometry.

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