How is magnesium interference avoided when measuring calcium with o-cresolphthalein complexone?

When measuring calcium levels using o-cresolphthalein complexone, magnesium interference is effectively avoided by adding 8-hydroxyquinoline. This compound acts as a chelating agent, specifically binding to magnesium ions. By doing so, it prevents magnesium from interacting with the o-cresolphthalein complexone, which would otherwise lead to an overestimation of calcium levels due to the similar reactivity of both ions.

In this method, o-cresolphthalein complexone forms a colored complex in the presence of calcium, which is measured spectrophotometrically. Magnesium can complicate this measurement since it can also react with the complexone, altering the absorbance readings. The inclusion of 8-hydroxyquinoline effectively prevents this interference, allowing for a more accurate determination of calcium concentrations.

Thus, while other options may influence the measurement process or relate to the chemistry involved, they do not specifically target the issue of magnesium interference in the context of this assay method.