The Micro Determination of Citric Acid by the Thunberg Methylene Blue Method

The method depends on the specific property of citric acid as a hydrogen donator in accelerating the decolorization of methylene blue by an enzyme found in cucumber seed. It gives reliable results only when enzyme extracts are prepared in the same way and used at the same age. Sodium, calcium and hexose-diphosphate do not interfere unless present in concentrations higher than those found in the blood. Oxalic acid does interfere because it changes the minimum amount of citrate necessary for maximum speed of decolorization. The sensitivity is directly proportional to the amount of methylene blue present in the reaction tube