In the colorimetric determination of orthophosphate, what is the purpose of forming the molybdenum blue complex?

• A. To quantify nitrate in the sample.
• B. To provide a colored complex that correlates to orthophosphate concentration.
• C. To neutralize the sample.
• D. To sterilize the sample.

Answer: (B)

The key idea is to create a colored species that can be measured to determine how much orthophosphate is present. In this method, orthophosphate reacts with molybdate under acidic conditions to form a phosphomolybdate, which is then reduced to a blue-colored molybdenum complex. The intensity of this blue color is proportional to the orthophosphate concentration, so measuring the color (by eye with a standard, or more precisely with a spectrophotometer) lets you quantify how much phosphate is in the sample. This approach isn’t about nitrate, neutralizing the sample, or sterilizing it—the purpose is to produce a measurable color that reflects phosphate levels.