When mixed with phosphotungstic acid, which compound causes reduction to a tungsten blue complex?

When mixed with phosphotungstic acid, which compound causes reduction to a tungsten blue complex?

• A. Urea
• B. Ammonia
• C. Creatinine
• D. Uric acid

Answer: (D)

The test hinges on a redox reaction where phosphotungstic acid is reduced to form a blue tungsten complex when a reducing substance is present. Uric acid can readily donate electrons under the assay conditions, becoming oxidized and reducing the phosphotungstate to the blue tungsten complex. This distinctive color change signals a positive result for a reducing substance in the sample. The other substances listed—urea, ammonia, and creatinine—do not act as reducing agents strong enough to reduce phosphotungstic acid here, so they do not produce the tungsten blue color.

The test hinges on a redox reaction where phosphotungstic acid is reduced to form a blue tungsten complex when a reducing substance is present. Uric acid can readily donate electrons under the assay conditions, becoming oxidized and reducing the phosphotungstate to the blue tungsten complex. This distinctive color change signals a positive result for a reducing substance in the sample. The other substances listed—urea, ammonia, and creatinine—do not act as reducing agents strong enough to reduce phosphotungstic acid here, so they do not produce the tungsten blue color.