MCAT Chemistry Question #6: Answer and Rationale

6. A test for proteins in urine involves precipitation but is often complicated by precipitation of calcium phosphate. Which procedure prevents precipitation of the salt?

A) Addition of buffer to maintain high pH

B) Addition of buffer to maintain neutral pH

C) Addition of calcium hydroxide

D) Addition of sodium phosphate

Answer Analysis

The correct answer is B.

Rationale: You must identify a change in experimental approach that will eliminate the frequently encountered complication of calcium phosphate precipitation, which will give a false positive. In order for calcium phosphate to precipitate, there must be sufficient amounts of calcium and phosphate ions in solution. Options C and D, adding more of either ion, will only increase the chances for calcium phosphate precipitation and a false positive. For option A, maintaining a high pH, means that there will be a low concentration of hydronium ions, which will increase calcium phosphate precipitation rather than decrease it. Option B, using a buffer to maintain neutral pH, is the only answer choice that will help to prevent calcium phosphate precipitation.

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