Chem/Phys Question

[I95bandit1]

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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

The correct answer is B.

Why?

 

[El-Rami]

It is because the precipitation is pH-dependent. If you examine the terms for free energy under non-standard state conditions, there is a term for pH for reactions that are pH-dependent. Free energy also happens to be related to the equilibrium constant, K. Most places in the body are neutral pH (except part of the GI system where you neutralize acidic contents emptying from the stomach), hence why you have to buffer at neutral pH.

A. High pH would just change the concentration of protons and make you have to take the pH term into consideration, thus affecting equilibria (hence precipitation).

C. Calcium hydroxide is readily soluble in water. The calcium ion is just a spectator ion, but the hydroxide would affect pOH and thus affect pH. See A for the same rationale (making it more basic will cause it to precipitate).

D. Adding phosphate will definitely result in precipitation because you are pushing it past saturation.

Even if you didn't know why B is correct (or that it is pH dependent), you can still reach the correct answer by eliminating the wrong ones. A and C essentially say the same thing (you are making it alkaline in both), so that automatically tells you that you can eliminate them. D is definitely wrong and should be pretty quickly eliminated. This only leaves B as your answer.

^This is a strategy that you must learn to use on the MCAT. You can't know everything, but if you can reason your way through, you can do well.

 

[aldol16]

C and D are immediately out because of the common ion effect. They increase the ion product such that it approaches the solubility product constant and thus make CaPO4 more likely to crash out of solution. A is the more challenging one to rule out because you have to understand that phosphate is part of a tri-protic acid and thus has three characteristic pKas. If you raise the pH of the solution, you will shift the equilibrium towards phosphate because the pH will exceed pKa3 (or at least get closer to pKa3) and thus the phosphate will exist in the completely deprotonated form. This will also raise the ion product and thus has the same effect as D.

A. High pH would just change the concentration of protons and make you have to take the pH term into consideration, thus affecting equilibria (hence precipitation).

It's not just the concentration of proteins but rather specifically that pH will make it so that pKa3 is more likely to be exceeded.

 

[El-Rami]

Ignore what I said about C where Ca2+ is just a spectator ion and is thus unimportant. The solubility constant definitely depends on it, so I wasn't thinking straight when writing out my response. I still stand by the thing about the hydroxide ion affecting pH and thus eliminating A and C because they are essentially saying the same thing. However, like @aldol16 said, you can eliminate C and D for the same reason (common ion effect).