Chem question

[ofi2016]

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 is B. I understand why it's not C or D, but what is the reasoning that it is B instead of A?

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[basophilic]

If anything, I'd expect lower pH to avoid precipitation since PO4^3- will readily take up H+, but I'm guessing significant pH deviations would affect the tertiary structures (at least) of the proteins being tested. Also, high pH would mean competition b/w precipitation of Ca(OH)2 and Ca3(PO4)2 - off the top of my head, the latter salt involves fairly hard ions which will pack more tightly in a lattice than the former (I know the hard-soft rule doesn't apply universally to all solids, but with some knowledge of solubility rules, you can still compare weakly soluble salts)

 

[sazerac]

We discussed this question before.

http://forums.studentdoctor.net/threads/protein-involving-precipitation.1122493/

I don't think we came up with a reasonable answer though. If you don't want to precipitate, you need to remove ions.

Adding a neutral solution will remove phosphate ions as they become HPO4, H2PO4 etc. Adding a basic solution will pull Ca ions from solution since CaOH2 is not soluble.

The Ksp for calcium phosphate will be equal to Ca^3 times PO4^2, so one could argue that you will get more bang for your buck by removing calcium ions with a high pH solution than by removing phosphates.

Without a doubt, the scientific literature focuses on high pH solutions increasing the concentration of unprotonated PO4, with no mention of removing Ca ions from solution. But this is outside knowledge and not really fair to test on for undergraduate bio / biochem.

Overall I think this is just a crappy question from the AAMC, which I find surprising because they normally produce the best standardized exam questions I have ever seen in my 25+ year academic career.