protein-involving precipitation

[m25]

This is from 2015 MCAT content description. I have a hard time understanding this one particular problem:

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. This is a Skill 3 question and relates to Content Category 5B,Nature of molecules and intermolecular interactions. In this Skill 3 question, you must identify a change in an
experimental approach that would eliminate a frequently encountered complication. The complication in
this case is related to the test for protein-involving precipitation. The test will give a false positive if
calcium phosphate precipitates. To answer this Skill 3 question, you need to reason about how changing
experimental parameters will eliminate the complication.

So why is the answer B? The provided explanation doesn't really help me...

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[sazerac]

Well C and D are out, because if you want to prevent the precipitation of calcium phosphate, the last thing you want to do is add more calcium or phosphate ions.

Was there more to the passage? Or was this a discreet?

 

[m25]

Well C and D are out, because if you want to prevent the precipitation of calcium phosphate, the last thing you want to do is add more calcium or phosphate ions.

Was there more to the passage? Or was this a discreet?

Yes this is a discreet question from here https://www.aamc.org/students/download/377882/data/mcat2015-content.pdf
It's on page 109. I was able to cross out C and D too, but was not sure between A and B.

 

[sazerac]

In a medium to low pH solution, the phosphates will form phosphoric acid, removing phosphate from solution. In a high pH solution, them hydroxides be stealin' all your protons, increasing the concentration of phosphate.

 

[m25]

In a medium to low pH solution, the phosphates will form phosphoric acid, removing phosphate from solution. In a high pH solution, them hydroxides be stealin' all your protons, increasing the concentration of phosphate.

So is this my train of thought below correct?
The overall purpose is to decrease the concentration of phosphate in the solution that are available to react with calcium (because it will form a precipitate when reacted with calcium), so by lowering the pH and increasing concentration of H+, it would react with phosphate to form a phosphoric acid instead, increasing the solubility of phosphate in the solution(although phosphoric acid is a weak acid and therefore not that soluble, it is more soluble than phosphate it self?? I am not sure about this) AND leaving less phosphate to react with calcium at the same time. If pH is increased, the hydroxide will pluck off more protons from phosphoric acid, generating more phosphate that can then react with calcium to form precipitate. Since the lowest pH in the answer choice is B, the answer is B.

 

[sazerac]

The H+ will react with the phosphate to produce other ions or phosphoric acid itself. The important point here is that it removes phosphate from solution.

The only thing that causes calcium phosphate to precipitate is a high enough concentration of calcium ions and phosphate ions, so removing phosphates is a good thing. HPO4, H2PO4, or H3 PO4 all mean less PO4.

Other than that, yeah you got it.

 

[sazerac]

You know, the more I think about it, the more I am not happy with my answer. I mean, what's to prevent a high pH environment from forming Ca(OH)2, precipitating it out, and removing calcium ions.

I know that high pH urine forms calcium phosphate crystals, because I am a third year medical student and we had to memorize stuff like that. But the MCAT should be solvable from first principles, not memorizing human medical trivia.

Maybe one of this year's experts like @Cawolf can help us out?

 

[Cawolf]

Haha @sazerac, I don't know if I qualify for "expert" status.

I like the reasoning about a neutral pH keeping phosphate in it's neutral form. As you said, C and D are definitely out as they would promote the precipitation of the salt. Choice A, buffering to a high pH, would drive the phosphate equilibrium to it's anionic form; then bonding to calcium ions and precipitating out of solution.

Calcium hydroxide is a highly soluble hydroxide salt, so that precipitating should not be a big concern. I always tried to look at the mot basic explanation for solving MCAT questions, that is great advice.

 

[m25]

Haha @sazerac, I don't know if I qualify for "expert" status.

I like the reasoning about a neutral pH keeping phosphate in it's neutral form. As you said, C and D are definitely out as they would promote the precipitation of the salt. Choice A, buffering to a high pH, would drive the phosphate equilibrium to it's anionic form; then bonding to calcium ions and precipitating out of solution.

Calcium hydroxide is a highly soluble hydroxide salt, so that precipitating should not be a big concern. I always tried to look at the mot basic explanation for solving MCAT questions, that is great advice.

So if calcium hydroxide is a highly soluble hydroxide salt, wouldn't raising the pH be a better choice(choice A) then?? But the answer explanation says it's B...?

 

[Cawolf]

I think the question is really looking to see if you can see the effects of pH on phosphate.

Ca(OH)2 being soluble has no real effect on what the question is asking. That was an aside discussion of something @sazerac said.

 

[sazerac]

Ca(OH)2 is soluble, and when it is in solution it breaks up into Ca2+ and OH-. So raising the pH will just add a lot of OH- ions but not create a lot of Ca(OH)2 which would have pulled calcium ions out of solution.

On the other hand, turing PO4 into HPO4, H2PO4, or H3PO4 really will pull PO4 ions out of solution and turn them into something else entirely, therefore making calcium phosphate less likely to precipitate.

Clever question, really.

 

[m25]

Ca(OH)2 is soluble, and when it is in solution it breaks up into Ca2+ and OH-. So raising the pH will just add a lot of OH- ions but not create a lot of Ca(OH)2 which would have pulled calcium ions out of solution.

On the other hand, turing PO4 into HPO4, H2PO4, or H3PO4 really will pull PO4 ions out of solution and turn them into something else entirely, therefore making calcium phosphate less likely to precipitate.

Clever question, really.

Oooh, so we basically need to form a covalent bond(as opposed to ionic bond that easily dissociates in water) when we want to prevent something from reacting with something else?

 

[Cawolf]

No, not at all.

It all relates to the solubility of the salt. The problem tells us that Calcium phosphate precipitates, which means it is not very soluble.

 

[m25]

No, not at all.

It all relates to the solubility of the salt. The problem tells us that Calcium phosphate precipitates, which means it is not very soluble.

Ah I see. Oh my god this is one challenging question.

 

[m25]

So using the concept of solubility, how does the below line work?

high pH urine forms calcium phosphate crystals

My guess is that raising pH doesn't affect calcium in the solution at all since Ca(OH)2 is very soluble and therefore we don't have to worry about Ca(OH)2 precipitating out, but high pH will pluck off protons from HPO4, H2PO4, or H3PO4, leaving negatively charged phosphate that will react with calcium which will then form precipate?

 

[m25]

So using the concept of solubility, how does the below line work?

My guess is that raising pH doesn't affect calcium in the solution at all since Ca(OH)2 is very soluble and therefore we don't have to worry about Ca(OH)2 precipitating out, but high pH will pluck off protons from HPO4, H2PO4, or H3PO4, leaving negatively charged phosphate that will react with calcium which will then form precipate?

Actually, I just looked up on the solubility of Ca(OH)2 and it turns out that it is only slightly soluble in water, so having high pH would precipitate out some Ca(OH)2 as well as calcium phosphate?

 

[sazerac]

Actually, I just looked up on the solubility of Ca(OH)2 and it turns out that it is only slightly soluble in water, so having high pH would precipitate out some Ca(OH)2 as well as calcium phosphate?

Yeah I thought so...

Here's what still bugs me. If you add phosphate and remove calcium ion, you know the Ksp equation will have Calcium CUBED vs the phosphate SQUARED. So you'd think it would lower the amount of calcium phosphate precipitated.

We are probably missing something obvious here, I wonder what.

 

[Cawolf]

My advice @m25, is to stop focusing on these little nuances that don't help you answer the questions and just answer what they are asking.

Group II hydroxide salts are soluble as far as anyone is concerned if we don't have any concentrations. The question is only looking to see if you understand how pH and phosphate are related. That is the best advice I can give anyone looking to succeed on the MCAT. You need to realize that what actually may happen is not important at all, and focus on what the question wants you to understand.

Adding hydroxide will likely form Calcium hydroxide, which is slightly soluble, and can precipitate out of solution. How can you make this prediction without concentrations and ksp values though? You cannot. So take a look at the only things you are given. Calcium phosphate is said to be insoluble, and you know what phosphate looks like. How does pH effect the formation of the salt? Well, high pH will deprotonate and promote Calcium phosphate formation, which helps us answer the question. We want a more acidic/neutral solution, boom, choice B and move on.

Separate what is important, and focus on that.

 

[m25]

Yeah I thought so...

Here's what still bugs me. If you add phosphate and remove calcium ion, you know the Ksp equation will have Calcium CUBED vs the phosphate SQUARED. So you'd think it would lower the amount of calcium phosphate precipitated.

We are probably missing something obvious here, I wonder what.

But if you add phosphate and remove calcium ion, the Ksp would still stay constant, since [Ca2+]^3 [(PO4)-3]^2 =Ksp and increasing concentration of phosphate would decrease the concentration of phosphate to maintain the same Ksp. Since Ksp is only affected by temperature(or is it? I might be missing something here as well)

 

[m25]

Okay, so is the way to approach the problem like this:

cross out choice C and D for obvious reasons

looking at choice A, since high pH will likely deprotonate more phosphate to react with calcium& and because having more OH- would PROBABLY precipitate out some Ca(OH)2 (which is not what we want) this probably isn't the best choice

looking at choice B, this is answer choice with the lowest pH. What does having more H+ do to calcium phosphate? It would probably form CaH2 and HPO4, H2PO4, or H3PO4. But I am not familiar with the molecule CaH2 and MCAT probably does not expect me to know that, so I will leave it aside and look at HPO4, H2PO4, and H3PO4. Since none of them is the strong acid that I am supposed to know for the MCAT, they are all weak acids, and since weak acids doesn't ionize well in aqueous solution, H+ will bond with phosphate pretty strongly, removing phosphate from the solution. And since phophoric acids are pretty soluble in water, I don't have to worry about them precipitating out. This choice definitely sounds like a better choice then A. Pick B.

 

[Cawolf]

Your reasoning is sound, but if you go through all of those scenarios on the actual exam you will never finish.

 

[m25]

Your reasoning is sound, but if you go through all of those scenarios on the actual exam you will never finish.

Okay, then I should just focus on the effect of forming phosphoric acid, a familiar weak acid on MCAT, and move on?

 

[Cawolf]

I would.

The simpler the better. It is definitely good to be able to explore all possibilities as you did, but for MCAT problem solving - simpler is better.

 

[m25]

I would.

The simpler the better. It is definitely good to be able to explore all possibilities as you did, but for MCAT problem solving - simpler is better.

Okay, but it's hard for me to simplify problems because sometimes it's really hard to get the essence of the question. How did you do it? Is it through doing a thorough analysis of each question and simplifying the question in your own words and try to pick up some sort of pattern, or is it just something you picked up without really thinking about it?

 

[Cawolf]

I just practiced it on every question I saw. I would do the passage and then review the explanation to see what they were hoping I would learn. I would then just try to pick out "the essence" of every question. It took me a couple months of practice to be able to do it effectively. I had never taken a standardized test after high school (not even the SAT), so I don't know if that is something you would develop then, but I had to for the MCAT.

 

[Labrat07]

Great discusion. Thank you, everyone.