# Homework Help: Phosphate buffer and pH

1. Oct 14, 2016

### ReidMerrill

1. The problem statement, all variables and given/known data
Phosphate, present to an extent of 0.01 M, is one of the main buffers in blood plasma, whose pH is 7.45. Would phosphate be as useful if the plasma pH were 8.5?

2. Relevant equations
pH = pKa+log[A/HA]
Ka1 = 7.11x10^-3 pka1= 2.148
Ka2=6.34x10^-8 pka2= 7.198
Ka3 = 4.22x10^-13 pka3=12.375
3. The attempt at a solution
Since the pH would still be between pka2 and pka3 so the phosphate would not gain or lose a hydrogen because of that change and the henderson-hasselbalch equation would still use the same pka. Does this mean it would be just as effective at pH 8.5 or am I missing something?

2. Oct 14, 2016

### Staff: Mentor

3. Oct 14, 2016

### ReidMerrill

4. Oct 14, 2016

### Staff: Mentor

You don't have to follow the derivation, but part of the text on that page contains exactly information that you need. Scroll down to the plot and the explanation.

5. Oct 14, 2016

### ReidMerrill

Okay so that phosphate wouldn't be as good of a buffer at a higher pH because it's greater than the pKa2? 8.5 is closer to pKa3 but since its ka3 is so much smaller than Ka2 that dissociation/association would be much lower.

6. Oct 15, 2016

### Staff: Mentor

I feel like you are still missing the point. How effective a buffer is depends on how far pH is from the respective pKa value. 7.45 is quite close to 7.2.

7. Oct 15, 2016

### epenguin

You must have seen the titration curve for a weak acid or base. Sketch or look up what it looks like.
You might have to turn it sideways for best understanding.

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