Phosphate Buffer and pH: Would it be Effective at pH 8.5? - Homework Discussion

[ReidMerrill]

Homework Statement

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?

Homework 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

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?

 

[Borek]

Since the pH would still be between pka2 and pka3

Nope, that's not how buffers work.

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

 

[ReidMerrill]

Nope, that's not how buffers work.

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

Okay I know that but I still don't know how to answer the problem.

 

[Borek]

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.

 

[ReidMerrill]

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.

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.

 

[Borek]

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.

 

[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.