buffering capacity concept

[theonlytycrane]

my understanding: the ability of a buffer solution to resist changes in pH.

Let's say we have the following buffer: Ha <- pKa = 5 -> A- + H+

and we have a nice buffer solution with 1 mol of Ha and 1 mol of A- in 1 L of solution. Diluting the buffer by addition of 100L of water won't change the pH, but will it change the buffering capacity?

Even though the concentrations of Ha and A- decrease, we still have the same mol of Ha and A- to neutralize incoming species- so my thought is it wouldn't change the buffering capacity. Am I thinking about this correctly?

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[aldol16]

and we have a nice buffer solution with 1 mol of Ha and 1 mol of A- in 1 L of solution. Diluting the buffer by addition of 100L of water won't change the pH, but will it change the buffering capacity?

Why do you think it won't change pH?

 

[theonlytycrane]

@aldol16 I was basing this off of the h.h. equation since pH would still equal pKa because [Ha] = [A-] even after dilution

 

[aldol16]

This is a common misconception - you're being guided by the equation instead of using it to understand. The equation tells you the buffering capacity of the solution. When you add a little bit of base, the buffer absorbs that pH change (and water is basic relative to, say, citrate buffer). But when you add a lot of water, you have basically trashed the buffer. See the table here: http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

Basically, when the buffer becomes dilute, the solution becomes dominated by the auto-ionization of water.

 

[theonlytycrane]

@aldol16 Thanks for the link! I was thinking that water would not affect Ha or A- but it helped me to think about how auto-ionization of water would affect the buffering ability, especially with more and more dilution.

 

[BerkReviewTeach]

Be very, very, VERY careful making the generalizations going on in this thread. As in be VERY careful not to make an extreme special case the foundation of your reasoning.

In the opening question you did not mention the volume of the solutions, so there is ambiguity.

If you have one liter of solution that contains 1.00 moles HA and 1.00 moles A-, then the pH will be pKa. If you add 100 mL of water to that solution, the pH will still be 5.0. It will not change to any detectable extent. The only way you see a change in pH upon adding water is when the amount of water severely exceeds the volume of the solution (a pool added to a beaker of buffer for instance). That is such an unrealistic special case that you do yourself a disservice when studying for the MCAT to consider it.

The HH equation will be perfectly fine for realistic questions. Keep it simple.

 

[aldol16]

Be very, very, VERY careful making the generalizations going on in this thread. As in be VERY careful not to make an extreme special case the foundation of your reasoning.

In the opening question you did not mention the volume of the solutions, so there is ambiguity.

Yes he/she did. See quote below. Diluting a 1 L solution with 100 L of water will most definitely change pH by any measure. This is the metaphorical pool into a beaker you speak of. One should always know what data one is given in the question - otherwise, one could fall into the trap of making an assumption that a case is "normal" when in fact it is "extreme."

and we have a nice buffer solution with 1 mol of Ha and 1 mol of A- in 1 L of solution. Diluting the buffer by addition of 100L of water won't change the pH, but will it change the buffering capacity?