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Buffer capacity

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Buffer capacity

Postby azmee » Mon Feb 23, 2009 2:26 pm

Hi,
Anybody can help me. I am looking for the method to determine the buffer capacity of the solution (ie.plant extract) can you guide me a detail procedure.

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Re: Buffer capacity

Postby blcr11 » Mon Feb 23, 2009 4:56 pm

Buffer capacity is defined as the number of moles of strong acid or strong base needed to change the pH of the solution by 1 pH unit. The simple experiment would be to measure the pH of a known volume of solution; then add a known amount of strong acid or strong base (HCl or NaOH, typically) and re-measure the pH of the resulting solution. The buffer capacity (beta) will be:

beta = (the change in the #moles of strong acid or base in solution)/ (the change in pH)

Say your pH of 5 ml of extract is 7.04. You add 0.01 mol of NaOH and see that the pH is now 7.12. The buffer capacity is 0.01/0.08 = 0.125 (which I just completely made up—I have no idea what you will see if you do the experiment.) If you now add another 0.01 mole of NaOH and the pH changes to 7.24: now the buffer capacity is 0.01/0.12 = 0.083. The change in the equivalents of base added to the solution is the same, but the change in pH is different, so the buffer capacity is different. Again, I completely made up these numbers, so you may get something very different when you actually do the experiment.

Here is a chemist's view on buffer capacity (be prepared for things that look messier than they really are):

http://www.chembuddy.com/?left=pH-calcu ... r-capacity


And here is something a little easier to swallow-have to jump to p 16 for the part on capacity; previous link didn't work, sorry:

http://www.umt.edu/medchem/teaching/Lec ... 202005.pdf
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