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about Henderson-Hasselbalch equation

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sdekivit:
see my modified last reply.

Amy:
Dear All, I like to find out if I can prepare a range of buffer solution only using potassium dihydrogen phosphate and dipotassium hydrogen phosphate? I want to prepare a solution with pH ranging from 6 to 8 in 0.5 increment. Using the Henderson-Hasselbach equation, the pKa need to be selected near to the pH values desired. There are only 3 different pKa for Phosphate buffer which are 2.15, 7.2 and 12.33. For pH 6, 6.5,7.5 and 8, what pKas that I can use? I only want to prepare using phosphates! Please explain me a little further? Is my understanding false?

thank you.

DevaDevil:
Amy,

the pKa is a given value for acid/base couples. It just determines which couple is the easiest to use in the buffer range you need

The by far easiest way to prepare buffers for (potassium)phosphates is to take the couple whose pKa is closest to what you need (in your case indeed hydrogenphospahte/dihydrogenphosphate) and use a pH electrode to get the pH right (by either adding phoshoric acid or potassium hydroxide)

If you do not have the possibility of using a pH electrode you'll have to calculate the exact concentration ratio of hydrogenphosphate and dihydrogen phosphate and make a solution of them using this ratio.

Finding the ratio is straightforward with the HH equation (pKa for this couple is 7.2)

Amy:
Dear Devadevil,

Meaning, if I want to prepare solution with pH 6,6.5,7,7.5, and 8, use pKa 7.2. Then I adjust the pH with phosphoric acid or potassium hydroxide? Can I use hydrochloric acid too as acid to adjust the pH? I tried to countercheck with buffer design calculator (http://bioinformatics.org/JaMBW/5/4/index.html), it does not allow me to use pKa 7.2 for pH 6 but only pH 6.5, 7,7.5, 8 are allowed to use pKa phosphate at 7.2? Why is that?

DevaDevil:
Hi Amy,

the reason it will not give a recipy is because the wanted pH is too far removed from the buffer's pKa value. And the further the actual pH is removed from it, the weaker the buffer strength will become.
In short, you can make a solution with pH 6 with the phosphate couple, but it will not be a good buffer to keep the pH at 6, small additions of acid can shift the pH negative decently fast at that point.


you said you wanted just potassium ions and (hydro-)phosphate ions in solution, hence my suggestion in using KOH and phosphoric acid to complete the buffers if you have a pH electrode to see the pH. HCl would add chlorides to your solution. If chlorides arent a problem, by all means use hydrochloric acid.

remember, if you do not have a way to read the pH of the solution (using that pH electrode) the best way is probably by calculating the desired ratio of monohydrophosphate and dihydrophophate.


Hope this helped you.

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