8 replies to this topic

[schaapje_86]

How do I make a 5mM potassium buffer pH=7.5?

[bob1]

Which potassium salt?

[schaapje_86]

potassium phosphate

[HomeBrew]

Make two 5 mM solutions, one of the K2HPO4 (dibasic potassium phosphate) and one of the KH2PO4 (monobasic potassium phosphate). The pH of the dibasic solution will be alkaline (around pH 8), and the monobasic solution will be weakly acidic (around 6.5, maybe 6.8).

Take a volume of monobasic close to what you want your final volume to be, and add dibasic to it dropwise with stirring until the pH is 7.5.

You'll likely wind up with slightly more than you need (which is much better than winding up with slightly less).  Filter sterilize if required.

[phage434]

And, for future reference, no one calls this "potassium buffer".  It's called "phosphate buffer".  This is because it is the phosphate ion that is doing the buffering.  The buffering would work fine with the sodium salts (but there might be other problems with the sodium present).

[schaapje_86]

Thank you very much for your help!!  

[Rebirth]

HomeBrew, on 19 November 2010 - 04:30 AM, said:

Make two 5 mM solutions, one of the K2HPO4 (dibasic potassium phosphate) and one of the KH2PO4 (monobasic potassium phosphate). The pH of the dibasic solution will be alkaline (around pH 8), and the monobasic solution will be weakly acidic (around 6.5, maybe 6.8).

Take a volume of monobasic close to what you want your final volume to be, and add dibasic to it dropwise with stirring until the pH is 7.5.

You'll likely wind up with slightly more than you need (which is much better than winding up with slightly less).  Filter sterilize if required.

Is there a reason why you specifically said to add the dibasic dropwise? Or could it be reversed (that is, take a volume of the dibasic close to the desired final volume and then add the monobasic dropwise).

[HomeBrew]

I used "dropwise with stirring" because that's how one typically adds an acid or base to a solution when adjusting the pH.  You can add the KH2PO4 acid to the K2HPO4 base or vice versa.

[Rebirth]

HomeBrew, on 31 January 2011 - 03:37 AM, said:

I used "dropwise with stirring" because that's how one typically adds an acid or base to a solution when adjusting the pH.  You can add the KH2PO4 acid to the K2HPO4 base or vice versa.

Ok thank you