physiologigal buffers at pH 2-5?

Nick Theodorakis nicholas_theodorakis at
Thu Sep 13 13:45:14 EST 2001

In <20010913145827.19281.qmail at>, Mathias Holpert 
>Hi all,
>I'd like to do some acid resistance studies on Toxoplasma gondii and 
>need a suitable buffer system that is buffering at around pH 2 to 5 to
>incubate my parasites for 30 min max (room temperature). Of course, 
>buffer would have to be physiological as I don't want to disrupt them right
>away. Any suggestions?

One of the pKa's for phosphate is pretty low (2.12), and acetate has a pKa 
of 4.76. Somebody else already mentioned citrate, which has one pKa at 
3.13, and another at 4.77.  Also, glycine can be used at pH of 2-3, but it's 
likely to be metabolized.

Calbiochem publishes a nice booklet on buffers (where I got those 
numbers). To cover a pH range of 2.6 to 7.0, they suggest a 
citrate-phosphate system: make 0.1M citric acid and 0.2M dibasic sodium 
phosphate, and titrate against each other to the desired pH.

If your guys need free Ca(2+), keep in mind that citrate and phosphate 
bind calcium, especially phosphate.


Nick Theodorakis
nicholas_theodorakis at

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