Why is a sodium phosphate buffer used for the pH 6.24 buffer?

Answer 1

Phosphate ions are used as a buffer because there are three protonated forms (H3PO4, H2PO4-, and HPO42-) that have pKa in the correct range. The pKa for the three listed forms of phosphate are 2, 7 and 12 respectively.

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Answer 2

A sodium phosphate buffer is used because it has a pKa around 6.8, which means it can effectively buffer solutions around pH 6.24. By using a buffer with a pKa close to the desired pH, the buffer capacity is maximized, making it more effective at maintaining the desired pH.

Answer 3

In general, a buffer works optimally near its pKa. Buffercapacity is usually defined as +/- 1 pH unit of the pKa (cite). With a pKa of ~7.0, one wouln't want to use phosphate buffer at a pH of 5. It would have no buffering capacity at this pH.

I don't know what your experimental set-up is, but it might also be possible that certain functional groups will interfere with your experiment--maybe phosphate will not interfere/react in your reaction mixture, whereas something like TRIS (w/ an amine group) will. Good luck.