I bought a phosphate buffer from Sigma/Fluka (73212-1L) containing potassium dihydrogen phosphate ($\ce{KH_2PO_4}$) and disodium hydrogen phosphate ($\ce{Na_2HPO_4}$).

It is at pH 8.0 but they do not provide the phosphate concentration. Can I determine it from pH via titration?

This table shows the concentrations for a 0.2M solution: http://www.sigmaaldrich.com/life-science/core-bioreagents/biological-buffers/learning-center/buffer-reference-center.html#phosphate

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    $\begingroup$ I don't get the product when I enter the catalog number on Sigma-Aldrich. How long have you had the bottle? $\endgroup$ – LDC3 Mar 22 '15 at 18:11
  • $\begingroup$ The closest product # I could find was 73208, sigmaaldrich.com/catalog/product/fluka/…, for sodium potassium phosphate buffer at pH 7.00, not 8. The concentration of potassium phosphate monobasic in that product is listed as 7.08 g/L. If your catalog # is correct, you should be able to contact Sigma Aldrich to get certificates of analyses for your product, which should include the confirmation of concentration. $\endgroup$ – Curt F. Mar 22 '15 at 18:25
  • $\begingroup$ I've had the bottle since the beginning of last year. Now it looks like it's discontinued. Back then, I called them and they said there was 0.004 mole/0.63 mole, but they sounded unsure and weren't able to give me any documents. I have already decided to make my own, but my previous experiments have been failing and I wanted to find out if incorrect phosphate concentration was the cause. $\endgroup$ – bravetang8 Mar 22 '15 at 18:35
  • $\begingroup$ The pH of the buffer can easily change since it is at the end of its range of it's buffering. Being more than 6 months old, you should check the pH of the buffer before using it. If it's critical that the pH is held near 8.0, I would see if a TRIS (Trizma) buffer would work. If it doesn't, there are several other chemicals (in the table above) that have a pKa near 8. $\endgroup$ – LDC3 Mar 22 '15 at 19:06
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    $\begingroup$ I just checked the pH and it's still at 8.0. Anyways, my experiments were done a while ago. Confirming the phosphate concentration experimentally could help with troubleshooting old experiments. Just wondering if there are too many unknowns. $\endgroup$ – bravetang8 Mar 22 '15 at 19:29

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