Can I bring the pH to 8 using NaOH? What are the pros and cons of that? Will it work if I add only sodium dihydrogen phosphate for the phosphate ions and skip the disodium hydrogen phosphate?
You can dissolve whatever you want (H3PO4, Na2HPO4, NaH2PO4) to produce your buffer, adjusting your pH with HCl or NaOH. Do not surpass the volume used for your calculations. For instance estimate the amount of any PO4-containing molecule to produce 1 L of buffer, dissolve the molecule using up to 900-950 ml, and adjust the pH without surpassing 1 L of final volume. Finally make up to the 1 L mark.
Adding NaOH to NaH2PO4 is equivalent to making Na2HPO4, so you can do it that way if you wish. I find it more convenient to mix equimolar solutions of NaH2PO4 and Na2HPO4 in the right proportion to get the desired pH. There are tables available to give the correct proportions for each pH. Here, for example:
http://www.sigmaaldrich.com/life-science/core-bioreagents/biological-buffers/learning-center/buffer-reference-center.html#phosphate
Thank You Omar and Adam!
@ Adam- The fact that NaOH will combine with NaH2PO4 to make Na2HPO4 makes sense, but what about the ionic strength. How do I keep that in control?
Good question. I think its rather complicated because of all the different species, the distribution of which depends upon the pH. Also, the relatively high concentrations of the stock solutions brings chemical activity into the picture. For your particular application, however, I don't think it is that important to know the precise ionic strength.
- Taking equimolar conc. In right ratio as suggested by Adam will bring the right pH or will require little addition of acid or base. In turn the ionic strength will be as required or with negligible change, respectively. Phosphoric acid and NaOH is recommended for pH adjustments
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