I happened to prepare 1L of 20mM Na-acetate buffer, pH 5. I calculated volume equivalent to 20mM Acetic acid for 1L buffer and adjusted the pH to 5 using NaOH solution.
Then I wanted to prepare 20mM Na-Phosphate buffer, pH 7. I calculated weight of Monobasic salt equivalent to 20mM NaH2PO4 for 1L buffer and adjusted the pH to 7 using NaOH solution.
However, I looked into many websites and all of them have used mono- and di- basic salts for preparing Phosphate buffer. Few of them mentioned that, adding too much of NaOH for pH adjustment leads to change in buffer concentration. I really don't understand how it matters! Ain't adding NaOH itself will form conjugate base? and since amount of Phosphate ions (or Acetate ions) are still 20mM, how it's going to affect the final buffer concentration?
Please if someone can help...
You must review the concept of Buffer capacity. As far as I remember, using conjugated bases the capacity of buffer to resist pH changes is higher that using only one salt and NaOH. In some cases you will need high amounts of NaOH to reach an specific pH. Review books as for example Physical Pharmacy by A.N. Martin that is a classic and very comprehensive book to learn the basics of physical chemistry applied to the Pharmaceutical Sciences.
A buffer will resist a change in pH, but only so much, hence the term "buffer capacity". So if you add too much of a base (or an acid), you may overshoot your available buffer capacity and then you're in trouble. For a pH 7.0 phosphate buffer the recipe is 50.0 mL 0.1M KH2PO4 (or NaH2PO4) + 29.1 mL 0.1M NaOH; or, if you dislike phosphate, take 50.0 mL 0.1M tris(hydroxymethyl) aminomethane + 46.6 mL 0.1M HCl.
To review buffers, look at any undergraduate analytical chem book, such as Harris, Quantitative Chemical Analysis.
Yes! A phosphate buffer contains both phosphate and hydrogen phosphate and can be converted into one another based on the pH or concentration of the Hydronium (Hydrogen) ion. Thus, you use Phosphoric Acid (H3PO4) for the conjugate ion in acidic media AND NaOH for the conjugate ion in alkaline conditions.
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