Answer to Question #95217 in Biochemistry for anon

Question

A - the sodium salt of a weak acid with pKa = 7.12, Mr = 120.0

B - the disodium salt of the conjugate base of A, Mr = 142.0

C - a synthetic amine (NH2 form), Mr = 121.4. The pKa of the
protonated amine (NH3+) is 8.3.
Using any one or combination of the unknowns above, as well as other required solutions available in the laboratory, prepare 100mL of 0.1M buffer pH 7.6.

Answer 1

There is a multitude of variants how to prepare the target buffer especially taking into account that it is unnecessary to use all 3 unknown compounds. It is evident that A and B are the NaH₂PO₄ and Na₂HPO₄ respectively (exact molar masses and pKa of HPO4^- anion). So the easiest way of solving the task is to refer to the standard protocols to prepare the phosphate buffer solution with pH7.6

For example Cold Spring Harbor Protocols give the following recipe:

http://cshprotocols.cshlp.org/content/2006/1/pdb.rec8303

15.5ml 1M solution of A + 84.5ml 1M solution of B results in 100 ml of 1M buffer pH7.6

To obtain the 0.1M solution 10ml of 1M buffer should be diluted with water and the volume adjusted to 100ml. It is important to note that dilution slightly interferes the pH, so it is necessary to adjust the pH by adding a few drops of sodium hydroxide or phosphoric acid solutions available in the laboratory, and to control the pH by using the pH-meter.