# Help with BuffersBiochem

1. Sep 11, 2012

### WSR

Question

You are asked to prepare 700 mL of PBS (phosphate buffered saline). The PBS needs to be pH 7.40 with 25mM phosphate and 140 mM NaCl. You are given a bottle of NaCl (58.45 g/mol), 1 M Na2HPO4 stock solution and a 1.00 M NaH2PO4 stock solution. ( H3PO4 pk1=2.12, pk2=7.21, pk3=12.66) Describe how to prepare this buffer.

Equations

I'll use the Henderson-Haeelbalch Equation so pH= pka +log(base/acid)

The thing that really screws me up in this problem is how to incorporate the M concentrations. The pH has to be 7.40 so 7.40 = pka + log (base/acid). I don't know where to go with the concentrations.

Also, a separate question that I don't know if I did correctly.
At what pH will 62% of x be in the protonated form if pka = 8.10. So I did, pH= pka +log(base/acid) and got pH= 8.10 + log (.38/.62) with an answer of 7.89. Does that makes sense?

I'd appreciate any guidance.

Thanks

2. Sep 12, 2012

### Staff: Mentor

You know sum of concentrations of hydrogen and dihydrogen phosphates must be 25 mM. That, plus Henderson-Hasselbalch equation give you two equations in two unknowns - solve for both concentrations. Then calculate how much of the original solutions you need to dilute to produce 700 mL of the PBS.

Approach to the other question looks OK, I haven't checked numbers.