Buffered solutions (or simply buffers) are used in many processes in biochemistr

Question

Buffered solutions (or simply buffers) are used in many processes in biochemistry/biotech labs. Many of the common buffers like phosphate buffered saline (PBS) or Tris/Boric Acid/EDTA (TBE) are commercially available as concentrated stock solutions at 10X the working concentrations. For the first buffer, PBS, the buffering system is a result of the equilibrium between the acid monobasic sodium phosphate (NaH2PO4) and its conjugate base, dibasic sodium phosphate (Na2HPO4) which has a Keq of 3.98E-7.

If the 10X stock at pH 7.20 is 250 mM total phosphate, what is the concentration (in mM) of the monobasic species.

Explanation / Answer

pKa of this buffer = -log Keq = 6.4

pH = pKa + log [Na2HPO4/NaH2PO4]

7.2 = 6.4 + log [Na2HPO4/NaH2PO4]

or, [Na2HPO4/NaH2PO4] = 6.31

[Na2HPO4] = 6.31 /1+6.31 = 0.86 M

concentration of monobasic species = 250mM * 0.86 = 215.8 mM

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