Biochemists frequently use “phosphate” buffers in their work. Here, “phosphate”
ID: 504241 • Letter: B
Question
Biochemists frequently use “phosphate” buffers in their work. Here, “phosphate” refers generally to the class of phosphate derived ions: H2PO4- , HPO42-, PO43- . (a) Which of these ions best represents the conjugate acid in a “phosphate” buffer at physiological pH? Explain. (b) A buffer used in an experiment was described as “a 0.2 M phosphate buffer with pH = 7.25.” Based on the conjugate acid you identified in (a), determine the concentration of conjugate acid in this “0.2 M phosphate buffer.”Biochemists frequently use “phosphate” buffers in their work. Here, “phosphate” refers generally to the class of phosphate derived ions: H2PO4- , HPO42-, PO43- . (a) Which of these ions best represents the conjugate acid in a “phosphate” buffer at physiological pH? Explain. (b) A buffer used in an experiment was described as “a 0.2 M phosphate buffer with pH = 7.25.” Based on the conjugate acid you identified in (a), determine the concentration of conjugate acid in this “0.2 M phosphate buffer.”
Biochemists frequently use “phosphate” buffers in their work. Here, “phosphate” refers generally to the class of phosphate derived ions: H2PO4- , HPO42-, PO43- . (a) Which of these ions best represents the conjugate acid in a “phosphate” buffer at physiological pH? Explain. (b) A buffer used in an experiment was described as “a 0.2 M phosphate buffer with pH = 7.25.” Based on the conjugate acid you identified in (a), determine the concentration of conjugate acid in this “0.2 M phosphate buffer.”
Explanation / Answer
a) Note down the dissociation of phosphoric acid (H3PO4) as below:
H3PO4 (aq) <====> H+ (aq) + H2PO4- (aq); pKa1 = 2.15
H2PO4- (aq) <====> H+ (aq) + HPO42- (aq); pKa2 = 7.20
HPO42- (aq) <====> H+ (aq) + PO43- (aq); pKa3 = 12.35
The physiological pH is assigned a value of 7.4.
An acid is defined as a species which can donate a proton and a base is a proton acceptor. Since pKa2 of phosphoric acid is close to the physiological pH, therefore, H2PO4- is the conjugate acid at this pH (ans).
The conjugate base is HPO42-.
b) Determine the ratio of the conjugate acid and conjugate base of phosphoric acid at the physiological pH by using the Henderson-Hasslebach equation.
pH = pKa2 + log [HPO42-]/[H2PO4-]
===> 7.25 = 7.25 + log [HPO42-]/[H2PO4-]
===> 0.0 = log [HPO42-]/[H2PO4-]
===> [HPO42-]/[H2PO4-] = 1.0
===> [HPO42-] = [H2PO4-] …..(1)
The total phosphate concentration is given as 0.2 M at the physiological pH. This would mean that
[H2PO4-] + [HPO42-] = 0.2
===> [H2PO4-] + [H2PO4-] = 0.2
===> [H2PO4-] = 0.2/2 = 0.1
The concentration of the conjugate acid is 0.1 M (ans).
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