The concentration of acetylcholine (a neurotransmitter) in a sample can be deter

Question

The concentration of acetylcholine (a neurotransmitter) in a sample can be determined from the pH changes that accompany its hydrolysis. When the sample is incubated with the enzyme acetylcholinesterase, acetylcholine is converted to choline and acetic acid, which dissociates to yield acetate and a hydrogen ion: In a typical analysis, 15 mL of an aqueous solution containing an unknown amount of acetylcholine had a pH of 7.65. When incubated with acetylcholinesterase, the pH of the solution decreased to 6.87. Assuming there was no buffer in the assay mixture, determine the number of moles of acetylcholine in the 15 mL sample.

Explanation / Answer

pH = -log[H+]

pH at start = 7.65

So, [H+] = 2.24 x 10^-8 M

pH at end = 6.87

So, [H+] = 1.35 x 10^-7 M

Difference [H+] = end - start = 1.13 x 10^-7 M

So moles of acetylcholine present in 15 ml (0.015 L) solution = 1.13 x 10^-7 M x 0.015 L = 1.695 x 10^-9 mol

(or mols present = 1.7 x 10^-9 mol)

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