91Ó°ÊÓ

Practise interconverting pH values and proton concentrations. Express all answers to three significant figures. (a) What is \(\mathrm{pH} 7.4\) expressed as \(\left[\mathrm{H}^{+}\right]\)in \(\mathrm{mol} \mathrm{L}^{-1}\) ? (b) What is \(\mathrm{pH} 4.1\), expressed as \(\left[\mathrm{H}^{+}\right]\)in \(\mathrm{mol} \mathrm{m}^{-3}\) ? (c) What is the \(\mathrm{pH}\) of a solution containing \(\mathrm{H}^{+}\)at \(2 \times 10^{-5} \mathrm{~mol} \mathrm{~L}^{-1}\) ? (d) What is the \(\mathrm{pH}\) of a solution containing \(\mathrm{H}^{+}\)at \(10^{-125} \mathrm{~mol} \mathrm{~L}^{-1}\) ? (e) What is the \(\mathrm{pH}\) of a solution containing \(\mathrm{H}^{+}\)at \(2.8 \times 10^{-5} \mathrm{~mol} \mathrm{~m}^{-3}\) ?

Decide on a suitable buffer to use. In the following instances, choose a buffer that would be suitable: (a) Maintaining the \(\mathrm{pH}\) at \(8.5\) during an enzyme assay of a cell- free extract at \(25^{\circ} \mathrm{C}\). (b) Keeping a stable \(\mathrm{pH}\) of \(6.5\) in an experiment to measure the uptake of radiolabelled glucose by a dense suspension of \(E\). coli. (c) Carrying out an assay of photosynthetic activity at \(\mathrm{pH} 7.2\) at temperatures of \(10^{\circ} \mathrm{C}, 20^{\circ} \mathrm{C}\) and \(30^{\circ} \mathrm{C}\) (d) Stabilising \(\mathrm{pH}\) at \(5.5\) during enzyme extraction, in a solution where you intend to measure total protein concentration at a later stage.

Practise using the Henderson-Hasselbalch equation. What are the relative proportions of deprotonated \(\left(\mathrm{A}^{-}\right)\)and protonated \((\mathrm{HA})\) forms of each substance at the following \(\mathrm{pH}\) values: (a) acetic acid \(\left(\mathrm{pK}_{2}=4.8\right)\) for use in an experiment at \(\mathrm{pH} 3.8 ?\) (b) boric acid \(\left(p K_{a}=9.2\right)\) for use in an experiment at \(\mathrm{pH} 9.5 ?\) (c) HEPES \(\left(p K_{a}=7.5\right)\) for use in an experiment at \(\mathrm{pH} 8.1 ?\)