Measurement
The most direct way of measuring an activity of a species is to measure its partial vapor pressure in equilibrium with a number of solutions of different strength. For some solutes this is not practical, say sucrose or salt (NaCl) do not have a measurable vapor pressure at ordinary temperatures. However, in such cases it is possible to measure the vapor pressure of the solvent instead. Using the Gibbs–Duhem relation it is possible to translate the change in solvent vapor pressures with concentration into activities for the solute.
Another way to determine the activity of a species is through the manipulation of colligative properties, specifically freezing point depression. Using freezing point depression techniques, it is possible to calculate the activity of a weak acid from the relation,
where m' is the total molal equilibrium concentration of solute determined by any colligative property measurement(in this case ΔTfus, b is the nominal molality obtained from titration and a is the activity of the species.
There are also electrochemical methods that allow the determination of activity and its coefficient.
The value of the mean ionic activity coefficient γ± of ions in solution can also be estimated with the Debye–Hückel equation, the Davies equation or the Pitzer equations.
Read more about this topic: Activity (chemistry)
Famous quotes containing the word measurement:
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—John Dos Passos (1896–1970)