Buffer lectures - introduction three component buffers
As we have shown earlier, by clever selection of the initial solution and of a way of adjusting the pH, we can keep the ionic strength of the buffer constant within some range. It can be also done using more complicated buffers made of several substances, assuming their pKa values are well separated, and their concentrations are correctly selected (see for example Buffers of Constant Ionic Strength for Studying pH-Dependent Processes, Keith J. Ellis, John F. Morrison, Methods in Enzymology, vol. 87, page 405). The most common multicomponent buffers are those made of three substances.
It is perhaps worth of noting here that name of these buffers is slightly misleading, for two reasons. First - what we call a three component buffer is actually a four component buffer, as after mixing three main components, we will use a fourth compound - strong acid (like HCl) or strong base (like NaOH) - to adjust the pH.
Second reason is of a slightly different kind. Imagine preparing buffer from acetic acid and NaOH and KOH - these are definitely three components. We can easily calculate how much base we need, but every solution in which sum of amounts KOH and NaOH is correct will have the same pH - so there is no single recipe, but infinitely many recipes. So the name implies something that doesn't make sense - buffer, that can't be uniquely calculated. But this is not what we mean by three component buffer, as in the multicomponent buffer ratio of concentrations of the main components is preset and doesn't change, which puts another restriction on the system and makes calculations possible.
Technically calculation of the recipe of the multicomponent buffer is not much different from the calculations we did in the case of the pH 7.0 phosphoric buffer - we check which component has pKa that is closest to the pH, then we calculate amount of acid or base that has to be added to get the correct ratio of the acid and conjugate base - remembering, that we may have to protonate or neutralize other components. It works nicely only if the separation of pKa values is high enough, at the same time we don't want the separation to be too high, as that would mean we can't use the buffer to cover continuous range of pH. So sometimes we may need to do some fine tuning, and calculations are not as easy as expected, in which case it is better to use our program, Buffer Maker.