Each buffer has its own buffering capacity. Beyond that it may have some change in pH. 1:10 dilution will change the molarity of the buffer and buffering ability will change. Better to prepare MES buffer of 0.05 Molar to avoid such changes.

In a buffer solution there is a balance between two populations of compounds. One population captures the H+ ions (the anion of your weak acid), the other (the protonated form of your weak acid) the OH- ions. By diluting ten times you reduce the populations at equilibrium of ten times, therefore, you will have a lower buffer power, i.e. your system will be less efficient at buffering both against added

acids and bases. It depends on how or what you need to buffer and how efficiently. For example, if the increase in protons (or OH-) is constant in time, diluting, you will buffer for a lesser time. The pH range in which your buffer will works is: pH range = pKa ±1. Outside this range your buffer is no more working. Thus your functional pH range depends on the pKa of your weak acid while the buffering power from the concentration. Diluting, the change of pH range will be negligible. If you wish to buffer at extreme values of pH, you can use a solution of strong acid (eg. HCl) or base (e.g. NaOH).

By naïve application of the Henderson-Hasselbalch equation, the pH should not change after dilution (ignoring the small amount of carbonic acid present even in dd water), as it depends only on the ratio between acid and corresponding base, and this isn't changed by dilution. However, it is not the concentrations, but the activities that determine this ratio, and activities increase by dilution (see a textbook of physical chemistry for an explanation of these terms). Thus, in practice, the pH of your diluted buffer will be different from that of your stock solution, by several tenths of a pH-unit, especially if your stock solution is very concentrated (more than 100 mM, or so).

Of course, Dear Engelbert, but in daily laboratory practice, after dilution, the slight pH variation could be brought back to the original value, using a pH meter. You just have to pay a "biological attention" to the new ions you might introduce. What is important to him is knowing that the new solution will have less buffering power. Very correct what you say, but I prefer to leave the "activity" to the chemical-physicists or to analytical chemists, in biology, in the vast majority of cases, it is not necessary. Of course, if you have to manage a technique where small pH changes make the difference, you need to be careful and know how to behave. Jain's need is just to create an environment where to put something biological with a pH "around" 5.6. If he asked that question, we should not confuse him but give only simple elements to think about. If he doesn't want problems he can use the calculator for buffers at

http://www.reachdevices.com/Protein/BiologicalBuffers.html

where he will find the slight correction to do. The rest comes much later, if I remember correctly, in the third year of the five-year chemistry degree at university. Do not you agree?

Greetings

Giovanni

Giovanni Colonna : In many cases, the changes in pH after dilution of a buffer are indeed ignored. However, this was not the question of the OP, who asked simply if the pH would change. And the answer to that question is: Yes, it would; but only slightly.

Is that change of practical relevance? That depends on the specific application. I once published results on an inhibitor ([Co(NH3)4PO4], an exchange-stable analogue of the Mg·PO4 complex), and got contacted by a colleague that he obtained completely different results as he repeated the experiments. We traced the reason to a small difference in pH (his 7.2 instead of my 7.5). It turned out that the inhibitor is a weak base with a pKa around 7. Apparently, the base and the corresponding acid had completely different effects on the enzyme (Na/K-ATPase). Only the paranoid survive!