Reaction buffers?

Question

Why are reaction buffers supplied at concentrations? And why are reaction buffers needed? Why can't the reaction be run in pure water?

Answer 1

Enzymes work optimally at a certain pH. You need buffers to keep the pH steady at the value where the enzyme works best.

If you just adjusted the pH to the optimum value using some strong acid or base instead of a buffer then the pH of the reaction mixture would probably change a lot when you add the substrate (dilution and also the substrate could be acidic or basic) or because of the reaction (H+ forming or being depleted depending on the reaction), of the absorbance of atmospheric CO2, etc.

Moreover sometimes the enzyme needs some additional factors to work well like specific ions, small molecules or a certain ionic strength (salt concentration) so in such cases the buffer has more ingredients and is involved in more than just adjusting the pH.

They are always supplied concentrated so that when you add the other reagents (which have a volume of their own), the final volume will be such that the working concentration of the buffer is correct.
If they provided the buffer directly at the concentration that it should be used then adding the reagents would dilute it and the final concentration would be less than it should. This would reduce the ability of the buffer to keep the pH steady and also in the case that the buffer has more ingredients, their concentration would also be below the desired level.