Select an appropriate buffering compound from a buffer table (see References 1). Each buffer has a characteristic constant, pKa. Your choice of a buffer depends on this pKa value, the pH value that you want to maintain and whether you expect the pH to rise or fall during the course of the experiment. If you expect it to fall, choose a buffer with a pKa just lower than the desired pH value, such that pH - pKa < 1. If you expect it to rise, choose a buffer with pKa just above the pH value, such that pKa - pH < 1. For example, acetic acid may be chosen to maintain the pH around 5.5 when expected to fall, because the pKa of acetic acid is 4.75.
Calculate the number of moles of buffer required by multiplying the final volume by the concentration. Typical concentrations for buffers are between 0.001 mol/L and 0.2 mol/L. For example: 0.5 L x 0.1 mol/L = 0.05 mol, where 0.5 L is the final volume, and 0.1 mol/L is the desired concentration.
Calculate the number of grams needed of the buffer by multiplying the number of moles by the molecular weight. For example, the molecular weight of acetic acid is 60.1 g/mol, so 0.05 mol x 60.1 g/mol = 3.0 g.
Weigh out the calculated amount of buffer and add it to 90% of the final volume of distilled water in a volumetric flask. Stir the solution slowly as you go. In our example 0.5 L is the final volume, so 90% of that is 0.45 L distilled water.
Titrate the solution to the desired pH using the pH meter. If your buffer is an acid, titrate it with NaCl; otherwise, use HCl. Make sure the temperature is the same as what it will be when conducting the experiment, because pH is temperature sensitive.
Add more distilled water until the final volume is met.