The buffer maintains the requisite pH of the solution even with addition of small amounts acid or base in the solution. This protects the integrity of the protein which is greatly influenced by pH of the solution. This is helped by the fact that a buffer is comprised of a weak acid (HA) with its conjugate base (A-) that attenuate the effects of the added acid or base.
As a general rule of thumb, the relative amounts of acid and base in a buffer should not differ by more than tenfold. This is because as this ratio changes in the endeavor to change the pH of the buffer, the less effective the buffering capacity of the buffer becomes. The best buffer is one where the pH of the buffer is equal to its pKa.
The range of sodium phosphate buffer is 5.8 to 8. This means that within these pH ranges, the buffer can easily moderate the pH of the solution by maintaining a relatively constant levels of H+ and OH- ions even with the addition of a small amount of strong acid or base.
