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The water turns red with one drop of HCl and blue with one drop of NaOH.
Large amounts of HCl are added to the buffer solution and the color does not change. Large amounts of NaOH are added in another buffer solution and the color still does not change. It should be noted that this will not go on forever.
Buffers lessen or absorb the drastic changes in pH that occur when small amounts of acids and bases are added to solution. In this case, the buffer solution is made of water, acetic acid and sodium acetate. The acetate ions shift the equilibrium, depressing the ionization of the acetic acid. The pH will remain essentially constant as long as the ratio of the concentrations of acids and bases are more or less constant. When enough acid or base is added to exceed the buffer capacity of the solution, the pH will change significantly and its color will change.
OAc - (aq) + H3O+(aq) ----> HOAc (aq) + H2O (l) (addition of acid)
HOAc (aq) + OH - (aq) ----> OAc - (aq) + H2O (l) (addition of base)
This material is based upon work supported by the National Science Foundation under Grant Number 1246120