Importance of Acid–Base Indicators

A suitable indicator for a given titration may be defined as one which has as narrow a pH range as possible that lies entirely on the upright part of the titration curve. Phenolphthalein, litmus and methyl orange may be used as indicators for acid-base titrations.

In an acid-base titration the base solution can be added gradually from a burette into an acid solution contained in a receiver flask. When the amount of the base added equals the amount of the acid in the flask, the equivalence point or the end-point is reached. The end-point of a titration is shown by colour change of an indicator previously added to the acid solution in the receiver flask. An acid-base indicator is an organic dye that signals the end-point by a visual change in colour.

Phenolphthalein and methyl orange are two common examples of acid-base indicators. Phenolphthalein is pink in base solution and colourless in acid solution. Thus when added to the acid solution in the receiver flask, it shows no colour. As the added base is in slight excess, it becomes pink. Thus phenolphthalein signals the end-point by a colour change from colourless to pink. Similarly methyl orange indicates the end-point by a colour change from red (in acid) to yellow (in base).

During an acid-base titration the pH of the solution in the receiver flask changes with the addition of the titrant from the burette. A plot of pH against the volume of the solution being added is known as pH curve or titration curve. As NaOH is added, the pH of the solution increases slowly at first, then rapidly in the vicinity of the equivalence point and again slowly. The equivalence point lies in the middle of the vertical portion of the curve (pH = 7). It must be clearly understood that equivalence point is the theoretical end-point of a titration. The end-point of a titration determined by a colour change of the indicator in titration solution is the experimental estimate of the equivalence point.


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