Finding the equivalence point indicators

When enough base has been added to react completely with the hydrogens of a monoprotic acid, the equivalence point has been reached. If a strong acid and strong base are titrated, the pH of the solution will be 7.0 at the equivalence point. However, if the acid is a weak one, the pH will be greater than 7 the neutralized solution will not be neutral in terms of pH. For a polyprotic acid, there will be an equivalence point for each titratable hydrogen in the acid these typically occur at pH values that are 4-5 units apart. 

The key to a successful titration is knowing when the equivalance point has been reached. The easiest way of finding the equivalence point is to use an indicator Aye] this is a substance whose color is sensitive to the pH. One such indicator that is commonly encountered in the laboratory is phenolphthalein it is colorless in acidic solution, but turns intensely red when the solution becomes alkaline. If an acid is to be titrated, you add a few drops of phenolphthalein to the solution before beginning the titration. As the titrant is added, a local red color appears, but quickly dissipates as the solution is shaken or stirred. Gradually, as the equivalence point is approached, the color dissipates more slowly the trick is to stop the addition of base after a single drop results in a permanently pink solution. 

Different indicators change color at different pH values. Since the pH of the equivalance point varies with the strength of the acid being titrated, one tries to fit the indicator to the particular acid. One can titrate polyprotic acids by using a suitable combination of several indicators. 

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