Monday 3 September 2007

ACID-BASE INDICATORS

In acid base titration, we wish to determine when the equivalence point is reached, the equivalence point means where the reaction is theoretically complete. In this point the titration process should be terminated. The other, the point at which the reaction is observed to be complete is called the end point.

We take a measurement in such way that the end point coincides with or is very close to the equivalence point. The most obvious way to determine end point is to measure the pH at different points of the titration, but usually more convenient to add indicator to the solution and visually detect the color change.

An indicator for acid-base titration is a weak acid or weak base that highly colored. The color of the ionized is markedly different from that of the un-ionized form. Since the indicator is a weak acid or base, the amount added should be kept minimal so that it does not contribute appreciably to the pH and so that only the small amount of the titrant will be required to cause the color change.

That is the color change will be sharper when the concentration is lower, because less acid or base required to convert it form one to another. Generally, a few tenths percent solution of the indicator is prepared and two or three drops are added to the solution to be titrated.

Phenolphthalein is an example of an indicator which establishes this type of equilibrium in aqueous solution:




Several acid-base indicators are thymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin yellow, tropeolin O, nitramine, and trinitrobenzoic



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