- these are the substances which have different colors at various value of PH .
- acid-base indicator(weak organic acid and base) having different color depending upon the concentration of hydrogen ion in the solution.
- for acid-base titration we select an indicator which will show a distinct color change at PH close to equivalance point .
- acid-base indicator also known as visual indicator.
types of acid-base indicator theory's :
[1] - litmus theory
[2] - ostwald theory
[3] - quinoiod theory
LITMUS THEORY :-
- according to litmus theory indicator are similar to litmus in their behaviour.
- litmus is a weak acid showing red color in unionised state and blue color is ionised state.
- in the presence of hydroxide ion ,litmus solution shows reaction between hydrooxide and hydrogen ion (litmus).
- this reaction released ionised litmus ion which gives blue color.
- the litmus solution in the presence of H+ ion excess amount results in the formation of unionised litmus which gives a red color.
OSTWALD'S THEORY :-
it state that;
- color change of the acid base indicator is the result of its ionisation because ionised and ionised form are differ in color.
- acid base indicator is either weak acid or weak base ,it's ionisation is highly affected in the presence of acid and base.
- if indicator is weak acid then it's ionisation in acid is very low (due to common H+ ion)and fairly well is alkali .in similar manner if the indicator is weak base it's ionisation in acid is high and low in alkali.
* ostwald's theory can be explain in following manner by taking phenophthalin.
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