Acid-base equilibria
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What is the best indicator for an acid alkali reaction: methyl orange or methyl red?
Rev writes …
The choice of indicator depends upon the acid and the alkali used. If you are reacting a strong acid with a strong alkali the solution produced at the end point will have a pH of about 7. If you are reacting a strong acid with a weak alkali the resulting solution at the end point will have a pH below 7, and if you are reacting a weak acid with a strong alkali the resulting solution has a pH greater than 7.
Indicators are weak acids that have an acid of one colour and a conjugate base of a second colour. Thus in excess acid they will be present as the free acid and in alkali they will be present as the base. The point they change colour is the point at which the free acid and the base are there in equal amounts. e.g. methyl orange has two colours red and yellow so its change colour is equal amounts of each i.e. orange. The point where the two are equal depends on the value of Ka for the indicator. Methyl orange and methyl red have very similar Ka values, and it does not matter which you use provided you are reacting a strong acid with a weak base.
Risk assessment
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updated: 21 August 2003
