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Chemical Equilibrium, Law Of Mass-action : Theory Of Indicators |
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The action of acids and alkalies in changing the colour of indicators has since the time of Boyle been utilised in testing for these two groups of compounds. Many natural colouring matters may be used for this purpose, the most important being litmus, a colour prepared from certain lichens, and turmeric, from the ground root of the Curcuma longa of India (used in making curries) which is yellow, turned reddish-brown by alkalies or boric acid. Many synthetic organic substances are now used as indicators. Methyl-orange is turned yellow by alkalies and red by acids; para-nitrophenol is colourless in acid solution, yellow in alkaline solution; methyl-red is turned red by traces of acids,, and yellow by alkalies; phenolphthalein is colourless in acid solution, and is turned bright red by traces of alkali; alizarin red is turned deep purple by alkalies, yellow by acids. According to Ostwald's theory of indicators (1891) these substances are weak acids or bases, one radical of which in the ionic state has a different colour from that in the undissociated molecule. Thus, para-nitrophenol is a weak acid, colourless in the undissociated state. A trace of strong acid drives back the slight dissociation of the weakly acidic indicator, and the pale yellow solution becomes colourless. If an alkali is added, the OH' ions combine with the H۰ ions of the indicator to form H2O molecules, and further ionisation of the indicator occurs. The anion of the indicator then exists in appreciable amounts, and exhibits a strong yellow colour. Phenolphthalein functions as a very weak acid; its salts, formed by the action of alkalies, are largely dissociated giving an intensely red anion. Its action is similar to that of p-nitrophenol. Methyl orange is the sodium salt of a sulphonic acid and is largely ionised in solution. The anion,  is yellow. In presence of acids this behaves as a weak base, attaches + a hydrogen ion H+, and rearranges to form a red ion:  which has opposite charges on different parts but is electrically neutral as a whole (a hybrid ion):  + OH' <=>  + H2O
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