Chemical Equilibrium, Law Of Mass-action : Sensitiveness Of Indicators

An indicator requires a definite concentration of H^۰ or OH' ions to produce its characteristic colour change: this concentration varies with different indicators. Thus, methyl-violet is turned blue by a definite small concentration of strong acids (e.g., H₂SO₄), whereas it is unchanged by the weak acetic acid at any concentration, since the latter can never produce the requisite concentration of H^۰ ions.

The ionic product [H^۰] x [OH'] is constant in all aqueous solutions on account of the equilibrium: H₂O <=> H^۰ +OH', and is equal to the dissociation constant of water: [H^۰] x [OH'] = 10^-13.8. The OH' concentration required to produce a colour change of an indicator may, therefore, always be represented by the equivalent H^۰ concentration: [OH'] = [OH'] x [H^۰]/[H^۰] = 10^-13.8/[H^۰]. At the neutral point the H^۰ and OH' concentrations are equal, each being equal to its concentration in pure water: [H^۰] = [OH'] = √10^-13.8 = 10^-6.9. If [H^۰] is greater than 10^-6.9 the solution is acid; if it is less than 10^-6.9, e.g., 10^-8, the solution is alkaline. The concentration of H^۰ ions may be represented by minus the exponent of the concentration, and is then usually written pH; e.g., if [H^۰] = 10^-8.1, pH = 8.1. An ideal indicator, which shows the exact point of neutrality, corresponds with pH = 6.9.

The values of pH required to produce colour changes of various indicators are given in the table below, compiled from the data of Kolthoff (1936).

The gaps are to be filled in with the colour next adjoining, e.g., phenolphthalein is colourless with all H^۰ concentrations greater than 10^-8, red for all less than 10^-9.5. It will be seen that a solution reacting neutral to litmus is actually neutral: [H^۰] = [OH'] = 10^-6.9; with phenolphthalein the solution would be still faintly alkaline; with methyl-orange it is slightly acid (0.0005N); whilst methyl-violet requires 0.01N-acid to produce a colour change.

Fig: Colour change pH intervals of indicators

Y=yellow. V = violet. B = blue. C = colourless. R = red. P = purple. Br = brown. O = orange.

The colour change for each indicator occurs over a definite range of pH, shown by the shaded rectangle. If this range is small, the colour change is sharp.

Expt. 9. - Three rows of flasks, each containing 100 c.c. of "conductivity" water, are supported on a rack

Fig: Experiment on indicators

with milk-glass shelves. To the flasks of each row are added solutions of phenolphthalein, litmus, p-nitrophenol (pH 5-7), methyl-red and methyl-orange, respectively. To the top row (A) a drop of baryta water is added, when the indicators give the alkaline reaction. To the bottom row (C) a drop of NH₂SO₄ is added, when the indicators give the acid reaction. To the middle row (B) 1 c.c. of very dilute baryta water is added from a burette and then, by means of a series of small tubes fastened to a board as shown, 1 c.c. of methyl formate freshly distilled over dry potassium carbonate is poured simultaneously into all the flasks of this row The methyl formate slowly hydrolyses, giving formic acid and metnyl alcohol (neutral):

HCOOCH₃ + H₂O = HCOOH + CH₃OH.

The H^۰ ions of the formic acid neutralise the OH' ions of the baryta, and then excess of H^۰ ions is formed. The solutions therefore change over from alkaline, through the point of exact neutrality, to acid. If the point of neutrality is taken as that corresponding with the colour change of litmus, the reactions of the other indicators, which change at different times, may be compared (Nernst, 1908).

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