THE ANALYST. 85 THE CONSTITUTION OF INDICATORS USED IN ACIDIMETRY. BY JOHN THEODORE HEWITT, D.Sc., PH.D., &LA. (Read at tlic Meeting, Febrziary 5 , 1908.) IN the course of the last few years the question of indicators has received con- siderable attention, for not only does the matter appeal to the chemist from the practical point of view, but affords material for much scientific investigation. Two hypotheses have been employed in attacking the problem of why 8 substance behaves as an indicator. On the one hand we have the physico-chemical considerations of Ostwald, which more particularly concern the dissociation-constants of the indi- cators; on the other hand, the possible changes in the constitution of the indicating substance when it undergoes salt formation. From whichever standpoint we start, we have to remember that our indicators are either acidic or basic substances.0 s t wald's remarks concerning indicators (Le hrbuc h der a1 lgeme in e u Chemie, 1891, i. 799, and Wissenschnftliche Grundlagen der analytischen Chenzie, 1901,86 THE ANALYST. p. 117) show plainly how the subject must be treated from this standpoint. ing to this view, methyl orange is an acid of medium strength. (CH,),N.C,H,.N : N.C,H,.S03' and H., of which the former exhibits a yellow cdour. The undiesociated acid is, however, red; so that in an aqueous solution the colour observed is an intermediate one. Addition of a strong acid (increase of H' ions) diminishes the dissociation, and the red colour appears. Alkalies and alkaline salts of weak acids, on the other hand, necessitate the removal of hydrogen ions, and the solution becomes yellow.Evidently the addition of very little caustic alkali will effect the removal of hydrogen ions at a rapid rate, and the colour-change will be sharp, since the methyl orange will pass rapidly into the dirnethylaminoazobenzene-sulphonate ion (yellow). Similarly, addition of strongly dissociated acids must bring about a rapid change to undissociated methyl orange (red). By the addition of weak acids to the alkaline solution the change will be gradual. This property has been utilised by Veley in an ingenious manner for the estimation of the strengths of different acids (Zeitsch. plysikal. Chenzie, 1906, 57, 147; Trnns. Chenz. Soc., 1907, 91, 155; Proc. Chenz. SOC., 1907, 23, 284). Friedenthal has also used a colorimetric method for the estimation of the concentration of hydrogen ions (Zeitsch.f. Elektroch., 1904, 10, 114), which has undergone considerable extension by Salm (Zeitsch. f. Elektroch., 1904, lo? 342 ; 1906, 12, 99 ; and Zeitsch. physiknl. Chemie, 1906, 57, 471)) who refers all his results to the concentration of the hydrogen ions, the limits of such concentrations falling between 2-normal for 6.034-normal hydrochloric acid, and 5 x 10 -15-normal for 6.744-normal potassium hydroxide (2-normal for OH'). The purely chemical view, which regards the indicator as a tautomeric sub- stance (pseudo-acid or pseudo-base, according to Hantzsch), the substance and its salts having essentially different constitutions, is well exemplified by Green's recent work on phenol- and quinol-phthale'ins (Berichte, 1907, 40, 3724, and JOUI*IZ.SOC. Chem. Ind., 1908, 27, 4). The isolation of a coloured quinonoid carboxylic methyl ester of the former substance leaves no doubt but that the coloured sodium salt of phenolphthalein is related to the parent substance in the following way : Accord- Its ions are : C'HGH4- CO ONa The explanation is in this case purely chemical. Green looks on the two views as being opposed to one another. I n a paper on the constitution of phenolphthalein, published jointly with A. G. Perkin (Tram. Chem. SOC., 1904, 85, 398), he concludes from the fact that phenolphthalein is entirely decolorised by an excess of alkali, and that the colour is not restored immediately by neutralisation with acetic acid, that the ionic hypothesis is thereby disproved, and refers to this matter again in the recent paper in the Journal of the Society of Chemical Industry. In the discussionTHE ANALYST.87 following Green's paper, Smithells observed that the ionic and chemical theories were not incompatible, and the author of this communication is strongly convinced that the two views are complementary. For a substance to act as an indicator, it must be a weak acid or a weak base ; but the ion which it forms must have a different constitution to the parent substance--i.e., the latter is a pseudo-acid or a pezcdo-base, as the case may be. Phenolphthalein and other weakly acidic indicators can be considered as showing the following equilibrium in solution : where Sn and X, are isomeric compound radicals.In the particular case of phenol- phthaleln, XU and Xv represent respectively- Xu is the more stable configuration, and the acid is very weak; therefore S;O.H and its ions are only present to a very small extent in neutral solution, and the absorption due to XU is observed (in the case of phenolphthalein this is in the ultra-violet, and the substance is colourless). Addition of bases (hydroxyl ions) remove6 the hydrogen ions, and the colour due to the XV configuration makes its appearance. Phenolphthalein actually has a dissociation constant, which Saliii (loc. cit., p. 493) has evaluated at 8.0 x 10-lO. In agreement with this low-value, phenolphthalein, as is well known, turns for a feeble concentration of the hydrogen ions, and is therefore used for the titration of weak acids.On the other hand, a caustic alkali is necessary to get a sharp reading. For weak bases, a weak base must be used as indicator. Thus, as Salm (Zoc. cit.) shows, aniline may be titrated, using dimethylaminoazobenzene (butter yellow) as indicator. The ions of salt8 of such a base should possess a different structure to the base itself. I n the case of butter yellow and its salts, the following constitutions are highly probable : C,H,.N : N.C,H,.N(CH,), and C,H;.NH.N : C,H, : N(CH,),Cl. Its sodium salt is yellow, like butter yellow. NaO,S.C,H,.N : N.C,H,.N(CH,), ; but the 6' acid " itself is violet, like butter yellow hydrochloride, so that, correspond- ingly, one may assign to the free '' acid " in the solid state or in acid solution the constitution of an internal salt : We thus come to the nearly related methyl orange (p-dimethylaminoazobenzene- p-sulphonic acid).C,H,.NH.N : C,H, : N(CH,), I [SO, 088 THE ANALYST. The aqueous solution will be an equilibrium of the internal salt, the real dimethylaminoazobenzene-sulphonic acid, and the ions of the latter : C,H,.NH.N : C6H, : N(CH,), r+ HSO,.C,H,.N : N .C,H,N (CH,), c3 I so,-- 0 Red. Yellou . H’ + ’SO,.C,H,.N : N.C6H,N(CH,),. Addition of even a weak base will remove hydrogen ions, and so produce the yellow colour. The question now arises, “Can an indicator be designed ?” To some extent this seems possible, for, as pointed out by the author in conjunction with H. V. Xitchell (Trans. Chem. Soc., 1907, 91, 1251), the oscillation frequency of a substance is slower the greater the number of alternate double and single linkages. Evidently one must synthesise a substance which is capable of tautomerism, its acid or basic functions must be weak, and, in addition, the number of conjugate double linkages must markedly differ in the configurations of the pseudo-base or acid and its ions.Paranitrobenzeneazo-a-naphthol (Meldola, Tq-ans. Clzem. Xoc., 1985, 47, 661) gives yellow or brown solutions in neutral solvents, which turn violet on addition of alkali, the free substance and its potassium salt possessing the constitutions : N02.C6H,.N : N.C,,,H,.OH and KNO, : C,H, : N.N : C,,H, : 0 ; and as it is a very weak acid, it might be used as a substitute for phenolphthalein.I t is not, however, an ideal indicator, in that its solubility in water is extremely small, and the violet colour of its alkaline salts is not very vivid. Hewitt and Mitchell hoped that p-nitrobenzeneazo-/3-methyl-a-naphthacournarin would be a better substance for the purpose, since its neutral solution is a bright yellow (the absorption spectrum shows blotting out of the blue and violet), whilst the potassium salt is a bright blue. Unfortunately, since the substance is a lactone, it takes some little time to hydroIyse, so that it cannot be used as an indicator in titrating an acid with a caustic alkali. The reverse titration can be fairly sharply effected, but the results are rather low. I find, however, that a monosulphonic acid of p-nitrobenseneaso-a-naphthol of the constitution [ Yellow.OH N NOTHE ANALYST 89 C.C. KOIi. gives extremely sharp results, the acid and its monobasic salts giving pale yellow solutions, Addition of excess of alkali turns such yellow solutions sharply to a magnificent purple. The results agree with those given by phenolphthalein, as shown by the following table, exhibiting the number of C.C. : l'henolphtlialein. ')-Nitrobenzel'e- am-a-naphthol, 7 ACETIC ACID REQUIRED TO NEUTRALISE KOH SOLUTIONS. Ni trosul phobenzene- azo-a-iiaphthol. Nitrobenzeneazo- itiethylnrtphtha- coumarin. 10 20 30 10.02 20.02 30.06 10.0 20.05 30.06 10.0 20.03 30.05 9.9 19.8 29.7 The dissociation constant (for the = N<gH group) is evidently not far different to that of phenolphthalein, since 10 C.C. Na,CO, were '' neutralised " by 4.86 C.C.;- acetic acid, using phenolphthalein as indicator ; and using the nitrosulphobenzene- aso-a-naphthol, 4-9 C.C. acetic acid were required to get an intermediate shade, and 5.4 C.C. for a complete yellow. Evidently the substance may be used as a substitute for phenolphthalein, and the colour change from yellow to purple is so marked as to be no less easy to observe than the colourless to purple of the phenolphthalein. The purple colour is not discharged by an excess of alkali, whether hot or cold; neither is it affected by alcohol, even in strongly alkaline solutions. In this respect the substance shows to distinct advantage when compared with phenolphthalein. I desire to thank Mr. A. G. A. Miller for help in examining the behaviour of this indicator towards alkaline carbonates.' DISCUSSION. Mr. CHAPMAN said that the phenolphthalein colour change had been studied by Stieglite, among others, who considered that the probability was that it was an intramolecular change, phenolphthalein being a lactone, and the salts being derivatives of a carboxylic acid. In the case of phenolphthalein, the destruction of the red colour, which occurred when alcohol or acetone was added to the aqueous solutions, was undoubtedly to be attributed to ionisation change. On the other hand, the silver salt was itself coloured in the dry state, in which case there could clearly be no question of ionisation. Both explanations would have to be taken into account in considering the colour changes of phenolphthalein, and probably other indicators.The PRESIDENT remarked that the case of corallin, which acted like litmus with phosphoric acid and some of the weaker acicls, but like phenolphthalein with sulphurous acid, showed that it was not always possible from the constitution and general behaviour of an indicator to predict what its action would be. Dr. HEWITT said that he had not meant to suggest that any indicator could be found which would be useful for all acids and alkalies, because evidently neutrality90 THE ANALYST. must be a relative term. Salm particularly quoted the case of phosphoric acid, which was monobasic to methyl orange, dibasic to phenolphthalein, and could even with trinitrobenzene be titrated as a tribasic acid. He (Dr. Hewittj thought that the greater delicacy of phenolphthalein was largely due to the great difference between the absorption spectra of undissociated phenolphthalein and of the phenol- phthalein ion. In the undissociated condition all the bands were somewhere in the ultra-violet, whereas in the case of the ion the absorption was in the green. I n the case of methyl orange no sharp change was to be expected, because the absorption of the two forms was to a considerable extent in the ~ a m e part of the visible spectrum. The free ‘‘ acid ” of methyl orange exists in solution in a state of equilibrium, and the change to yellow (complete ionization) takes place with a very small increase of concentration of the hydroxyl ions.