SOME ORGANIC INDICATORS USEFUL in ANALYTICAL OXIDATION-REDUCTION REACTIONS ALLEN G.GRAY University of Wisconsin, Madison, Wisconsin
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N A consideration of the oxidizing agents most valuable in quantitative analysis, there is little hesitancy in deciding that potassium permanganate and potassium dicbromate are the most valuable in use. Ceric sulfate is now being used to a large extent but as yet has not supplanted permanganate and dichromate solutions. Potassium permanganate, by virtue of its high oxidation potential and its readily visible purple color, is a very desirable oxidizing agent. The permanganate color is so sensitive that it is reported that 0.05 cc. of a
0.03 molar solution is plainly visible in a volume of 600 cc. Perhaps it is this latter factor that has enabled permanganate to hold its place as an oxidant because therein lie the chief disadvantages of other oxidants, regardless of which would prove superior to permanganate. Potassium dicbromate is an excellent oxidizing agent, but in practice it requires the use of some type of an indicator. In iron determinations potassium ferricyanide has been used as an outside indicator, when the method of the spot plate is used. This titration is necessarily slow, and hazardous because of the danger of
the oxidation of the ferrous iron by the oxygen of the air. The slowness and uncertainty of the method has, for a long time,greatly discouraged theuseof dichromate as an oxidant in iron determinations. Ceric sulfate also presents wonderful possibilities as a strong oxidizing agent, the difficulties encountered being that some sort of indicator must be used. Only since organic indicators have been developed has ceric sulfate been used to any extent in oxidation-reduction titrations. Although permanganate is widely used as an oxidizing agent in analytical work, i t possesses many disadvantages in actual practice. It is in no wise a primary standard. Crystals of the dried salt contain traces of manganese dioxide. The dioxide can and does act as a catalyst for the reduction of the permanganate, causing a continuous as well as an increasing lowering of the permanganate concentration of the solution. Care must be exercised in making up a solution of permanganate that is to be standardized, and even after its concentration has been determined the standardization must be repeated a t frequent intervals. Permanganate cannot be used in the presence of hydrochloric acid without resorting to certain special procedures. In the presence of a large amount of free acid permanganate is slowly decomposed, and thus cannot successfully be used in back titration procedures if a large amount of free acid is present. Traces of organic matter may easily reduce permanganate. Even small amounts of ammonia in distilled water may result in some of the permanganate being reduced to the dioxide, which, as has been noted, may serve to catalyze further reduction. Potassium dichromate can be thoroughly purified by crystallization and thus serves as a first order primary standard oxidant. After it has once been purified and dried the salt may be preserved indefinitely. Dichromate solutions will serve in the presence of hydrochloric acid, and may be used successfully where back titration procedures are more desirable than direct titrations. Finally, the dichromate solution is stable, and is only very slowly attacked by the organic matter occurring in the water or which may be introduced from other unavoidable sources. Ceric sulfate is usually used in the form of the double salt, ceric ammonium sulfate. Ceric sulfate solutions are not subject to decomposition on long standing, and are resistant to decomposition, even on boiling. It may be used in acid medium. Ceric sulfate presents great possibilities as a strong oxidant, in acid solution, either hot or cold, and these possibilities are beginning to manifest themselves with the advent of suitable organic indicators. Early workers in the field of oxidation-reduction indicators soon came to recognize that if good internal indicators could be found for the lesser used and much more advantageous oxidants, they would become very valuable in oxidation-reduction determinations. An oxidation-reduction indicator is a substance possessing a different color when present in the oxidized form than when present in the reduced form. The oxidation and reduction as well as the color change
should take place between two fairly definite oxidation potentials. The correct oxidation-reduction indicator may be chosen when the oxidation potential a t the equivalence point in the titration and the transition color change interval of the indicator are known. Potentiometric methods are useful in choosing the correct indicator, as well as studying the relative sensitivity of various indicators. Knop (1)in 1923 suggested the use of diphenylamine as an internal oxidation-reduction indicator. ~ oxidant Diphenylaminediphenylbenzidine (irreversible) oxidant
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diphenylbenzidine violet Diphenylbenzidine is very insoluble. This is obviously a disadvantage because the indicator may precipitate a t this point and react, only slowly during subsequent addition of oxidant. The indicator cannot be used in the presence of mercury salts or in the presence of tungstates. Diphenylamine is useful in the titration of strong oxidizing agents with ferrous sulfate. The color change is from violet-blue to colorless. The indicator is also useful in permanganate titrations in the determination of traces of ferrous iron, since in dilute solutions the color of permanganate is less sensitive than that of the indicator. Diphenylamine sulfonic acid was first prepared and used by Sarver and Kolthoff (2). The indicator has properties somewhat similar to those of diphenylamine. The introduction of the strongly polar HS03 group in the molecule greatly increases its solubility over that of diphenylamine. The indicator is used as the barium salt, which is sufficiently soluble that i t may be used in water solution. The color change is very sharp. In oxidized form it gives a reddish purple color, resembling that of permanganate, instead of the characteristic diphenylbenzidine violet produced with diphenylamine. It can be used in the presence of mercury salts and tungstates. The color change takes place a t an oxidation potential of +0.83 volt (reference to normal hydrogen electrode), and is readily reversible. Diphenylamine sulfonic acid gives very good results in the titration of ferrous iron by dichromate. The coloration produced in the solution is so intense that titrations can sometimes be made in colored solutions. Also, the indicator is very useful in direct titrations of oxidizing agents, such as dichromate, permanganate, and ceric sulfate, with ferrous sulfate. The indicator is particularly useful in quantitative work with alloy steels, in its application to the determination of chromium andvanadium occurring with tungsten, the possibilities of which have been studied by Willard and Young (3). The writer has found the indicator particularly useful in titrations involving the quantitative determination of tungstates using metal reducers. The properties of ortho-phenanthroline ferrous complex as an oxidation-reduction indicator have been studied by Walden, Hammett, and Chapman ( 4 ) , and
seem to be those of an ideal indicator for powerful oxidizing agents. Phenanthroline will combine with many inorganic salts. For use as an indicator i t is converted either into its ferrous sulfate complex or ferrous perchlorate complex. Ferrous Complex +Ferric Complex red color violet-blue color The color change is readily and easily reversible as well as vivid and unmistakable, and takes place a t a very high potential, namely, +1.2 volts. The color change is so sensitive that it may well be used in permanganate titrations for the detection of the endpoint, rather than depending on the color of the permanganate to indicate the termination of the titration. Experience has shown the deviation to be much less when the indicator is used. o-Phenanthroline ferrous complex can very successfully be used as an internal indicator in the titration of ferrous compounds with either dichromate or ceric sulfate, or the reverse titration may be used equally as well. The indicator finds a very useful application in the standardization of ceric sulfate in which the primary standards potassium dichromate and sodium oxalate, are used. Ferrous sulfate solution mav be standardized with dichromate using o-phenanthroline complex. The standard ferrous sulfate solution is employed to standardize the ceric sulfate making use of the same indicator. The method gives reproducible results. The indicator properties of diphenylbenzidinesulfonic acid strongly resemble those of diphenylamine sulfonic acid, as has been shown by Sarver and Fischer (5). Its use in slightly acidic solutions gives a reversible violet coloration on oxidation. The indicator action is not hampered in the least by the presence of tungstates, and is catalyzed by the presence of traces of ferrous iron. The indicator is very useful in work with dichromate in which case it is best to add an excess of dichromate and hack titrate with ferrous sulfate. Nitrophenanthroline ferrous complex has been studied (6) and findings report a slightly higher oxidation potential than that exhibited by the more familiar ophenanthroline ferrous complex. The indicator possesses the advantage that i t is stable in acid solutions. The color change is sharp, going from red to blue. Likewise the substituted diphenylamines (6) show oxidation potentials higher than the original diphenylamine. Para-nitrodiphenylamine has a potential high enough that i t may be used in the oxidimetric titration of ferrous iron, where diphenylamine can be used only when the ferrous potential has been reduced by converting it into a complex. The oxidized form of the indi-
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cator gives a very sharp violet coloration. Because of the irreversible oxidation of the indicator the reverse titration of ceric ion with ferrous ion is unsatisfactory. Naphthidine, para-tolylphenylamine and meta-tolylphenylamine are particularly useful in dichromate titrations (7). Both the ortho- and para-tolylphenylamines have been used in the determination of iron and chromium using dichromate. The characteristic color change is to a deep blue, but the indicator action is rendered worthless in the presence of mercury salts. Naphthidine (7) is used in concentrated sulfuric acid solution, and is best prepared by the method of Cummings and Steel (8). The oxidized indicator gives a deep red color, as against the green color of the chromic solution, a decided contrast and a very sharp and easily visible endpoint when used in dichromate determinations. It works well in the presence of mercury salts, a fact of recognized importance in the ferrous-ferric system, if stannous solution is to he used as the reducing agent. The properties of phenanthranilic acid (o-diphenylamine carboxylic acid) as studied by Suirokomskii and Stepin (9) present an indicator particularly useful in oxidation-reduction determinations when ceric sulfate is the oxidant. The oxidation potential is only slightly lower than that of o-phenanthroline complex, and it exhibits an extremely delicate color change, from colorless in the non-oxidized form to pink in the oxidized form. In solutions of excess oxidant the indicator possesses much greater stability than the diphenylamine derivatives. Phenanthranilic acid may be used successfully in all standard oxidation-reduction volumetric procedures using the oxidants dichromate, ceric sulfate, and permanganate. It may be used in the ferric-ferrous system without lowering the potential of the system by the usual formation of a complex. For use with dichromate in the determination of vanadium and in solutions other than those containing ferrous ion, best results are realized in a rather strongly acid solution. The indicator can be particularly recommended for all titrations with ceric sulfate, and in the standardization of ceric sulfate against sodium oxalate, for in actual use with this strong oxidant the color changes are extremely sharp and indicator errors arenil. The possibilities and limitations of some oxidationreduction indicators have been reviewed. With increasing and widening interest in the application of these indicators to analytical determinations, many very desirable oxidants that are not able to furnish their own endpoints in titrations, may become extremely useful and almost indispensable in this work.
LITERATURE CITED
(1) RNoP,1.Am. Chem. Soc., 46,263 (1924). (2) SARVHR AND KOLTBOPF, ibid., 53,2906 (1931). Ind. Erik'. Chem., Ed.* 5, (3) WILLAnn AND (1933). HAMME:TT, AND CHAPMAN, J. Am. C h m . Soc., 55, (4) WALDEN, 2649 (1933). (5) SARvEn AND FISCHER, hi Eng. Chm., Anal. Ed., 7, 271 (1935).
(6) HAMME=, WALDEN. AND EDMONDS, J. Am. Chem. Soc., 56, 1092 (1934). (7) STROKA AND OESPER, Ind. Eng. C h . , Anal. Ed., 6, 405 (1934). (8)CUMMINGS AND STEEL, I.C h . SOC.,123,2464 (1923). (9) S ~ ~ O K O M SAND K I ISTEPIN,J. Am. Chem. Soc., 58, 928 (1936).
