Investigation on the Possibility of Differential Reduction from the Standpoint of Oxidation-Reduction Potentials Citation Ewing, Frederick Junior (1927) Investigation on the Possibility of Differential Reduction from the Standpoint of Oxidation-Reduction Potentials. Bachelor's thesis, California Institute of Technology. doi:10.7907/JSKE-7E31. https://resolver.caltech.edu/CaltechETD:etd-12102004-141151 Abstract No abstract. Item Type: Thesis (Bachelor's thesis) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: None, None Defense Date: 1 January 1927 Record Number: CaltechETD:etd-12102004-141151 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-12102004-141151 DOI: 10.7907/JSKE-7E31 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 4928 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 10 Dec 2004 Last Modified: 03 Oct 2019 23:39 Thesis Files Preview PDF (Ewing_fj_1927.pdf) - Final Version See Usage Policy. 466kB Repository Staff Only: item control page

INVESTIGATION on the POSSIBILITY OF DIFFERENTIAL REDUCTION from the STANDPOINT OF OXIDATION-REDUCTION PCTENTIALS. Thesis by Frederick J. Ewing In partial fulfillment of the requirements for the degree of Bachelor of Science in Chemistry. CALIFORNIA INSTITUTE OF TECHNOLOGY Pagedena, California 1927 The object of this work was the invest- igation of the possibility of differential raduction by maintaining a known reduction potentiel in the solution of reductants. The compounds investigated belonged to the irreversible systems which haye been investigated by J. B. Conant end co-workers, and whose apparent reduction potentials have been tabulated. In previous work of Conant and others, the reduction of this class of compounds had been studied by following the reaction with a bare platinum electrode, whose potential against a calomel half cell would vary as to the relative amounts of the reduced and oxidized reagent changed in the solution surround- ing the electrode. It was thus possible to determine the rate of reduction by various reagents of known re- duction potential, and to stipulate as the Apparent Reduction Potential of a compound the reduction poten- tial of that reegent which would give appreciable reduction in a given length of time. With the data thus obtained it seemed possible to devise a mechanism for the accurately con- trolied differential reduction of a mixture of these compounds, since each compound would remain unreduced until the reduction potential of the solution became less than the 4. R. P. of the compound. | Various methods were tried for the study and application of this principle. They were essen- tially the same in that the reduction-oxidation poten- tial of the solution was elways followed by a bare platinum electrode and calomel half cell with 4 sat- urated KCl bridge. Variations consisted in the differ- ent ways of mixing the reducing agent and the solution of the compounds, and consisted of {1) the use of a buffer or catalytic agent in the form of a reversibly oxidized and reduced organic compound, (2) the absence of a buffer, (3) the alternate and suitably regulated inflow of both solutions into the cell, (4) the very slow continuous addition of the reducing egent, and (5) the rapid mixing of both reductant and reducing agent. From the &. H. F. curves thus obtained it was possible to draw certain conclusions and recommenda- tions concerning the optimum conditions. The cell employed was a two liter bottle with a mechanical stirrer and mercury seal, and pro- vided with an inlet and exit for nitrogen, inlets for two burettes, a platinum wire electrode and a KCl bridge to a calomel half cell. 4 potentiometer was used to measure the electromotive force developed. The nitrogen was bubbled through several bottles containing pyragallol and brought into the cell through glass tubing. The exit was closed by a water seal. The cell was always swept with nitrogen for fifteen minutes before any reducing agents were intro- duced, and an appreciable flow continned throughout the entire experiment. The reducing agent was a O.1N solution of TiClg in 0.2NHC1. | The first compounds to be investigated were the f&z0 dyes: a M2 CO "OC . ARB ON8S ve 3oNq / ¥ ARP. = OFFI Cn $0, Na FAY N=IN ARP = v, Sor } " )x7 303N Ch; (The values for the A. R. P. given here are ageinst the hydrogen electrode, and hence do not check the potential ranges given later). Dye No. 24 was yellow, while the other two were red, so that the color changes gave a check on the electrometric titrations. Dye No. 5 was found to give a colloidal gel at a concentration of e0002 molal in 0.2NHC1, and was soon discarded in favor of No. 6. The solution finelly adopted was a mixture of .OOlmolal No. 24 and .001 mois] No. 6 in 0.2 NHCl. Each dye was standardized separately against TiCls. , Chi oresbéteso acid was used in a buffer, the plan being to reduce the chlorani #éic acid until it hed reached the proper potential, (see Fig. I), and then to add such an increment of dye that the chloranilwic acid would reduce the more easily re- ducible of the dyes, then to reduce the rapidly re- acting chlorani tic acid which was again in the oxidized form by more TIiCl sz. The presence of the reversible system formed by the oxidized and reduced portions of the chlorani fic acid would thus keep the potential controlled, and prevent the titanium from arriving at such a potential as would reduce the red dye. The results, using a solution .0006M in dye No. 24, and .0002 molal in dye No.w 5, are in Table L. Increments Sel 1.5 0.9 1.0 1.5 2.0 1.8 1.8 220 12.5 8.7 TABLE I Increments cc. Dya 100 ea. of '.001 molal ght grani Yric 50.0 50.0 50.0 100.0 100.0 100.0 100.0 100.0 650.0 Range of Voltage .244,.116 0128, .056 .105, 055 -108, 055 «121, .064 +113, 057 118, .069 .118, .070 e119, .051 -065,.004 Time of Cycle 43 min. 20 15 10 20 16 . LO Thus we had established an &. M. F. curve with the chlorani tric acid, and by alternate additions of dye and TiCl 3 had slid up sand down on it from 120 to 60 millivolts, reducing dye No. 24 ag it was added to the solution. The potential fell very rapidly when TiClz wes introduced, but it took about fifteen minutes for the dye to oxidize the chlorani tric acid back up on each cycle. When these cycles were finished, the red colored residue of dye No. 5 was titrated till colorless, the potential renge dropping to 65 - 4 millivolts. The ratio of the volumes of TiClg in these two titrations was 1.35, while the ratio of the two concentrations was 3.0. Apparently the chloranitric acid did not completely reduce the dye No. 24, so that there was a cumulative residue from each cycle. The next step was to conduct a suffi- ciently slow reduction to be able to perceive precisely what happened throughout the entire titration. Solutions .001 molal in dye No. 24 and dye No. 6 were prepared. chloranitie acid was reduced, 420 CC. of dyestuff introduced, and TiCl, allowed to slowly flow in until the potential dropped rapidly. ‘the TiClg was then added in 0.1 ce. increments at 5 min- ute intervals to allow the system to come to equili- briun. When the potential no longer fell rapidly, the rate of the inflow of TiCl= was incfeasea, with occasional five minute stops to ascertain what the equilibrium potential was. The curve thus developed (Fig. IT) had several significant features. The inflection point on the curve corresponded to roughly half the total volume of TiCl, required to decolorize the solution, the ratio being 0.525, while the ratio of each dye to the total amount of dye present was 0.500, Hence there is a very sharp and definite separation in the reduction of these two dyes, whose Aa. R. P. differ by only 100 millivolts. Furthermore, the curve drawn through the equilibrium peaks is approximately parallel to the curve of the potentials that have been depressed by @ rapid inflow of TiClz, so that the character of the reduction can be studied even though the potentials of the true equilibrium are not attained. It was next interesting to investigate whether or not the reversible part of the irreversible system which was postulated by J. B. Conant would be sufficiently influential in stabilizing potentials to do away with the necessity of the presence of chlora- ntiete acid. 4ccording to this hypothesis the irre- versible cleavage of the ¢azo dye is preceded by a reversible reduction, and the latter might well assume 811 the functions of the reversible system hitherto furnished by chloranitfie acid. The experiment was carried out as above except for the absence of chlora- ni ffic acid, with the discovery of a curve (Pig. III) similar in all respects to that of Fig. II. In order finally to acquire a completely uniform curve that would be unbroken by equilibrium intervals, a titration extending over six hours was - undertaken, in which 0.1 cc. increments of TiClg were added every minute to a solution similar to that of Fig. III, and the potentials read 45 seconds after their addition. This curve (Fig. IV) shows without any ambiguity the sharpness of the change from one potential level to the other as one and then the other dye is reduced. The rise and subsequent constancy of potential at the end of the titration is unexplained. A short investigation of the differential reduction of the nitro groups in 3 - 5 dinitro benzoic acid was undertaken. & solution containing 1.0 grams of the acid was heated to 90 degrees to speed up the reactions and 6 equivalents of TiCl, were added, over & time of 3 hours, at which time the potential began to drop noticeably. fo a similar solution 6 equiva- lents were added in about 20 minutes, the potential eurve being about 50 millivolts lower. Both solutions were evaporated to dryness and the residue extracted with 200 cc. of boiling water. The solution was brought to an acid concentration of 16" | and extracted with 50 cc. of ether. The ether upon evaporation geve 0.50 grams of 3 amino 5 nitro penzoic acid for the carefully reduced solution and 0.34 grams for the rapidly reduced, which ratio is assumed comparable to the relative concentrations of | the mono-amino acids, since at that acid concentration the diamino compound is very soluble in water. There was no evidence of the dinitro compound since there was no product insoluble in strong acid, but the presence of the aiamino benzoic acid was evinced by the development of a green decomposition product in both water phases. In conelusion, it has in certain cases been shown to be eminently possible to effect differ- ential reduction merely by ea careful wateh of the oxidation reduction potential as reported by a bare platinum electrode, and also that the potential may be controlled by varying the rate of addition of the reducing agent. Furthermore, it has been shown possible to electrometrically titrate a mixture of azo dyes having sufficiently different "apparent reduction potentials”. The time necessary to effect a sharp differential reduction of course depends on the repidity LO ef the reaction. With these types of compounds the time required is in general from 1 to 3 hours at 20 degrees Centigrade, which time may be considerably shortened at higher temperatures, since the ordinary titration of an azo dye by reduction takes but a few minutes. If the reducing agent is introduced too rapidly it appreciably attacks the compounds which are desired unreduced, as occured in the second re- auction of the dinitro benzoic acid. SUMMARY A mixture of two azo dyes was sub jected to successful differential reduction by following the reduction potential of the solution with a bare platinum electrode. A partial study of the differential reduction of 3,5 dinitro benzoic acid was made. In conclusion I wish to thank Professor James B. Conant, who, in his visit to the Celifomia Institute of Technology, introduced me to that field of organic electro-chemistry in which he stands unique, and to whom the essence of this paper is due in its entirety. 190 i709 /50 130 90 Fo 70 SO | Electrometric Titration -. of Chloranilic Acid with Ti Ci, 49 2.0 TCH; OAL m by o/ ° OF oA of 7 9% or o gh -—~ o'ce oF &¢ ajs0p0s" ‘“ \ ‘ Wb N oof —™ au OF of of oz or o oer T 19h —t "$697 40/03 \ ~— ow Wh \ Ost Tm °61 4 006 oot a0 ¥ 70L6 ssopeo {92 AT & T tory os SYA - Wty on