118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions.Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. OXIDATION AND REDUCTION POTENTIALS OF ORGANIC COMPOUNDS. BY EINAR BIILMANN. ReceZ'vtd November 8th, I 9 2 3. I. THE QUINHYDRONES. (a) Reduction Potenrial of Benmequi9zhydrme. The affinity of organic chemical processes has only been measured in very few cases.In organic chemistry there has been much talk about affinity, but very little exact investigation, which could give numerical values for the affinity of distinct transformations. However, since I 9 2 0 several investigations of this kind have been carried out, concerning the reduction-oxidation poten- tials of couples of organic compounds, especially of the type quinone-hydro- quinone, which was examined by Haber and Russ as far back as 1904. A series of American investigators have determined the reduction potentials of several quinones, and at the same time, but independently of them, I have been working on the same problem. Still there has been a difference in the scope of our work, for while the American investigators have extended their experiments over a vast number of quinones, I have tried to study the reduction potentials of some few quinones in electrolytes with different hydrion concentration.In consequence of this difference in plan the American scientists have given us a very valuable impression of the influence of the chemical constitution on the reduction affinity of quinones, and, on the other hand, my investigations have shown that by means of quinhydrone, which is a combination of one molecule hydroquinone and one molecule quinone, an electrode may easily be made up, which in many cases may be used as a very good hydrogen electrode corresponding to a hydrogen pres- sure which is exceedingly feeble, but in the case of dilute electrolytes con- stant and well defined at a given temperature.This was defined by measurement of a series of cells of the type Pt I quinhydrone, a-nH+ I a-nH+, H21 atm. 1 Pt where a-nH+ represents an a-normal, dilute acid electrolyte, whose molar concentration does not exceed 0-1. The measurements, which were camed out at 18" C. and at 25' C. are shown in Table I. and Table 11. 676ORGANIC COMPOUNDS 67 7 TABLE I. QUINHYDRONE-HYDROGEN AT 18' C. Concentration Quinhydrone. Electrolyte. of Potential. 0'7043 0.7043 0'7046 HCI 0.1 IL 0.005 0'7038 0'7038 0'7038 Sodium citrate {:::; PH = 4'96 0'00 j 0'7045 Phosphate mixture pH = 6.81 Average 0-7044 TABLE 11. QU~NHYDRONE-HYDROGEN AT Concentration Quinhydrone. Electrolyte. of HCI 0.1 za solid solid H,SO, 0.1 $1 H,SO, 0.02 t~ Sodium citrate pH = 4-96 Phosphate mixture 1:::; pH = 6.8~ 25' C.Potential. 0.6987 0.6988 0-€& 0.699 I 0.6ggO 0.6992 0'6988 0.6993 0.6987 0.6985 0.6991 0-6ggo 06992 0.6989 Average 0.6g& In the cells the platinum in the quinhydrone solution is positive to the If we send a current from right to left inside the chain, the process at platinum of the hydrogen-electrode. the hydrogen-electrode will be and at the quinhydrone-electrode H, + 2F + 2H+ (F = 95,540 coulombs) C,H,Og + 2H+ + 2F + CGH,O?Ht, quinone hydroquinone so that the total change in the celI consists in the transformation of quinone in hydroquinone. A current in the inverse direction will produce the in- verse transformation, or the chemical process in the cell will be C6H402 + H2 fs C,H,02H2. Accordicg to the law of mass action we have which says that at a particular temperature the hydrogen concentration in the quinone-hydroquinone electrode is exclusively dependent on the ratio betwden the concentrations of hydroquinone and quinone.And to this hydrogen concentration in the solution must correspond a hydrogen pres- sure in the solution, which is678 OXIDATION AND REDUCTION POTENTIALS OF Therefore, if we consider the electrode as a hydrogen-electrode with the hydrogen pressure P,, the potential of the cell will be = o-oooo992~. log P,, + 0*0000gg2 . T . log P,, where P,, is the hydrogen pressure in the hydrogen-electrode. From the figures in Table I. and 11. we can calculaie the hydrogen pressure in solu- tions of quinhydrone, that is in solutions containing quinhydrone and quinone in equal molarity, and the calculation gives us the values P, I O - ~ ~ * ~ ~ at 18" C.P, 1 0 - 2 ~ ~ ~ ~ at 25" C. These hydrogen pressures correspond to less than one single molecule of hydrogen in I litre, and as for free hydrogen in the solutions of quin- hydrone there will not be one single molecule present in the quantity of this solution contained in the electrode vessel. As a matter of course we cannot attribute to these figures any physical significance. However, the calculated values of hydrogen F ressures corresponding to the normal potentials may be very useful characteristics of the reducing powers of the substances examined. As for the organic chemist who desires to work with these problems, and who usually may not be very familiar with thermo- dynamics, I think he will eliminate many errors by referring oxidation- reduction electrodes to the type of hydrogen electrodes.In the quinhydrone-electrode we do not employ a fortuitous mixture of hydroquinone and quinone but the compound quinhydrone, which in aqueous solution dissociates widely in to equal molecules of its components. I n this special case we have [CsH402Hd = [Cd&Od and consequently [H,] = K and P, = K'. The constancy of P,, independent of the quinhydrone concentration, is of very great importance for the practical applicability of the quinhydrone electrode, and the figures in Tables I. and 11. show that this independence is realised in the electrode, for solutions which are 0.005 molar in regard to quinhydrone produce the same potential as solutions which are saturated with quinhydrone (" solid ") corresponding to 0.018 molar solutions at 2 5 O c. (6) Influences on the PoteittiaZ.We shall now consider the influence which chemical compounds present in the quinhydrone electrode may be able to exercise on the potential. First we have to consider that one of the components of the quin- hydrone, the hydroquinone, is itself a feeble acid. v. Euler and Bohlin have found that its first dissociation constant is 1-1 x I O - ~ O . In markedly acid solutions, therefore, the hydroquinone will not be able to alter the hydrion concentration. But as the hydrion concentration of a saturated solution is calculated to be c . 10 -6, the addition of quinhydrone to a feebly acid solution without buffer effect may affect the hydrion concentration.On the other hand, in the case of a buffer mixture, the acidity of which does not differ much from pH6, the influence may be negligible. So I found in a phosphate buffer mixture the value 6-80 for pH with quinhydrone electrode, and 6-79 with the hydrogen electrode. Another influence of the dissociation, which may be important in alkaline solutions, is that it alters the ratio between hydroquinone andORGANIC COMPOUNDS 67 9 quinone, which in the markedly acid solutions of quinhydrone is unity. I f t is the total concentration of hydroquinone and G' is the concentration of undissociated! hydroquinone, and I O - ~ O the approximate value of the dis- sociation constant of hydroquinone, we have C 10-** - - - I f - Ct P + l ' and the potential between a quinhydrone electrode with only undissociated hydroquinone and one in which the ratio between total and undissociated hydroquinone is c : t' is AT = 0~0000g92 .T . log -;. t C By means of this equation we find W+l. c : c'. AT. 10-9 1-01 0'00012 volts. IO-~ 1'10 o'ooIIg ,, We see that this influence should, in the first place, be important in markedly alkaline solutions. Next we shall regard an influence of quite another kind, namely the influence of acids and salts on the solubilities of hydroquinone and quinone, that is to say, on their activity. After my first publication on the quin- hydrone electrode at the Oersted-meeting in Copenhagen in September, 1920, S. P. L. Soerensen, Margrethe Soerensen and Linderstroem Lang very carefully studied this problem.By determinations of the solubilities of hydroquinone, quinone and mixtures of these components of the quin- hydrone in acid solutions of sodium chloride and by measurement of the potentials of electrodes made by means of such solutions, the authors have studied the activity theory and at the same time have made a very interest- ing contribution to the chemistry of the quinhydrone electrode in salt-rich solutions. They find that the potential decreases when the salt concentra- tion increases and that this is due to the influence of the salts on the solubilities of hydroquinone and quinone, which both diminish but not in the same degree. The hydroquinone is affected more than the quinone, and in consequence the ratio between the activities of hydroquinone and quinone has a value higher than unity.Therefore the quinhydrone electrode in a solution rich in salt is more negative than that of a quin- hydrone electrode in a solution with a lower salt concentration. The authors have only examined the influence of sodium chloride in 0-01-n HCl. We therefore know very little about the nature of the salt effect on the quinhydrone electrode. But at a meeting at Gothenburg last summer, Mr. Linderstroem Lang communicated some researches he had done on the influence of the kations Li+, Na+, K+, Rb+ and Cs+ and the anions Cl', Brj, and I+ on the solubilities of hydroquinone, quinone and some other substances. These experiments seem to indicate a relation between the reducing and oxidising effect of hydroquinone and quinone on one side and the electrical charges and the size of the ions on the other side.As a matter of course, these effects are of great importance for the practical use of the quinhydrone electrode, I n Table 111. is shown the influence of salt- and acid-concentration on the potentials of chains of the type- Pt I Quinhydrone, Electrolyte, H, I Pt at 18" C. and I atm. hydrogen.680 OXIDATION AND REDUCTION POTENTIALS OF TABLE 111. Electrolyte. 0.01 HCI . . . . . . . 0'7044 Volts. 0'01 HCI + o'og NaCl . . . . . 0.7042 ,, 0.01 HCl + 1.99 NaCI . . . . . . 0,6978 ,, 0-01 HCl -t 3-99 NaC . . . . . 0.6921 ,, 0.50 HCl . . . . . . . 0.7029 ,, 0.50 HCI f 0.50 NaCl . . . . 0 - 7 q ,, 0.50 H,SO, . . . . . . 0'7039 ,, The figures, are taken from the paper of Biilmann and Lund on the quinhydrone electrode.The potentials of the first two electrolytes are quite consistent with the figures in Table I., and the values for the a-molar and the 4-molar solution of hydrochloric acid + sodium chloride agree well with the observations of Soerensen and his collaborators. (6) 2% Quinhydrone Elecfrode, th Quino-guiahydrone Electrode and the H j d ~ ~ - p t h h ~ d ~ o ~ Electrode. The chemical process in the quinhydrone electrode is a transformation of dissolved hydroquinone in dissolved quinone and hydrogen, and vice R FIG. I. versa. We must not suppose that the affinity of this transformation and in consequence the potential of the electrode should not be affected by the nature and the composition of the solvent. Only experiment can tell us the magnitude of this influence, but as a matter of course, if we make up an electrode in which the process is a transformation of a solid substance into another solid substance, we may suppose that the affinity and the potential will be found to be independent of the composition of the solvent, and this may easily be tried by means of electrodes saturated with quinhydrone and with the one or other of its components.Together with Mr. Lund I have examined electrodes of these types, and I think that it may be useful to describe the practical arrangement of these electrodes as well as of the quinhydrone electrode itself. As for the quinhydrone electrode we employ electrode vessels of theORGANIC COMPOUNDS 68 F form usual for calomel electrodes (see Fig. IA). The platinum electrode is a blank platinum foil or a platinum wire.For making the electrode ready for use c. 15 C.C. of the electrolyte are shaken for t minute or so with some c. gm. of quinhydrone, the mixture is poured in the vessel and the platinum electrode fitted in place. As soon as the solution has the desired temperature, the electrode is ready for the measurement of potentials. I n the electrodes containing saturated solutions of quinhydrone and quinone or hydroquinone-we have called them the quino-quinhydrone electrode and the hydro-quinhydrone electrode-it is only that part of the solution in contact with the platinum which needs to be saturated with quinhydrone and with quinone or hydroquinone. By means of the electrode vessel shown in Fig. I B we have succeeded in preparing these saturated solutions in the electrode vessel itself.For this purpose we shake c. 15 cc. of the electrolyte for + minute with a mixture of 0.1 gm. quin- hydrone and 0.5 gm. quinone (for the quino-quinhydrone electrode) or with a mixture of 0-1 gm. quinhydrone and I gm. hydroquinone (for the hydro- quinhydrone electrode). The mixtures of solutiop and solid substance are poured into the electrode vessel, and for platinum electrode is used a platinum wire as shown in Fig. IB. The figures in Tables 1V.-VII. show that by means of this arrangement the solution moistening the platinum very rapidly attains a complete and well-defined saturation. All the measurements were carried out at 18" C. and with 2 electrodes. TABLE IV. Pt. I Quinone., Quid drone Electrolyte, Hz I Pt.&ids): Electrolyte. t. lr. Hrr. Mins. 0.1 ti HCl 0 45 0'7563 0.7563 z:;;?} O'75% 1 15 - 0.7561 0.756 I 0.5 ti H,SO, 1 25 0'7558 0.7560 - 1 55 0.7558 0.7560 - 4 15 0.7562 0.7558 - 5 I5 0.7565 0.7561 0.7562 0.7561 0.7562 0.7561 0.5 n HCI 0 15 0.5 ti NaCl 0 30 I 00 Average 0.7562 TABLE V. (Solids). pt. I Hydroquinone, Quinhydrone, HCl, Ho I Pt. Electrolyte. t . m. 0.1 10. HCI I 00 0'6175 0.6174 - 1 45 0.6174 0.6174 0.1 rz HCl 0 30 0.6178 0.6181 - I 0 0 0.6177 o617g 0.5 ts HCI 2 25 0.6178 0.6173 - 3 15 06178 0'6175 - 6 0 0 0.6177 06175 - 18 00 0.6176 0.6174 1.0 n HCl I 00 0.6172 0.6171 - 2 0 0 0.6174 06174 - 3 30 0.6174 0.6174 Hrs. Mins. 0.6177 06176 1 1 0.6174 } Average 0.6176 A capillary quinhydrone electrode, working with 2-3 drops of the electrolyte, has See also the April been described by Biilmann and Lund (Annales dc Chimie, 1921). number, 1924, of the 3!'owrnal of Agricultural Science.682 OXIDATION AND REDUCTION POTENTIALS OF TABLE VI.Pt. I Hydroquinone, Quinhydrone, HCl + NaCl, H., I Pt. (Solids). Electrolyte. t. ?r. Hrs. Mins. 0.6181 0 30 0.6183 1 30 0.6 182 0'6180 0'6 I 80 0.6180 ~6 00 0.6180 0.1 11 HC1 + 0.9 n NaCl 0.6176 1 '5 0.6174 0.6176 1 30 0.6176 0.6177 2 I5 06176 0.6178 0-5 IL HC1 i- 0.5 12 NaCl Average 0.6179 TABLE VII. I Hydroquinone, Quinhydrone, HzSOp, HZ I pt. P t . (Solids). Electrolyte. t . 7r. Hrs. Mins. 0.6180 0.6181 0 45 Od180 0.6181) 0'6181 1 I5 I 00 0.6183 0.6185 0.6182 0'6184 1 45 0.6181 0.6184 2 45 0.6176 1 15 06177 - 1 30 0'6179 - 2 00 0.6180 0.1 n H,SO, 0.5 F,SO, - 0.6183 - - 0.6 I 78 r o n H,SO, 0 45 - '9 15 0.6 I 76 20 0 0.6176 Average 0.6181 The constancy of the potentials is striking.As for the chemical re- actions in these electrodes, in the quino-quinhydrone electrode we measure the affinity of the transformation 2 mol quinone (solid) + H, (gas) + I mol quinhydrone (solid) and in the hydro-quinhydrone electrode the affinity of the reaction I mol quinhydrone, (solid) + H, (gas) 4 z mol hydroquinone (solid). I t remains to say that the platinum electrodes must be blank. They are cleaned by treatment with a hot mixture of chromic acid and strong sulphuric acid, then washed with distilled water and heated to a red heat in an alcohol steam lamp or in a benzine blow lamp, but not in a gas flame. As for the quinhydrone it has been found suitable to prepare it as follows : roo gms.ferric ammonium alum are dissolved in 300 C.C. water at c. 65' C., and this solution is turned into a warm solution of 25 gms. hydroquinone in 300 C.C. water. The quinhydrone precipitates in fine needles. The mixture is cooled in ice and filtered by suction and the pre- cipitate then washed four or five times with cold water. The yield is 15-16 gm:. The preparation may contain a feeble trace of iron, which is without serious effect. Quinone may easily be prepared by using a double quantity of ferric ammonium alum and distilling with water-steam.ORGANIC COMPOTJNDS 683 (d) ApgZications of the Quhhyd~ow Elecirodts. The quinhydrone electrode may mainly be applied for two purposes : as an electrode of comparison and for the determination of hydrion con- centrations.As for the first of these applications there are many cases where we have to measure the potential of an acid electrode, and by using a calomel electrode for the comparison we introduce a liquid junction potential. In such cases it may often be advantageous to apply a quinhydrone electrode with an electrolyte corresponding to that of the examined electrode. Of course, we could also use a hydrogen electrode, but the quinhydrone electrode is so much easier to prepare than a hydrogen electrode. Here in my laboratory we have now no calomel electrodes at all. We always use quinhydrone electrodes as comparison electrodes, and Mr. Stig Veibel has examined in my laboratory a quinhydrone electrode with 0.01-n HC1+ 0-09-t: KC1, which is reproduced so quickly and so constant that it may be a very good standard electrode.For the determination of hydrion concentrations the quinhydrone electrode has the advantage that it is very quickly prepared and its hydrogen pressure is so feeble that it gives well defined potentials in many cases where the usual hydrogen electrode does not work, because it reduces one of the chemical compounds in the electrolyte of the electrode. In Table VIII. are shown values for pH determined by means of the quinhydrone electrode and, for comparison, the values determined by means of the hydrogen electrode or calculated from the dissociation constants. As examples, unsaturated organic acids are mainly taken or else halogen-substituted organic acids, that is to say, chemical compounds which are reduced in the hydrogen electrode and which do not give well-defined values for the dissociation constant by means of the conductivity.I only give the values of pH, not the measurements or other particulars. But I may say that all the electrodes presented well defined and very stable potentials. TABLE VIII. pH-VALU ES. Substance. ________ ~ Phosphate mixture . . Citrate mixture . . . Acrylic acid . . , . Crotonic acid . . . Fumaric acid . . . Pheny propiolic acid . . Ch:oraceticacid . . . Iodpropionic acid . . . B bromosuccinic acid . . 180-bibromo6uccmic acid. . Quinhydrone Electrode. 6-80 2'93 3-10 2'28 2'65 1.71 1-57 4'96 2-12 2'12 6 79 4-96 2-89 3-10 2-29 2'28 2-18 3-18 1-82 1-77 (Hydrogen electrode) ( Yoerensen) (0 st wald) (White and Jones) (O&dd[' " (Chz&3ler) (Holmberg) 9, 1 Stig Verbel, I.c., p.2004. Very striking results were obtained by measuring o - ~ n HCl, mixtures of these in chains of the type Quinhydrme, HClor HNO, KCl KCI HgCl Pt I o*oog-n or HCl+HNO, 1 3-5-72 13-5-72, solid HNOa and H g684 OXIDATION AND REDUCl'ION POTENTIALS OF TABLE IX. Acid. Normality. o Hrs. o Hrs. 30 Mins. H C1 0'1 0.3880 0.3880 ,? 0.3879 0.3880 9 , 9 9 0'3881 0.3877 HCI Y ? 0'3882 0.3880 3 , 9 9 0.388~ 0.3880 9 , 9 1 0.3882 03881 H NOs 9 , 0.3880 0'3877 o Hrs. o Hrs. 40 Mins. 0.3882 - HNO,'; HCl :; 0.3882 0.3881 28 Hts. 30 Mins 0'3878 volts. 0'3875 1, 0'3874 ? I - $ 9 21 Hn. 0.3880 volts. 0.3880 ,, - l ? $ 9 - 0.3879 9 , These figures do not call for comment.Further, we have found that the quinhydrone electrode may be applied for the determination of hydrion concentrations in mixtures of soils and water. Together with Mr. Hakon Lund I have studied the application in some few characteristic cases and then the method has been examined on a large scale by Mr. Tovborg Jensen of the Danish State Laboratory for Plant Culture (Statens Planteavls laboratorium, Lyngby, Denmark, Director Mr. Harald R. Christensen), and it was stated that electrodes prepared simply by addition of a feeble quantity of quinhydrone to the mixture of soil and water give sufficiently exact pH-values up to pH 8.4. This means that the electrode may be applied to the entire interval of pH-values which are of practical interest in soil-investigations. For these applications it is important that the electrode be not affected by nitrates as is the hydrogen e1ectrode.l But, of course, the quinhydrone electrode is by no means a universal electrode, and there may be soils containing constituents which affect the electrode potential.For determining pH-values by means of the quinhydrone electrodes we have to measure the potential difference between these electrodes and a standard electrode. Taking as basis the figures used by Soerensen in his work on standard buffer solutions we have at 18' C :- if the standard electrode is the 0.1-n calomel electrode for the quinhydrone electrode 0'4183 - T for the quino-quinhydrone electrode pH = 0'0577 ' 0.2800 - z €or the hydro-quinhydrone eIectrode pH = and if the standard electrode is the quinhydrone electrode described by Veibel (electrolyte 0.01 -n HC1- o-og-n KC1) 0 .0 5 7 7 ' 7T for the quinhydrone electrode pH = 2.04 + ___ "'"577' In all the formulae T is the potential of the electrode examined relatively to the standard electrode. I n the last combination we measure the potential difference between two quinhydrone electrodes with different hydrion concentrations. As the " hydrogen pressure " is supposed to be the same in the two quinhydrone electrodes, the constant 2-04 in the formula is equal to the value of pH in the standard electrode.2 1 See the April number, 1924, of the Journal of Agricultural Science. a For applications of the quinhydrone electrode not described here see the papers of La Mer and Parsons, Kolthoff, Bodforss, Schreiner, Harris, and of Larsson recorded in the Bibliography.ORGANIC COMPOUNDS 685 do.0.7124 0.7120 0-7230 0.7228 0.7088 (e) Reduchbn Potentials of Dz;fJerent Quinones. In the preceding we have dealt exclusively with the simplest of all quinones and hydroquinones, the benzoequinone and the benzoehydro- quinone. I t has already been mentioned that these compounds were examined by Haber and Russ (1904). These investigators used as electrolyte a mixture of water and dilute sulphuric acid, and they could not obtainan exact reproduction of the potentials. All the same, their work must always be considered a very important contribution to the chemistry of the reduction potentials. Until my first paper in xg20 and a communi- cation made by Granger and Nelson (I 92 I) no further work on the problem seems to have been carried out.Other quinones were examined by Victor K. Lamer and Lillian Baker (I 92 2), who successively oxidised solutions of hydroquinones with chromic acid and reduced solutions of quinones with titanous chloride and measured the potentials of the solutions at different stages of the oxidation or reduction. The values obtained by this method seem to be very exact. Conant, Kahn, Fieser and Kurt (I 92 2) examined in the same way anthraquinone derivatives and Conant and Fieser made a comparative study of the potentials in aqueous solutions and in mixtures of water and alcohol. Together with Mr. Langseth Jensen I am trying to determine the reduction potentials of quinones in a very simple way, namely by measuring electrodes made by means of equimolar quantities of a hydroquinone AH2 and a quinone B, against a quinhydrone electrode A,AH, or B,BH9 with known reduction potential.In the mixture of AH, and I3 there will be formed the quinone A as well as the hydroquinone BH, :- a AH2 + aB -a (a - x)AH2 + xA + (a - x)B + xBH2. At the equilibrium the potential a produced by the mixture (a - x)AH2 -+ xA may be equal to the potential produced by the mixture (a - x)B + xBH2. If a'o is the normal potential of the quinhydrone AH2,A and rO is the normal potential of the quinhydrone BH2,B, we have ~~ ~ Other Determinations. 0'7125 (La Mer) 0.7152 ,, 0.7210 ,, As we do not measure the potential of the electrode against a hydrogen electrode but against a quinhydrone electrode BH2,B with the normal potential x0, the measured potential 4 is equal to I + TO or a = + x,,.Then we have In the Table X. are shown the values of w'o found in this way for a series of quinhydrones at 25' C. = a. + 24. TABLE X. Hydroquinone. Monochlorhydroquinone . . Monobromhydroquinone . . 2, 5-Dichlorhydroquinone . . 2, g-Dibromhydroquinone . . Tribromhydroquinone . . . Toluhydroquinone . . . Quinone. Benzoquinone ,, ** 1 9 9 9 9 9 I Xylohydroquinone . . . I Toluquinone686 OXIDATION AND REDUCTION POTENTIALS OF 11. REDUCTION POTENTIALS OF ALLOXANTHINES. In the dialuric acid and the alloxane we have two compounds which in many accounts resemble the hydroquinone and the quinone. One is easily converted into the other by oxidation or reduction, and one molecule of the dialuric acid combines with one molecule of the alloxane to alloxanthine, which is sparingly soluble in water.Of course, the quinone is yellow and the alloxane is colourless, and in the constitution there are also great differences. For instance, the quinone contains the group : C : 0 and the alloxane contains the group : C\OH),. But, as Biltz has shown, the alloxane forms an anhydride, which is yellow, and, together with Mrs. Bentzon, I have found that a concentrated, aqueous solution of alloxane, if heated, has a yellow colour. We may therefore suppose that at ordinary temperature also a part of the alloxane in an aqueous solution is present in the anhydride form. logether with Mr. Hakon Lund, I have determined the potentials of solutions containing alloxanthine, which in aqueous solution dissociates in H .N - C : O 0 : C - N . H dialuric acid and alloxane, and of tetramethyl alloxanthine, which dissociates in CH3: N-C : 0 0 : C - N . CH3 HO 0 : c CH3. N - C : 0 dimethyldialuric acid and dimethylalloxane. 0 C - N . CH3 For the experiments we used the vessel shown in Fig. 2 , which permits B FIG. 2. the experiments to be carried out in an atmosphere of carbon dioxide. The electrolyte was o-zn and 0.05" H,S04.ORGANIC COMPOUNDS 687 0'3944 0.4055 0.4169 0.4274 0.4386 0.4582 For chains of the type Pt 1 alloxanthine (solid), dilute sulphuric acid, H2 (I atm.) 1 Pt we found the following potentials :- TABLE XI. - 0.098 0'094 0.094 0.093 0'102 ~ 'I Hg-Pressure." I I' I I Alloxanthine. . . 0.10 0.3696 volt 0,3664 volt 10-l~~ Tetrakethylalloxanthin; I z::: 1 0.3657 :: 1 1 1 - .. 0.3699 111. REDUCTION POTENTIAL OF Azo COMPOUNDS. Together with Mr. Jakob Blom, I am studying the reduction potentials of azo compounds. The reaction R . N : N . R + H, +R. NH. NH. R is analogous to the transformation of a quinone to a hydroquinone, and so the treatment of the problem is analogous to that of the quinhydrones. But the properties of the hydrazo compounds occasion some special difficulties. First, the azo- and hydrazo-benzene are not soluble in water, acids, or alkalies. We are therefore compelled to apply derivatives con- taining amino groups or acid groups. Further, the oxygen of the atmos- phere oxidises many hydrazo compounds, and the experiments have therefore to be carried out in absence of free oxygen.And last, but not least, many hydrazo compounds are converted into benzidines or semidines by the action of acids and other reagents, so that the concentration of the hydrazo compound may decrease during the experiment. Here I only shall mention one example, namely, the reduction of the azo-toluidine H,N . . NH, H,C . o N = N - a . CH, to the corresponding hydrazo compound. We have measured the chain azotoluidine hydrazotoluidine HCl quinhydrone 1 Pt, pt I o.oo4o8 m o.oo4o8 m 0.1 n Particulars will be communicated in a special paper. In Table XII. we give the potentials 1 of the azo-hydrazo electrode against a hydrogen electrode, that is to say, the normal potential of the quinhydrone electrode (at 18') minus the measured potentials ; f is the time in minutes.TABLE XII. t. 21 31 41 50 60 72 81 - 1'0 ' - 1.4 - 1.8 - 2'2 - 2.6 - 3'3 - 3.6688 OXIDATION AND REDUCTION POTENTIALS OF We see that T increases, and this means that the hydrogen pressure in the azo-hydrazo electrode decreases in consequence of the transformation of the hydrazo compound into a semidine. The curve in Fig. 3 shows the relation between time and potential, and we see that from f = 22 to near t = 72 the relation is almoit linear.. An extrapolation to t = o gives value 0.3763. To this value we have to add 0.0044 volt, because the the .Mih!&?J FIG. 3. amino groups neutralise a part of the acid in the azo-hydrazoelectrode. We have then the normal reduction potential of the examined azotoluidine is r0 = 0.3807 volt, corresponding to a hydrogen pressure of 10 + 1 3 2 atm.If At, At’, Ht and Hp are the concentrations of the azo- and hydrazo compound at the times fland t’, we haveORGANIC COMPOUNDS 689 as the concentration of the azo compound is not altered. As for the transformation of the hydrazo compound in a semidine, this reaction may be supposed to be of the first order, catalysed by the hydrions. We find, therefore, that the velocity constant K at the hydrion concentration existing in the electrode is where Ht = a and Hp = (a - x). have By combination of (I) and (2) we K ( i - t). RT vF 7rp - 7rt = - As Y =I 2 and T = 291, the velocity constant is 7rtf - .rrt K =I 80 x -. t' - f The values of K are shown in Table XII. The velocity is exceedingly I think it should be dficult to determine the constant by quantita- high.tive analysis. From the equations (I) and (2) we derive a - x log - - Z ? L 2 7 t e a 0'028 The values of log (a - 'x) : a show, that after a short time we have to do with exceedingly feeble concentrations. IV. FINAL REMARKS. We have here dealt with some special cases of oxidation and reduction reactions of organic compounds. Some few others (indophenol, indigo- sulphonates and methylenblue) have been examined by W. M. Clark in 1920 and 1921. I think it useful to say that measurements of this kind should not be done exclusively by physical chemists, who may in most cases prefer to examine substances which are suitable for their special purposes and at the same time easily procured. But the examination of many other compounds has to be done by organic chemists, in the first place, because the prepara- tion and purification of organic compounds may take weeks or months and demand in many cases a high degree of exercise in practical organic chemical work.Further, it may demand the full skill of an expert in organic chemistry to estimate all possibilities of side reactions interfering with the reversibility, which is the condifzr'o sine qua non for the measure- ment of the potential of a particular reaction. For instance, it may seem very easy to examine electrometrically the reaction CH, : CH . COOH + Br, CH2Br. CHBr . COOH, but the organic chemist knows that this reaction need not be reversible in aqueous solution, as the main reaction of bromine in water on a double linking is an addition of HOBr.But there may be other reactions which render it possible to measure by means of a potential the influence of composition and constitution on the affinity VOL. XIX-T26690 OXIDATION AND REDUCTION POTENTIALS OF of reactions with double linking. For instance, between ethylene derivatives and mercury salts we have a series of reactions of the type ) C : C ( + Hg++ + OH + 2 > C(0H) . C<Hg+, that is to say, formation of organic mercury compounds, the complexity of which may be supposed to depend on the constitution of the organic molecule. Some years ago I studied the affinity of processes of this kind by measuring the potentials of elements containing the mercury compounds of allylalcohol, allylacetic acid, crotonic acid and maleic acid (Biilmann and Hoff, 1917).We are now continuing these investigations in my laboratory. For many years past synthesis has been at the basis of the development of organic chemistry. I think we are now entering an epoch in which the study of the laws governing the transformations of organic compounds and their properties may prove to be as important in organic chemistry as the still very important synthetic work and study of constitutions. As a matter of fact, since Kopp’s investigations on the relations between boiling-point and composition a great deal of useful work has been done dealing with problems of this kind. But just this important problem of affinity has until recent years been but little studied by means of organic compounds, and in most cases the term “affinity ” has been applied in organic chemistry in rather a confusing manner, quite different from the classical definition of affinity.In many cases it may surely be difficult or impossible to determine the the affinity of organic reactions by the electrometric method. But the examples treated in the preceding pages show that the measurement of the affinities of important types of transformation is possible using one of the best working methods of physical chemistry. As, moreover, we also get- by very simple calculations-the equilibrium constants, the heats of trans- formation and in special cases (as shown here for an azo compound) a velocity constant, I think it useful to draw the attention of organic chemists to this problem and this method. BIBLIOGRAPHY.Baker, Lillian E. : The effect of substitution on the free energy of reduction of benzo- Biilmann, Einar and Agnes Hoff : Sur la complexit6 de quelques combinaisons organiques Biilmann, Einar : Kinhydroners Brintning, Annual ofthe University of Copetrhagetz, 1920. Biilmann, Einar : Sur I’hydrogCnation des quinhydrones, Ann. de Chimie, g’s., IS, ~og, Biilmann, Einar and Hakon Lund : Sur I’Clectrode A quinhydrone, Ann. dc Chimic, Biilmann, Einar and Hakon Lund : Sur le potentiel d’hydroghation des alloxanthines, Bodforss, Sven : Ueber die Beeinflussung von verschiedenen chemischen Reaktionen Conant, J. B., Kahn, Fieser and Kurtz: An electrochemical study of the reversible re- Conant, J. B., and Fieser : Reduction potentials of quinones, ibid., 45, 2194, 1923. Clark, M. W. : The determination of hydrogen ions, Baltimore, 1922. Granger, F. S., and J. M. Nelson : Oxidation and reduction of hydroquinone and quinone from the standpoint of electromotive-force measurements, Am. Chem. SOC., 43, 1401, 1921. Haber, F., and R. Russ: Elektrische Reduktion, 2. phys. Ch., 47, 257, 1904. Harris, Leslie J.: Use of the Quinhydrone Electrode for the Estimation of Amino- acids and of Acid and Basic Functions, your. Chem. SOL, 123, 3294, 19-23. Kolthoff, F. M. : Die Verwendung der Chinhydron-statt Wasserstoffelektrode bei potentiometrischen Aciditatsbestimmungen, Rec. Trav. Chim. &s Pays-Bas, XLII., quinone, Dissert, New York, 1922. de mercure, Rcc. Trav. Chim. Pays. Bas., 36, I, 1917. 1921. g’S., 16, 321, 1921. Ann. & Chimie, g’s., 19, 137, 1923. durch Substituenten, 2. phys. Ch., 10% 51, 1922. duction of organic compounds, Am. Chem. SOC., #, 1382, 1922. 186, 1923.ORGANIC COMPOUNDS. 691 La Mer, V. K., and L. E. Baker : The effect of substitution on the free energy of oxida- tion and reduction reactions, Am. Chem. Soc., 44, 1954, 1922. La Mer, V. K., and T. R. Parsons : The Application of the Quinhydrone Electrode to electrometric Acid-Base Titration, The yourn. o Biol. Chemistry, LVII., 613, 1923. Larsson, Erik : Zur elektrolytischen Dissoziation d zweibas. SPuren, 2. anorg. Ch., 125, 287, 1922. Schreiner, E. : Die Hydratation des Wasserstoffions, 2. phys. Ch., 121, 321, 1922. Id. ; Der Dissoziationszustand von Mittelstarken Sauren, besonders der Dichloressig- saure, in Wasser und in Salzlosungen, 2. phys. Ch., 122, 201, 1922. Soerensen, S. P. L. S., Margrethe Soerensen and K. Linderstroem Lang : Sur l’erreur de sel inherente A 1’Clectrode de quinhydrone, Ann. de Chimie, g’s., 16, 283, 1921. Veibel, Stig : The quinhydrone electrode as a comparison electrode, ywr. Chm, SOC., 123, 2203, 1923.