Studies on Hypophosphorous Acid. Part I V. Its Reaction with Cupric Chloride

View Article Online / Journal Homepage / Table of Contents for this issue 1624 MITCHELL : STUDIES ON HYPOPHOSPHOROUS ACID. PART IV. CXC1V.-Studies on Hypophosphorous Acid. Part I V. Its Reaction with Cupric Chloride. By ALECDUNCAN MITCHELL. IT has been shown in previous communications (T., 1920, 117, 1322; 1921, 119, 1266) that hypophosphorous acid effects reduction by means of an " active " form, which is designated, for purposes of formulation, as the fully hydroxylated compound H,PO,, but the precise constitution will, it is hoped, be eluci- dated by some projected experiments with alkyl derivatives-the dialkylphosphinic and alkylphosphinous acids. The active form reduces iodine and mercuric chloride rapidly, and the velocity of reduction is independent of the concentration of iodine or mercuric chloride (unless it is very dilute) and is conditioned solely by the rate of formation of the active form from hypophosphorous acid molecules. It follows that the molecules are normally in Published on 01 January 1922. Downloaded by Cornell University Library 04/10/2016 02:07:31. equilibrium with a small proportion (probably much less than 1 per cent.) of this active form. Baly has suggested (Ann.Reports, 1921, 18, 41) that this time-reaction is due to the activation of the molecules by the catalyst, which is giving to them the required energy increment, but this is a physical generalisation which doubt- less constitutes a stage in all reactions, and in this case a purely chemical explanation for the time-reaction is quite possible. The oxy-acids of phosphorus exhibit a marked tendency to assume the quinquevalent form, as is shown by the monobasicity of hypophosphorous acid and the dibasicity of phosphorous acid, and Arbuzov (J.Russ. Phys. Chem. Xoc., 1906,38,687)has shown that the trialkyl oxygen-esters of phosphorous acid readily pass into the quinquevalent form, P(OR), -+ PRO(OR),, so that the possible existence of the tervalent form of hypophosphorous acid, HP(OH),, which would be very reactive, supplies a chemical explanation. View Article Online ITS REACTION WITH CUPRIC CHLORIDE. 1625 The reaction between hypophosphorous acid and copper sulphate is interesting in that the salt is reduced ultimately to the hydride, Cu,H,. The products of this reaction in weakly acidic solution have been studied by Firth and Myers (T., 1911, 99, 1329), who showed that one obtained a mixture of hydride, phosphate, and suboxide, and it appeared to be of interest to follow the course of the reaction kinetically. A few preliminary experiments showed, however, that it was even more complicated than anticipated, and, in the hope of elucidating the earlier stages of the reaction, cupric chloride was studied instead, since, in the dilutions used, this is reduced only to cuprous chloride, and the hypophosphorous acid is oxidised only to phosphorous acid. It is found that, in this case also, the velocity of reduction is independent of the concentration of the copper if it exceeds decimolar, and is almost identical with the -rates of reduction of iodine and mercuric chloride in similar circumstances, being limited by the rate of change of hypophosphorous acid to its active form. The chief interest centred, therefore, in the examination of the reaction in very dilute solutions of copper chloride, where this reaction becomes so slow that it is of a lower order than that of the change H,PO,-+H,PO,, and is then the only measurable reaction. The change is repkesented stoicheiometrically by the equation : H,PO, + 2CuC1, = H,PO, + Cu,Cl, + 2HC1, but mathematical analysis shows that this reaction also is composed of two stages. The results are necessarily very complex, and their interpretation rests, to some extent, on circumstantial evidence, since their complete mathematical treatment (although fairly confirmatory so far as it goes) is limited by lack of data as to the two-stage ionisation of cupric chloride. It is hoped, however, Published on 01 January 1922. Downloaded by Cornell University Library 04/10/2016 02:07:31. that the results will prove of use in the further study of the reaction with copper sulphate, which is relatively so slow that it is independent of the hypophosphorous acid change, and depends only on the reaction between “ active’’ acid and copper sulphate. The discussion can be divided into three parts : (a) Comparatively strong copper chloride solutions in which the slowest reaction is that concerning the change of hypophosphorous acid to its active form, and in which this constitutes the measurable reaction ; (b) Weak copper chloride solutions, in which the foregoing reaction is relatively so fast as not to affect the measured velocity, which concerns only the subsequent changes ; and (c) Solutions intermediate in strength between these two classes, in which the reactions are of the same order and are superimposed. Gase (a)-For copper solutions of this class, the method of treat- VOL. CXXII; 3L View Article Online 1626 MITCHELL: STUDIES IN HYPOPHOSPHOROUS ACID. PART IV. ment adopted in previous communications (be.cit.) i6 applied, with a slight modification due to the effect of the copper chloride on the ionisation of the hydrochloric acid. The “total ion” hypothesis, of which use has previously been made, assumes that, in a mixture of two binary electrolytes having a common ion, each has the same degree of ionisation that it would have if present alone in the same total ion Concentration. This has not been extended to ternary electrolytes with any certainty, but is here adapted to the case of copper chloride as follows : the hypothesis is obviously equivalent to stating that the degree of ionisation for each electrolyte is the same as it would have been if present alone with the same concentration of the common ion, and in this form it is applied to the case under discussion. This modification is equally probable on theoretical grounds, in view of the theories of isohydric solutions, and is simpler in its application ; moreover, as the effect of the copper chloride in depressing the ionisation of the hydrochloric acid, as found by graphical means on this hypothesis, rarely exceeds 7 per cent., it is unlikely to cause an actual error of more than 1 per cent., but the error in the case of copper chloride itself may be much greater-a point which is discussed more fully later. As before, for the reaction H,PO,-+H,PO, catalysed by hydrogen ions, one deduces the equation since the “active” acid is being utilised as fast as it is formed and the reaction is therefore direct and not reversible ; l is the initial concentration of hypophosphorus acid and a its degree of ionisation, whence Z(l - a) is the concentration of the undis- sociated molecules ; h is the total hydrogen-ion concentration Published on 01 January 1922. Downloaded by Cornell University Library 04/10/2016 02:07:31. initially, and k is the velocity coefficient for the direct equilibrium change (kl,that for the reverse change, not being concerned in these cases). After a time t, s is the concentration of phosphorous acid formed or of hypophosphorous acid oxidised ; and the foregoing symbols have suffix t added : h, however, is nearly a linear function of s, and may be replaced by (h + ms), m being the mean coefficient of increase of h with s in any particular experiment; (1 - oca) is the value of (1 - a) half-way through each particular period. The integration of (1) gives : (1 - a=)(h + ml)kt = log& + m)/h+ logel/(l - s) = log, htth + log, J/lt (2) The values of k in case (a) are moderately concordant and the mean (111 x 10 - 6) is in fair agreement with those obtained in the cases of iodine and mercuric chloride, 128 x 10- and 134 x 10- 5, View Article Online ITS REACTION WITH CTJTRIC CHLORIDE. 1627 respectively, there being a, variety of causes which makes the lower value less accurate. Experiment 25 is shown in detail and the remainder are summarised in Table I. Experiment 25. a = 99.7, m = 1.438, aCuCl,= 0-517, UHCl = 0.850. f. 2s. 1-8. H'HCl, a- 1 -UH' H*p. h. kx 105. 0 - 21.06 - 0.553 - 12.10 12.10 - 10 2-88 19-62 2-45 0-527 0-460 11.53 13.98 109 20 5.84 18.14 4-97 0.500 0.473 10.95 18.92 106 30 9.04 16-54 7.69 0-478 0.487 10.46 18-15 105 45 14.16 13-92 12.04 0.447 0.505 9.79 21.83 106 60 19-96 11.08 16.97 0.417 0-522 9-13 26.10 106 75 25.32 8.40 21-52 0.391 0.538 8.56 30.08 107 90 30.52 5-80 25.94 0-370 0.553 8-10 34.04 110 Mean 107 X TABLEI. Expt. a. 1. a. R.p = h. k x 105. uAc1. acuci* *26 19.76 21-96 0.544 12-22 104 0.913 0.700 30 39.72 20.60 0.553 11-83 111 0.885 0.637 33 79.76 21.06 0.549 11-83 107 0.859 0.551 33a 78.56 21-96 0.544 12.22 113 0.839 0.551 36 30.00 20.60 0.553 11.83 110 0.895 0.668 37 49.84 20.60 0.553 11.83 111 0.878 0.616 Mean 109 x Experiment 65. t. 28. 1-8. H' HCl- U. I-uH. H*p. h. k x 105. 0 - 10.58 - 0-650 - 7-02 7.02 - 5 0.27 10.44 0.23 0.646 0.352 6.98 7.21 104 10 0-58 10.29 0.50 0-640 0.354 6.91 7.41 106 20 1.18 9-98 1-03 0.630 0-360 6.80 7-83 106 30 1.83 9.66 1.57 0.618 0.365 6-68 8.25 105 45 2-88 9.14 2.48 0.602 0.372 6.50 8-98 107 Published on 01 January 1922.

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