PAPERS READ. LAWS GOVERNING THE DECOMPOSITION OF EQUIVALENT SOLUTIONS OF IODIDES UNDER THE INFLUENCE OF ACTINISM. By ALBERT R. LEEDS, of Hoboken, N. J. In a paper published in the Philosophical Magazine for June, 1879, I have given a brief review of the controversy as to whether potassium iodide, in very dilute solution, is decomposable by sulphuric acid. I likewise pointed out that the explanation of the opposite views, entertained by experimenters upon this question, was due to their having overlooked the essential part played by air or oxygen in this reaction. This last was brought to view by Baumert', in the course of experiments by which he showed that Andrews2, in the famous investigation undertaken to prove that Baumert's hypothesis that electrolytic ozone is a teroxide of hydrogen3 was false, had himself fallen into an error. For Baumert showed that when a stream of electrolytic ozone has been deprived of all of its active oxygen, by passage through a neutral solution of iodide of potassium, it may bring about a liberation of iodine in an acidified solution, placed later in the series, many times greater (from four to ten in the experiments tried) than that effected by the ozone itself in the first instance. So the curious fact remains, that while Andrews' main conclusion is true, all the results by which he succeeded in establishing it are affected by a constant error and are in excess of their true values. The triumph of Andrews' opinion (1856), that ozone contains no hydrogen whatsoever, but in its substance-matter is identical with the matter of ordinary oxygen, probably explains why the permanently valuable part of Baumert's work has been generally lost sight of; and why the erroneous method, of titrating ozone with an acidified solution of potassium iodide, has been persisted in even down to the present day. Ten years after the facts above stated were made known by 1 Pogg., Ann., XCIX, p. 88. 2 R. Soc. Proc., VII, p. 475; Pogg., Ann., XCVIII, p. 435. Baumert, they were rediscovered by Payen 4, who extended their application to the action of nitric, acetic, oxalic and other acids. upon dilute solutions of potassium iodide, in and out of contact with air. In former papers 5, a summary of which is given in the Phil. Mag. (loc. cit.), I have shown that the presence of oxygen not merely facilitates, but is absolutely essential to, the occurrence of the reactions in all cases, and whether the reaction occurs in open or closed vessels, in the heat or cold, in darkness or in light. The only exception to this law is in the case of an acid like nitric, which, under the influence of light (the action of heat alone in the absence of light and oxygen is being investigated), spontaneously breaks up, and supplies by internal change the essential oxygen. The experiments made to determine the rates of decomposition, when various iodides in the presence of different acids were submitted to the influence of the magnesium, electric and solar rays, were made with solutions of known though not of chemically equivalent strength. To supply this defect a new series of experiments were performed immediately after the publication of those cited above, and it is to make known certain remarkable laws of actino-chemical change deduced from these latter experiments, that the present article is written. The solutions were of such strength that 1 cc. of each of the acids employed was chemically equivalent to 12.6 cc. of a normal caustic soda solution; the iodides were each exactly equivalent to the 20 per cent. solution of the potassium iodide. 1 cc. of the iodide and 1 cc. of the acid were used in each trial, the volume of the test being made up to 100 cc. by the addition of distilled water. The tests were contained in "comparison-tubes" made of thin, perfectly colorless glass of uniform bore and dimensions, which were supported on frames in such a manner that each tube should be normal to the incident rays, and in the case of the magnesium and electric lamps, six inches from the focus of the light. The first set of trials was made to determine, whether the addition of starch as an indicator facilitated the decomposition under the influence of light, as had been originally supposed, or whether, as later on, there had been reason to think, it retarded the reaction. 4 Compt. Rend., LXII, p. 254. Proc. Amer. Chem. Soc., 1878, II, No. 4; Jour. Amer. Chem. Soc., 1879, I, p. 18; ibid., P. 65. In this, as in succeeding experiments, the amounts of iodine liberated are given in milligrammes. Effect of Starch upon the Rate of Change (March 18, 1879). The experiments of March 18th, performed as they were with a sun approaching the horizon, having been less decisive than could have been desired, they were repeated upon the following day, but with the disadvantage of feeble sunlight, with the result of showing that six times more iodine was set free in the absence than in the presence of starch. The suspended precipitate of starch iodide cut off the light except upon the superficies of the solution. Henceforth the employment of starch was abandoned. The next set of trials was instituted in order to observe the influence upon the rate of change, of larger access of oxygen than that derivable from the air already dissolved, or in contact with the solution at its upper surface. To this end tests were prepared in duplicate, and through one pair of these duplicate solutions, the comparison-tubes being connected together in the manner of wash-bottles, a slow current of oxygen was passed. Influence of increased supply of Oxygen. March 19, 1879. 11 A. M.-12. Feeble sunlight. The absorption of oxygen, when sulphuric and hydrochloric acids are employed, is expressed by the equations: (1) 4 KI+2 H, SO, +0,2 K2SO, +2 H2O+2 I, and (2) 4 KI + 4 IICI + O2 = 4 KCl + 2 H2O+ 2 Ig. And since 100 cc. of water, when saturated with air under the ordinary circumstances of temperature and pressure, would hold in solution only about 1 mgrm. of oxygen, the maximum amount of iodine which could be liberated during one of these tests, in case no fresh absorption of oxygen took place from the upper surface, would be 16 mgrms. The influence of these conditions upon the accuracy of the estimations made with the Iodo-acid Actinometers, is being submitted to further investigation. In the case of nitric acid, the reactions become much more complicated. For in the first place, of the three mineral acids, nitric is the only one which spontaneously decomposes, when subjected to sunlight in closed vessels. This is true both of the concentrated acids, and when diluted with 500 times their volume of water. Moreover, in the latter case, the presence of starch had no influence except in the nitric acid, in which it nearly doubled the rate of decomposition. The same effect of starch (whether it is true of organic matter in general has not been determined) is to be noted in the above table of decompositions for March 18th, in which it will be seen that, in the trials where starch was present, the amounts of iodine liberated by nitric acid were largely in excess of those set free by equivalent amounts of the other acids. In fine, while nitric acid conforms to the general law of actinic change as expressed in the equation, 4 KI+4 IINO2+0,= 4 KNO3+2 II,O+2 I, it is likewise subject to the special decomposition, 4 II NO, (in sunlight) = 2 N,O,+2H2O+0. For these reasons, at an early stage of the inquiry, the actinometric use of nitric acid was discontinued, until the exact influence of temperature, actinism, oxygen and organic matter, upon its special rate of change, had been established, and the nature of the accompanying reactions.6 In order to study the influence of organic acids, trials were made as above, the solutions being of such strength that 1 cc. of 6 Gay-Lussac states (Ann. de Chem. et de Phys., 1816, p. 317), that no decomposition of dilute acid takes place m the light except in the presence of a certain quantity of concentrated sulphuric acid. Also that the decomposition is into nitrous acid. each was chemically equivalent to the same amount of the mineral acid. Nor was there any change with acetic acid during this three hour interval. To determine more precisely the rate of change effected by the organic, as compared with the mineral acids, it will be necessary to make simultaneous estimations, but even with oxalic acid, the most active of them all, it is evident that the amount of decomposition was relatively small. An actinometric measurement of the solar ray was made with the equivalent solutions of various iodides and acids, the principal object being to note the effect upon the rate of decomposition of different acids in the presence of the same base, and of different basic radicals of the iodides in the presence of the same acid. The sunlight, which was good during the early part of the day, declined towards noon, and shortly after the sky became overcast. ACTINOMETRIC MEASUREMEnt of the SOLAR RAY, March 26, 1879. 1.81 2.1 2.06 1.87 1.7 1.6 0.75 1.18 1.28 1.41 1.35 1.13 0.96 0.93 0.93 1.15 ISO: HCI 1.83 1.9 2.03 2.0 1.56 1.25 1.58 1.4 1.69 1: 1.47 |