T H E DECOL9RIZATION OF CARBON DISULPHIDE SOLUTIONS OF IODINE BY R E D PHOSPHORUS BY RALPH N. TRAXLER AND FRANK E. E. GERMANN*
Sestinil, working in 1871, observed that when he added I O g. of red phosphorus, previously washed with water and ether, to I O O cc. of carbon disulphide containing I g. of iodine the violet color of the carbon disulphide solution changed to reddish brown and later to yellowish red. After 24 hours he filtered off the red phosphorus and washed it with carbon disulphide until the solvent showed no more color. He recognized the presence of phosphorus iodide in the filtrate. Upon treating this red phosphorus with warm water he obtained a solution which gave a test for iodine with nitric acid. He also found that 6 or 8 g. of freshly washed red phosphorus completely decolorized 30 cc. of carbon disulphide containing “a little” iodine. From these experiments he was led to believe that red phosphorus removed iodine from carbon disulphide solution, just as charcoal and other finely divided solids remove other substances from solution. It has been suggested2 that it would be of interest to determine adsorption isotherms using red phosphorus and iodine in carbon disulphide. With such an aim in view the following experiments were performed, the results of which show that adsorption is not the primary cause of the changes observed by the previous inve~tigator.~ When the above experiments were repeated, similar results were obtained. A more complete examination was made, however, of the aqueous solution resulting from the washing of the red phosphorus which had effected the whole or partial decolorization of a carbon disulphide solution of iodine. This examination showed that the aqueous solution contained a large amount of hydrogen iodide and phosphorous acid but no free iodine. Since the washing with carbon disulphide before treatment with water would remove practically all phosphorus iodide, the hydrogen iodide and phosphorus acid must have resided upon the red phosphorus before treatment with water. The presence of hydrogen iodide and phosphorous acid on the red phosphorus can be explained as follows: The slightly moist red phosphorus which Sestini used reacts with the iodine present to form phosphorus iodide which is soluble in carbon disulphide giving a reddish brown color. The iodide is then acted upon rather slowly by the small amount of water adhering to the red phosphorus, yielding hydrogen iodide and phosphorous acid both of which being * Contribution from the Department of Chemistry
of the University of Colorado. Gam. chim. ital. 1, (1871) 323 (1873). Bancroft: Nat. Res. Council Reprint No. 1 3 problem ~ No. 53. Gordon and Krantz: J. Am. Pharm. Assoc. 13, 906 (192 ) carry on a series of experiments of decolorization of iodine solutions from various schents and assume they get adsorption as Sestini did. They have, however, missed entirely the real explanation of the phenomenon.
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insoluble in carbon disulphide are precipitated upon the red phosphorus, Washing with carbon disulphide removes any phosphorus iodide which is not decomposed by the water but not the hydrogen iodide and phosphorous acid. These are then removed by the treatment with water. Thus the presence of iodine in washed red phosphorus after adding to carbon disulphide-iodine solutions depends primarily upon the insolubility of hydrogen iodide in carbon disulphide and not upon the adsorbing power of red phosphorus toward iodine. It should be noted at this point that 0.15g. of water will react completely with the I g. of iodine used in Sestini’s experiments. This is an amount which could very easily be present in I O g. of washed red phosphorus. A series of experiments were performed using dry red phosphorus which had not been opened to the air for any length of time. The solutions of iodine used varied in strength from O.OI;% to ro.ooo% by weight. Only in the case of the most dilute solutions was complete decolorization obtained, the color of the other solutions after treatment with red phosphorus varied from yellow to dark reddish brown according to the strength of the original iodine solution. The decolorization of the very dilute solutions may have been due ( I ) to the presence of traces of moisture or, (2) to the fact that phosphorus iodide does not have 2,s high a coloring power toward carbon disulphide as does iodine. The second hypothesis is suggested by the fact that the decolorized dilute solutions gave, upon concentration, a pink colored solution and finally upon complete evaporation a small but dark colored residue. YThen dry red phosphorus was added to iodine in carbon disulphide contained in a flask which had been rinsed but not dried the red phosphorus flocculated and was found t o contain hydrogen iodide and phosphorous acid which could be removed by washing with water. Water is practically insoluble in carbon disulphide and thus would be taken up by a substance such as red phosphorus which it is capable of wetting. This water adhering to the red phosphorus reacted with the phosphorus iodide present to give hydrogen iodide and phosphorous acid which in turn are left adhering to the phosphorus due to their insolubility in carbon disulphide. , Samples of the dry red phosphorus used were washed very thoroughly with carbon disulphide and small amounts of iodine added to the washings. No decolorization or change in color was noted. The washed red phosphorus reacted with iodine as readily as the unwashed. Carbon disulphide could not remove from red phosphorus any material capable of reacting with iodine in carbon disulphide and therefore the possibility was eliminated that a portion of the red phosphorus had changed to ordinary white phosphorus after leaving the factory. A number of experiments were performed to determine the percentage of the red phosphorus which would react with iodine in carbon disulphide. Weighed amounts of dry red phosphorus were added to weighed amounts of iodine in this solvent. After allowing sufficient time for reaction to take place the red phosphorus was filtered off, washed with carbon disulphide and again added to a fresh iodine solution. This procedure was followed for each sample
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of red phosphorus until the phosphorus would no longer affect a solution of iodine. It was found that the red phosphorus would react with iodine until 18.jYc of its original weight had been used, beyond that point it became inactive toward iodine under the conditions of the experiments (22°C and 630 mm. Hg). This was later repeated at 2 2 ” and 760 mm. with 18.5% reacting. Since the so-called red phosphorus prepared by heating white phosphorus, varies in its properties (color, vapor, pressure, etc.) with the temperature to which and the length of time during which it is heated, it seemed advisable to attempt other methods than mere solution of any residual white phosphorus by carbon disulphide. Red phosphorus was therefore heated in a tube, provided with a very small opening, by a Meeker burner. This drove off considerable yellow phosphorus vapor, which burned at the opening. After about two hours of heating the tube was allowed to cool and opened under carbon disulphide which dissolved out any residual white phosphorus which might have condensed in cooler parts of the tube. The remaining red phosphorus mas somewhat darker than the original. It reacted to the extent 0: 18.5% with the iodine. Ordinary red phosphorus was boiled with sodium hydroxide followed by hydrochloric acid according to Chapman’s method. Not enough of the violet phosphorus mas obtained for a quantitative determination, but a qualitative test showed that it acted quite strongly on iodine. The treated material as a whole reacted to the extent of 18.7% with iodine. Other experiments of a similar nature show that bromine reacts with red phosphorus almost quantitatively. This must be due to the greater chemical activity of bromine. It would therefore seem that our experiments with iodice exclude the possibility of any pseudo-equilibrium, but rather point to the existence of at least two constituents in violet phosphorus in the percentages of 18.5 and 85.5. The progressive addition of fresh solutions of iodine to the treated red or violet phosphorus certainly exclude any possibility of a mass action equilibrium. The inactive red phosphorus had a slightly darker color than the original, the difference being so slight however as to escape notice except under close comparison with the original material. In all of the above experiments the reddish-brown solution resulting from the addition of red phosphorus to iedine in carbon disulphide yielded upon evaporation of the solvent red crystals which fumed in moist air. These crystals upon analysis gave an average of 7.5 2 % phosphorus and g 2 . 5 % iodine. Upon treatment with distilled water the crystals dissolved, giving a water white, acid solution which contained hydroiodic acid and phosphorous acid. The crystals melted at 55°C. From the analyses, action toward water and melting point the crystals were identified as phosphorus tri-iodide. Phosphorus tri-iodide was prepared by dissolving I O g. of yellow phosphorus and 123 g. of iodine in carbon disulphide adding the solutions and evaporating. The crystals melted at 55°C. This is the value given by Abeggl ~~
“Handbuch der anorganischen Chemie” 3, I11 p. 419. Quoting L. Ouvrard: Ann. Chim. P h F . (71, 2 , 224 (1894). 1
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for the melting point of phosphorus tri-iodide. Carbon disulphide was used as the crystallizing medium in the preparation quoted by Abegg. 55' C. is also the melting point given by Snapel for phosphorus tri-iodide prepared by the following reaction : 3 KI PC13 = 3KC1 -k PI3 followed by extraction with carbon disulphide and crystallization from that medium. The commonly accepted value for phosphorus tri-iodide is 61' C. given by Landolt-Bornstein based upon the work of Besson2 and Jaeger and Doornbosch3. Besson prepared phosphorus tri-iodide by passing hydrogen iodide into carbon tetrachloride which contained phosphorus trichloride. Carbon disulphide was not used by Besson for recrystallization. Jaeger and Doornbosch4 prepared their phosphorus iodide by Besson's method. They also sublimed the material and recrystallized from carbon disulphide which had been shaken with mercury and then thoroughly dried. To remove all adhering carbon disulphide they heated the iodide to 65' C. under reduced pressure meanwhile bubbling nitrogen through the molten mass. A number of experiments were performed to ascertain the cause of the lowering of the melting point of phosphorus tri-iodide when crystallized from ordinary carbon disulphide. Consideration was given to the possibilities that . from carbon disulphide the phosphorous tri-iodide crystallized in a form different from that obtained from carbon tetrachloride or that some of the carbon disulphide ordinarily remained in loose combination with the iodide. Freshly prepared phosphorus tri-iodide crystallized from carbon disulphide was placed in a tube connected with a vacuum pump. The crystals were melted and kept just above the melting point for 20 minutes, meanwhile maintaining a quite low pressure in the tube, After cooling the mass was ground to a powder in an agate mortar and melting points taken immediately. The determinations gave a melting point of 55' C. for the powder. From this it is to be concluded that the lowering of the melting point is not due to a particular crystal form resulting from crystallization from carbon disulphide or to the presence of solvent loosely combined with the iodide. The presence of slight impurities in the phosphorus iodide crystals obtained from ordinary carbon disulphide was next considered as a cause of the lowering of the melting point. Upon shaking carbon disulphide with mercury, a black powder is formed which can be recognized as mercuric sulphide. Since carbon disulphide always contains small amounts of free sulphur such a reaction would be expected. Highest grade carbon disulphide when freed from sulphur by this method and used as the solvent for preparing phosphorus triiodide yielded crystals which melted at 61' C. The carbon disulphide must be
+
Chem. News, 74, 27 (1896). 1347 (1897). 3 Z. anorg. Chem. 75, 270 (1912). 4 Personal communicn,t,ionfrom Prof. Jaeger.
* Compt. rend. 124,
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used soon after purification for slight decomposition of the solvent will take place on standing, giving a liquid containing small amounts of free sulphur. By adding 0.1 g. of sulphur to IOO cc. of purified carbon disulphide and using this as the solvent for the preparation of phosphorus tri-iodide a product was obtained which melted at 53" C. Crystals prepared by Besson's method from carbon tetrachloride when recrystallized from impure carbon disulphide gave a product which melted at 5;" to j6" C. The conclusion is reached that the low melting point obtained by Corenwinder, Ouvrard, and Snape for phosphorus tri-iodide is due to the presence in the crystals of small amounts of sulphur compounds, very probably one or more of the compounds of phosphorus, sulphur, and iodine, and that these impurities may be avoided by the use of carbon disulphide freshly purified by shaking with mercury. Gmelin-Kraut lists PSI,PzS212, P2SI2, PzSI4, PS,I3 and P48312. The melting point of the P2S12is given at 75' C. and of the P&I2 is given at I 19.5' C. P2S& and P4S312can each be made by dissolving the three elements in carbon disulphide in molecular proportions. P2S12,PzSz12and P4S312are all soluble in carbon disulphide, but P&I2 is given as being less soluble than P~S212. Since phosphorus is present in excess, comparatively small amounts of iodine, and only traces of sulphur, we would expect to have either PzSIZor P4S& formed. The former, being more soluble in carbon disulphide would probably not be as readily formed as the less soluble P4S312. It therefore becomes evident that PI3crystals melting below 61" C. must be contaminated with a salt of P, S,and I, this salt probably being P4S312, which when pure melts at 11g.5Oc. The preparation of pure phosphorus tri-iodide directly from purified carbon disulphide is much shorter and less complicated and tedious in execution than the usual method of preparation from phosphorus trichloride in carbon tetrachloride. For these reasons it is believed that, knowing the cause of the difficulties usually encountered and the means for their removal, the method will be of distinct value to any one wishing to prepare the pure compound. Finally the method offers another verification of the now well established melting point of phosphorus tri-iodide.
Summary I. Sestini's work dealing with the action of washed red phosphorus on iodine in carbon disulphide was repeated and checked. The red phosphorus was found to contain hydrogen iodide and phosphorous acid but nofreeiodine. 2. ildsorption has little or nothing to do with the appearance of these substances on the red phosphorus. Their insolubility in carbon disulphide offers the best explanation. 3. Experiments using dry red phosphorus yielded decolorization only in very dilute solution, More concentrated solutions gave phosphorus tri-iodide in abundance. 4. Experiments using wet containers yielded results quite similar to those obtained by Sestini.
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5 . Carbon disulphide cannot remove any substance from red phosphorus which is capable of reacting with iodine in carbon disulphide. 6. Red phosphorus agitated with iodine in carbon disulphide was found to lose 1 8 . 5 7 ~of its original weight before becoming inactive toward iodine, This was also true for red phosphorus heated for two hours. 7 . Phosphorus tri-iodide prepared using both red and yellow phosphorus in carbon disulphide gave a melting point of 55°C. This melting point was not changed by melting the original crystals in a vacuum, cooling and grinding to a powder. Thus the low melting point of such phosphorus tri-iodide is not caused by the presence of a different crystal form or to the presence of carbon disulphide loosely held by the phosphorus iodide. 8. Phosphorus tri-iodide prepared in carbon disulphide which had been purified by shaking with mercury gave a melting point of 61OC. 9. The lowering of the melting point of phosphorus tri-iodide as usually prepared from carbon disulphide was found to be due to the presence of sulphur probably in the form of P4S& which melts at I 19,j"C. I 0 . The preparation of phosphorus tri-iodide using carbon disulphide freshly purified with mercury is recommended as a distinctly shorter and more convenient method of preparation than those usually used to obtain the pure product. Boulder, Colorado
