STUDIES O X T H E PHYSICAL AND CHEMICAL PROPERTIES O F T H E PLATIXOCYASIDES. I The Hydrates of Lithium Platinocyanide* BY FRANK E . E , G E R X A N S AND 0. B. MUENCH
Potassium platinocyanide, or Gmelin’s Salt, the first of the platinocyanides known, was discovered by v. Ittner. Under the heading of double cyanides, Berzeliusl says in part: “The ability of iron to combine prussic acid with bases was long thought to be a property possessed by this metal only, and Porret and Robiquet claimed that iron is thereby combined with the constituents of prussic acid, to a new acid. In the meantime v. Ittner showed that this property of iron is shared by gold, silver, platinum and copper, whose cyanides (or iron-free prussic acid salts) are dissolved by the cyanide of potassium, among others, to real double cyanides or double prussic acid salts. These properties are shared according to Leopold Gmelin2 by palladium, mercury, zinc and cobalt, and according to Woehler, by nickel. Gmelin described a new method for preparing the double cyanides of platinum. The older method consisted in mixing platinic chloride with potassium ferrocyanide, and evaporating to crystallization. Gmelin’s method consists in mixing platinum sponge sbtained from ammonium ehlorplatinate, with an equal part of pot’assium ferrocyanide, and heating just up to incandescence, but no more. By this process a part of the ironls replaced by platinum, and a mixture of both salts is obtained. The double cyanide of potassium and platinum can be crystallized from the saturated water solution of the above salt mixture, and can be purified by repeated crystallization.” It would appear from the above that v. Ittner prepared potassium platinocyanide by the following reaction: K4Fe(CN)& PtCl, = K2Pt(CN)4 zKC1 FeC12 (CY), However, the simplified reaction which usually bears v. Ittner’s name does not involve iron, and is written as follows: 6 KCN PtC14 = H,Pt(CS), 4KCl (CX)t Gmelin’s methodI3involving the use of platinum sponge can be represented by the following equilibrium reaction: K4Fe(CS)e Pt K2Pt(CN), ZKCX Fe. We have, at present, a t least five distinct methods for the preparation of the various piatinocyanides which may be tabulated as follows : I. By neutralizing the free acid with the base of the desired salt as indicated by the following equation:
+
+
+
+
+ e
+
+
+
+
+
* Contribution from the Department of Chemistry of the University of Colorado. Berzelius Jahresber., 3, 95 (1824). Gmelin: J. Chem. Physik, (2) 6 , 230 (1822). Gmelin: 1.c. See also Gmelin: Handbuch, 1st ed., p. 1456,
416
FRANK E. E. GERMASN AND 0. B. M U E N C H
+
+
+
+
+
+
HzPt(CN), 2KOH = KzPt(CN)4 2H20. By double decomposition of the metallic sulphate with barium platinocyanide in solution as follows: BaPt(CN)4 K2S04 = K2Pt(CiT)4 BaS04. 3. The metallic platinocyanides, when insoluble in water may be formed by the following reaction: KzPt(CN), C U S O ~= CuPt(CN)4 KzS04 4. The alkali or alkaline earth cyanide solutions may be warmed with platinum salts, when the following reaction proceeds: 6 KCS PtC14 = KzPt(CX)4 4KCl (CN)z 5 . The most recent method consists in passing an alternating electric current through platinum electrodes immersed in aqueous solutions of the cyanides of the alkali or alkaline earth metals, and is probably represented by the following equation: Pt 4KCN 2Hz0 = KzPt(CK), zKOH Hz Lithium platinocyanide seems first to have been described by Schabus,’ and has received but little attention since that time. Weselsky2 claimed to have converted lithium platinocyanide to lithium platinicyanide, but his work was proved to be in error by Hadow3 and later by Levy.4 He does not, however, describe lithium platinocyanide, nor tell how he obtained it. Martius5 passes over the salt with the following brief statement: “Lithium platincyanur (LiCy;PtCy 3HO) erhielt ich beim Fallen von Barium platincyanur mit schwefelsaurem Lithon als ein leicht krystallisirendes Salz von milchweisser Farbe und blauen Flachenschiller.” Sir James Dewar6 made certain observations on the color changes which he had observed when a salt, which had been supplied to the Laboratory of .the Royal Institution as “Lithium platinocyanide,” was cooled in liquid air. Dewar gave J. Emerson R e y n ~ l d s ~ s o mofethe salt with which he had worked. Reynolds refers to the salt as “this nearly white crystallized substance” and later states that “Chemical examination of the Royal Institution specimen led to the conclusion that it was a mixture of the hydrated chloride, cyanide and sulfate of lithium with a platin-cyanogen salt of lithium, and that the proportion of the latter salt ‘cvas small.-Hence the percentage of platinum compound present could not exceed 5 per cent of the mixture of salts.--I prepared afresh some pure lithium plat,inocyanide and obtained the salt in fine grass-green crystals when fully hydrated. On completely analysing these crystals they gave data agreeing well with the formula Li,Pt(CX),, gHz0.” 2.
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1 Schabus: “Bestimmung der Krystallgestalten in chemischen Laboratorien erseugter Produkte,” page 43 (1855). 2 Weselsky: Sita. Akad. Wiss. Wien., 20, 282 (1856); J,prakt. Chem., 69, 276 (1856). a Hadow: J. Chem. Soc., 13, 106 (1861). Levy: J. Chem. SOC., 101, 1081 (1912). 5 Martius: “Ueber die Cyanverbindungen der Platinmetalle,” Inaug. Diss., Gottingen, page 45 (1860). Dewar: Proc. Roy. Inst., 1895, 667. ’Reynolds: Proc. Roy. SOC.,82A, 380 (1909).
STUDIES ON T H E P R O P E R T I E S O F T H E PLATINOCPASIDES
417
Levy1 converted lithium platinocyanide to lithium platinicyanide, but states nothing of the preparation or properties of the former salt. Terry and ,Jolly2state that Baumhauer3 described lithium platinocyanide, but this is not true, since the original article deals only with double salts of lithium platinocyanide with potassium platinocyanide and rubidium platinocyanide. From this small amount of experimental work, the various chemical dictionaries have obtained their data. Many errors have crept in, so that we find the degree of hydration for what is apparently the same hydrate given as 3H,O, 3?H20, 5H20and xHBO. Some state that the salt is slightly soluble in water, while others state that it is very soZubZe. The color of the crystals is given as grass green, greenish yellow, rose red, blue, and milk white. I n view of the above chaos, it. was thought desirable to subject the salt to a systematic revision. Experimental
B a r i u m platinocyanide. A very high grade of barium platinocyanide can be obtained on the market, which can readily be made more pure by several re-crystallizations. Accordingly, the second method, which involves the double decomposition of barium platinocyanide and lithium sulfate, was used in the preparation of the lithium platinocyanide. Lithium suljate. The highest grade lithium sulfate obtainable was spectroscopically free of potassium salts, but showed traces of sodium. For its further purification, advantage was taken of the fact that the solubility of lithium sulfate4decreases about 10% in heating from 20 to 9 j"C. Accordingly a saturated solution was made a t room temperature and heated t o boiling and the precipitated salt filtered off while hot. In order to keep down the sodium content, it was necessary to use Pyrex in all the operations. Even with its use, the sodium content in the lithium sulfate was decreased with one precipitation, but increased if several precipitations were used. Lithium platinocyanide. X slight excess of lithium sulfate solution was added to one gram of barium platinocyanide in seventy-five C.C. of hot conductivity water. After standing on the hot plate over night, the barium sulfate was filtered off. The clear filtrate gave negative tests for both sodium and barium. The filtrate was evaporated almost to dryness and the lithium platinocyanide allowed to crystallize. The crystals were then re-crystallized four times from alcohol and finally from conductivity water, to insure the absence of alcohol of crystallization. All operations mere carried out in Pyrex with the result that the final product showed only traces of sodium as evidenced by the faintness of the D line. The hydrate thus prepared crystallized in long pointed needles having a grass green appearance. Parallel to the long axis (probably the C-axis) 'Levy: J. Chem. SOC.,101, 1091 (1912). Terry and Jolly: J. Chem. Soc., 123, 2217 (1923). 3Baumhauer: Z.Kryst. Min., 49, 113 (1911). Etard: Ann. Chim. Phgs. ( 7 ) , 2, j47 (1894).
418
F R A S K E . E . GERMANN AND 0. B. MUENCH
the crystals are bluish-green in appearance, while perpendicular to this axis the color appears to be more of a canary yellow with a slight tinge of green. It is very soluble in water. Determination of the Hydrates Thin films of the saturated solution of lithium platinocyanide were painted on microscope cover glasses by means of a rubber policeman. These samples were put in desiccators over the following per cents of sulfuric acid; ( I O , 20, 30, 40, 50, 6 0 , io, 80). A sample of the dehydrated salt was also put in each one of these desiccators. These were allowed to stand a t z j°C., and observed from time to time. It was soon evident that the samples over the I O , 2 0 and 30 per cent acid were in solution, while the rest seemed to form stable salts. The samples over Soyc acid appeared to be anhydrous. (Determinations had been made over concentrated sulfuric acid and phosphorus pentoxide, and it was found that each of these readily and completely dehydrated the salt). The anhydrous salt had a bright canary yellow color. This then gave a rough qualitative det,ermination of approximately the concentration of sulfuric acid over which certain forms of the salt are stable. More than this, after standing a while over the acids, there were certain color changes in the samples, which were proved to take place without decomposition. Whenever the anhydrous salt is but momentarily exposed to the air, or over a lower concentration of acid (707~ or less), the color immediately chaliges to tan. The grass green hydrate also begins to change to the tan after a few days, especially when over 6 0 7 ~ - - 7 0 7acid. ~ \$‘hen the tan hydrate is left a week or more, over the 707~ acid, it changes to a dark green and finally becomes almost black with a purple metallic sheen. There was no abrupt change from one color to another over certain strengths of acid. To determine the hydrates quantitatively, use was made of the micro method devised by the authorsLof this paper and was found especially suitable for the determination of the hydrates of lithium platinocyanide. The densities of the sulfuric acid solutions which were used, were determined by means of accurate hydrometers, and after each determination the density was checked by means of a sensitive Westphal chainomatic balance. This latter was necessary because of the change in density due to the acid taking up water from the salt and from the air. The percentages and vapor pressures of the acid solutions were obtained from the results of careful determinations by various experimenters.2 After equilibrium conditions had been reached, for example, in a desiccator or the balance case, some of the acid was taken out and checked by means of the Westphal balance at the time a determination was finished. X11 the quantitative work reported in Chis paper was carried out at’ zs°C. . Concentrated zulfuric acid completely dehydrated lithium platinocyanide. Several determinations leare no doubt on this point. The fact that the salt Germann and SIuench: J. P h p . Chem., 32, 1380 (1928). Lundolt-Bornstein-Koth: “Tabellen,” p. 2 6 j ( 1 9 1 2 ) ; K. E. Kilson: J . .im, Chem. SOC.,43, 721 ( 1 9 2 1 ) ; J. Ind. Eng. Chem., 13, 326 ( 1 9 2 1 ) . 1’
STUDIES O S T H E PROPERTIES O F T H E P L A T I S O C T A S I D E S
4'9
i$ readily dehydrated over concentrated sulfuric acid, gives us a convenient stsrting and also reference point for the calculations of the amount of hydration of the hydrates after a constant weight has been reached. Li2Pt(C?1')4.4H20, Hydrate (a). The tan salt is readily obtained by exposure of the anhydrous (canary yellow) salt to the air, or a low concentration of acid. When this experiment is performed in the balance, the color change is seen to be accompanied by an increase in weight until equilibrium conditions are reached. h saturated solution of lithium platinocyanide mas painted in thin films on carefully weighed microscope cover glasses. These were dehydrated over concentrated sulfuric acid and the weights taken. The acid in the balance was then changed to 4opc sulfuric acid (later joyc,607, and ;omc were tried with the same result). One sample of the anhydrous salt was put in the balance in turn with each of the above concentrations of acid and after equilibrium had been nearly established, was weighed, and re-weighed until constant weight was obtained. The detailed record of each of these weighings will not be recorded in this paper, but below is given a summary in the form of a table, of a few of the results and the calculations made from the weight of the anhydrous salt and the hydrate in question. On each side of it are given the weights that would have been obtained if the compound had consisted of a hydrate containing three and five molecules of water. This shows that the tan hydrate is the tetrahydrate. I n the determination of each experimental figure, the procedure outlined above was followed.
TABLE I Weights in milligrams so.
11-t.of Anhydrous Salt
11-t.of Tan Hydrate
Calculated
Calculated to
Calculated
to
4H2O
3H2O
5H20
11.02
1 2 . IO
12.42
13.64 18.08 5.60 4.76 18.08 18.06 19.84 18.34 16.91
5. 6.
I4,05
17.25
11.56 13.04 17.28 5.35 4.55 17.28
7. 8.
I4,04 15.42
17.26 18.90
18.87
9.
14.25
17.50
17.53
I3
16.10
16.16
I.
9.40
I1.jO
2.
IO.60
3.
14.0s
13. IO 1 7 ,I9
4.
IO.
4.35 3 ' io
'
1.1
5.34 4.60
17.27
16,47 5.10
4.34 16.47 16.45 18.07 16.jo 15.40
to
The conclusion drawn form the above summary of the this work is that the tan hydrate is: IJinPt(CX),.4H20. (tan)
S o record of a tan hydrate can be found in the literature.
420
FRANK E. E. GERMANN AND 0 . B. MUESCH
Li?Pt(CN)4.4H?0,hydrate (b) .-In working with the tan hydrate, it was observed that the color gradually became darker and some samples became almost black. By putting samples of the tan hydrate into the series of desiccators, it was seen that a concentration of about roc; acid favored the most rapid formation of the black modification. The change was gradual, taking almost two weeks for completion. Over 5 5 % acid, it was obvious that transformation to the black was taking place after two months, hut it was by no means complete. The transformation was so gradual that no definite strength of acid could be considered as the limit of the change. The formation of the dark salt mould indicate either decomposition, the formation of another hydrate, or another modification of the same hydrate. Quantitative determinations showed that there was no change in weight and that we were again dealing with a salt with four molecules of water of crystallization. That there was nc decomposition was shown by transferring a black sample to a desiccator containing 307~ acid and allowing it to take up moisture to form a solution, and then putting it over 40% acid. Under these conditions the green salt reappeared] which even under the microscope showed no traces of impurities. The weight of the green crystals obtained was identical t o that of the original black form. Since the change to the dark hydrate is very slow, even over 70% acid, attempts were made to accelerate the change, or to find a catalyst that would speed up the transformation. Levy1 has shown that hydrogen, hydroxyl and cyanide ions are influential in producing such changes, when these ions are in contact with the solution from which the crystals are coming. In the present work it was found that the vapor from concentrated hydrochloric acid caused a rapid darkening, hut there was also a change in weight. When 7 0 7 ~sulfuric acid containing four ten-thousandths of one per cent of hydrochloric acid was used bhe rate of change to the dark salt was about double that when pure rocc sulfuric acid was used. In this case there was no change in weight. A slightly greater increase in speed was obtained when a small lump of potassium cyanide was dropped into the 70% sulfuric acid. The HCX thus generated was volatile and was responsible for the change. Here again there was no change in weight. Hydrochloric acid added to 40% sulfuric had no effect, the tan going directly to the light green just as in the case of 40% sulfuric acid alone. A drop of piperidine or of ammonium hydroxide placed near the dark modification brought about decomposition with increase in weight. The samples could not be restored to their original weights by heating and subsequently placing over lower concentrations of acid. Solutions of sodium hydroxide having the same vapor tension as 7oC; sulfuric acid produced effects identical to those of acid. Since this is non-volat,ile, the effect is one of pressure and not of OH ions. When the tan hydrate was placed in the presence of 70% acid and exposed to bright sunlight, darkening was very rapid, a decided green appearing in the 1
Levy: loc. cit.
STUDIES O S THE PROPERTIES O F T H E PLATIXOCYANIDES
421
course of fifteen minutes. Cltraviolet light produced in the laboratory also had a very rapid effect. It would, therefore, appear that there are several factors which control the format,ion of the black modification from the tan. Light when allowed to strike the salt in t,he presence of a low pressure of water vapor causes a rapid change. Light in the presence of high vapor pressure of water has little or no effect. Hydrogen and cyanide ions seem t o accelerate the change. The dark hydrate has a purple metallic sheen by reflected light. It seemed amorphous under the microscope. Its composition is: Li2Pt(CN)4.4H20(black). Li2PtfCS)4.4H?0,Hydrate (c).-This is the very soluble grass green hydrate which is obtained by allowing solutions of lithium platinocyanide to crystallize from an aqueous solution of the salt. In order to find the highest vapor pressure over which it is stable, a systematic process of elimination was used, and it was found that a t z j"C, the grass green crystals formed and remained stable over 38.67' sulfuric acid, while over 38. jccacid, the sample remained liquid. From these facts, the vapor pressure at equilibrium of the system saturated solution-green hydrate a t 2 5°C can readily be established. .kccording t'o Wilson1 the vapor pressure of a 40'7~ sulfuric acid solution is 13.46 mm. Hg. The vapor pressure of 3 j . S i % acid is 14.47 mm. Hg. Then the calculated vapor pressure for 3 8 . ~ acid 7 ~ is 14.17 mm. Hg., and for 38.6Y0 acid is 14.12 mm. Therefore at 2jDC, the vapor pressure of the system saturated solution of lithium platinocyanide-green crystals is between 14.12 mm. and 1 4 . 1j mm. The most probable value, the average is 14.14mm. Hg. This, then a t 2 j°C, is the upper limit of vapor pressure at which the grass-green hydrate is stable, and is the vapor tension of the saturated solution at z 5°C. The lower limit of vapor pressure at which the green hydrate is stable is arrived a t in much the same general way. At z j°C in the presence of acid of a concentration greater than 73.4'3, the sample lost weight, while over 73.zYc acid it was stable. This makes the vapor pressure of the system green hydrate-anhydrous salt at 2 j ° C , 1.02 mm. Hg. This is generally called the vapor pressure of the grass green hydrate. Deter~ninntionof the Composition of the Grass Green Hydrate.-Using the same method as before, it was found that over 4 0 7 ~sulfuric acid the green salt crystallized and came to constant weight. This weight showed the hydrate to have four molecules of water of hydration. This strange result was a t first doubted, for the tan hydrate was definitely found to be the tetrahydrate. The same plan of procedure was followed as in the case of the tan hydrate, only in this case most of the samples were also taken to the tan hydrate, and the weight found to check with that of the green hydrate. The general plan was this: a sample from the saturated solution was painted on the cover glass and this immediately put in the balance over 407' sulfuric acid, and allowed to come to equilibrium, the loss in &ght being followed Wilson: J. Am. Chem. SOC., 43, 721 (19211.
4 22
FRAXK E. E. GERMANS A S D 0 . B. M U E S C H
from time to time till constant weight was obtained. To be absolutely certain that there would be no further loss over this strength of acid, many of the samples were left in t,he balance for several days after constant ~veiglit had been obtained, and it was found that they neither gained, nor lost weight by as much as one-hundredth of a milligram. This same green hydrate \vas then dehydrated over concentrated sulfuric acid, and with concentrated acid in the balance, the weight of the anhydrous salt obtained. This anhydrous salt was then allowed to take up moiqture, in some cases from bhe air, in others over (407, 50%;, 607~)sulfuric acid, again to constant weight producing the tan hydrate. Thus nearly every sample was carried thru these three stages (grass green, anhydrous and t a n ) and in that way a weight of each modification obtained. Again, only a summary of a fex of the experimental results of the work are given in Table 11. Khile the table gives a few more values confirming the formula for the tan hydrate, its chief interest is in the fact that the weight of the green hydrate is identical (within the limits of experimental error) with the tan hydrate. bIixtures of the two modifications of the tetrahydrate, forming on the cover glass in the balance under certain conditions, also shorv the same weight as either the green or the tan salt.
TABLE I1 Keight in hlilligrams w t . of
11-t.of
Anhydrous Salt
Grass-green Hydrate
Tan Hydrate 4Hz0
I.
11.02
13. j z
2.
11.50
3.
I3 ’ 50 17.34 14.60
14.28 16.60
13’52 14.18 16.jj 21.30 17.98 14.j6
Wt. of KO.
4.
5. 6. 7. 8. 9. IO.
21.32
17.96
12.00
14.78
19.65
24. I9
24.17
20.42
2j.16 19.I 2
2j.16 19.I O 22.90
15.54 18.60
22.88
Calculated to
4Hz0
13.55 14.I 4
16.60 21.33 17.96 14.76 24.17 25.12
19. 11 22.88
The conclusion drawn from the work summarized in the above table is, that the green as well as the tan salts are tetrahydrates, having the same general formula : Li2Pt(Clu’)c./rH20 (grass green) This is the well known hydrate which has been assigned varicus amounts of water of crystallization. Strangely enough, no one has given it the value of four molecules, which the above results warrant.
STUDIES O S T H E P R O P E R T I E S O F THE PLATINOCYASIDEG
423
summarg The early history of the platinocyanides is traced and a summary of the methods which have been used in their production is given. A complete bibliography of all work done on lithium platinocyanide 2. is collected and critically reviewed. 3. Pure lithium platinocyanide has been prepared and its hydrates studied by means of a vapor tension method. 4. Anhydrous lithium platinocyanide is formed a t 2 j"C when the pressure of water vapor is less than 1 . 0 2 mm. Hg. It has a canary yellow color. j . Lithium platinocyanide tetrahydrate is tan when formed a t 25'C from the anhydrous salt by exposure to a pressure of water vapor greater than 1.02 mm. Hg. 6 . Lithium platinocyanide tetrahydrate in a black modification is produced when the tan salt is exposed to bright sunlight, ultraviolet light, or is kept in contact with water vapor at very low pressures, but above 1.02 mm. Hg. Light alone n411 not cause this change if the pressure of water vapor is high. Hydrogen and cyanide ions catalyzed the change. 7 . Lithium platinocyanide tetrahydrate of a grass green color is the familiar salt which crystallizes from water solutions. It crystallizes in needles, whose structure could not be determined even when grown under the microscope. The equilibrium pressure of the water vapor of the system lithium platinocyanide tetrahgdrate-saturated solution at 2 j°C was found t o be 14.14 mm. Hg. 8. It has been proved that lithium platinocyanide may be obtained as the anhydrous salt or with four molecules of water of crystallization. The other hydrates which have been reported do not exist. I.
