THE SOLUBILITIES OF SODIUM ETU. FERROUYANIDES. 49 XI.-The Solubilities of Sodium Potassium and Calcium Perrocyanides. Part I. By MONICAFARROW. THISinvestigation was undertaken on account of the uncertainty of the solubility data for the ferrocyanides of sodium and pohsiurn and the lack of data for the calcium salt. Early workers e.g. Michel and Kraft (Ann. Chim. Phys. 1854 41 482) &ard (ibid. 1894 2 546) and Schiff (Annalen 1860 113,350) obtained values varying from 25 to 40 g. of potassium ferrocyanide in 100 g. of water at 20". Determinations for the potassium and sodium salts were made at a few temperatures by Conroy (J. Eoc. Chem. Id. 1898 1'7 104) by Harkins and Pearce (J. Amer. Chem. Soc. 1916 38,2716) at 25" only by Fabria (Gaxxetta 1921 51,ii 374) over the whole range from the cryohydric point to loo" and by Vallance (Chem.News,1922 125,7) for the potassium salt only at low temperatures. Puri;ficaion of the SctZts.-The purest sodium and potassium ferrocyanides obtainable were further purified by rapid crystalliaation from hot water. By simple qualitative tests the absence of the most likely impurities caleium and ammonium was proved. The purest calcium salt obtainable for which we are indebted FARROW THE SOLUBILITIES OF SODIUM to Mi. W. G. Adam of the Gas Light & Coke Co. was sliBhtly tinged with blue. It was purified by precipitation with absolute alcohol in the cold (to avoid oxidation) from an air-free saturated solution. The pure lemon-yellow finely crystalline product was centrifuged dried in vacuum desiccators first over charcoal for 24 hours then over the partly dehydrated salt and stored in the dark in dark glass-stoppered bottles.No discoloration appeared after 18 months. No tmce of sodium potassium or ammonium could be detected in the residue left after decomposition of the salt with concentrated sulphuric acid. Determinations of calcium as oxalate in the original and the purified salt proved that no measurable change of composition had resulted from the purification process Orig. salt 16.2 16.2 16.2 16.3 16.25 16.1 16.2 16.2. Mean 16.2% Ca. Recryst. salt 16.2 16.25 16.2 16.2 16.2 16.2 16.2 16.3. Mean 16.2% Ca. There has been some doubt about the precise number of molecules of water in the calcium salt.The above determinations which lie much nearer to the theoretical value for a hydration of 11H20 (1603% Ca) than to that for 12H20 (1543% Ca) support the con- clusion of Berkeley Hartley and Burton (Phil. Trans. 1908 209A,177) that the lower figure is correct. The lower figure is also in accordance with the recent careful determinations of Cumming (J. 1924 125 240). Method of Determining Solubility.-A solution was heated in contact with excess of the solute to the required temperature and after the attainment of equilibrium a definite volume was withdrawn in a weight pipette to be weighed and analysed and the density calculated. The ferrocyanide in all three salts waa estimated volumetrically with permanganate by the simple method of de Haen (Treadwell “ Quantitative Analysis,” p.632). This method after practice gave concordant results; it is essential to standardise the solution used against standard potassium ferrocyanide and not against the customary oxalic acid. The calcium ferrocyanide was also deter- mined independently as oxalate. hming’s method of estimating ferrocyanides with benzidine hydrochloride (J. 1924 125 240) was tried but it did not appear so suitable for these metallic ferrocyanides as the simple per- manganate method and was therefore not employed. Pure dilute boiling sodium oxalate solution was used in the deter- mination of the calcium salt as oxalate because ammonium oxalate and oxalic acid as precipitants produced insoluble double calcium POTASSIUM AND CALCIUM FERROCYANIDES.PART I. ammonium ferrocyanide and insoluble hydrof errocyanic acid respec- tively. In the first experiments in which an alundum cone was used as a filter no sharp end-point could be obtained on the addition of permanganate owing to slow diffusion of oxalate from the inner pores of the cone. Gooch crucibles were substituted for the cones but even then the permanganate readings were not concordant. The asbestos used had no reducing action on permanganate and experiment also showed that no measurable occlusion occurred at the dilutions used. The cause of the error was traced to the appre- ciable solubility of calcium oxalate in water. The washing mas therefore limited to three fillings of the FIG.1. Gooch and this procedure was kept constant throughout the experiments.Apparatus.-The solubility vessel consisted of a wide tube of about 200 C.C. capacity fitted with a I rubber bung and stirrer. In order to hold the pipette in place in the solubility vessel the bung carried a short glass tube which was closed mhen not in use. The temperature of the thermostat was constant to within 0.2". The weight pipette which had a volume of about 4.5 C.C. (Fig. l),was of a design which has been used for several investigations in this laboratory but has 2 not previously been described. It is warmed to the temperature of experiment and introduced into the solution with its shorter arm fitted with a glass-wool filter adapted to the ground end. The solution can then be admitted by opening the ground-in plug A; when it has reached the required level which can be accurately read by means of the graduations the plug is replaced.The pipette is then removed the filter immediately replaced by a ground-on cap and the whole cleaned and weighed after attaining room temperature. The pipette was repeatedly calibrated with pure mercury to give accurate volume readings for the density determin- ations. Owing to the high solubilities of the calcium salt these solutions were very viscous and consequently difficult to introduce into the pipette. To hasten the filling slight pressure was applied to the solution in the solubility vessel. The determinations of solubility and density were in almost every case repeated at least once and as the data of the table show the individual solubility experiments very seldom differed from the mean by more than 0.1 unit or the densities by more than 0.002 except at the highest temperatures FaRROW THE SOLUBILITIES OF SODIUM when the experimental difticulties already mentioned make the determinations somewhat less concordant.Solubilities and Densities. 9 = grams of anhydrous sait in 100 gams of solution. d = density of EKLturated solutions. The mean values are printed in it,alics. Temp. 24.9' !7. 23-96 d. 1.1731 Na,Fe(CN) g.17.10 d. 1.1330 Ca,Fe(CN)B. 9. 36.62 d. 1.3674 23-83 1.1731 17-16 1.1296 36-42 1.3652 24.00 1.1734 17.08 1.1312 36*46* 1.3563 24-01 1.1729 17-11 1.1312 36.36* 23.95 1.1731 36.44 34.9 28-00 1.2026 20.68 1-1676 39-11 1.3633 28.08 1.2014 20.68 1.1691 39.30 1.3690 27.95 28-02 1.2016 1.2018 20.58 1.1584 39*09* 39.38" 1,3662 39.22 49.8 33.26 1.2360 26.16 1.2009 41.98 1.3976 33.00 1-2340 26.23 1.2000 41.99 1.3964 33.13 1.2350 26.20 1.2004 42*12* 1.3970 42.06* 42.04 64.7 37.06 1.2627 3 1.46 1.2432 44.28 1.4071 36-83 1.2644 31.41 1.2421 44.27 1.4077 36.94 1.2635 31.43 1.2426 44.72* 1.4074 44*48* 44.44 79.6 40.36 1.2858 36-87 1.2794 40.54 1.2860 36.83 1.2779 40.45 1.2854 36.85 1.2786 84-7 37.96 38.33 38-15 1.2891 1.2942 1.2916 * As oxalate the rest as ferrocyanide.89-6 38.08 1.2969 94.7 38.26 1.2870 99.7 43.83 1.3148 37.46 1.2928 43.72 1.3082 37.66 1.2849 43.78 1.3115 37.55 1.2806 37-65 1.2861 37.47 37.53 Results of Experiments.-The values obtained for the solubilities of calcium and potassium ferrocyanide lie on two smooth curves (Fig.2). The values obtained by Vallance and by Fabria (loc. cit.) for the potassium salt which are plotted on our curve for purposes of comparison are in good agreement with our measurements except at temperatures above 84" where Fabria's values are considerably higher. To what extent the divergence is due to his having kept his solutions at these high temperatures for as long as 3 hours we are unable to say but we have repeatedly noticed that solutions of the calcium salt show green coloration on POTASSIUM AND CALCIUM BERROCYANIDES. PART I. cooling owing presumably to incipient decomposition.The experimental difficulties with the calcium salt solutions at high temperatures were considerable. The solutions had both high viscosity and high density; the former made the rate of flow into the pipette very slow and also prevented the very small particles of solid salt from settling rapidly. It was almost impossible to prevent the capillary entrance to the pipette from being blocked and although five determinations were made for the solubility at FIG.2. A Conroy. El Vullance. x Fabyin. 0Authors. 1 40 50 B. 1-100 1.200 1.300 1.400 1.500 A. Grams of anhydrous salt in 100 grams of solution. B. Density of saturated solution. I. Density of sodium fprrocyanide solutions. 11. Density of potassium ferro -cyunide solutions. 111. Solubility of sodium ferrocyanide.IV. Solubility of potassium ferrocyanide. V. Density of calcium ferrocyanide solutions. VI. Solubility of calcium feirocyanide. 79.6" these are not given as they show almost the same value as that obtained at 65". As may be seen from the graph the solubility curve of sodium ferrocyanide shows a sharp break at 81.7". This transition tem- perature does not appear to have been observed before though it is apparently responsible for the reported transition in the curve for the potassium salt of fitard (Zoc. cit.). Work is in progress on the three-component system calcium ferrocyanide-sodium ferro- cyanide-water and the identity of the lower hydrate of the sodium FARROW THE SOLUBILITIES OF SODIUM ETC. salt will be investigated.The four points given by Conroy (Zoc. cit.) for the solubility of the sodium salt lie almost exactly on our curve though as the number of his determinations was small the t,ransition point evidently escaped his observation. The density results (Fig. 2) require little comment. No other work appears to have been done on this property of the saturated solutions. The three curves are of the same general form as the corresponding solubility ones. In the case of the sodium salt the points evidently lie on two curves and are consistent with the transition at 81.7" as given by the solubility curve. Verijication of the Transition Temperature of Sodium Ferrocyanide. -The transition point was verified by the two following methods 1. The cooling curve.On allowing the salt to cool from about 90" to 75' in a jacketed test-tube carrying a copper stirrer and a sensitive thermometer arrests were observed in three experiments at 81.0" 81.2" 81.3",giving a mean value of 81.2". 2. Dilatometer. The dilatometer used was of the ordinary form and had a graduated capillary neck about a foot long. The bulb contained the hydrate (lOH,O) and sufficient paraffin to rise to a convenient level in the capillary when the temperature was in the region of 81". Before the dilatometer was introduced into the thermostat it was heated above 81" so as always to have some of the higher-temperature hydrate present to facilitate the change. In all six experiments were carried out at temperatures ranging from 80-6"to 82.6".At the highest temperature 82.6" there was a rapid rise of 1 inch in nearly 3 minutes and at 81.9" the level of the oil still rose although at the rate of only Q inch in 8 hour. In the next three determinations at 81-2" 81.3" and 81-3" respectively no change in the oil level could be observed. Finally at 80.6" there was a fall in the level of 4 inch in 2 hour. These experiments show that the change from the higher-temperature hydrate to the lOH,O-hydrate occurs with a volume decrease and vice versa and that the most probable value of the transition temperature is 81.5". The values obtained by the less accurate methods namely 81.7"by the solubility and 81.2"by cooling curves give as a mean value one very close to the most trustworthy value of 81.5".Summry. 1. The solubility of sodium potassium and calcium ferrocyanide in water has been determined together with the density of the saturated solutions. 2. Sodium ferrocyanide has a transition point at 81.5". Neither the potassium nor the calcium salt shows any transition over the NORRISH AND JONES STUDIES OF VALENCY. PART VII. 65 range investigated the transition point of the potassium salt recorded by Btard being probably due to contamination with sodium ferrocyanide. IKORGANIC CHEMICALLABORATORY OXFORD. [Received October 12th 1925.1