NOTE ON SODIUM THIOCYANATE BY C. K. BUMP
In the preparation of very concentrated solutions of sodium thiocyanate, it was found that solutions saturated a t about 60°C and cooled to room temperature, were highly supersaturated. When such a metastable solution
TEMPERARJE
too
IO
'C
25 40 55 SOLU8lLITV OF SODIUM THIOCIANA TE If( 0 HUGHES AND M E A D n BUMP
70 WAn-ER
l
FIG. 1
crystallized out, long needle-shaped crystals were formed which were quite different in appearance from the regular sodium thiocyanate crystals (anhydrous). A consultation of the literature' showed that a monohydrate NaCNS.H20 is known, the melting-point of which is 30%. The melting point of our crystals was 30°C. Fig. I is the solubility curve for the stable form taken from Hughes and Mead' with the metastable portion taken from our data. It was found that on standing in the open the crystals effloresced, while the usual experience is that sodium thiocyanate takes up moisture. The vapor Hughes and Mead: J. Chem. SIX., 1929,2282-2284.
1852
C.
K. BUMP
TABLE I Solubility of Sodium Thiocyanate in Water Hughes and Mead T e m p T gms. NaCNS/Ioogms. H?O
Temp% I5
10.7
112.7
17.3 21.3
127.5
20
139.3 167.5 172.4 178.9 189.5 196.2
25
29.2 33.8 46.1 65.8 73.8 81.8
Bump
gms. NaCNS/IoogmsH20
164.5 165.6 167.8
202 . o
pressures of stable, saturated solutions were measured and compared with the vapor tension of water in the room. At I ~ Oand C 2 5 O C the vapor pressures of saturated solutions were j.2 and 7.j mm Hg, respectively. At 25OC the vapor tension of the water in the air was 4.8 mm Hg. With the given humidity the crystals must effloresce since they have the higher vapor pressure. Because these measurements were made in a steam-heated room they serve as a check on the statement that artificially heated rooms are exceptionally dry unless some means of humidifying the air is supplied.2 4.8 mm H g corresponded to 20.4% relative humidity, while normal relative humidity varies from 30% to 60%. I n England the average humidity is about 7 5% and rarely goes below The vapor pressure of the saturated solution at 2 5°C being 7.j mm Hg, the humidity would have to be 32% or better at this temperature for the salt to be deliquescent. Therefore, since this is more liable to be the rule than the exception, sodium thiocyanate is generally noted as deliquescent. Microscopic examination of the crystals (hydrate) gave the following information : The crystals are monoclinic showing clinopinacoid faces. The anhydrous form is rhombic.‘ The optic axial angle, z V,is not more than 30°. There is strong dispersion, the optic axial angle for violet light being greater than for red, (v>r). a is very near the refractive index of the mother liquor. The refractive index of the mother liquor a t 27OC is 1.4954. The refractive index in the other direction is higher than that of the mother liquor, but the solubility of the crystals prevents its determination by immersion methods. The crystals are optically positive. The axial plane is perpendicular to the plane of symmetry (BXoto “b”). The extinction angle is about 12’ and the angle p is about 78’. Cornell Unioersily. P 8
‘
Grierson: “Some Modern Methods of Ventilation,” 63 (1917). Stevenson and Murphy: “Treatise on Hygiene,” 1, 48 (1892). Friend: Text Book of Inorganic Chemistry, 2, 147 (1924).
