Note 50 bis — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

Pickering (1890) showed (a) that dilute solutions of sulphuric acid containing up to H2SO4 + 10H2O deposit ice (at -0°·12 when there is 2,000H2O per H2SO4, at -0°·23 when there is 1,000H2O, at -1°·04 when there is 200H2O, at -2°·12 when there is 100H2O, at -4°·5 when there is 50H2O, at -15°·7 when there is 20H2O, and at -61° when the composition of the solution is H2SO4 + 10H2O); (b) that for higher concentrations crystals separate out at a considerable degree of cold, having the composition H2SO44H2O, which melt at -24°·5, and if either water or H2SO4 be added to this compound the temperature of crystallisation falls, so that a solution of the composition 12H2SO4 + 100H2O gives crystals of the above hydrate at -70°, 15H2SO4 + 100H2O at -47°, 30H2SO4 + 100H2O at -32°, 40H2SO4 + 100H2O at -52°; (c) that if the amount of H2SO4 be still greater, then a hydrate H2SO4H2O separates out and melts at +8°·5, while the addition of water or sulphuric acid to it lowers the temperature of crystallisation so that the crystallisation of H2SO4H2O from a solution of the composition H2SO4 + 1·73H2O takes place at -22°, H2SO4 + 1·5H2O at -6°·5, H2SO4 + 1·2H2O at +3°·7, H2SO4 + 0·75H2O at +2°·8, H2SO4 + 0·5H2O at -16°; (d) that when there is less than 40H2O per 100H2SO4, refrigeration separates out the normal hydrate H2SO4, which melts at +10°·35, and that a solution of the composition H2SO4 + 0·35H2O deposits crystals of this hydrate at -34°, H2SO4 + 0·1H2O at -4°·1, H2SO4 + 0·05H2O at +4°·9, while fuming acid of the composition H2SO4 + 0·05SO3 deposits H2SO4 at about +7°. Thus the temperature of the separation of crystals clearly distinguishes the above four regions of solutions, and in the space between H2SO4 + H2O and +25H2O a particular hydrate H2SO44H2O separates out, discovered by Pickering, the isolation of which deserves full attention and further research. I may add here that the existence of a hydrate H2SO44H2O was pointed out in my work, The Investigation of Aqueous Solutions, p. 120 (1887), upon the basis that it has at all temperatures a smaller value for the coefficient of expansion k in the formula St = S0/(1 - kt) than the adjacent (in composition) solutions of sulphuric acid. And for solutions approximating to H2SO410H2O in their composition, k is constant at all temperatures (for more dilute solutions the value of k increases with t and for more concentrated solutions it decreases). This solution (with 10H2O) forms the point of transition between more dilute solutions which deposit ice (water) when refrigerated and those which give crystals of H2SO44H2O. According to R. Pictet (1894) the solution H2SO410H2O freezes at -88° (but no reference is made as to what separates out), i.e. at a lower temperature than all the other solutions of sulphuric acid. However, in respect to these last researches of R. Pictet (for 88·88 p.c. H2SO4 -55°, for H2SO4H2O +3·5°, for H2SO42H2O -70°, for H2SO44H2O -40°, &c.) it should be remarked that they offer some quite improbable data; for example, for H2SO475H2O they give the freezing point as 0°, for H2SO4300H2O +4°·5, and even for H2SO41000H2O +0°·5, although it is well known that a small amount of sulphuric acid lowers the temperature of the formation of ice. I have found by direct experiment that a frozen solidified solution of H2SO4 + 300H2O melted completely at 0°.