The properties of colloidal systems. III.—The osmotic pressure of electrolytically dissociated colloids

In a previous paper I showed that the osmotic pressure of solution of Congo red, as measured directly in an osmometer with a membrane of parchment-paper, is about 90—95 per cent. of that which they should have if the dye were present as undissociated single molecules, such as those of glucose or urea. Attention was chiefly directed, in the paper referred to, to the fact that a body behaving as a colloid gives as high an osmotic pressure as if it existed in solution as single molecules and not as aggregates. It is to be remembered, however, that Congo red is the sodium salt of a fairly strong acid and as such must be dissociated to a considerable degree in solutions of the concentration employed. On this account, the interpretation of the experimental results required further work. Subsequent investigations have shown that there are many difficulties in the way of a satisfactory explanation. As will be seen later, the close correspondence between the osmotic pressure found and that of the dye if undissociated must be due to the chemical nature of this particular dye as a disodium salt of a dibasic acid. Other dyes of a similar constitution, but of different sodium content, such as Chicago blue, do not shoe this property. I regard it as a somewhat unfortunate accident that Congo red was chosen as the object of the first investigation. Attention was thereby diverted from the more essential facts. As it will frequently be necessary to refer to the osmotic pressure as it would be shown by a body present in solution in undissociated single molecules, I propose, for convenience, to speak of it as the “molecular” osmotic pressure, although of course the expression is not strictly correct.