1911.] Ttie Properties of Colloidal Systems. 



235 



pressure could only be 155 mm. Hg, if it were quadrivalent only, and it may 

 be higher in valency.* 



On the other hand, benzo-purpurin, which is of similar constitution to 

 Congo red, but with tolidine in place of benzidine, gave me values of osmotic 

 pressure only about 0"87 of the molecular." The preparation used was that 

 supplied by Ivahlbaum, " reerystallised " by me five times. An analysis of 

 its sodium content showed that it only contained - 81 of what it should have 

 contained if it had been the neutral (disodium) salt. It was probably a 

 mixture of the mono- and di-sodium salts or of free acid and neutral salt-f- 

 its osmotic pressure corresponded to that of a Congo-red solution of the same 

 conductivity, as would be expected if the acid salt were in large colloidal 

 aggregates and osmotically inactive, as was no doubt the case. 



Tliese sodium salts of non-diffusible organic acids appear to give osmotic 

 pressures related to their sodium-content, although less than would be 

 expected if they were dissociated normally.^ 



* See Laqueur and Sackur, ' Hofmeisters Beitrage z. Cheni. Phys. und Path.,' 1903, 

 vol. 3, ]). 199. The fact that its osmotic pressure is double the "molecular," similar to 

 that of Chicago blue, suggests that it is tetrabasic, like the acid of the dye. 



t Au interesting fact worth mention is that, if to a solution of Congo red there be 

 added half that amount of a strong acid which is necessary for complete decomposition 

 of the salt, instead of the acid salt being formed, as in the case of sodium sulphate, half 

 the salt is decomposed with production of the free acid, while the other half remains in 

 its original state of disodium salt. The solution becomes at first turbid and of a purple- 

 brown colour. On standing, the blue free a^id is deposited, leaving a clear solution of the 

 bright red colour of the neutral salt. This behaviour appears to be due to the fact that 

 the free acid is insoluble in water. If the whole be evaporated to dryness, the deposit 

 extracted with methyl alcohol, in which the free acid is slightly soluble, the extract 

 filtered, the filtrate evaporated again to dryness and taken up in water, a permanent 

 colloidal solution of the acid salt is formed, turbid and of a purple-brown colour. It is 

 difficult to say, however, whether this is a true salt or a colloidal complex, since it has to 

 be remembered that it is in very dilute solution and contains no foreign electrolyte, facts 

 which favour its stability. 



As regards the free acid itself, it has been stated that the sodium salt may possibly not 

 be decomposed even by strong acids, owing to the presence of the two sulphonic acid 

 residues. I have recently tested this by precipitating the salt, by addition of sulphuric 

 acid, and determining the sodium in the two phases. It was found to be entirely in the 

 fluid phase as sulphate. The precipitate was merely the free acid, containing no salt. 



i I have made one or two experiments recently with a dye known as " Priruulin,'' 

 which may be shortly described as the monosodium salt of a poly-thiazol-sulphanilic acid. 

 It has a molecular weight of 608'5, and passes through parchment paper with extreme 

 slowness. Its osmotic pressure appears to be between the " molecular " and half this 

 value. In a dilution of 168 litres the pressure observed was 84 mm. Hg, the " molecular " 

 being 1L8 mm. The sodium content of the preparation was, however, greater than 

 corresponded to that of the salt of a monobasic acid. Its conductivity was that of a 

 Congo-red solution giving an osmotic pressure of 140 mm. Hg. Further experiments 

 with this dye are in progress. 



