1911.] 



The Properties of Colloidal Systems. 



231 



to be described below. Tbe dye was Kahlbaum's best preparation, but was 

 found to contain appreciable quantities of sodium chloride and a small 

 amount of sodium sulphate. It was therefore purified by " recrystallising " 

 from dilute alcohol. The hot saturated solution on cooling deposited a 

 considerable part of its contents. Although this deposit did not seem to be 

 actually crystalline, it was possible to purify the dye in this way, naturally 

 with considerable loss. After repetition of the process for five times, the 

 conductivity of solutions of equal concentration from two successive treat- 

 ments became identical, so that no foreign electrolyte was present. Several 

 determinations of conductivity in successive dilutions were made and a curve 

 made for future use in order to obtain the concentration corresponding to a 

 known conductivity. 



The following numbers (Table I) will serve as an example of an 

 experiment : — 



Table I. 



Dilution in 

 litres. 



Concentration 

 in millimols. per 

 litre. 



Specific 

 conductivity in 

 recip. ohms x 10 s . 



Molar 

 conductivity in 

 recip. ohms x 10 4 . 



Dissociation, 

 per cent. 



14 



VI -4 



6772 



950 



45 -6 



28 



35 -7 



3782 



1060 



51 



56 



17 -86 



2086 



1166 



56 



112 



8-93 



1154 



1293 



62 



224 



4-46 



662 



1482 



71 -2 



448 



2-23 



367 



1645 



79 



896 



1-12 



201 



1800 



86 -5 



4480 



0-223 



47 



2083 



100 



The determinations were made at 25° C, the same temperature at which 

 the osmotic pressure measurements were made. The dilutions were made in 

 quantities of 40 c.c. in a series of flasks, as it was found that the usual 

 method of removing solution from the conductivity vessel and replacing with 

 water was apt to lead to inaccuracy with the more concentrated solutions, 

 owing to their viscosity. Further dilution beyond 4480 litres gave only 

 slightly increased values of the molar conductivity, and from curves 

 it appeared that the limiting value at infinite dilution would be 

 2100 x 10~ 4 recip. ohms, which was accordingly taken as the basis of 

 calculation for the degree of dissociation of the various dilutions. 



It will be noticed that, although the acid of Congo red is stronger than 

 acetic acid, the degree of ionisation in the more concentrated solutions is less 

 than that of sodium acetate. For example, in a dilution of 32 litres sodium 

 acetate is dissociated to the extent of 86 per cent., whereas Congo red at the 



VOL. LXXXIV. — B. T 



