200 MO-CHEMICAL JOURNAL 



This being so, the powerful action of such salts on adsorption is not 

 tj be wondered at. In fact, I find ."^ z n o, to have an equal effec^ 



to that of KCI. 



200 



This action of zinc sulphate gives the opportunity for testing by 

 experiment whether in this case, as in Picton and Linder's 1 cases of pre- 

 cipitation of colloids by electrolytes, the precipitating ion is carried down 

 with the colloid to become attached to the paper. Since the 'precipitating' 

 agent in the adsorption of a negative colloid is the positively-charged 

 kation, and the most convenient method of detecting a disappearance 

 of an ion, is the use of a concentration battery in the manner of Nernst, 

 it is plain that we cannot conveniently determine the question where 

 salts of alkali metals are concerned. A concentration battery in 

 Zn++ ions is, on the contrary, easily arranged. In the present case I 

 took two vessels, each containing an amalgamated zinc electrode 

 immersed in Znso 4 solution and connected together by an inverted 

 U-tube filled with the Znso 4 solution. Such a battery being sym- 

 metrical has no E.M.F. A piece of filter paper was now placed in one of 

 the vessels, the E.M.F. began to rise and attained a value nearly equal 

 to what it would be by the Nernst formula, if all the Zn+ ions had 

 disappeared from this vessel, viz., 0*0257 volt. This effect was, in all 

 probability, due to adsorption by the paper. On now adding 5 c.c. 

 of i per cent, congo-red in ^ ZnSO 4 the E.M.F. went down again, due 

 to the addition of more z n + ions. Presently, as the dye become 

 adsorbed, the E.M.F. commenced to rise again and when plotted on 

 squared paper showed the usual form of the adsorption-curve. As 

 far as it goes, then, the experiment shows a diminution in concentra- 

 tion of Zn++ ions. But there are two circumstances which deprive it 

 of much value. In the first place I found that next day the E.M.F. of 

 the battery had risen to 0*094 volt, a value above that possible by 

 removal of all the Zn++ ions ; there must have been, therefore, some 

 secondary process set up by the congo-red. In the second place the 

 possibility must be taken into account, that when congo-red and zinc- 

 sulphate are mixed, a zinc-salt of the congo-red may be formed by 

 double decomposition which might not be completely dissociated, this 



i. Lie (it. 



