ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 9 
Under the conditions of Odén’s experiments, sulphur is a 
negative colloid, and the precipitation is therefore due to an 
adsorption of cations. The first thing to be noticed is that 
hydrogen ion is not adsorbed strongly by sulphur, the precipitat- 
ing power of hydrochloric acid beimg much less than that of 
lithium, ammonium, sodium, potassium, rubidium, or cesium 
chloride. Instead of these univalent cations precipitating at the 
same concentration, the required concentration of lithium 
chloride is, in round numbers, one hundred times that of cesium 
chloride. The liminal values for barium and strontium are 
nearly equal, but calcium chloride requires a distinctly higher 
concentration. If we take the different bivalent ions the values 
range from 0°0756 for zinc to 0°0022 for barium, a ratio of over 
thirty toone. The univalent cesium.ion has a greater precipitat- 
ing power than the bivalent zinc, cadmium, nickel, and uranyl 
ions; and about the same precipitating power as the bivalent 
copper, manganese, and magnesium ions. ‘The trivalent 
aluminum ion has about the same precipitating power as the 
bivalent calcium ion, and distinctly less precipitating power than 
bivalent strontium and barium ions. The specific nature of the 
adsorption comes out extraordinarily clearly with sulphur, about 
the only orthodox thing being that nitrate, chloride, and sulphate 
behave practically alike, though even here Odén considers that 
sulphate has a slight protecting action. This specific nature 
appears more clearly, perhaps, if we arrange the cations in order, 
the one with the greatest precipitating power coming first: Ba, 
Sr > Ca, Al > Mg, Cs, Mn, Cu > UO, > Rb > K > Ni, Cd, Zn > 
Na > NH, > Li > H. 
Sulphur is admittedly an extreme case, but Freundlich® 
gives data for colloidal platinum from which I deduce the order: 
Al, Pb > Ba, VO, > Ag > K, Na. Bivalent lead has practically 
the same precipitating power as trivalent aluminum. Univalent 
silver is nearer to bivalent uranyl and barium than to univalent 
potassium and sodium. If more cations had been studied we 
should very likely have got more distinct evidence of specific 
action. As it is, it takes 130 millimols NaOH per litre to 
coagulate the platinum, and only 2°5 millimols NaCl. The change 
from chloride to hydroxide has a more marked effect than the 
change from sodium to barium. It seems very probable that 
barium hydroxide would have no greater precipitating power 
than sodium chloride. From Pappada’s experiments with col- 
loidal silver®® I deduce the following order of adsorption: Al 
> Ba, Sr, Ca>H>Cs>Rb>K>Na>Li. From these data 
Pappada concludes that the migration velocity is the determining 
factor with the univalent cations; but this cannot be true. The 
_ difference between aluminum and hydrogen is not very great, 
one drop of M/10 HCl producing a coagulation and one drop 
M/20 AICI,. In tenth-normal solutions potassium iodide, 
nitrate, and sulphate produce no coagulation. The reason given 
by the author is that these anions react with the colloidal silver, 
65 Kapillarchemie, 352 (1909), 
