260 CHEMISTRY OF THE PROTEIDS CHAP. 



This view, expressed by the author in 1902, he believes still to 

 hold good for a colloid such as that of gold, which moves towards the 

 positive electrode or anode when its watery solution is subjected to 

 an electrical current. When Bredig makes his colloidal gold solutions l 

 by passing a current of 10 to 12 amperes through gold electrodes 

 which are immersed in acid-free water and which are kept 1 to 2 mm. 

 apart, he finds the negative electrode or kathode to break up into 

 colloidal particles of gold. According to the author the + hydrogen- 

 ions of the water transfer their electrical load at the kathode to the 

 gold, which now passes into solution as + gold-ions. These gold-ions, 

 having a very low electro-affinity, then unite with the - OH-ions of the 

 water, which have a stronger electro-affinity, and there are formed 

 new electro-negative ions, namely, [AuOH]', which travel towards the 

 positive anode. The negative colloidal gold-ion has as its partner a 

 positive hydrogen-ion of the water. This explanation helps us to 

 understand why the addition of minute traces of alkali, i.e. OH-ions, to 

 the water in which the colloidal gold solution is being made, greatly 

 facilitates the formation of the colloidal gold, and why, on the other 

 hand, any free acid will at once precipitate the colloidal gold for 

 [AuOH]' + H - Au + H 2 0. 



In the case of gold-solution the problem is comparatively simple, 

 but it becomes much more complex if we are dealing with a substance 

 such as arsenic sulphide, As 2 S 3 . This compound is formed by passing 

 a stream of sulphuretted hydrogen gas, H 2 S, through a solution of 

 arsenic trioxide, As 2 3 , and then removing the excess of H 2 S by means 

 of an inert gas. Arsenic trioxide when in solution gives rise to 

 arsenious acid, As(OH) 3 , which by dissociating into [As(OH) 2 0] / + H 

 liberates the acid H-ion, and hence gives rise to the acid reaction of 

 an arsenic trioxide solution. Sulphuretted hydrogen in watery 

 solutions also dissociates to a slight extent into H + SH', and hence 

 again the solution gives an acid reaction. 



We are dealing, therefore, with the interaction of two substances 

 which are potential acids, and which for this reason have the 

 tendency of mutually precipitating one another. 



Now, As 2 O 3 + 3H 9 S give rise to As 2 S 3 + 3H 2 0. The arsenic 

 trisulphide formed in this way undergoes in its turn a hydrolytic 

 dissociation into As 2 3 + 3H 2 S. The possibility of such a dissociation 

 was pointed out to Freundlich 2 by Dr. Bottger, and Freundlich 

 actually found that H 2 S, being a gas, can leave a colloidal arsenic 

 sulphide solution, and thereby lead to a disintegration of the colloidal 



1 Georg Bredig, Zeit.f. angew. Ohem. 1898, p. 951. 

 2 Herbert Freundlich, Zeit.f. physik. Chem. 44. 129 (1903). 



