COLLOID CHEMISTRY. 2tl 



but it seems to indicate a certain initial resistance which must be broken 

 down, and which is probably connected with the surface-tension between 

 the particles and the liquid, or possibly with the stability of the electrical 

 double layer, with which the surface tension is closely connected, as shown 

 by the work of Quincke and Lippmann. The resistance to electrolytes 

 is somewhat affected by the ra+^e at which they are added, being greater 

 when the addition is slow or in small quantities than when the necessary 

 amount is added at once. 



It can hardly be doubted that flocculation, whether by added electro- 

 lytes or by other colloids, is dependent on the neutralisation of the charges 

 of the particles. In the case of electrolytes, the ions of which the charges 

 are much greater than those of the particles, probably act as nuclei round 

 which a number of particles are aggregated. The greater efficiency of 

 polyvalent ions may thus depend on the larger aggregates which their 

 larger charges are able to neutralise, and which naturally fall sooner 

 under the influence of gravity and of cohesive attraction. As the 

 particles probably owe their charges to attached ions, both Hocculation 

 and adsorption are in a sense chemical reactions. As one ion probably 

 neutralises many particles, the quantity carried down with the gel is 

 relatively very small. 



The action of ' protective ' colloids has been already mentioned, and is 

 usually most marked in the effect of organic colloids on inorganic sols, but 

 certain inorganic sols have also protective effect. Biltz ' found that 

 zirconium hydroxide exerted a protective action on gold sols even stronger 

 than that of gelatine ; and Ruer,'^ that zirconium and ferric sols contain- 

 ing traces of chlorine gave no precipitate with silver nitrate, the silver 

 chloride remaining in colloidal solution. Kuspert has also produced gold 

 and silver sols by reduction in presence of silica jelly. The efficiency of 

 protective colloids extends to both positive and negative sols, though 

 usually in difJ'erent degrees, and is marked in a large number of organic 

 colloids, many of which, it may be remarked, are amphoteric, and take 

 different charges according to the acidity or alkalinity of the medium. 

 Thus gelatine and its peptones are among the most powerful with regard 

 to gold sols, less than 1 nigr. being sufficient to protect a litre of gold sol 

 from precipitation by salt, while 80 mgr. were required to exert an equal 

 protective effect on a (much more concentrated) AsgSj sol. Most other 

 colloids are less powerful in their effects, 10-20 mgr. of egg albumen, a 

 somewhat larger amount of gum arable, and from 0-6 to 2 grm. of dextrine 

 or starch being required to proiiuce on gold sol an equal effect to 1 mgr. 

 of gelatine. Zsigmondy •' has taken advantage of this difference of eflfect 

 for the detection and identification of colloids in mixtures. He calls the 

 number of mgr. of a colloid required to protect 10 c.c. of a stable gold sol 

 containing 0005:3-0-0058 per cent, of gold from the precipitating action 

 of 1 c.c. of a 10 percent, salt solution the 'gold-value' of the colloid. 



The cause of protective action is not fully explained. It is not due to 

 the viscosity of the colloid, since the effect is produced by quantities too 

 hmall to have any appreciable influence either on the vis.-osity of the 

 liquid or on the pedetic motion. IS^either does it appear due to direct 

 influence on the charges of the particles, since many tollods protect both 

 positive and negative sols. It is more probably due to an actual coating 

 of the particles of the sol with the protective substance. Bechhold * 



' Ber., 1902, 35, 4431-S. "- Z. atwrg. Ch., 1905, 43, 8C-93. 



• Z. anal. Ch., 1902, 40, 697-719, •• Z.phys. Ch., 1904, 48, 385-423. 



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