290 COLLOIDS 



From this table it may be seen that the comparatively 

 slight precipitating power of the kations, Mg and NH 4 -, is 

 completely neutralized by the anions -C 2 H 3 O 2 or -Cl, while 

 the more powerfully inhibiting anions -NO 3 and -C1O 3 are 

 able to neutralize the precipitating power of the kation K as 

 well as that of Mg and NH 4 -. Similarly the powerfully in- 

 hibiting anions -Br, -I and -CNS, are able to counteract the 

 precipitating power of sodium as well. 



The salts of the metals belonging to the second class 

 produce irreversible precipitation, owing, no doubt, to the 

 formation of new compounds by chemical reactions. The 

 case of zinc is peculiar, inasmuch as very dilute solutions of 

 zinc salts produce irreversible precipitation of egg albumen, 

 whereas strong solutions may either not produce a precipitate, 

 or else cause one already formed to dissolve.* 



The precipitating power of the anions when combined with 

 one of the metals of the alkaline earths is exactly the reverse 

 of that observed when the same anions were combined with 

 the alkali metals. Thus the precipitating power of the anions 

 increases in the order C 2 H 3 O 2 >Cl>NO 3 >Br> I>CNS, 

 whereas when combined with the alkali metals the inhibiting 

 power increased in this same order. 



Irreversible precipitation of proteins is also brought about 

 by nitric acid and by the alkaloidal reagents, such as picric, 

 tannic, phosphomolybdic, phosphotungstic acids, etc. 



(_/) The Precipitation of Colloids by Other Colloids of Oppo- 

 site Electric Sign. This phenomenon was first observed by 

 Linderand Picton, who found that certain solutions of organic 

 dyes, on mixing, produced precipitates. Further investiga- 

 tions have shown conclusively that only oppositely charged 

 colloids could mutually precipitate; thus, arsenic sulphide, 

 which is negatively charged, is not precipitated by any other 

 negatively charged colloid, but is precipitated by ferric hyd- 

 roxide, which is positive. The resulting gel is described as 

 an adsorption compound (see below under Adsorption). 



This mutual precipitation of colloids has many very im- 

 portant practical applications ; for example, the use of ferric 

 salts in the purification of sewage water is probably due to the 



*Pauli; " Beitr. z. chem. Phys. u. Path.," 1905, 6, 233, 259. 



