LABORATORY WORK 179 



Mutual Precipitation of Oppositely Charged Colloids 



Take a series of test-tubes containing equal quantities of 

 arsenious sulphide, add gradually increasing quantities of ferric 

 hydroxide. At a certain relative proportion, dependent on the 

 concentration of the colloids, there will be complete precipitation 

 of both. At other proportions, a compound colloid will be pro- 

 duced with excess of one or the other sign, and will remain more 

 or less completely in suspension. As a rule, the ferric hydroxide 

 solutions used will be found to be more concentrated than the 

 arsenious sulphide. 



Take also a 5 per cent, solution of egg-white and filter it. 

 Make a part acid to neutral red by the addition of acetic acid, and 

 another part alkaline with sodium hydroxide. The former will 

 be precipitated by arsenious sulphide, not by ferric hydroxide ; 

 the latter, the converse. Thus we have produced an electro- 

 positive colloid by excess of H-ions, and an electro- negative one 

 by excess of OH-ions. 



Excess of Electrolyte. If we add a solution of a precipitating ion 

 in excess, it may happen that, instead of obtaining precipitation, the 

 particles have conferred upon them a charge of the sign opposite to 

 their original one, so that the concentration of the requisite precipi- 

 tating ion is insufficient. They remain, in such a case, suspended. 

 The experiment is rather difficult, on account of the fact that the 

 actual amount required can only be found by trials. It varies with 

 the dimensions of the particles. It may be tried with a suspension 

 of gamboge obtained by pouring a small amount of an alcoholic 

 solution of the gum-resin into a large amount of distilled water. 

 Adding 0.0016 molar cerium chloride to an equal volume will 

 precipitate it, whereas 0.16 molar will probably not do so, or not 

 so rapidly. If no difference is found, try intermediate concentra- 

 tions. The distinction is seen best by shaking again after the 

 first deposition and noting the second effect. 



Staining and Electrical Adsorption. Take some circles of filter 

 paper of 9 or 10 cm. in diameter. For good results, the paper 

 should be the purest analytical preparation, and the dyes should 

 be free from mineral salts. Congo-red may be dialysed, since it 

 usually contains sodium sulphate and chloride. Immerse a circle 

 of paper in a weak solution of crystal-violet and another in a weak 

 solution of congo-red. The former rapidly takes on a deep colour, 

 the latter very little. Add 0.5 per cent, sodium chloride to each 

 of two fresh samples. The staining will be much increased in the 

 case of congo-red, decreased in that of crystal-violet. 



Rate of Chemical Reaction between Colloids. The best way to 

 observe the comparative slowness of this is to prepare a colloidal 



