436 Blake — Behavior of lied Colloidal Gold Solutions. 



cipitation. A considerable amount of an electrolyte up to a 

 definite limit can be added to a permanent suspension without 

 causing any appreciable sedimentation within a reasonable 

 lenoth of time. This portion of the electrolyte is concerned 

 in bringing about " coagulation." The further addition of an 

 electrolyte after the above limit has been reached causes sedi- 

 mentation to take place, the rate of settling varying with the 

 amount of electrolyte thus added. This portion of the electro- 

 lyte is concerned in bringing about " precipitation." In work- 

 ing with colloidal gold solutions it is necessary to distinguish 

 five effects : 



(1) Coagulation of red gold solutions. 



(2) Precipitation of red gold solutions. 



(3) Coagulation of blue gold solutions. 



(4) Precipitation of blue gold solutions. 



(5) Transformation of red gold solutions into blue gold solu- 

 tions. 



Transformation of red colloidal goUl solutions into hlue 

 colloidal gold solutions^ loith sid)seqnent sedimentation. 



The most obvious change brought about in red colloidal 

 gold solutions by the addition of electrolytes is the change of 

 color from red to blue, with subsequent subsidence of the 

 gold. The change of color was investigated to some extent by 

 Hardy, ^ who used a dilute red colloidal gold solution produced 

 by the action of phosphorus in ether on an aqueous solution of 

 gold chloride. The difference in the stability of his solution 

 and the one used in these ex])ei"iments, as indicated by the 

 strengths of the various electrolytes necessary to produce the 

 change of color, is so great that the results here recorded may 

 be found none the less interesting. A red gold solution pre- 

 pared according to tlie method already described and diluted 

 to contain 0-0490 grams of gold per liter was titrated with 

 electi'olytes of known strength, the change of color from 

 purple to violet which ensues soon after the original red color 

 has changed to purple being taken as the end-poiut. The time 

 required for titration varied from two to five minutes, but the 

 time factor is not active until the end-point is nearly reached. 

 Consequently only the last portions of the electrolyte were 

 allowed to drop regularly from the burette. The gold solu- 

 tion changed from violet to pure blue in about two niinutes 

 after the titration was completed and the gold all settled out in 

 four or five hours — the maximum ratef of subsidence in water 

 noted in any experiment, including the work on absorption. :j: 

 The results are given in the following table. 



*Proc. Eoy. Soc, Ixvi, 110; Zeitscb. Phys. Cheiii,, xxxiii, 385. 

 f Cf. Durham, Chem. News, xxxvii, 47. 

 ^This Journal, xvi, 381. 



