46 COLLOIDS IN BIOLOGY AND MEDICINE 



Though the concentration is increased twenty fold, the boiling 

 point rises only two and one-half times. In a solution of 19.5 per 

 cent sodium stearate, F. KRAFFT found absolutely no rise in the 

 boiling point as compared to pure water. 



Let us examine the conclusion of W. BILTZ and A. v. VEGESACK 

 based on their critical study of the osmotic method. True colloids, 

 like iron oxid and tungstic acid show a small osmotic pressure, only 

 so long as they contain electrolytes. As the electrolyte vanishes, 

 the colloid particles aggregate to larger complexes which then cease 

 to show any osmotic pressure. For the existence of these colloids, 

 some electrolyte content is an absolute necessity. 



When these investigators studied " colloid electrolytes," particularly 

 colloid color salts (congo red, night blue and benzopurpurin) whose 

 constitution, molecular weight, etc., were determined by chemical 

 methods, they obtained results which especially in the case of congo 

 red must be closely examined. Congo red has the formula C32H 22 N 6 - 

 S20 6 Na2, and being a sodium disulphonate, is a strong electrolyte. 

 Its molecular weight (M) = 696. On account of its electrolytic dis- 

 sociation into 3 ions (2 crystalloid and 1 colloid) we would expect 

 an osmotic pressure three times as much as its molecular weight 

 would indicate. Instead of this W. M. BAYLISS and also W. BILTZ 

 and A. v. VEGESACK as well as DONNAN and HARRIS obtained by 

 dialysis against pure water a pressure which was approximately 5 per 

 cent lower than would have been obtained had the undissociated 

 molecule been active. The explanation is not difficult. Let us desig- 

 nate by R, the acidic color radical of congo red, then congo red has the 

 formula R.Na 2 . In solution a portion becomes ionized into R and 

 Na Na, of which some, even though possibly only a small fraction 

 forms with the H and OH ions of the water RH (color acid) and 

 NaOH. This occurrence would be without much influence in chang- 

 ing osmotic pressure if the measurement was made in a closed vessel 

 in which the equilibrium was undisturbed. As a matter of fact the 

 measurement is made in a membrane permeable for electrolytes. 

 Consequently the NaOH which has been formed diffuses away and 

 some fresh color acid (RH) may be formed. This process continues 

 until practically only color acid remains in the membrane. Con- 

 sequently in this instance we have not measured the high osmotic 

 pressure of the electrolytically strongly dissociated color salt but that 

 of the practically undissociated color acid. If measured against outer 

 water containing electrolytes, it yields a very much lower osmotic 

 pressure, equivalent to a value for M of 2088. We shall thoroughly 

 understand this occurrence when we have become more familiar with 

 the equilibria of membranes. See page 59. 



