38o PRACTICAL ORGANIC AND BIO-CHEMISTRY 



PROPERTIES OF COLLOIDAL SOLUTIONS. 



A. Suspensions and Suspensoids. 



(1) Concentration. These colloidal solutions are generally very 

 dilute and contain only a fraction of I per cent, of solid matter in 

 suspension. 



(2) Osmotic Pressure. They have a low osmotic pressure. The 

 freezing-point of the continuous phase is lowered very slightly and the 

 boiling-point is raised very slightly. 



(3) Viscosity. The viscosity of a suspensoid sol is only slightly 

 higher than that of water and is proportional to the amount of solid 

 matter present 



(4) Behaviour to Electrolytes: 



(a) The suspended particles are precipitated immediately or in a 

 short time by the addition of a small quantity of an electrolyte, e.g. : 



If a few drops of saturated sodium sulphate be added to colloidal 

 ferric hydroxide solution, or of metallic silver, the solid matter is 

 precipitated. 



(b) Salts containing divalent ions are more effective than salts with 

 monovalent ions; salts with trivalent more than those with divalent. 



2 mol. NaCl = i mol. BaCl 2 ; i*nol. A1C1 3 = 3 mol. NaCl. 

 The particles are probably discharged by the oppositely charged 

 ion so that they no longer repel one another, but coalesce to form larger 

 aggregates. 



(5) Behaviour to other Suspensoids. 



Positively charged Suspensoids will precipitate negatively charged 

 Suspensoids. Both Suspensoids are precipitated together. If the two 

 colloidal solutions contain an equal number of particles with a suitable 

 number of electric charges both are completely precipitated, e.g. : 



Varying quantities (i, 2, 3 c.c.) of ferric hydroxide sol may be 

 added to varying quantities of arsenious sulphide sol (3, 2, I c.c.). 

 Precipitation will occur. The excess of either sol remains and the 

 precipitate contains both substances, as can be seen from the colour of 

 the precipitate and of the solution. 



(6) Influence of Emulsoids. 



Emulsoids protect Suspensoids from precipitation by electrolytes. 

 It seems that a layer of emulsoid particles is formed round the 

 suspensoid and so alters its properties ; e.g. if ferric hydroxide sol or 

 arsenious sulphide be diluted with (a) an equal velume of water, (b} an 

 equal volume of albumin solution, and sodium chloride solution be 

 carefully added to each, the amount required to precipitate in (b} will 

 be considerably greater than in (a). 



