THE COLLOIDAL STATE 



be concentrated at the interface, giving rise to an electrical charge. The 

 experiments of Lachs and Michaelis (1911) show that, when a charge is already 

 present on a surface, ions of the opposite sign are adsorbed there ; but whether 

 a process of this kind can confer a charge on an uncharged surface is uncertain. 



Investigation on the electric charge can be 

 made by Perrin's method (page 71 above), when 

 the substance can be made into a plug, such as 

 paper, sand, etc. In the case of colloidal solu- 

 tions which can be dialysed free from electrolyte 

 the method of Whetham (1893, pp. 342-345) is the 

 best. The solution is run slowly into the bottom of 

 the bend of a U-tube (Fig. 41) which is already half 

 filled with distilled water or the final dialysate, which 

 was in equilibrium with the colloidal solution, to 

 which a little alcohol may be added in order to lower 

 its density slightly. A sharp boundary surface is 

 thus formed in both limbs of the tube. When 

 electrodes, having between them a potential difference 

 of 100-200 volts, are placed in the water, one at the 

 top of each limb, the boundary surface rises in one 

 limb and falls in the other, the colloidal particles being 

 carried towards the electrode of opposite sign to them- 

 selves, and their rate of movement can be measured. 



ACTION OF ELECTROLYTES 



Since many of the properties of colloidal 

 particles depend on their electric charges, it is 

 to be expected that the charged ions present in 

 solutions of electrolytes would have a consider- 

 able effect upon these properties. Such is 

 found to be the case. 



The presence of H' or OH' ions was found 

 by Perrin (1904, p. 625) to exercise an enor- Fio. 41. APPARATUS FOB DETERMIN- 



mous effect on the potential difference at the 

 contact of inert solids with water. Naphtha- 

 lene, for example, is electro-positive in 0-0002 

 molar hydrochloric acid and negative in sodium 

 hydroxide of the same concentration. This 

 seems to be a law which applies to the great 

 majority of insoluble bodies, but not to all. 

 Cellulose is negative even in 0'002 molar 

 hydrochloric acid, though less so than in alkali. 

 Univalent ions, other than H' and OH', such 

 as Na 1 and Cl', have comparatively little effect. 

 Multivalent ions, on the other hand, have a 

 powerful effect. Suppose that a substance is 

 in contact with a weak alkaline solution, so 

 that it has a negative charge, the addition of 

 a multivalent electro positive ion will greatly 

 reduce, annul, or even reverse the sign of the 

 charge on the surface, and this in very low 

 concentrations. Similarly, mutatis mutandis, 

 will the presence of a multivalent electro- 

 negative ion reduce the charge of an electro- 



INO THE SIGN OF THE ELECTRICAL 

 CHARGE OF COLLOIDAL PARTICLES. 



(Hardy's modification of Whe- 

 tham's method of measuring the 

 migration rate of coloured ions 

 Jour. Physiol., 33, p. 289) 

 The upper part of each limb of 

 the U-tube is filled with water, 

 which has been dialysed into 

 equilibrium with the diffusible 

 electrolytes of the colloidal solu- 

 tion under investigation. The 

 lower part (shaded obliquely) con- 

 tains the colloidal solution. This 

 solution has been run in slowly 

 from the bottom under the water. 

 Large platinum electrodes are in- 

 serted in the water at the top of 

 each limb, and connected with a 

 potential difference of 100-200 

 volts. The position of the two 

 menisci in the figure is such as 

 would be shown by an " electro- 

 negative colloid" after exposure 

 to the electric field for two hours 

 or so. 



positive surface. 



What will be the effect of such alterations of charge on the suspended particles 

 of colloidal solutions 1 



The fact that salts precipitate gold hydrosols was known to Faraday (1858, 

 p. 165), and it was this action of salts which first attracted the attention of 

 investigators. Schultze (1882) noticed that the power of various electrolytes 

 was greatly increased by valency, indeed much beyond relation to the increased 

 number of electric charges. Hardy (1900, i. p. 241), by more quantitative 



