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the Adsorption of Ions. 335 



chemical reactivity of the hydroxyl group. The presence of 

 the potentially tetravalent oxygen atom possibly leads to a 

 selective adsorption of hydroxyl ions by most surfaces. Thus 

 glass and quartz have a marked negative charge in contact 

 with water (cp. Elissafoff). On the addition of an acid the 

 electrostatic forces will produce a diminution of the charge. 

 The electrical adsorption of hydrogen ions by hydroxyl ions 

 cannot be distinguished from the recombination of hydrogen 

 and hydroxyl ions to form neutral molecules of water. This 

 is confirmed bv the fact that the equation of electrical ad- 

 sorption (cp, previous paper) satisfactorily represents the 

 diminution of the charge. 



Perrin [loc.cit) found that, excepting alumina and chromium 

 chloride, all other substances (naphthalene, silver chloride, 

 boric acid, sulphur, salol, carborundum, gelatine, and cellulose) 

 show a preferential adsorption of hydroxyl ions. The sur- 

 faces have a negative charge even in contact with acid 

 solutions. He also found that at higher concentrations of 

 the acid the surface acquired a positive charge. Elissafoff, 

 McTaggart, Ellis, Powis, and others could not observe this 

 reversal in their investigations. Electrical adsorption of 

 hvdrooen ions cannot lead to a reversal of the charge. The 

 reversal (or the non-reversal) of the charge becomes intelli- 

 gible if it is assumed that the considerations set forth in 

 deducing equations (8) or (9) are correct. 



In contact with pure water the surface has a layer of 

 adsorbed water and a number of hydroxyl ions. The amount 

 of hydroxyl ions adsorbed by the surface will, in general, be 

 small, as the concentration of the hydroxyl ions is very 

 small in pure water. If, however, the adsorption is very 

 strong the surface will have a considerable negative charge. 

 On the addition of an alkali the negative charge of the 

 surface will increase, due to two reasons: 



(1) the preferential adsorption of hydroxyl ions will 

 increase, and 



(2) the number of hydrogen ions being formed at the 

 surface will be more and more neutralized by hydroxyl ions 

 in the liquid (cp, scheme 2 {a)). A maximum will be 

 reached when the surface is saturated by preferential ad- 

 sorption and when — \ in equation (9). The maximum 

 charge per unit area can be written as 



E ro =# + y (for alkali), .... (12) 



where " os" corresponds to the charge when = 1 in 

 equation (8) and " y" is proportional to the number of 



