172 ATOMS, IONS, SALTS, AND SURFACES 



amount of salt (see Table V). Thus it requires twelve hundred times more equiva- 

 lents of potassium chloride than of thorium chloride to reduce the cataphoretic ve- 

 locity from 3.1 to 2.5 fjL (which corresponds with the lowering of (</>) from 69 to 55.5 



millivolts). 



TABLE V 



Lowering Effect of Salts upon the Cataphoretic Mobility (y), the Zeta 



Potential (f), or the Phi Potential (0) for Droplets of Oil in Water 



No. of Milliequivalents of Salt 

 Electrolyte Required to Lower f from 46 to 37 



Millivolts (or ^ from 6g to 55.5 Millivolts) 



KCl 24 



BaCU o. 90 



AICI3 03 



ThCl4 0.02 



The same relationship concerning the valence of the ion of opposite charge is ap- 

 parent in the flocculation values. Thus, for the negatively charged arsenic trisulphide 



TABLE VI 



Flocculation Values; Number of Milliequivalents of Salt per Liter 

 Required to Flocculate the Arsenic Trisulphide Sol 



c 1. Ajj J Flocculation Value Milliequivalents 



Salt Added ^f g^H pg^ Liter 



Monovalent Cations 



LiCl 58 



NaCl 51 



KCl 50 



KNO3 SO 



^K.S04 63 



HCI 31 



Divalent Cations 



MgS04/2 1.62 



MgC]2/2 1.44 



CaCl2/2 1.30 



ZnCl2/2 1.36 



Trivalent Cations 



AICI3/3 0.279 



Al(N03)3/3 285 



Ce(N03)3/3 0-24 



Tetravalent Cation 

 Th(N03)4 0.36 



sol the number of millimols per liter of salt required to cause flocculation decreases 

 very rapidly with the valence of the positive ion (Table VI). 



The work thus far done upon the carrying down of ions by precipitates shows 

 that with a negatively charged sol flocculation is produced by about the same number 

 of equivalents of one positive ion as another (Table VII). This indicates that inor- 

 ganic ions of high charge (valence) are much more highly absorbed than those of low 

 charge (valence). 



