loidal gold and of arsenious sulphide, no coagulation occurs, but when the suspensions of ferric hydroxide and arsenious sulphide, which we have seen from their behavior on migration have opposite electric charges, are poured together, there is immediate coagulation, and in a few minutes the precipitate settles, leaving the liquid clear above.
Crystalloid substances are also to be divided into two classes with respect to their effect in coagulating colloids. Non-electrolytes, whether organic or inorganic, have no tendency to produce coagulation; indeed, we have seen that organic substances, like ether, glycerine or sugar, often increase the stability of the suspension. On the other hand, strong electrolytes, that is, substances which are themselves largely dissociated into electrically charged particles or ions, cause coagulation, when their concentration in the solution becomes sufficiently great. Although complete coagulation does not occur suddenly as the quantity of electrolyte is increased, yet the interval between the concentration at which, in a given time, the turbidity becomes visibly greater and that at which the particles have become large enough to settle out or to be retained by a filter is usually so small that a fairly definite concentration can be specified at which each electrolyte causes a certain, experimentally determinable, degree of coagulation in a definite time. Now recent investigations have demonstrated the remarkable fact that this coagulation-concentration is nearly the same for different ions having the same electric charge (or valence), but that it diminishes enormously with increase of the electric charge of the ion of unlike sign to that of the colloid, while it is not affected by a change in the electric charge on the ion of like sign.
These principles are well illustrated by the results given in the table, which were obtained by Freundlich, on the one hand with the negative colloid, arsenious sulphide, and on the other with the positive colloid, ferric hydroxide, by determining the concentration of various salts which in two hours caused such coagulation as would prevent any of the colloid from passing through a hardened filter.
| Coagulation-concentration in milli-equivalents per liter | |||||||||
| of AS2S3, a negative colloid, by | of FeO3H3, a positive colloid, by | ||||||||
| NaCl | 71 | MgCl2 | 2.0 | AlCl3 | 0.39 | NaCl | 9.3 | K2SO4 | 0.41 |
| KNO3 | 70 | Ba(NO3)2 | 1.9 | Al(NO3)3 | 0.41 | KNO3 | 11.9 | K2Cr2O7 | 0.39 |
| K2SO4 | 91 | MgSO4 | 2.3 | Ce2(SO4)3 | 0.40 | BaCl2 | 9.6 | MgSO4 | 0.43 |
It will be seen by comparing the first two values in each column that so long as the electric charge or valence of the ions of the salt remains the same, their chemical nature has no influence. It will be further seen from the third value in each column that a variation of the electric charge of the negative ion has no great influence upon the
