According to the other of the two theories, here considered, the electrification of the colloid may result from what is known as contact or surface electricity.[269] At the surface of two different substances there is always a potential difference.[270] In the case of finely divided suspensions, like the colloids, the contact surfaces are enormous, as compared with the surfaces involved in ordinary contact. Whether metals (Au, Pt, Ag) owe their charges to simple contact effects or to their (minimal) tendency to ionize (Chapters XIV and XV) is not known. In fact, no exact knowledge of the source of electrification of any colloid has yet been obtained.
Precipitation of Colloids by Electrolytes and by Colloids.
The purple precipitate (purple of Cassius), which is formed when stannous chloride is added to gold chloride (p. [126]), contains colloidal gold,[276] which, in suspension, is charged with negative electricity, and colloidal stannic acid,[276] which in acid solution, presumably, carries a positive charge[277]: these two colloids mutually precipitate each other in the presence of hydrochloric acid.[278]
When the precipitate is treated with an alkaline liquid (ammonium hydroxide solution), the charge on the stannic acid becomes negative, both colloids acquire the same charge and the precipitate dissolves, to form the beautifully tinted suspensions of colloidal gold (p. [126]). In this condition the colloids (gold and stannic acid) are sensitive to precipitating electrolytes, and the solution is more sensitive to a mixture of magnesium nitrate and ammonium nitrate than to ammonium nitrate alone (exp.), as is to be expected from a negative suspension (see below).
The characteristic difference in behavior between ortho-, pyro- and metaphosphoric acids toward albumen, which is used as a characteristic analytical test to distinguish metaphosphoric acid from the other two acids,[279] appears to be due to similar relations[280]: metaphosphoric acid, which precipitates (coagulates) albumen, is colloidal, ortho- and pyrophosphoric acids are not. The [p135] coagulation seems to be the result of the union of the negative colloid metaphosphoric acid (or a complex negative colloidal ion thereof) with the positive colloid albumen (or a positive colloidal ion thereof).
The Precipitating Power of Electrolytes and the Valence of their Ions.
Thus, colloidal As2S3, carrying a negative charge, is precipitated by the positive ions of added electrolytes. The addition of a few drops of a molar solution of ammonium nitrate (the precipitating ion is NH4+) to 20 c.c. of the colloidal suspension[282] produces a slight precipitate; complete precipitation requires 3.5 to 3.6 c.c. of the ammonium nitrate solution.[283] Only 0.06 c.c. of an equivalent solution[284] of magnesium nitrate (the precipitating ion is Mg2+) is required, and as little as 0.015 c.c. of an equivalent solution of aluminium nitrate[285] (the precipitating ion is Al3+) has the same effect (exp.). An increase in valence of the negative ion, which is not the precipitating ion in this case, does not affect the result appreciably: 3.5 c.c. of a solution[286] of ammonium sulphate, (NH4)2SO4, equivalent to the solution of NH4NO3, is also required for the complete precipitation of the colloidal As2S3 (exp.), although the one contains the univalent ion, NO3−, the other the bivalent ion, SO42−.
Conversely, a positively charged colloid, like ferric hydroxide, may be precipitated by much smaller quantities[287] of bivalent negative ions than of univalent ions, etc. [p136]