Peptising ions

In addition there is still a complication in this case which causes doubt as to whether the correct particle size was determined even by osmotic methods In fact the colloid sols in water always contain substances which are dispersed as ions graham already knew the necessary presence of the peptising ions in addition there are always the so-called counterions present Therefore the osmotic pressure of the ions present, as well as thit of the colloid, is measured elimination of thQ former was attempted by using membranes which are semipermeable to the colloid but not to the free ions, and these osmotic investigations led DoHnan to his theory on the osmotic pressure under such complicating circumstances. It is only after this theory that it has been possible to draw conclusions with any certainty with regard to the size of the kinetically active particles from osmotic measurements made under these circumstances. 

Graham found that gels may be converted into sols by the addition of suitable substances, and he called the process peptisation, since it resembles the action of pepsin on proteins. It can be brought about by ions (e.g. alkali on stannic acid) which can then be partly dialysed out but a small residue of ion seems to confer stability. W. D. Bancroft collected evidence to show that every adsorbed substance tends to peptise the adsorbing substance he included liquids such as water peptising tannin, etc., and in some of his examples (e.g. alkali hydroxides and gelatinous metal hydroxides) there is probably compound formation rather than adsorption. 

The hypothesis, that the electric charge on irreversible hydrosols is due to the adsorption of ions on, or the giving up of ions by, the particles, seems capable of explaining an enormous number of experimental facts. The principles of this point of view, arising out of Hardy s work, were first presented by Bredig, and more fundamentally dealt with by Billitzer. The author had independentl3r employed the same hypothesis to explain peptisation and the reactions of the purple of Cassius. These considerations were not published until 1904. 

The Taking up and the Giving off of Ions.— It is the concensus of opinion among a large number of investigators that the adsorption of ions plays an essential role in the charge of colloids. Since the publication of the fundamental work of Hardy, the author has been much occupied with this question, and has come to the conclusion that the adsorption and dissociation of ions by the particles offer the only rational explanation of the coagulation and peptisation by electrolytes. 

In the following paragraphs a few specific examples will be given to show how peptisation and a number of other colloidal reactions may be explained in a simple manner on the basis of adsorption of ions. 

If alkali is added to a gel of stannic acid without doubt a small portion of each unite to form a stannate. It is equally certain that a part of the stannate will be adsorbed by the stannic acid. If now the mixture is diluted with water the dissociation of the stannate occurs, and one has but to assume that the stannate ion is more adsorbed by the ultramicrons than the potassium ion in order to explain the peptisation. The stannate ion imparts the negative charge to the particles and the subdivision goes on in a manner very similar to that. of diffusion. The potassium ion diffuses to portions of the liquid where the concentration is not so great taking in its wake the stannate ion with its attendant ultramicrons, just as in the case of hydrochloric acid the hydrogen ion leads the way to places of lower concentration and the chloride ion follows. The reaction may be expressed by means of the following equation ... 

Under certain circumstances a very small amount of alkali is sufficient to cause complete peptisation without any further dilution. The direct adsorption of the stannate ion may be assiuned. In this case the following reactions come to pass. 

The peptisation of most oxides, sulfides and salts may be explained on the same basis.- According to Graham the peptisation of ferric hydroxide hydrogel takes place on treating it with ferric chloride. One may assume that the ferric ion attaches itself to the ultramicrons of the colloidal ferric hydroxide, thus imparting to the latter the positive charge. 

Arsenious sulfide, like many other sulfides, is peptised by hydrogen sulfide. It is probable that the hydrogen sulfide ion, SH, is responsible for the negative charge. 

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