Acid, stannic peptisation

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. 

With the irreversible colloids of the second class, on the other hand, it is possible to reform the hydrosol by the addition of a small amount of a suitable reagent provided that the residue has not been too thoroughly dehydrated. A too complete dehydration causes a continuous change to take place which may proceed so far that the dry residue will no longer undergo peptisation. (Example, colloidal stannic acid.)... 

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 ... 

If now an amount of alkali insufficient to cause peptisation is added most of it is taken up by the stannic acid, without entailing any apparent outward change. On adding somewhat more alkali a cloudy liquid is formed, the ultramicrons of which are composed of a number of amicrons. When an excess of the alkali is now added the amicions may suffer a further subdivision, and finally be completely transformed into stannate. This process has nothing whatever to do with true peptisation. 

Not only are the reactions of metastannic acid partially hidden by the mixture, but also those of iron oxide. If sufiKcient stannic acid is in the precipitate, ammonia will cause peptisation. This is contrary to the behavior of iron oxide, of course, because the latter is thrown out of solution by ammonia. 

Zoltai, T., and M. J. Buerger, 1960. The relative energies of rings of tetrahedra. Z. Krist. 114 1. Zsigmondy, R., 1914. Influence of the quantity of peptising agent on the properties of hydrosols. Hydrosols of stannic acid. Z. Anorg. Chem. 89 210. 

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