Sulphate colloids

On the lower curve we find Na-agar, K-chondroitin sulphate and Na-carrageen all having ester sulphate groups. We will call them sulphate colloids. 

Also chondroitine sulphate and mucoitine sulphate usually considered as relatively small molecules, so that our definition of these substances as colloids (Chapter I, p. 3) might be in danger, lay on the same curve characteristic of sulphate colloids as the truly long chain molecules of Na-agar and Na-carrageen. 

In the next section, we shall discuss at some lei th the reversal of chaise of colloids of acidic nature with a greater number of inoi anic cations in neutral or slightly acid media, the ionogenic groups of all three kinds of colloids (phosphate, carboxyl, sulphate colloids) thus being practically wholly ionised. 

Before discussing these results, we shall first consider the reversal of charge behaviour of carboxyl and sulphate colloids. 

Fig. 15 gives the cation spectra of three sulphate colloids. In the upper spectrum the usual monovalent ions are not recorded, as they have not been determined. They have been determined in media containing 40% acetone, and on the floccules then obtained the same order (Ag [Pg.285]

Fig. 15. Reversal of charge spectra of some sulphate colloids (see text).

Comparing the results with those of the phosphate colloids, we may state, that in all four points the reverse is obtained. The differences with the carboxyl colloids are also considerable, as only point 4 and the sequence of the alkali — and alkaline earth cations in point 2 are alike. Still we may consider the ion spectra of the carboxyl colloids as intermediate between the two extremes of phosphate and sulphate colloids, though the carboxyl colloids stand much nearer to the phosphate colloids than to the sulphate colloids. 

Comparison of the cation spectra of phosphate and sulphate colloids... 

It is however interesting to note that in one case (the position of Ag in certain sulphate colloids) in which theoretically a difficulty in explanation exists, this difficulty also exists in the relatively small solubility of silver sulphate. See 2h, p. 290. 

In sulphate colloids the expected reversal position tends to be present or is actually present in the case of the divalent ions, the reversal of charge concentrations of the ions of the B subgroups lying at the same order of concentration as those of the A subgroups (chrondroitin sulphate, carrageen) or actually at higher concentrations (agar). See p. 285, Fig. 15. 

The very low reversal of charge concentration of UO characteristic of phosphate colloids and not found in carboxyl or sulphate colloids, finds its analogy in the relative solubilities of uranyl phosphate, tiranyl acetate and uranyl sulphate. 

Now turning to the valency of the cation, it rnust be expected that in phosphate colloids increase of valency would decrease the reversal of charge concentration, in sulphate colloids however the contrary should occur. 

In sulphate colloids, the tendency to the reversal sequence shows itself in the fact, that the groups of mono, di and trivalent cations of the A subgroups lie in nearly the same range of concentrations. In every case, the very marked valency influence shown in phosphate colloids has disappeared here altogether. 

After having discussed at some length the cation spectra of phosphate and sulphate colloids, we can be brief in discussing those of carboxyl colloids. 

The results obtained show that from a physico-chemical point of view the division of the biocolloids considered accordii to the composition of rhe ionised groups into phosphate, carboxyl and sulphate colloids is a fertile one. Their behaviour depends not only on their equivalent weight, but also on the said composition. 

It is even by this choice of 6, 5, 4, and 3 valent complex cations, that in 1 d the sulphate colloids differ only quantitatively from the phosphate and carboxyl colloids. For in this case the fundamental difference in polarisability between sulphate groups on the one hand and phosphate and carboxyl groups on the other has no, or practically no, influence on the results. 

If it were possible to use in Table 2 (p. 270) only ideal 6, 5, 4, and 3 valent mono-atomic cations (La and Ce being the only ideal cases of 3 valent cations, A1 and Th already behaving as large complex ions, see p. 291, and the still higher valent cations not existing at all) then the sulphate colloids would behave totally otherwise than the phosphate and carboxyl colloids (horizontal rows consisting presumably only in minus signs). 

For increase in valency of small monoatomic cations may have as a result in sulphate colloids, that reversal of charge is shifted to higher concentrations and flocculation becomes more difficult. 

The reversal of charge spectrum of SiO resembles fairly much that of sulphate colloids (p. 285, Fig. 15), that of TiOa the reversal of charge spectrum of carboxyl colloids (p. 284, Fig. 14). 

By inspection of these ion spectra, it appears that the sequences of the alkali and alkaline earth cations are those also found in carboxyl and sulphate colloids. However as regards the relative positions of the cations of the B subgroups and of the A subgroups a marked difference exists in both spectra. 

The ion spectrum of SiOs resembles nearly that of a sulphate colloid, the ion spectrum of TiOo that of a carboxyl colloid. It follows from the ion sequences that SiOg is less, TiOg is more polarisable than water. 

Fig. 25. Reversal of charge spectra of two carboxyl and two sulphate colloids with hydrochlorides of quinine, strychnine, procaine, guanidine LiCI, NaCl and KCl.

For this investigation a sulphate colloid (Na agar), a carboxyl colloid (Na pectinate) and three phosphate colloids (Na yeast nucleate, purified egg lecithin, and a soya bean phosphatide fraction soluble in alcohol) were used. 

Further also all reversal of charge points could be reached with SiOg particles (without colloids). As we have already seen that the ionogenic surface of SiOg very much resembles the sulphate colloids in polarisability (see p. 296 2 m), the results obtained on Si02 have also been given in Fig. 29 to serve as a substitute for the cation sequences in sulphate colloids. 

Fig. 29. Reversal of charge spectrum of SiOg particles (substitute for sulphate colloid) with substituted ammonium cations.

The points 2 and 3 show in the first place that these relatively large organic cations do not exert a noticeable polarising power, for the sequences obtained are the same for sulphate, carboxyl and phosphate colloids (in distinction to the smaller inorganic alkali cations, which show in phosphate colloids the reverse sequence of that in carboxyl and sulphate colloids). 

The experimental results show that a simultaneous resemblance of ephedrine and Ca on the one side and of acetylcholine and K on the other side is present neither in sulphate colloids (agar) nor its substitute (Si02), nor in carboxyl colloids (pectinate) nor in the phosphate colloid nucleate but only in phosphatides. 

Summarizing the findings surveyed in 4 b and 4 c we may generally say that polarisation effects do not play a role in the fixation of organic ions on the ionised groups of colloids. This is evident for organic cations from the similar behaviour of phosphate, carboxyl and sulphate colloids towards amine hydrochlorides discussed in 4 b. 

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