Hexol nitrate

Ordinates and abscissae as in Fig. 16. A = KCl, B = BaCla, C = Co(NH3)eCl3 and D = [Pt(en)3] (NOa)4 fit completely with the behaviour in Fig. 16. E = hexol nitrate (composition, see note 1 on page 208) gives a curve with a minimum. At this minimum reversal of charge of the sol takes place from negative to positive. Flocculation does not take place with any of the salts, even not around the minimum of the hexol nitrate curve. 

This state of affairs has been shown to exist in the sol of soluble starch using hexol nitrate (a complex cobalt salt with hexavalent cation ). 

Fig. 19 shows the results obtained. The curves for the salts with mono- di-tri and tetravalent cations, have the same relative positions and shapes as with the agar sol (Fig. 16). Hexol nitrate — in accordance with the still higher valency of the cation — initially lowers the relative viscosity still more than the complex Platinum salt, and shows further the above predicted curve form.

Though the underlying mechanism of the reversal of charge with proteins by charing the pH is quite different, the minimum of the well known viscosity-pH curve is comparable to the minimum discussed for soluble starch with hexol nitrate. This minimum, occurring at the I,E,P.y is here quite easily detectable, because of the still low values of x. 

It will not be difficult to understand also the occurrence of viscosity minima at reversal of charge points (Fig. 20 and hexol nitrate in Fig. 19) from analogous points of view. We shall however consider this point in more detail in 9. 

Reversal of charge with hexol nitrate of negatively charged macromolecular sols — first met with in the case of amylum solubile (see p. 207 Fig. 19) — has been shown to occur very generally. In general they do not lend themselves to a vis-... 

If such a colloid and such salts are chosen, that form favourable combinations for viscosimetric investigation, — favourable here meaning that no flocculation or coacervation accompanies the reversal of charge — then it may be expected that, similar to the combination amylum solubile + hexol nitrate minima in the( / — o )/>/ curves must also occur. 

The viscosimetric detection of these minima for salts, which bring about reversal of charge at not very low concentrations, will however not be very easy. For at these higher salt concentrations (where activity coefficients are already low) it may be expected that the branch ascending from the minimum at the side of the higher salt concentrations will be still much less steep than for hexol nitrate in Fig. 19. 

The behaviour of soluble starch sol towards reversal of charge with hexol nitrate, without flocculation accompanying it, is rather an exceptional case (p. 208). 

As a rule negative colloids of acidic nature are flocculated or coacervated by small amounts of hexol nitrate, rhodochrome chloride and Pt(en)f (N03)4, that are salts with 6, 5 or 4 valent complex cations (see p. 270, Table 2)... 

That these flocculations (or coacervations) with high valent cations are of a quite different nature is clearly shown in the case of gum arabic. Its sols, as PoHL has already shown, cannot be salted out even by the highest concentrations of salts commonly used for this purpose. Nevertheless a 1% sol is coacervated by hexol nitrate in concentrations higher than 5 m. eq. p. 1, and more diluted sols at proportionally lower concentrations, the latter fact already indicating that the expression "concentration is here of a doubtful use (for fuller information see p. 262, Chap. IX lb). 

Still another example may be quoted to illustrate the quite different nature of salting out and of flocculations with hexol nitrate and other salts with polyvalent ions in small concentrations amylum solubile sols are readily salted out e.g., with certain sulphates, whereas its sols do not flocculate with the named 6, 5, or 4 valent complex cations. 

Fig. 6. Flocculation or coacervation with Na nucleate -j- hexol nitrate, a At room temperature flocculation is produced. 140 x lin.

As was already stated in Ch. VII, p. 223, hexol nitrate, a complex cobalt salt with hexavalent cation, flocculates or coacervates most biocolloids of acidic nature, thus also gum arabic and sodium arabinate sols. Here the remarkable situation is encountered, that the hexol nitrate concentration needed just to start coacervation is nearly proportional to the arabinate concentration, this fact already indicating that the expression hexol nitrate concentration is here of a doubtful value the hexol cations cannot be freely present in solution but must combine with the arabinate. 

Using graphs, in which the electrophoretic velocity is plotted as a function of the hexol nitrate concentration the reversal of charge concentration of the latter can be accurately determined, owing to the steep character of the curves. 

If now dilute sodium arabinate sols of different concentrations are investigated, it appears that the hexol nitrate concentration required to reach exactly the reversal of charge point, is a strictly linear function of the colloid concentration, as shown in Fig. 2. The straight line obtained extrapolated to zero colloid concentration intersects the ordinate axis at very small positive value (4.10- N) thus indicating that we obtain two different kinds of information regarding the reversal of charge phenomenon. 

Fig. 2. Reversal of charge of Na arabinate with hexol nitrate as a function of the sol concentration.

Thus the hexol nitrate concentration needed for an arabinate sol of given concentration consists always of two parts viz, the real reversal of charge concentration and a fictitious concentration — the quantity fixed by and proportional to the arabinate present. 

At moderate arabinate concentrations (right side of the figure) the hexol nitrate concentration is thus for the greatest part fictitious, only at very small arabinate concentrations, (to the extreme left in Fig. 2) it would be practically equal to the real reversal of charge concentration. 

The figure shows the general effect in choosing a lower valent cation, which consists in a nearly parallel displacement of the straight line towards higher con-centrations. " The nearly equal slope of both lines means that the reciprocal La number will indeed be practically the same as the reciprocal hexol number. The main effect consists thus in an enormous increase of the real reversal of charge concentration, the latter being 7 X 10 N for hexol nitrate and 3.6.10 for La(NOg)3. 

For this number must now be obtained from relatively not very different gross reversal of charge concentrations, which latter should for this purpose be known with much greater accuracy than in the case of hexol nitrate. Now in general choosing a lower valent cation, the slope of the electrophoretic velocity-concentration curve decreases, so that in fact the gross reversal of charge concentration with La(N03)s can be determined with less accuracy than with hexol nitrate. 

Summarising we may say that hexol nitrate is at the moment the salt best suited for the experimental method discussed in this subsection, the hexol cation by its high valency giving very low real reversal of charge concentrations and by its nature of a strong complex ion being hardly hydrolysed at low concentration (see also p. 300, note 1). 

Still hexol nitrate has its inconveniences, its solutions decomposing as time proceeds. Therefore hexol nitrate solutions should always be used perfectly fresh. 

Na-arabinate + hexol nitrate —hexol arabinate + NaNOs,... 

That the latter appear negatively charged if not enot h hexol nitrate is added, that is so long as Na arabinate is present in excess, indicates that at the surface of the coacervate drops Na arabinate is adsorbed in the way indicated in A . 

The positive charge of the droplets is in the same way caused by adsorption of hexol nitrate at the surface of the coacervate droplets as indicated in... 

The reversal of charge point can then be formulated by E, in which the negative charge of D is just compensated by the positive charge of hexol nitrate adsorbed at the surface. 

This extra adsorbed hexol nitrate on the coacervate surface corresponds however to a coacervate which no longer exclusively contains hexol arabinate but also a certain amount of hexol nitrate (analogous to C). Thus at the reversal of charge point (E, or simplified to symbol F) the proportion hexol arabinate is higher than the equivalent one (D). 

In contrast to this the coacervation of Na arabinate with hexol nitrate occurs already at very low concentrations and is the consequence of the mutual electric attraction between the negative arabinate ion and the polyvalent hexol cation. 

This is shown in the following Table 2, in which fourteen colloids are listed in the order of decreasing reciprocal hexol number. The table contains information on the flocculability with six different salts, viz., hexol nitrate (6—1) rhodochromium chloride (5—1) platinium triethylenediamine tetranitrate (4—1) luteocobalt chloride (3—1) CaCl2(2—1) and NaCl(l—1). 

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