Reversal of charge concentrations

The reversal of charge concentrations were also determined electro-phoretically on the movements of suspended SiOa particles in the arabinate sol, see Fig. 32. 

Comparing the curve minima with the reversal of charge, a direct correlation is plainly visible, see Fig. 33. The order in which the minima follow from left to right is the same as that of the reversal of charge concentrations, but in general the latter lie somewhat (mean 0,2 units) to the right. 

Fig. 33. Relative positions of the viscosity minima of Fig. 31 and of the reversal of charge concentrations of Fig. 32.

The reversal of charge concentrations lie in general somewhat higher (average 0.2 logarithmic units). 

With the cations studied here the arabinate sol still remains perfectly stable, but with 4 and 6 valent cations flocculation or coacervation occurs. These higher valent cations have lower or even very low (true) reversal of charge concentrations (see p. 259 Chapter IX 1 and 2), that means ... 

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. 

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. 

From the slope of the straight line one calculates that the reciprocal hexol number (number of grams which binds one equivalent of hexol ions at the reversal of charge point) amounts to 294. The true reversal of charge concentration (segment of the ordinate axis cut off by the straight line) amounts to only 0.6.10- N. 

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. 

Now we have taken as an example a still very favourable case, phosphatides, showing relatively low gross reversal of charge concentrations with La(N03)s. In other colloids the latter are as a rule much higher, so that the applicability of La for obtaining a reciprocal La number is experimentally quite excluded. 

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). 

This may possibly explain the fact that the viscosity minima in Fig. 33 (see p. 221) lie at lower concentrations than the reversal of charge concentrations. 

As no flocculation, occurs the reversal of charge concentrations needed for the calculation of R.H.N. were determined on suspended particles (for instance Si02) which become covered with a complete colloidal film. For particulars see p. 277 2 b. 

We shall see that for these reversal of charge concentrations the simple rules no longer hold, which we met in studying the suppression of the electroviscous effect at very low salt concentrations, (see p. 203 Ch. VII 5b) Indeed the valency of the cation is no longer the only factor of primary importance, as very marked specific differences occur between cations of the same valency. Other properties of the cations viz., volume and polarising action come into play next to valency. 

It is the aim of this section to compare for a number of colloids the relative positions of the reversal of charge concentrations of a large number of inorganic ions. 

We may thus write very generally C = -f C, and it will thus depend on the relatively magnitudes of Ct and Cfy what kind of information the reversal of charge concentration mainly gives. 

In this section we shall on the contrary be interested only in the true reversal of charge concentration Cf. For this can be considered as a measure of the affinity of the cations towards the ionised groups of the colloids. The general equation C = Cf + C/ approximates to the form C = Cf if Cf is very small compared to Cf. This is practically always the case for mono and divalent cations at the small sol... 

Thus already the determination of the reversal of charge concentrations at one sol concentration gives us the true reversal of charge concentrations for mono and divalent cations, and the experimental material thus obtained may serve as a basis for the discussion of the relative affinities of these cations for the ionised group of the colloid considered. 

It is easily shown that for this pupose the absolute values of Q for tri, quadri and even hexa valent cations need not be known, but that the gross reversal of charge concentrations obtained at only one definite sol concentration may serve as well as in the case of mono and diavalent cations, as a basis for discussion. 

Thus the sequence of the cations 1, 2 and 3 is not altered if we compare at one sol concentration the values of the gross reversal of charge concentrations instead of the true reversal of charge concentrations. 

In the various figures which follow we must therefore always remind the reader, that the reversal of charge concentrations of 6, 4 and often 3 valent cations are no longer true reversal of charge concentrations, and that their positions depend on the sol concentration chosen in the experiment. Nevertheless their relative positions are the same as those of the corresponding true reversal of charge concentrations. 

Finely divided particles (e.g., Si02 particles) are introduced into the sol in which they become covered with a colloid film. By measuring the electrophoretic velocities (at 1/5 depth of the cuvette) of the particles as a function of the salt concentration the reversal of charge concentration can be determined in quite the same... 

Fig. 8. Coating curves of SiOa particles with an egg lecithin or with an arabinate film. Ordinates logarithm of the CaClg or Co(NH3)5Cl3 reversal of charge concentration in equiv. per 1. Abscissae logarithm of the sol concentration in % (thus 0 = 1% —1 = 0.1%, —2 — 0.01% etc.). Left hand graph.

The CaCl2 reversal of charge concentrations are taken from Fig. 7 (intersections of the U — log C curves with the level U = 0). Since the reversal of charge concentration of the completely naked SiOg particles lies at a much higher concentration, the coating curve here falls from the upper dotted level to the lower dotted level on increase of the sol concentration. 

According to data from a graph similar to that of Fig. 7 but in which the displacement of the U—log C curves on increase of the sol concentration is just the opposite. This is a consequence of the reversal of charge concentration of completely naked SiOa particles lying at a much lower concentration of the Co(NH3)eCl3 than that of the particles coated with arabinate. Consequently the coating curve here rises on increase of the sol concentration from the lower dotted level to the upper dotted level. 

Figs. 10 and 11 give for two different preparations of egg lecithin the interpolation graphs used for determining the reversal of charge concentration of a number of mono and divalent cations. (Li, Na, K, Mg, Ca, Sr and Ba as chlorides, the rest as nitrates). 

The intersections of the curves with the dotted level (U = 0) give the reversal of charge concentrations. It is only with KCl that this cannot be reached since in that case it lies higher than that of the saturated solution. 

The sequence of the cations is with the exception of one detail (interchange of Ba and Sr) the same as in Fig. 11. Only the reversal of charge concentrations lie lower, to which we return again on page 294. 

For the sake of surveyability the logarithms of the reversal of charge concentrations (can be read off from Fig. 11 and supplemented with similar data for UO2, 3, Co, Ni and cations of higher valency) are plotted not on one level but on several horizontal levels. The uppermost level contains data on monovalent cations, the succeeding three data on divalent cations and the lowest data on cations of higher valency. 

Increasing the valency of a cation of the A subgroups of the Periodic S37stem decreases considerably the reversal of charge concentration. Compare the relative positions of the monovalent Li, Na, K with the divalent Mg, Ca, Sr, Ba and with the trivalent Ce, La. 

Increasing the ion radius within the above named mono and trivalent ions (thus Li — Na —> K or Ce —La) increases the reversal of charge concentration. This does not apply to the divalent ions (Mg, Ca, Sr, Ba) in which irregular series appear. The relatively small Ca ion has here often the lowest reversal of charge concentration. These irregular series will be discussed further in 2 g (p. 288). 

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