Principles of General Physiology

There are certain facts, however, which cannot be neglected,- not readily to be explained on the basis of electrolytic dissociation. Quincke (1898, p. 217) noticed that a great variety of inert substances, paper, charcoal and so on, have a negative charge in water. The similar charge on drops of petroleum (Lewis) and of aniline (Ridsdale Ellis) has already been mentioned (page 53 above), and the difficulty of explanation on a purely chemical basis was pointed out. On the

other hand, as Hardy has shown (1912, p. 632), a mere trace of a chemicallyactive substance, present as impurity, such as an ester or olcic acid, is sufficient to cause the spreading on water of a heavy hydrocarbon oil which, when pure, does not do so. This being so, a chemical explanation of all the above cases must not be too hastily set aside. But also, it must not be forgotten that electrical charges can be conferred by other means than electrolytic dissociation in the usual sense. It will be sufficient to refer to the phenomena of frictional electricity. The separation of positive and negative electricity here, and the source of the electrical energy resulting, must be looked for in the mechanical work of tearing apart the constituents of the double layer, although the way the double layer itself is produced is not quite clear.

Rudge (1914) finds that dust, blown up so as to make a cloud, becomes highly charged, and that the sign of the charge depends on the chemical nature of the particles. "Acidic" substances, such as sand or molybdic acid, become negative, " basic " substances, such as coal, Hour, red lead or alkaloids, become positive. The facts show that the charges of frictional electricity may, after all, be due to electrolytic dissociation. The various phenomena connected with the electrification of gases and the action of ultra violet light are also to be remembered. It is possibly such facts that caused Lewis to suggest an " electronic " origin for the charge in certain cases. It appears to be the point of view taken by Perrin (1904 and 1905) in his work on electrification at the surface of contact between solids and liquids This observer found that no electrical charges are present except in ionising liquids, such as water, alcohol, etc. None was found in ether, chloroform, turpentine, etc. But, as Hardy points out (1910, p. 193), the absence of migration in a non-conductor does not necessarily prove the absence of potential difference between the phases.

Although many of the cases described by Perrin can be explained on the basis of electrolytic colloids, as stated above, it must be admitted that in such cases as charcoal, carborundum, cellulose, etc., the hypothesis of ionisation seems rather forced. It does not assist matters greatly to point to the existence of graphitic acid in the case of charcoal, while the sign of the charge on aniline is opposite to that which one would expect from electrolytic dissociation. The existence of any charge on petroleum drops is, moreover, a difficulty. How far the presence of impurities may account for some of these facts, as in Hardy's experiments on surface tension (1912, p. 632), is at present uncertain. It would he interesting to know whether Hardy's pure hydrocarbon oil, which does not spread on water, has any charge on its surface of contact with water.

An experiment of Gee and Harrison (1910, p. 46) is interesting in this connection. Alizarin (one part in 10,000) forms a colloidal solution in 2'5 per cent, alcohol, and a true solution in 50 per cent, alcohol. When a current is passed through this latter solution, no migration of the dye occurs, so that it is not ionised, nor has it any charge at all. In the colloidal solution, along with the formation of a contact surface, the particles have a charge and move in the electric field. Apparently, then, this charge cannot be due to electrolytic dissociation, since the molecules in true solution are not so dissociated. In strengths of alcohol intermediate between the above, the rates of migration of the particles showed all intermediate stages. The interpretation of this experiment, as it seems to nu . is not quite simple. Owing to the lower dielectric constant of alcohol, a less charge would be expected, and, moreover, I have found that the temperature coefficient of conduct i.vity of a suspension of well-washed alizarin in water amounts to 3'29, a value greater than that which would be given by a trace of foreign electrolyte, in fact 28 per cent, more than that of potassium chloride, and indicating some slight true solubility and; electrolytic dissociation of alizarin itself (see page 77 above). If this is so, the electric charge may well be due to surface ionisation, with production of colloidal negative ions, similar to those of silicic acid. We know, indeed, that alizarin does behave as a weak acid.

On the whole, the question of the origin of the charge in certain cases requires further investigation, although it seems that Perrin's view of contact electrification has considerable justification. In the majority of cases, there is no doubt that electrolytic dissociation is the cause of the charge. One possibility should be referred to, although the experiments of Elissafov, to be described in the next section, do not support it. If, at the contact of an " insoluble substance " with water, there is surface tension of the ordinary mechanical kind and a trace of an electrolyte be added to the water, it is conceivable that one of the ions into which the electrolyte dissociates may produce a greater diminution of surface energy than the other one. This ion would then

be concentrated at the interface, giving rise to an electrical charge. The experiments of Lachs and Michaelis (1911) show that, when a charge is already present on a surface, ions of the opposite sign are adsorbed there ; but whether a process of this kind can confer a charge on an uncharged surface is uncertain. Investigation on the electric charge can be made by Perrin's method (page 71 above), when the substance can be made into a plug, such as paper, sand, etc. In the case of colloidal solutions which can be dialysed free from electrolyte the method of Whetham (1893, pp. 342-345) is the best. The solution is run slowly into the bottom of the bend of a U-tube (Fig. 41) which is already half filled with distilled water or the final dialysate, which was in equilibrium with the colloidal solution, to which a little alcohol may be added in order to lower its density slightly. A sharp boundary surface is thus formed in both limbs of the tube. When electrodes, having between them a potential difference of 100-200 volts, are placed in the water, one at the top of each limb, the boundary surface rises in one limb and falls in the other, the colloidal particles being carried towards the electrode of opposite sign to themselves, and their rate of movement can be measured.

Since many of the properties of colloidal particles depend on their electric charges, it is to be expected that the charged ions present in solutions of electrolytes would have a considerable effect upon these properties. Such is found to be the case. The presence of H' or OH' ions was found by Perrin (1904, p. 625) to exercise an enor- Fio. 41. APPARATUS FOB DETERMIN- mous effect on the potential difference at the contact of inert solids with water. Naphthalene, for example, is electro-positive in 0-0002 molar hydrochloric acid and negative in sodium hydroxide of the same concentration. This seems to be a law which applies to the great majority of insoluble bodies, but not to all. Cellulose is negative even in 0'002 molar hydrochloric acid, though less so than in alkali. Univalent ions, other than H' and OH', such as Na1 and Cl', have comparatively little effect. Multivalent ions, on the other hand, have a powerful effect. Suppose that a substance is in contact with a weak alkaline solution, so that it has a negative charge, the addition of a multivalent electro positive ion will greatly reduce, annul, or even reverse the sign of the charge on the surface, and this in very low concentrations. Similarly, mutatis mutandis, will the presence of a multivalent electronegative ion reduce the charge of an electro-

— (Hardy's modification of Whetham's method of measuring the migration rate of coloured ions —Jour. Physiol., 33, p. 289) The upper part of each limb of the U-tube is filled with water, which has been dialysed into equilibrium with the diffusible electrolytes of the colloidal solution under investigation. The lower part (shaded obliquely) contains the colloidal solution. This solution has been run in slowly from the bottom under the water. Large platinum electrodes are inserted in the water at the top of each limb, and connected with a potential difference of 100-200 volts. The position of the two menisci in the figure is such as would be shown by an " electronegative colloid" after exposure to the electric field for two hours or so.

What will be the effect of such alterations of charge on the suspended particles of colloidal solutions 1 The fact that salts precipitate gold hydrosols was known to Faraday (1858, p. 165), and it was this action of salts which first attracted the attention of investigators. Schultze (1882) noticed that the power of various electrolytes was greatly increased by valency, indeed much beyond relation to the increased number of electric charges. Hardy (1900, i. p. 241), by more quantitative

methods, formulated a law according to which, if we call the precipitating power of a univalent ion, x, that of a bivalent ion will be a;2, and that of a trivalent one,, a;3. Whetham (1899) showed that this result could be deduced from the theory of probability. Suppose that the charge of a trivalent ion is required to precipitate a certain number of colloidal particles ; to obtain the same charge from bivalent ions, these particles will have to meet two instead of one ; and if from univalent ions, three will be necessary. Now the chances of meeting two or three separate ions, instead of one only, are proportional to the square and cube of their concentration.

Hardy proceeded further to show (1900, 1, p. 242) that the active ion is that one whose charge is of the opposite sign to that of the colloid precipitated. He gives the following general statement: "The coagulative power of a salt is determined by the valency of one of its ions. This prepotent ion is either the negative or the positive ion according to whether the colloidal particles move down or up the potential gradient. The coagulating ion is always of the opposite electrical sign to the particle." This is known as "Hardy's rule."

It may be asked, how do we know which is the active ion, since we cannot add one without the other ? This is possible by taking a series of salts with the same anion or the same cation respectively. We find, for example, that potassium chloride, sulphate, and phosphate, of the same concentration in K' ion, have the same effect on a negative colloid, say arsenious sulphide, although the valency of the anions is respectively one, two, and three. On the other hand, the chlorides of potassium, calcium, and lanthanum differ widely in their action. On a positive colloid the members of the latter series are equal, whereas the chloride, sulphate, and phosphate of the same metal are of greatly increasing potency in the order mentioned.

The following may be given as instances of electro-negative colloids : gold, platinum, arsenious sulphide, silicic acid, " insoluble " organic acids, such as caseinogen, mastic, or the free acid of Congo-red ; suspensions of most powders, charcoal, kaolin, etc., are electro-negative. The hydroxides of aluminium, thorium, iron, are electro-positive. The student is recommended to perform the following experiments on arsenious sulphide, made by passing hydrogen sulphide through a saturated solution of arsenious acid. The resulting hydrosol should be dialysed. On standing, the coarse particles will subside. Add to samples of this solution an equal volume of O'OOOOS molar lanthanum sulphate, 0'027o molar calcium chloride and 0*74 molar potassium chloride. The concentrations of the mixtures will then be as a; to a?2 to ar* in La-", Ca" and K' ions respectively. The precipitating powers will be found to be about equal. Experiments may also be made with varying amounts ; it will be found that a concentration of Ca" or K" equal to that of La"' used is quite inactive, while if the concentration of K' be taken equal to the active one of Ca", it also will be inactive. Corresponding experiments may be made with a hydrosol of ferric hydroxide, prepared by dialysis of a strong solution of ferric chloride, which is hydrolysed, so that the free acid is gradually removed by diffusion. Potassium chloride, sulphate, and phosphate may be used. The phosphate should be neutral and may be made by mixing ten parts of molar phosphoric acid with 17 '7 parts of molar sodium hydroxide and diluting to a concentration in PO/" ion of about 0'00057 molar (Prideaux, 1911). The corresponding solutions of sulphate and chloride may be O0067 and T35 molar in S04" and Cl' ions respectively. It will be found, however, that different preparations of colloids require different concentrations for precipitation, owing to their varying degrees of dispersion, as will be shown later. It may be added that lanthanum is used as a trivalent ion on account of the fact of the minimal hydrolytic dissociation of its salts.

Before proceeding further, it is necessary to remark that the two great classes of colloids, the suspensoid or lyophobe and the emulsoid or lyophile, differ widely in their sensibility to the precipitating action of electrolytes, the former class being very sensitive, the latter comparatively insensitive. The difference, however, is merely one of degree and not fundamental, as the following facts will show. Wiegner (1910, p. 235) showed that even potassium chloride in a concentration of 2*5 millimols to 1,000 of emulsoid (olive oil and water) caused obvious aggregation when observed by the ultra-microscope. Mines (1912, p. 211) finds that egg-white is at once precipitated by a simple trivalent ion, such as La-", even in a concentration of only 0*0016 molar, although comparatively insensitive to univalent ions. Hopkins and Savory (1911, p. 213), in their investigation of the

properties of Bence Jones protein, found that in the cold the precipitating effect of certain ions is very marked. The mechanism of the action of electrolytes must clearly be related to the neutralisation of the electric charge on the colloidal particles by the opposite charge on the precipitating ion. It was thought at one time that the charged colloidal particles were kept in suspension by the mutual repulsion of their similar charges ; in such a case, their stability should be least at the exact neutralisation point and, when excess of the precipitating electrolyte is added, so as to give the particles a new charge of the opposite sign to their original one, the condition should also be a stable one. Although in many cases this seems to be the case, in others the maximum stability has been found not to be exactly at this point. The presence of the Helmholtz double layer puts theoretical difficulties in the way of accepting the mutual repulsion of particles as being directly responsible for their permanent suspension. However this may be, it is clear that the presence of

FIG. 42. AGGREGATION BY ELECTROLYTES. — Photo-micrographs of blood corpuscles of Scyllium canicula, suspended in half-normal sodium chloride. The dilute trivalent ion causes aggregation hy reversing the sign of the charge of a part only of the corpuscles. The concentrated solution causes rapid reversal of the charge to the positive sign on all the corpuscles together. electric charges of the same sign is likely to be a hindrance to their mutual coalescence*.

The mechanism of the precipitation by electrolytes is well illustrated by the following experiment by Mines ( 1 91 2, p. 227). The blood corpuscles of Scyllium are agglutinated (aggregated) by cerium chloride in a concentration of O'OOOS molar, as shown in Fig. 42. In a concentration of 0'08 molar they remain in suspension. Tested by their direction of migration in an electric field, the corpuscles are found to have a negative charge, when in sodium chloride of a strength corresponding to that of the blood plasma of the fish. In the strong cerium solution the charge is completely reversed, and the corpuscles are electro-positive. When the dilute solution is added, certain of them have their charge reversed before others ; these positive ones will unite with negative ones, forming aggregates large enough to fall rapidly under the influence of gravity.

The fact that excess of electrolyte does actually reverse the sign of the charge on particles can be investigated by an apparatus on the plan of that of Ridsdale Ellis (1912, p. 339) which, by the use of non-polarisable electrodes, avoids the production of gas and other troublesome electrolytic disturbances. If the charge on particles is neutralised or reversed by the adsorption of ions of opposite sign, it follows that these ions must be carried down with the precipitate. This has been shown to be the case. Linderand Picton (1895, p. 66) found that when arsenious sulphide is precipitated by barium chloride, the Ba- • ion goes down with the precipitate, while the liquid becomes acid from the hydrochloric acid set free. This Ba' • ion is held fast to the precipitate by electrostatic forces, since it cannot be removed by mere washing with water, although it can be replaced by another cation, when washed with a solution of a salt of this latter. In connection with this fact, an observation by Paine (1912, p. 62) is of interest. Colloidal copper is electro-positive (probably due to a coating of hydroxide) and the precipitating ion is naturally the anion. When this is Cl', by repeated washing of the precipitate it can be removed and the colloidal solution formed anew. When bivalent, as SO4", mere washing will not remove it ; but, if first treated with sodium chloride in excess, so as to replace the SO4" by Cl', then water will restore the original colloidal solution. This illustrates the more powerful action of the bivalent ion.

In connection with this reversible coagulation, it is important to note that it has given the opportunity to Oden and Ohlon (1913) to investigate the dimensions of the aggregates before precipitation and after resuspension. Hydrosols of silver or of sulphur, after aggregation by ammonium nitrate or by sodium chloride, can be resuspended by washing with water. Investigated by the ultra-microscope, these new solutions are found to consist of particles of the same dimensions as the original ones. It would appear, therefore, that in the process of aggregation, no actual fusion takes place ; otherwise it is difficult to understand how separation into particles of the same size as before could be ensured.

The carrying down of the precipitating ion with the precipitate is explained by Linder and Picton (1905, p. 1914) as due to salt formation. That this is not so is shown by quantitative relations, e.g., Perrin (1905, p. 69) finds that one atom of lanthanum, as nitrate, will precipitate 425 atoms of arsenic, as sulphide. Further evidence of the same nature is given by Hopkins and Savory (1911) in the case of the Bence- Jones' protein and will be referred to under the head of proteins.

The actual number of ions carried down is of interest. Burton (1906) estimated the number of aluminium ions adsorbed by a particle of a certain preparation of colloidal silver to be 2 x 10". It is unfortunate that an aluminium salt was chosen, because these salts are hydrolytically dissociated ; lanthanum should have been used. But an approximate idea of the number of atoms in a colloidal particle can be obtained by combining this value of Burton's with that of Perrin given above. One La--- ion precipitates 425 atoms of arsenic in the sulphide, so that the number of atoms in such a colloidal particle is somewhere about 425 x 2 x 107 or 8-5 x 109. Of course this only refers to one individual hydrosol. The dimensions of the particles vary very widely. In the case of the free acid of Congo red, I found (1909, p. 283) by an ultra-microscopic method that the mass of each particle was approximately 2*3 x 10"11 mg. Taking the mass of the hydrogen atom to be l-6xlO~21 mg., that of the molecule of the acid (molecular weight = 652) is 1'04 x 10~18 ; so that there would be 2 x 107 molecules in each particle on the average. Each molecule contains 70 atoms, so that there would be 70 x 2 x 107 atoms in each particle, or about one sixth the number of those in the particle of arsenious sulphide.

Although it may be possible to represent by a chemical formula a long chain, say uf 400 ferric Irydroxine molecules with one of ferric chloride at the end, all united by bom Is. I am unable to see what advantage is gained. It seems rather to obscure the essential nature of chemical combination, as attended by change of properties, since such colloids behave chemically as mixtures only. Moreover, these ferric hydroxide colloids must be regarded as completely hydrolysed, since it is possible to remove all the chlorine by diah sis, although great instability results. If a compound is completely hydrolysed in solution, how does it differ from a mixture? Again, it is difficult to believe that an atom of chlorine at the end of a long chain can have a chemical effect on molecules 400 places away.

If the electric charge on colloidal particles is due to surface ionisation, the greater will be this charge the finer the particles into which a given mass is divided. So that, given equal solid content of two solutions, that one which contains the smaller particles will require more precipitating electrolyte to neutralise the charge and cause aggregation. This has been found to be the case by Sven Oden (1912, p. 123) for hydrosols of sulphur and of silver. A specimen

of the former, containing particles with a diameter of 90 yu/u,, required a concentration of hydrochloricacid of 1 molecule per litre. Another specimen with particlesof 2 10 fip. required only O5 molar. When the particles were too small to be resolved by the ultra-microscope, 0'3 molar sodium chloride was required, whereas particles of 210 fj.[ji only needed 0'07 molar solution of sodium chloride. It will also be noted that the smaller the particles, the greater the changes of surface energy involved in aggregation.

The fact that precipitation is due to inequality and irregular distribution of electric charges, as in the experiment of Mines related above, explains why the effect of a given amount of electrolyte depends on the suddenness with which it is added, as found by Freundlich (1903, pp. 145 and 151). If a quantity capable of precipitating, when added all at once, be added in small portions at a time, a process of acclimatisation or tolerance (" Gewohnung ") is established and no apparent effect is produced, because the particles have all been equally affected by the electrical changes.

When the electric charge is due to surface ionisation, the mode of action of an electrolyte may be analysed further in the following way (Freundlich and Elissafov, see Elissafov, 1912, p. 411): The charge is due to the different solution tensions of the ions of the comparatively insoluble matter of which the suspended particles consist. On the surface of such a substance as glass, for example, there is a layer of ionising silicate, tending to go into true solution in the water ; the K1 and Na* ions have a great solution tension and form an outer layer ; the almost insoluble, slowly diffusing, perhaps strongly adsorbed, silicate ions form an inner layer which, attached to the solid particle, give it the properties of a huge multivalent ion, the colloidal ion of Hardy. The essential diffei'ence between this and an ordinary ion is that, on account of the size of the colloidal ion, surface actions come into play, so that differences in concentration in its neighbourhood are produced by adsorption. Now, according to the law of mass action, there is a constant relation between the product of the concentrations of anion and cation on the one hand, and the concentration of the non dissociated electrolyte on the other hand. Or, as usually expressed : —

Applied to the multivalent colloidal anion of the case before us : — (multivalent anion) (cation) — K (non-dissociated salt). This implies that the concentration of the cation determines that of the multivalent anion, in other words, the charge on the surface, so that the cation of an electrolyte added will diminish or annul the concentration of the anion of the surface, and with it the electric charge. For further details of this point of view, the reader is referred to the paper quoted.

It is interesting to note that, according to the experiments of Dumanski (1910), substances which show all the signs of being in true solution can be converted, by the action of neutral salts, into the colloidal state. For example, solutions of molybdenum oxide showed no signs of heterogeneity under the ultra-microscope, not even a diffused illuminated cone ; the depression of the freezing point also showed that the molecules present were not polymerised. On the addition of ammonium or barium chloride, or other salts, a colloidal solution was formed by coalescence of the molecules.

There is a difficulty sometimes felt with regard to the precipitation of colloids by electrolytes which must be mentioned, since it is not satisfactorily explained. When one ion of the precipitating salt is carried down with the coagulum, the other ion must be left free. To take a case, it seems that Cl' ion must be left when calcium chloride acts upon arsenious sulphide. Even if we suppose that more water is dissociated to give the increase of H' ion shown by the acid reaction, there still remains the corresponding OH' ion to be accounted for.