scholarly journals HYDROPHILIC AND HYDROPHOBIC COLLOIDS AND THE INFLUENCE OF ELECTROLYTES ON MEMBRANE POTENTIALS AND CATAPHORETIC POTENTIALS

1924 ◽  
Vol 6 (3) ◽  
pp. 307-328 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Membrane Potentials ◽  
Ion Concentration ◽  
Protein Solutions ◽  
Depressing Effect ◽  
Hydrogen Ion Concentration ◽  
Donnan Equilibrium ◽  
Low Concentrations ◽  
Protein Gels ◽  
The Difference ◽  
Charging Effect

1. In order to be able to compare the effects of electrolytes on membrane potentials and cataphoretic potentials it seems necessary to distinguish between the charging and depressing effect of electrolytes on these potentials. Only low concentrations of acids and alkalies have a charging effect on the membrane potentials of proteins, while low concentrations of neutral salts have only a depressing effect; in the case of the cataphoretic potentials, low concentrations of salts have a charging effect as have also low concentrations of alkalies and in some cases low concentrations of acids. This difference finds its explanation in the difference of the origin of the two potentials and there can therefore be no common theory for the charging effect of electrolytes in the two cases. 2. There exists, however, an analogy in the depressing action of electrolytes on the two types of potentials inasmuch when the maximal P.D. is reached, all three kinds of electrolytes, acids, alkalies, and neutral salts, have a depressing effect on both types of potentials (taking into due consideration the effect of changes in the hydrogen ion concentration). 3. This depressing effect is adequately explained for the membrane potentials of protein solutions and protein gels on the basis of the Donnan equilibrium, and the question arises whether the same explanation may also hold for the cataphoretic potentials. 4. The active ion in the depressing action of electrolytes on membrane potentials as well as on cataphoretic potentials has the opposite sign of charge from that of the colloidal particle. It had been shown before that only the valency but not the chemical nature of the active ion determines the depressing effect in the case of membrane potentials and it is shown in this paper that the same is true for the cataphoretic potentials of particles of collodion, mastic, Acheson's graphite, and denatured egg albumin. 5. It is shown that the same valency rule holds also for the effect of acids on the cataphoretic potentials of collodion particles coated with gelatin, and that the ratio of the effect of dibasic to that of mono-basic acids is approximately 0.66, as Donnan's theory of membrane potentials would demand. 6. If we have a right to conclude from the validity of the valency rule for cataphoretic potentials that the depressing effect of electrolytes on the cataphoretic P.D. is determined by the Donnan equilibrium, we can understand the analogy between the depressing action of electrolytes on membrane potentials of hydrophilic colloids and on the cataphoretic potentials of hydrophobic colloids. We can also understand the analogy between the influence of electrolytes on the precipitation of hydrophobic colloids and on the depression of the values of all those properties of hydrophilic colloids which depend on the Donnan equilibrium, since the precipitation of hydrophobic colloids occurs when the cataphoretic P.D. is depressed below a critical value.

scholarly journals ON THE INFLUENCE OF AGGREGATES ON THE MEMBRANE POTENTIALS AND THE OSMOTIC PRESSURE OF PROTEIN SOLUTIONS

1922 ◽  
Vol 4 (6) ◽  
pp. 769-776 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Membrane Potential ◽  
Hydrostatic Pressure ◽  
Osmotic Pressure ◽  
Membrane Potentials ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Gelatin Solution ◽  
True Solution ◽  
Hydrogen Ion Concentration ◽  
Donnan Equilibrium

1. It is shown that when part of the gelatin in a solution of gelatin chloride is replaced by particles of powdered gelatin (without change of pH) the membrane potential of the solution is influenced comparatively little. 2. A measurement of the hydrogen ion concentration of the gelatin chloride solution and the outside aqueous solution with which the gelatin solution is in osmotic equilibrium, shows that the membrane potential can be calculated from this difference of hydrogen ion concentration with an accuracy of half a millivolt. This proves that the membrane potential is due to the establishment of a membrane equilibrium and that the powdered particles participate in this membrane equilibrium. 3. It is shown that a Donnan equilibrium is established between powdered particles of gelatin chloride and not too strong a solution of gelatin chloride. This is due to the fact that the powdered gelatin particles may be considered as a solid solution of gelatin with a higher concentration than that of the weak gelatin solution in which they are suspended. It follows from the theory of membrane equilibria that this difference in concentration of protein ions must give rise to potential differences between the solid particles and the weaker gelatin solution. 4. The writer had shown previously that when the gelatin in a solution of gelatin chloride is replaced by powdered gelatin (without a change in pH), the osmotic pressure of the solution is lowered the more the more dissolved gelatin is replaced by powdered gelatin. It is therefore obvious that the powdered particles of gelatin do not participate in the osmotic pressure of the solution in spite of the fact that they participate in the establishment of the Donnan equilibrium and in the membrane potentials. 5. This paradoxical phenomenon finds its explanation in the fact that as a consequence of the participation of each particle in the Donnan equilibrium, a special osmotic pressure is set up in each individual particle of powdered gelatin which leads to a swelling of that particle, and this osmotic pressure is measured by the increase in the cohesion pressure of the powdered particles required to balance the osmotic pressure inside each particle. 6. In a mixture of protein in solution and powdered protein (or protein micellæ) we have therefore two kinds of osmotic pressure, the hydrostatic pressure of the protein which is in true solution, and the cohesion pressure of the aggregates. Since only the former is noticeable in the hydrostatic pressure which serves as a measure of the osmotic pressure of a solution, it is clear why the osmotic pressure of a protein solution must be diminished when part of the protein in true solution is replaced by aggregates.

scholarly journals DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES OF PROTEINS

1921 ◽  
Vol 3 (6) ◽  
pp. 827-841 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
High Viscosity ◽  
Relative Volume ◽  
Ion Concentration ◽  
Hydrogen Ion Concentration ◽  
Egg Albumin ◽  
Low Viscosity ◽  
Donnan Equilibrium ◽  
Protein Ions ◽  
Order Of Magnitude ◽  
The Difference

1. Gelatin solutions have a high viscosity which in the case of freshly prepared solutions varies under the influence of the hydrogen ion concentration in a similar way as the swelling, the osmotic pressure, and the electromotive forces. Solutions of crystalline egg albumin have under the same conditions a comparatively low viscosity which is practically independent of the pH (above 1.0). This difference in the viscosities of solutions of the two proteins seems to be connected with the fact that solutions of gelatin have a tendency to set to a Jelly while solutions of crystalline egg albumin show no such tendency at low temperature and pH above 1.0. 2. The formulæ for viscosity demand that the difference in the order of magnitude of the viscosity of the two proteins should correspond to a difference in the relative volume occupied by equal masses of the two proteins in the same volume of solution. It is generally assumed that these variations of volume of dissolved proteins are due to the hydration of the isolated protein ions, but if this view were correct the influence of pH on viscosity should be the same in the case of solutions of gelatin, of amino-acids, and of crystalline egg albumin, which, however, is not true. 3. Suspensions of powdered gelatin in water were prepared and it was found, first, that the viscosity of these suspensions is a little higher than that of gelatin solutions of the same concentration, second, that the pH influences the viscosity of these suspensions similarly as the viscosity of freshly prepared gelatin solutions, and third, that the volume occupied by the gelatin in the suspension varies similarly as the viscosity which agrees with the theories of viscosity. It is shown that this influence of the pH on the volume occupied by the gelatin granules in suspension is due to the existence of a Donnan equilibrium between the granules and the surrounding solution.

scholarly journals DONNAN EQUILIBRIUM AND THE PHYSICAL PROPERTIES OF PROTEINS

1921 ◽  
Vol 3 (5) ◽  
pp. 667-690 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Osmotic Pressure ◽  
Chloride Solution ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Neutral Salt ◽  
Hydrogen Ion Concentration ◽  
Monovalent Anion ◽  
Donnan Equilibrium ◽  
The Difference ◽  
Fair Degree

1. It is shown that a neutral salt depresses the potential difference which exists at the point of equilibrium between a gelatin chloride solution contained in a collodion bag and an outside aqueous solution (without gelatin). The depressing effect of a neutral salt on the P.D. is similar to the depression of the osmotic pressure of the gelatin chloride solution by the same salt. 2. It is shown that this depression of the P.D. by the salt can be calculated with a fair degree of accuracy on the basis of Nernst's logarithmic formula on the assumption that the P.D. which exists at the point of equilibrium is due to the difference of the hydrogen ion concentration on the opposite sides of the membrane. 3. Since this difference of hydrogen ion concentration on both sides of the membrane is due to Donnan's membrane equilibrium this latter equilibrium must be the cause of the P.D. 4. A definite P.D. exists also between a solid block of gelatin chloride and the surrounding aqueous solution at the point of equilibrium and this P.D. is depressed in a similar way as the swelling of the gelatin chloride by the addition of neutral salts. It is shown that the P.D. can be calculated from the difference in the hydrogen ion concentration inside and outside the block of gelatin at equilibrium. 5. The influence of the hydrogen ion concentration on the P.D. of a gelatin chloride solution is similar to that of the hydrogen ion concentration on the osmotic pressure, swelling, and viscosity of gelatin solutions, and the same is true for the influence of the valency of the anion with which the gelatin is in combination. It is shown that in all these cases the P.D. which exists at equilibrium can be calculated with a fair degree of accuracy from the difference of the pH inside and outside the gelatin solution on the basis of Nernst's logarithmic formula by assuming that the difference in the concentration of hydrogen ions on both sides of the membrane determines the P.D. 6. The P.D. which exists at the boundary of a gelatin chloride solution and water at the point of equilibrium can also be calculated with a fair degree of accuracy by Nernst's logarithmic formula from the value pCl outside minus pCl inside. This proves that the equation x2 = y ( y + z) is the correct expression for the Donnan membrane equilibrium when solutions of protein-acid salts with monovalent anion are separated by a collodion membrane from water. In this equation x is the concentration of the H ion (and the monovalent anion) in the water, y the concentration of the H ion and the monovalent anion of the free acid in the gelatin solution, and z the concentration of the anion in combination with the protein. 7. The similarity between the variation of P.D. and the variation of the osmotic pressure, swelling, and viscosity of gelatin, and the fact that the Donnan equilibrium determines the variation in P.D. raise the question whether or not the variations of the osmotic pressure, swelling, and viscosity are also determined by the Donnan equilibrium.

scholarly journals THE ELIMINATION OF DISCREPANCIES BETWEEN OBSERVED AND CALCULATED P.D. OF PROTEIN SOLUTIONS NEAR THE ISOELECTRIC POINT WITH THE AID OF BUFFER SOLUTIONS

1922 ◽  
Vol 4 (5) ◽  
pp. 617-619 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Aqueous Solution ◽  
Aqueous Solutions ◽  
Isoelectric Point ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Protein Solutions ◽  
Protein Solution ◽  
Hydrogen Ion Concentration ◽  
Buffer Salt ◽  
Buffer Solutions

1. It had been noticed in the previous experiments on the influence of the hydrogen ion concentration on the P.D. between protein solutions inside a collodion bag and aqueous solutions free from protein that the agreement between the observed values and the values calculated on the basis of Donnan's theory was not satisfactory near the isoelectric point of the protein solution. It was suspected that this was due to the uncertainty in the measurements of the pH of the outside aqueous solution near the isoelectric point. This turned out to be correct, since it is shown in this paper that the discrepancy disappears when both the inside and outside solutions contain a buffer salt. 2. This removes the last discrepancy between the observed P.D. and the P. D. calculated on the basis of Donnan's theory of P.D. between membrane equilibria, so that we can state that the P.D. between protein solutions inside collodion bags and outside aqueous solutions free from protein can be calculated from differences in the hydrogen ion concentration on the opposite sides of the membrane, in agreement with Donnan's formula.

scholarly journals ION SERIES AND THE PHYSICAL PROPERTIES OF PROTEINS

1921 ◽  
Vol 3 (3) ◽  
pp. 391-414 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Physical Properties ◽  
Osmotic Pressure ◽  
Opposite Sign ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Depressing Effect ◽  
Protein Solution ◽  
Hydrogen Ion Concentration ◽  
Specific Effects ◽  
Protein Ions

1. Ions with the opposite sign of charge as that of a protein ion diminish the swelling, osmotic pressure, and viscosity of the protein. Ions with the same sign of charge as the protein ion (with the exception of H and OH ions) seem to have no effect on these properties as long as the concentrations of electrolytes used are not too high. 2. The relative depressing effect of different ions on the physical properties of proteins is a function only of the valency and sign of charge of the ion, ions of the same sign of charge and the same valency having practically the same depressing effect on gelatin solutions of the same pH while the depressing effect increases rapidly with an increase in the valency of the ion. 3. The Hofmeister series of ions are the result of an error due to the failure to notice the influence of the addition of a salt upon the hydrogen ion concentration of the protein solution. As a consequence of this failure, effects caused by a variation in the hydrogen ion concentration of the solution were erroneously attributed to differences in the nature of the ions of the salts used. 4. It is not safe to draw conclusions concerning specific effects of ions on the swelling, osmotic pressure, or viscosity of gelatin when the concentration of electrolytes in the solution exceeds M/16, since at that concentration the values of these properties are near the minimum characteristic of the isoelectric point.

scholarly journals ELECTRICAL CHARGES OF COLLOIDAL PARTICLES AND ANOMALOUS OSMOSIS

1922 ◽  
Vol 4 (4) ◽  
pp. 463-486 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Isoelectric Point ◽  
Salt Solution ◽  
Electrical Transport ◽  
Colloidal Particles ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Trivalent Cation ◽  
Hydrogen Ion Concentration ◽  
Donnan Equilibrium ◽  
Positively Charged

1. It has been shown in previous publications that when solutions of different concentrations of salts are separated by collodion-gelatin membranes from water, electrical forces participate in addition to osmotic forces in the transport of water from the side of the water to that of the solution. When the hydrogen ion concentration of the salt solution and of the water on the other side of the membrane is the same and if both are on the acid side of the isoelectric point of gelatin (e.g. pH 3.0), the electrical transport of water increases with the valency of the cation and inversely with the valency of the anion of the salt in solution. Moreover, the electrical transport of water increases at first with increasing concentration of the solution until a maximum is reached at a concentration of about M/32, when upon further increase of the concentration of the salt solution the transport diminishes until a concentration of about M/4 is reached, when a second rise begins, which is exclusively or preeminently the expression of osmotic forces and therefore needs no further discussion. 2. It is shown that the increase in the height of the transport curves with increase in the valency of the cation and inversely with the increase in the valency of the anion is due to the influence of the salt on the P.D. (E) across the membrane, the positive charge of the solution increasing in the same way with the valency of the ions mentioned. This effect on the P.D. increases with increasing concentration of the solution and is partly, if not essentially, the result of diffusion potentials. 3. The drop in the transport curves is, however, due to the influence of the salts on the P.D. (ϵ) between the liquid inside the pores of the gelatin membrane and the gelatin walls of the pores. According to the Donnan equilibrium the liquid inside the pores must be negatively charged at pH 3.0 and this charge is diminished the higher the concentration of the salt. Since the electrical transport is in proportion to the product of E x ϵ and since the augmenting action of the salt on E begins at lower concentrations than the depressing action on ϵ, it follows that the electrical transport of water must at first rise with increasing concentration of the salt and then drop. 4. If the Donnan equilibrium is the sole cause for the P.D. (ϵ) between solid gelatin and watery solution the transport of water through collodion-gelatin membranes from water to salt solution should be determined purely by osmotic forces when water, gelatin, and salt solution have the hydrogen ion concentration of the isoelectric point of gelatin (pH = 4.7). It is shown that this is practically the case when solutions of LiCl, NaCl, KCl, MgCl2, CaCl2, BaCl2, Na2SO4, MgSO4 are separated by collodion-gelatin membranes from water; that, however, when the salt has a trivalent (or tetravalent?) cation or a tetravalent anion a P.D. between solid isoelectric gelatin and water is produced in which the wall assumes the sign of charge of the polyvalent ion. 5. It is suggested that the salts with trivalent cation, e.g. Ce(NO3)3, form loose compounds with isoelectric gelatin which dissociate electrolytically into positively charged complex gelatin-Ce ions and negatively charged NO3 ions, and that the salts of Na4Fe(CN)6 form loose compounds with isoelectric gelatin which dissociate electrolytically into negatively charged complex gelatin-Fe(CN)6 ions and positively charged Na ions. The Donnan equilibrium resulting from this ionization would in that case be the cause of the charge of the membrane.

scholarly journals The potential difference occurring in a Donnan Equilibrium and the theory of colloidal behaviour

1923 ◽  
Vol 102 (719) ◽  
pp. 705-710 ◽  
Keyword(s):  
Second Law Of Thermodynamics ◽  
Great Emphasis ◽  
Potential Difference ◽  
Protein Solutions ◽  
Hydrogen Ions ◽  
Protein Solution ◽  
Strong Argument ◽  
Donnan Equilibrium ◽  
Viii And Ix ◽  
The Difference

J. Loeb, in a recent and stimulating work (1), has given a convincing, if somewhat over-emphatic, study of the colloidal behaviour of proteins in solution, based largely upon the theory of the Membrane Equilibrium first suggested by Donnan (4). In one important particular, however, his argument is incorrect. Loeb observed, by certain means (2) devised by himself, the potential difference (P. D.) between a protein solution on one side of a semipermeable membrane and a solution of acid, or of acid and salt, on the other side. He found this P. D. to vary as the concentration of hydrogen ions, or of salt, was varied, in the same manner as did a number of other factors (osmotic pressure, viscosity and swelling). He found also that this P. D. could be “calculated” from the observed difference of ρ -H (or of ρ -Cl) in the two solutions, on the basis of the theory of the Donnan Equilibrium, and he concludes that the excellent agreement between calculated and observed is a strong argument in favour of his explanation of other colloidal phenomena by that theory. This conclusion is not correct: the equality found by Loeb of the observed P. D., to that calculated from the difference of ρ -H is a necessary consequence of any mechanism which does not offend the Second Law of Thermodynamics, and in itself offers no support to the theory that the Donnan Equilibrium underlies the colloidal behaviour of protein solutions. That theory may rest on other and stronger ground; since, however, Loeb appears, throughout his book (and especially in Chapters VIII and IX) and in other places (2), (3), to lay great emphasis on this agreement of the observed P. D. with that “calculated” from the observed ρ -H’s it is necessary to point out that this agreement proves no more than that the system investigated was in equilibrium, and that the observations were accurately made.

Intracellular hydrogen ion changes and potassium movement

1963 ◽  
Vol 204 (5) ◽  
pp. 765-770 ◽  
Author(s):  
E. B. Brown ◽  
Bernard Goott
Keyword(s):  
Extracellular Fluid ◽  
Plasma Potassium ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Reverse Direction ◽  
Acid Base ◽  
Hydrogen Ion Concentration ◽  
Donnan Equilibrium ◽  
Extracellular Compartment ◽  
Direction Of Movement

Intracellular hydrogen ion concentration was determined on skeletal muscle by the DMO technique in dogs subjected to various acid-base alterations. The data verified the fact that a given alteration in Pco2 produces a larger hydrogen ion change in intracellular fluid than in extracellular fluid. In spite of this, however, the ratio (See PDF) decreased. On the basis of this change in ratio, the Donnan equilibrium would predict that potassium would move from intracellular to extracellular compartment and not in the reverse direction as had been assumed previously. Using the change in plasma potassium as the criterion of direction of movement of potassium between intracellular and extracellular fluids, the movement of potassium produced by any of the acid-base alterations which were studied was usually that which would be predicted by the Donnan equilibrium.

scholarly journals THE COLLOIDAL BEHAVIOR OF PROTEINS

1921 ◽  
Vol 3 (4) ◽  
pp. 557-564 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Aqueous Solution ◽  
Osmotic Pressure ◽  
Chloride Solution ◽  
Hydrogen Ion ◽  
Ion Concentration ◽  
Preliminary Note ◽  
Hydrogen Ion Concentration ◽  
Chloride Solutions ◽  
The Difference ◽  
Two Sides

1. It is well known that neutral salts depress the osmotic pressure, swelling, and viscosity of protein-acid salts. Measurements of the P.D. between gelatin chloride solutions contained in a collodion bag and an outside aqueous solution show that the salt depresses the P.D. in the same proportion as it depresses the osmotic pressure of the gelatin chloride solution. 2. Measurements of the hydrogen ion concentration inside the gelatin chloride solution and in the outside aqueous solution show that the difference in pH of the two solutions allows us to calculate the P.D. quantitatively on the basis of the Nernst formula See PDF for Equation if we assume that the P.D. is due to a difference in the hydrogen ion concentration on the two sides of the membrane. 3. This difference in pH inside minus pH outside solution seems to be the consequence of the Donnan membrane equilibrium, which only supposes that one of the ions in solution cannot diffuse through the membrane. It is immaterial for this equilibrium whether the non-diffusible ion is a crystalloid or a colloid. 4. When acid is added to isoelectric gelatin the osmotic pressure rises at first with increasing hydrogen ion concentration, reaches a maximum at pH 3.5, and then falls again with further fall of the pH. It is shown that the P.D. of the gelatin chloride solution shows the same variation with the pH (except that it reaches its maximum at pH of about 3.9) and that the P.D. can be calculated from the difference of pH inside minus pH outside on the basis of Nernst's formula. 5. It was found in preceding papers that the osmotic pressure of gelatin sulfate solutions is only about one-half of that of gelatin chloride or gelatin phosphate solutions of the same pH and the same concentration of originally isoelectric gelatin; and that the osmotic pressure of gelatin oxalate solutions is almost but not quite the same as that of the gelatin chloride solutions of the same pH and concentration of originally isoelectric gelatin. It was found that the curves for the values for P.D. of these four gelatin salts are parallel to the curves of their osmotic pressure and that the values for pH inside minus pH outside multiplied by 58 give approximately the millivolts of these P.D. In this preliminary note only the influence of the concentration of the hydrogen ions on the P.D. has been taken into consideration. In the fuller paper, which is to follow, the possible influence of the concentration of the anions on this quantity will have to be discussed.

scholarly journals THE INFLUENCE OF THE CHEMICAL NATURE OF SOLID PARTICLES ON THEIR CATAPHORETIC P.D. IN AQUEOUS SOLUTIONS

1923 ◽  
Vol 6 (2) ◽  
pp. 215-237 ◽  
Author(s):  
Jacques Loeb
Keyword(s):  
Aqueous Solutions ◽  
Chemical Nature ◽  
Positive Charge ◽  
Ferric Hydroxide ◽  
Solid Particles ◽  
Ion Concentration ◽  
General Tendency ◽  
Acid Dyes ◽  
Hydrogen Ion Concentration ◽  
Low Concentrations

1. The effect of eight salts, NaCl, Na2SO4, Na4Fe(CN)6, CaCl2, LaCl3, ThCl4, and basic and acid fuchsin on the cataphoretic P.D. between solid particles and aqueous solutions was measured near the point of neutrality of water (pH 5.8). It was found that without the addition of electrolyte the cataphoretic P.D. between particles and water is very minute near the point of neutrality (pH 5.8), often less than 10 millivolts, if care is taken that the solutions are free from impurities. Particles which in the absence of salts have a positive charge in water near the point of neutrality (pH 5.8) are termed positive colloids and particles which have a negative charge under these conditions are termed negative colloids. 2. If care is taken that the addition of the salt does not change the hydrogen ion concentration of the solution (which in these experiments was generally pH 5.8) it can be said in general, that as long as the concentration of salts is not too high, the anions of the salt have the tendency to make the particles more negative (or less positive) and that cations have the opposite effect; and that both effects increase with the increasing valency of the ions. As soon as a maximal P.D. is reached, which varies for each salt and for each type of particles, a further addition of salt depresses the P.D. again. Aside from this general tendency the effects of salts on the P.D. are typically different for positive and negative colloids. 3. Negative colloids (collodion, mastic, Acheson's graphite, gold, and metal proteinates) are rendered more negative by low concentrations of salts with monovalent cation (e.g. Na) the higher the valency of the anion, though the difference in the maximal P.D. is slight for the monovalent Cl and the tetravalent Fe(CN)6 ions. Low concentrations of CaCl2 also make negative colloids more negative but the maximal P.D. is less than for NaCl; even LaCl3 increases the P.D. of negative particles slightly in low concentrations. ThCl4 and basic fuchsin, however, seem to make the negative particles positive even in very low concentrations. 4. Positive colloids (ferric hydroxide, calcium oxalate, casein chloride—the latter at pH 4.0) are practically not affected by NaCl, are rendered slightly negative by high concentrations of Na2SO4, and are rendered more negative by Na4Fe(CN)6 and acid dyes. Low concentrations of CaCl2 and LaCl3 increase the positive charge of the particles until a maximum is reached after which the addition of more salt depresses the P.D. again. 5. It is shown that alkalies (NaOH) act on the cataphoretic P.D. of both negative and positive particles as Na4Fe(CN)6 does at the point of neutrality. 6. Low concentrations of HCl raise the cataphoretic P.D. of particles of collodion, mastic, graphite, and gold until a maximum is reached, after which the P.D. is depressed by a further increase in the concentration of the acid. No reversal in the sign of charge of the particle occurs in the case of collodion, while if a reversal occurs in the case of mastic, gold, and graphite, the P.D. is never more than a few millivolts. When HCl changes the chemical nature of the colloid, e.g. when HCl is added to particles of amphoteric electrolytes like sodium gelatinate, a marked reversal will occur, on account of the transformation of the metal proteinate into a protein-acid salt. 7. A real reversal in the sign of charge of positive particles occurs, however, at neutrality if Na4Fe(CN)6 or an acid dye is added; and in the case of negative colloids when low concentrations of basic dyes or minute traces of ThCl4 are added. 8. Flocculation of the suspensions by salts occurs when the cataphoretic P.D. reaches a critical value which is about 14 millivolts for particles of graphite, gold, or mastic or denatured egg albumin; while for collodion particles it was about 16 millivolts. A critical P.D. of about 15 millivolts was also observed by Northrop and De Kruif for the flocculation of certain bacteria.

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