HYDROGEN ION CONCENTRATION AND T H E PROPERTIES OF T H E EMULSOID COLLOIDS‘
Introduction Much has been written in the last few years upon the importance of a consideration of the hydrogen ion concentration with respect to the properties and behavior of colloids, and of the proteins in particular. Michaelis3 and Sorenson4 have studied the effect of the concentration of these ions on body tissues. R o b e r t ~ o nClark, ,~ and Zoller’ have studied particet UL.,’~ and Boguell ularly casein. Loeb,8 S h e ~ p a r d Davis ,~ have reported similar investigations on gelatin. Many other workers in this field have also made valuable contributions to the subject. In nearly all of these studies, however, the treatment of hydrogen ion concentration has been merely incidental to the main investigation, and a corollation of the several studies should be made with the focus of attention centred at that most interesting and important point. I n the present paper such a corollation is attempted, and in many places where the evidence has seemed to be contraPublished by courtesy of the American Chemical Society. Presented before the 63rd General Meeting of the American Chemical Society, Birmingham, Alabama, April 3 t o 7, 1922. * Industrial Fellow of the Mellon Institute of Industrial Research of Pittsburgh, and Research Chemist for Armour and Company, of Chicago. I,. Michaelis: Biochem. Zeit., 1909-1914. S. P. I,. Sorenson: Comptes rendus Lab. Carlsberg, 1909-1913. T. B. Robertson: “The Physical Chemistry of the Proteins,” (1920.) W. M. Clark: Jour. Ind. Eng. Chem., 12, 1182 (1920). H. T. Zoller: Ibid., 13, 510 (1921); Jour. Gen. Physiol., 3, 3G5 (1921). Jacques Loeb: Jour. Gen. Physiol., 1917-1921. S. E. Sheppard: Jour. Ind. Eng. Chem., 12,1007 (1920); 13,423 (1921). Io C. E. Davis, E. T.Oakes, and H. H. Browne: Jour. Am. Chem. SOC., 43, 1526 (1921). l1 R. H. Bogue: Chem. Met. Eng., 23, 5, 61, 105, 154, 197 (1920); Jour. Am. Chem. SOC.,43,17G4 (1921); Jour. Ind. Eng. Chem., 14,32 (1922).
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Robert Herman Bogue
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dictory or insufficient further experiments have been conducted with the hope of making clear or complete these deficiencies. Very little effort has been made in the past upon the linking up of the findings obtained in the laboratory with the applicability of those findings in plant practice or control. As a result, it is inevitable that the relative importance of the several discoveries should be many times incorrectly accentuated. This is a distinct disadvantage to the chemist. The latter may report very large variations in viscosity, for example, with hydrogen ion concentration, and a plant man, upon repeating the test, may find no variation whatsoever. The reason is obvious, for the chemist may have used a very delicate viscosimeter, sensitive to 0.001 centipoise, and the plant man an instrument sensitive only to about 1 centipoise. But these distinctions are not always apparent to the practical glue tester, and the result is an injured reputation for the chemist: not for that one alone, but, which is much worse, for the profession at large. Some points bearing on this aspect will be mentioned in this paper.
Investigations on Hydrogen Ion Concentration Hydrogel? Ioiz Concentration awd Viscosity: Solvation: Hyduation.-I,oebl has shown that the viscosity of any given sample of gelatin varies considerably with the hydrogen ion concentration of the material. If a pure isoelectric gelatin2 is obtained, the viscosity is found to be a t a minimum at that point. The hydrogen ion concentration, or CH,was found by N a t the isoelectric point, or in terms -Michaelis3 to be 2 X of Sorensen’s logarithmic symbol, pH = 4.74. Jacques loeb: loc. cit. Obtained by treating a finely granulated gelatin with N/128 hydrochloric acid a t 15” C, and washing out the excess of acid with water. It is then dissolved in water and made up to 1 percent solution, and the viscosity measured at 40’ C with a n Ostwald viscosimeter. Isoelectric gelatin is gelatin a t the point of electro-neutrality. 8 I,. Michaelis: “Die Wasserstoffionenkonzentration,” (1914). pH = l/log Ca. 2
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Hydrogen Ion Concentration
803
When the hydrogen ion concentration was increased (the pH decreased), as by the addition of more acid, the viscosity was found to increase rapidly until a pH of about 3.5 was reached, after which the viscosity fell upon further additions of acid. When the hydrogen. ion concentration was decreased (the pH increased), as by the addition of more alkali, the viscosity was found to increase somewhat more slowly, until a pH of about 9.0 was reached, after which the viscosity fell upon further additions of alkali. These results have been confirmed, on the acid side, by Davis, Oakes, and Browne,’ and on both the acid and the alkali side by Bogue.2 It is probable that the former also would have obtained the same characteristic rise on the alkali side of the isoelectric point if they had started with an ash-free gelatin. The presence of calcium as calcium gelatinate would greatly inhibit the tendency of increased viscosity a t pH values greater than 4.7. Similar results have been obtained by earlier investigators, but their results are expressed in terms of concentration of acid or base added, rather than of the hydrogen ion concentration to which the solution is brought by such a d d i t i ~ n . ~Loeb has pointed out the necessity of following the latter procedure. It has also long been known that gelatin swells to different degrees depending upon the amount of acid or alkali added to the water in which the gelatin is immersed. Curves illustrative of this are shown by Wo. Ostwald4 and by D. J. Lloyd,5 and Fischer and Hooker6 find similar results with polybasic acids and their salts. Bogue’ has shown that swelling and viscosity are parallel functions within certain limits, except Davis, Oakes and Browne: loc. cit. e
R. H. Bogue: loc. cit. See P. von Schroeder: Zeit. phys. Chem., 45,75 (1903). Wo. Ostwald (Fischer) : “Handbook of Colloid Chemistry,” p. 247
(1915). Dorothy J. Lloyd: Biochem. Jour., 14, 147 (1920). Martin Fischer and Marion Hooker: Jour. Am. Chem. SOC., 40,272
(1918). R. H. Bogue, Jour. Am. Chem. SOC.,43, 1764 (1921).
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Robert Herman Bogue
that the percentage of change in the former may be far greater than the percentage of change in the latter upon similar alteration in hydrogen ion concentration. Hydrogen Ioiz Conceiztratioiz and Jelly Strejigflz : Gelatioil : Sofutian.-flischer and Coffrnanl have pointed out that the solution of a protein is an essentially different process from swelling, and should not he regarded as an extreme case of swelling. They urge that hydration (swelling) is to be regarded as a change through which the protein enters into physicochemical combination with its solvent (water) ; and solution as the expression of an increase in the degree of dispersion of the colloid. Their findings show that there is a progressive increase in the tendency of gelatin to go into solution in mixtures of the salts of polybasic acids as the amount of acid or alkali in these mixtures is increased from a given low point. Bogue2has shown that the same tendency exists in mixtures of gelatin and sodium silicates upon increasing the hydroxyl ion concentration, either by the use of those silicates which are progressively more completely hydrolyzed in water, or by the direct addition of sodium hydroxid. The curves and data show that a t a pH of about 8.5, a 1 percent solution of pure gelatin will just fail i o form a firm jelly a t 10" C, while at a pH of about 9.5 the solution will remain quite liquid, At pH values between 4.7 and 8.5 a firm jelly is formed. On the acid side of the isoelectric point the jelly has been found to remain firm to a pH of about 3.5 and to again be quite liquid at a pH of about 2.5. Sheppard and his collaborators3 have studied the jelly strength of gelatins and glues by means of an instrument which measures both the breaking load and the percentage twist a t hreak. The €actor, breaking load X twist / cross section, is 1
Martin Fischer and W. D. Coffman: Jour. Am. Chem. Soc., 40, 303
(1918).
R. H. Bogue: Jour. Ind. Eng. Chem., 14,32 (1922). S. E. Sheppard, S. S. Sweet and J. W. Scott, Jr.: Jour. Ind. Eng. Chem., 12, 1007 (1920); S. E. Sheppard and S. S. Sweet: Jour. Am. Chem. SOC.,43, 539 (1921). 2
3
Hydrogen loa C o n c e n t r a t k
805
taken as the jelly strength. They report that there seems to be no simple relation between the jelly strength 0f.a glue and the concentration of glue in the jelly; nor between the jelly strength and the hydrogen ion concentration. The tensile strength of the dried material was also found to bear no simple relation to the jelly strength. They report that the cooking of a glue solution lowers both the tensile strength and the jelly strength, but not in equivalent amounts. Hydrogen Ion Covtceiztratioiz aiid Meltiizg Point: Setting Point.-In 1920 Boguel demonstrated that the melting point determination of gelatins or glues differentiated these products in a way that was more accurately representative of actual value (as determined by either gelatin content or joining strength) than either the jelly strength test or the viscosity (at 60" to 80" C) test. Sheppard and Sweet? have studied the setting and melting points of gelatins by means of an instrument which determines the temperature of an arbitrarily selected degree of rigidity of the jelly or gelatinous solution. They find that the melting or setting point curves bear no consistent relation to concentration, the curves for different gelatins a t varying concentrations sometimes cutting each other. The slope of these curves is very different from that of the jelly strength curves, these increasing in setting temperature very rapidly with the first increases in concentration, and showing only slight further rises in temperature at concentrations greater than 20 percent while the jelly strength curves increase slightly at first, but very rapidly a t concentrations above 15 or 20 percent. The type of curve represented by the melting and setting point determinations is also the type which obtains in actual joint strength tests. KO work has been reported bearing directly upon the relations of hydrogen ion concentration to melting point. Hydrogen Ion Concentration and Joiniizg Strength of G1w.s.The joining strength is to the joining and veneer trade the most R. H. Bogue: Chem. Met. Eng., 23,61, 197 (1920). S. E. Sheppard and S. S. Sweet: Jour. Ind. Eng. Chem,, 13, 423 (1921).
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Robert Herman Bogus
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important property of glues. It is of the greatest importance to that trade .that every means by which the actual joining strength may be raised should he understood and applied with intelligence. Much has been said and written upon the changes which may be brought about in the viscosity, the jelly strength, and the melting point by a variation in the hydrogen ion concentration, and upon the relations which these several properties bear to joininx strength. It has, for example, been pointed out that a change in the pH of a glue solution from 4.7 to 3.5 by the addition of an acid, or to 8.5 by the addition of an alkali will likewise result in an increase in the viscosity in either case. If the pH of the original sample is about 6.0 or 7.0 (which is the usual value), the addition of small amounts of acid, since that brings the pH nearer to that of the isoelectric point, 4.7, will decrease the viscositj. Larger amounts, since that will bring the pH beyond 4.7 and nearer to 3.5, will again raise the viscosity. Still greater amounts of acid will cause it once more to fall. Alkali added in any amount up to a pH of 8.5 will raise the viscosity, and beyond that produce a lowering. What has been said of viscosity is true, in a general sense, of jelly strength and of melting point. But it has also been shown that the joining strength may be indicated by the melting point, or by the viscosity a t 32" to 35" C1. Does i t follow that the joining strength may be caused to vary by alterations in the hydrogen ion concentration of the glue solution, or which is the same thing, by the addition of acids or alkalies? From an examination of the curves by Davis, Oakes, and Browne2 (Fig. 7 of original paper) it will be observed that with a 1 percent solution the viscosity increased from about 1.27 centipoises at pH 4.7 to about 1.62 centipoises a t pH 3.4. The difference between these two points is about 0.35 centipoise. This may be accepted as a fair indication of the variation obtained in 1 percent solutions of gelatin or high grade glue upon e
R. H. Bogue: Jour. Ind. Eng. Chem., 14,435(1922). C . E. Davis, E. T. Oakes and H. H. Browne: loc. cit.
Hydrogen Ion Concentration
807
altering the hydrogen ion concentration from its least favorable t o its most favorable value. The instrument most commonly used in glue laboratories for measuring viscosity is some kind of pipette. The time outflow of a definite volume of the solution at a stated temperature (60" to 80" C) is noted by a stop-watch. An instrument which allows 180 cubic centimeters of water to pass through in 42 seconds is often used. In such an instrument a difference in time of flow of one second is nearly equivalent to 10 centipoises. Its sensitiveness is not greater than one half second, or 5 centipoises. If a 20 percent solution of the glue is used for the test, the maximum variation in viscosity which may be expected would be 20 X 0.35 centipoise (the variation in a 1 percent solution) or 7.0 centipoises. In the pipette described, this difference would be observed by a variation of less than a second in time of outflow. It would be just within the limits of detectability. But it would scarcely impress the plant technologist as an operation of especial merit. The MacMichael viscosimeter is sensitive, in ordinary practice, to about one centipoise, and a variation of 7 centipoises would be inevitably observed. But the difference be tween any two grades (for example the Peter Cooper grades) is, in a 20 percent solution at 32" t o 35" C, about 10 to 20 centipoises. It is obvious, therefore, that the improvement in viscosity which may be obtained by a change in hydrogen ion concentration is of but slight importance in the practical control of grade. It is doubtful if such control could raise the value of the product more than one full grade in any case, and its effect would usually be much less than that. Hydrogen Ion Concentration and Gelatin Content of Gelatins and GZues.-Schryverl has recommended the use of 2 cubic centimeters of dilute (1 t o 4) sulfuric acid t o each 100 cubic centimeters of solution for the complete precipitation of protein from a solution with half saturated magnesium sulfate. S. B.Schryver: "Allen's Commercial Organic Analysis," vol. 8, p. 484
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Robert Hernzan Bogue
Boguel demonstrated that, with gelatins which he was testing, the maximum precipitation occurred upon the addition of only l/z cubic centimeter of the dilute (1 to 4) sulfuric acid t o each 100 cubic centimeters of solution. Either greater or lesser amounts of acid diminished the amount of protein thrown out. The reasons for this were not a t that time apparent, but it is now obvious that the greatest insolubility occurs at the isoelectric point, and that at that point the maximum precipitation will take place when treated nith salts. The content of undegraded gelatin does not vary with hydrogen ion concentration (except that large amounts of either acid or alkali promote hydrolysis) but the readiness with which it may be precipitated does vary. This is because the precipitation is essentially a dehydration process, and the more nearly the gelatin approaches this condition before the addition of the salt, the less will be the amount of salt required to finish the process. Or, a t a given salt concentration, the greater will be the amount dehydrated, or thrown out of solution. The amount of acid required to be added to bring the solution to the isoelectric point will, of course, depend upon the original hydrogen ion concentration of the gelatin, and any instructions which specify the amount must be regarded as dependable only insofar as they effect this condition. The Significance of Hydrogen Ion Covlceiztratioiz in Research; in tlze Plant; and in Eva1ztation.-There is probably no condition that i t is more necessary to control in any physico-chemical investigation of a protein water system than the hydrogen ion concentration. Nearly every other property of the system is in some degree variable with its content of hydrogen ions. The reason for this is that the behavior of the protein in the formation of salts, and the ionization of these salts is determined by the hydrogen ion concentration. At a pH of 4.7 there is in gelatin no tendency whatsoever toward salt formation, and any inorganic ions which may be R. H. Bogue: Chem. Met. Eng., 23, 106 (1920).
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Hydrogeia Ian Concentration
809
present are easily washed out of the gelatin with pure water, or with water brought to a pH of 4.7. If a little hydrochloric acid is then added, the pH will fall, and the gelatin will show a tendency to combine with the chlorid ion to form gelatin chlorid. This salt readily ionizes. According to the law of the Donnan Equilibrium,l if such a gelatin is allowed to dry out and then placed in a weak solution of the electrolyte, the concentration of the acid inside and outside of the jelly would, a t equilibrium, not be the same, but would be greater in the exterior solution. Working on this principle, Procter and Wilson2 have developed a theory to account for the swelling of gelatin, and L,oeb3 has deduced his “Occlusion Theory” which purports to account for the varying osmotic pressure, viscosity, etc., of gelatin water systems a t varying hydrogen ion concentraions. Bogue4 has studied particularly the sol-gel equilibrium, and urged a fibrilar structure theory for elastic gels and sols. If an alkali, as sodium hydroxid, were added to the isoelectric gelatin, the p H would be raised and sodium gelatinate would be formed. This also is readily ionized, and the effects produced on the acid side would be largely duplicated on the alkali side. The other papers quoted earlier in this report fully corroborate the importance of a control of hydrogen ion concentration in any investigational work with gelatins. In the plant, a control of hydrogen ion concentration within certain limits is very desirable, but for different reasons than in investigational work. An alkaline gelatin or glue is much more readily attacked by molds in the course of manufacture than a slightly acid material. This is af considerable F. G. Donnan: Zeit. Elektrochemie, 17, 572 (1911); Donnan and Harris: Jour. Chem. SOC.,99, 1554 (1911); Donnan and Garner: Ibid., 115, 1313 (1919). H. R. Procter: Jour. Chem. SOC.,105,313 (1914); Procter and Wilson: Ibid., 109, 307 (1916). Jacques Loeb: Jour. Gem Physiol., 3, 827 (1921); 4, 73, 97 (1921). 4 R. H. Bogue: Jour. Am. Chem. Soc., 44, 1313, 1343 (1922).
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Robert Her.man Bogue
importance to successful plant operation, especially in hot and humid summer weather. There is also a belief among joiners that an alkaline glue deteriorates in the joint, eventually resulting in a large number of failures. This point, however, needs confirmation. When the product is to be used for certain delicate operations, as the sizing of fabrics, it is often necessary to ensure the absence of acid or alkali as these may react with the color base. The improvement of grade by a control of the hydrogen ion concentration in manufacture is, however, of doubtful value, except in special cases. But it would certainly be the part of wisdom to avoid the region of the isoelectric point. Troubles of insolubility, opaque product, or the separation of a flocculent precipitate have many times been traced to this particular concentration of hydrogen ions. It bas been suggested that, since the viscosity, the jelly strength, and the melting point of gelatins and glues have been found to vary with the hydrogen ion concentration of the solutions, the tests for these properties should be made a t some selected pH which should be regarded as standard. There are two objections to this procedure. First, the test of the material should represent the value of the product as it is, rather than 'as it would be if it were treated with an arbitrarily selected amount of some acid or alkali. If it is argued that the test should represent the maximum obtainable value, we will add that dialysis to eliminate the ash, and precipitation to eliminate the hydrolytic products, might be included in the preliminary treatment, but we see no object in such extravagance of interpretation. Second, it has already been pointed out that although wide differences are found to exist, by the use of sensitive instruments, be tween gelatins at varying hydrogen ion concentrations, yet in significant grades the maximum variation will rarely be the equivalent of one full grade. These arguments make it clear that it is neither desirable nor useful to attempt to evaluate gelatins or glues a t a standard, but arbitrarily selected, hydrogen ion concentration.
Hydrogen Ion Concentration
811
Summary It has been shown that the various physical properties of the emulsoid colloids, including the viscosity, jelly strength, melting point, and joining strength, are at a minimum at a hydrogen ion concentration corresponding to the isoelectric point. Several exact studies have been cited. As the acidity or the alkalinity of the solution is increased from this point, these properties rise in value. It is shown that salt precipitations for gelatin contents should be made at the isoelectric condition if maximum precipitation is desired. The necessity for a careful control of hydrogen ion concentration in investigational work on proteins is emphasized and the desirability of a similar control in the gelatin and glue plant during manufacture is pointed out. The limitation of benefit from such control makes questionable, however, the practicability of such methods for the purpose of the improvement of grade. The estimation of hydrogen ion concentration as one of the tests in the evaluation of gelatin and glue is urged, but it is not recommended that all tests of viscosity, jelly strength, etc., be made at a specified pH value. Melloia Institute of Industrial Research University of Pittsburgh February I , 1922
