678
R. C. VOGEL AKD M. W. LISSE1
pH MEASUREMENTS O N THIXOTROPIC GEL SYSTEMS USING T H E GLASS ELECTRODE R. C. VOGEL’
AND
M. W. LISSE
Department of Agricultural and Biological Chemistry, The Pennsylvania State College, State College, Pennsylvania Received September 9, fO&
Thixotropy is the reversible isothermal gel-sol transformation which may be brought about by stirring, shaking, or similar procedures. A thorough historical treatment of thixotropy is not necessary here, in view of the splendid review article written by H. Freundlich (3). Within recent years the glass electrode and apparatus to be used in connection with it have been extensively developed. The glass electrode is the only instrument which is suitable for use in measuring the pH of materials in the gel state; accordingly, it is now possible to study the pH changes which may accompany the first gelation of a gel-forming material, the thixotropic liquefaction, and the subsequent re-gelation of the sol thus produced. The literature reveals a number of studies of pH changes during the processes preceding the first gelation of a gel-forming material (1, 5, 7 , 8 , 9 , lo), but none appears to have been made during the complete first gelation of a gel-forming material, the thixotropic liquefaction, and the re-gelation of the sol produced by the thixotropic liquefaction. We shall call the process up to and including the first gelation “primary gelation” and the gelation of any sol originally a gel but liquefied by shaking “secondary gelation.” THEORETICAL CONSIDERATIONS
The scaffolding hypothesis (4,6, 11) has been one of many which have been advanced to describe the structure of thixotropic gels. According to it the gel consists of a network of particles aggregated in the form of chains and cross chains, the particles being in actual contact. This structure is pictured as a scaffolding extending throughout the gel and providing complete molecular binding from one side of the container to the other (4). There i,s some experimental evidence supporting this hypothesis (4, 6, 11). It is a well-known fact that gels, sols, and suspensions of particles approaching colloidal dimensions have very large surfaces on which adsorption could occur. If wre consider the scaffolding hypothesis, might one not theorize that when particles link together there will be a decrease in surface? It is also a well-known fact that the hydrogen ion is often very actively adsorbed, and thus one might expect that a change in surface would bring about a change in pH of the dispersion medium, provided it is not buffered too much. If during primary gelation, thixotropic liquefaction, and secondary gelation changes in structure produce changes in surface available for adsorption, pH changes as 1 Present address: Department of Chemistry, Harvrtrd University, Cambridge, Massachusetts.
pH
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well as changes in ionic activity of other ions might possibly be expected. The change in pH might be sufficiently large to be measurable. The fact that other investigators have observed pH changes during the early processes of primary gelations of a number of gels (1, 5, 7, 8, 10) might also lead one to expect pH changes during the later processes of primary gelation, the thixotropic liquefaction, and the secondary gelation; however, the pH changes noted during the initial processes of primary gelations might have been due to the establishment of equilibrium conditions after the mixing of the gel components and not in any way connected with the formation of the gel's structure. EXPERIMENTAL PROCEDURE
The systems which were studied were thorium molybdate, bentonite, and ferric hydroxide. The gelation time of each system was determined using the inverted-tube method. It was realized that this method was quite arbitrary, but it has been used many times in investigations of thixotropy and is convenient for obtaining relative data. Broughton and Squires (2) discussed the invertedtube method and concluded that it was capable of giving consistent results. It was necessary to determine gelation times in vials of the same size as those in which the gel was to be placed during the pH determinations (inside height 2.8 cm., inside diameter 1.7 cm.), since the time of re-gelation of a thixotropic gel-forming material is dependent upon the amount of glass surface in contact with it and thus upon the dimensions of the vessel. G l w rods were made of the same size and shape as the glass and calomel electrodes, except in cases when a bridge made direct contact with the gel in place of the calomel electrode. Then glass rods were made of the same size and shape as the glass electrode and the end of the bridge dipping into the gel. The false glass electrode and the false calomel electrode or bridge, depending upon which was used during the pH determinations, were placed in rubber stoppers so that they could be held rigidly in the gel by placing the rubber stoppers in the gelation vials during the experiments used to determine the gelation times. In all experiments the temperatures of the room and of the solutions were kept a t 25OC. f lo. RESULTS
Thorium molybdate @ern The thorium molybdate gel-forming system was prepared by thoroughly mixing 2 ml. of a potassium molybdate solution (12.000 g. of KzMoOa per liter), 5 ml. of thorium nitrate solution (55.481 g. of Th(NO&.4HaO per liter), and 0.5 ml. of distilled water. When this mixture was allowed to remain undisturbed, a gel showing thixotropy was formed. The average primary gelation time of the gel was found to be 24 min. with a maximum variation of 3 min., and the average secondary gelation time 25 min. with a maximum variation of 3 min. In preliminary attempts to measure the pH changes during the primary gela-
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R. C . VOGEL AND M. W'. LISSE
tion, thixotropic liquefaction, and secondary gelation, the saturated calomel and glass electrodes were dipped directly into the gel-forming thorium molybdate. It was found that the gel-forming material would diffuse into the calomel electrode and solidify there, thus interfering with its operation. It was obvious that some way must be found to avoid direct contact between the calomel electrode and the thorium molybdate gel-forming system. To accomplish this the glass electrode was placed in the gel-forming material and the calomel electrode in a separate vessel containing a saturated potassium chloride solution. The gel-forming material and the saturated potassium chloride solution were connected by a bridge. An agar bridge containing a saturated potassium chloride solution did not give satisfactory results, but a bridge containing thorium molybdate gel, prepared in the same manner as the gel-forming material, did solve the problem. In pH determinations of the type using the glass and saturated calomel electrodes, there is always a liquid-junction potential between a saturated potassium chloride solution and the sample whose pH is desired. This liquid-junction potential is kept to a minimum, because the transference numbers of the potassium and the chloride ions are about the same and because the concentration of these ions is much larger than the concentration of the ions of the system in contact Tyith the saturated potassium chloride solution. For this reason the diffusion of the potassium and chloride ions chiefly controls the liquid-junction potential. In the experiments vith the thorium molybdate gel bridge, this liquid-junction potential occurred betmeen the thorium molybdate in the bridge and the saturated potassium chloride solution. In the use of the glass electrode we are required to correct for its asymmetry potential by standardization with a standard buffer. As nearly as possible the liquid-junction potentials in the standardization and in the actual pH determinations should be about the same and consist of one junction potential between the saturated potassium chloride solution and the sample vhose pH is being determined. If in the standardization the gel-forming thorium molybdate mas merely replaced with the standard buffer, this mould not have been the case, as there would have been junction potentials betveen the saturated potassium chloride solution and the thorium molybdate gel in the bridge and between the same material in the bridge and the standard buffer. For this reason, in the standardization the glass and calomel electrodes were dipped directly into the standard buffer, thus giving as nearly as possible liquid-junction potentials similar to those existing in the actual pII determinations. After the pH meter had been standardized, the gel components mere mixed, placed in the bridges, and allowed to solidify there. Then a fresh sample of the gel-forming material was made, 2 ml. of the mixture nas pipetted into a gelation vial, and the pH readings were taken at specific times. After the material had been a gel for 6 min., the vial with its gel was removed from the bridge and glass electrode. The gel was liquefied by shaking, and the first pH readings made 25 see. after liquefaction (removing, shaking, replacing of the liquefied gel, and taking of the first pH readings required about 1 min.). The readings
pH
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681
were then continued during the secondary geIation. After each experiment with this system had been completed, the pH meter was checked with the standard buffer to make certain that it had been giving correct readings. The data given in table 1 are representative of four experiments performed on the thorium molybdate system using this method. The maximum variation in each group TABLE 1 p H changes in thixotropic gel systems PH VALUES TIME
Thorium molybdate system
Ferric hydroxide system
Bentonite system
minules
1
2 3 4 5 6 7 8 9
10 15 20 25 30
2.57 2.58 2.59 2.61 2.61 2.62 2.62 2.62 2.62 2.62 2.62 2.62 2.62* 2.62
4.50 4.48 4.47 4.46 4.45 4.44 4.44 4.43 ’ 4.42 4.42 4.41 4.40* 4.40 4.40
4.87 4.87 4.87 4.87 4.87 Primary gelation 4.87 4.87 4.87 4.87 4.87 Thixotropic liquefaction of all gels
31 32 33 34 35 40 45 50 55 60
2.62 2.62 2.62 2.62 2.62 2.62 2.62 2.62 2.627 2.62
4.40 4.40 4.40 4.4Ots 4.40 4.40
4.87 4.87 4.87 4.87 4.87 4.87
Secondary gelation
4.87
* pH a t
t pH
approximate primary gelation time. a t approximate secondary gelation time.
of four pH readings taken at a specific time after each experiment was started is 0.02 pH.
Ferric hydroxide system The ferric hydroxide gel-forming system was prepared by mixing 10 ml. of 5 per cent “Dialyzed Iron” (Merck) with 5.3 ml. of distilled water and then adding to this 1 ml. of 0.493 N sodium hydroxide. This mixture was shaken vigor-
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R. C. VOOEL AND M. W. LISSE
ously, and when it was set aside a gel showing thixotropy was formed. The average primary gelation time was found to be 20 min. and the average secondary gelation time 10 rnin., both with a maximum variation of 4 min. When the pH readings during the gelations were taken by placing the electrodes directly in the gel-forming mixture 15 ithout any precautions, the initial readings of each experiment differed \I idely and non-reproducible pH changes upon thixotropic liquefaction ere obtained. These difficulties could not be corrected by using bridges in the manner described under the thorium molybdate system. Two things had to be done in order to obtain initial pH readings a hich were identical n-ithin the limits of error (+0.02 pH). If after each gelation experiment the glass and calomel electrodes were placed in concentrated hydrochloric acid for 5 min. and then washed off with distilled water, the initial pH readings when the electrodes were placed in the gel-forming material mere the same. It was thought that the concentrated hydrochloric acid dissolved the material which had been strongly adsorbed from previous experiments on the glass electrode and thus interfered with its operation. It m s also discovered that if the electrodes had been in standard buffer immediately preceding the gelation experiments erratic initial readings were obtained. This source of trouble was eliminated by allowing the electrodes to remain in a ferric hydroxide gel-forniing system for an hour after they had been in the standard buffer and before they were treated with concentrated hydrochloric acid. In preliminary experiments the procedure used during the thixotropic liquefaction giving inconsistent results nas the folloning: After the last pH reading during the primary gelation, the gel in its vial was removed from the electrodes and shaken. The electrodes were waslied off uith distilled water and the excess water removed with lens paper. The electrodes were then replaced in the liquefied gel and the pH readings were taken during the secondary gelation. However, when the electrodes were removed from the gel and replaced in contact with the liquefied gel without any treatment, the variations obtained in pH when the gel was shaken disappeared. The final experimental method used for the ferric hydroxide system was the following: Before each set of gelation experiments the glass and calomel electrodes were placed in concentrated hydrochloric acid for 5 min. and then washed off with distilled water. Using a standard buffer, the Beckman pH Meter M&S set to give correct readings. The electrodes which were used in the standardization were placed in the ferric hydroxide system for 1 hr., treated with concentrated hydrochloric acid for 5 min., and washed off with distilled water. They were then placed in 3 ml. of the freshly prepared gel-forming mixture, and pH readings were taken at intervals until the material had become a gel and constant pH readings were obtained. The gel and its vial were removed from the electrodes, shaken until the gel liquefied, and replaced in contact with the electrodes. The first reading during the secondary gelation was taken 25 sec. after the gel waa liquefied (the removing of the gel, shaking, replacing of the liquefied gel, and taking of the fimt reading required about 1 min.). The pH readings were continued for about 10 min. after the system had again become
pH
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683
a gel. After the experiment was complete, the electrodes were removed from the geT and washed off with distilled water, and the pH meter was again checked with the standard buffer to show that correct readings had been obtained. The complete process was repeated in each set of gelation experiments. The data given in table 1 are representative of four experiments performed on the ferric hydroxide system using the above method. The maximum variation in each group of four pH readings taken at a specific time after each experiment was started was 0.02 pH.
Bentonite system The bentonite system was prepared from a bentonite suspension made by dispersing 20 g. of bentonite (Volclay from the American Colloid Company, Chicago, Illinois) in 1000 ml. of distilled water. This dispersion was allowed to settle for not less than 6 days nor more than 3 weeks. It was then decanted from the settled material before use. To 900 ml. of this decanted suspension was added 11 ml. of 0.197 N hydrochloric acid. The unacidified bentonite suspension had a pH of about 7.80; thus it was desirable to use an a c g bentonite suspension to prepare the systems to be studied, since this avoided a possible error due to absorption of carbon dioxide from the air. The gelation times of a number of systems using various ratios of the suspension, 2.5 N potassium chloride, and distilled water were determined in order to find a ratio of reagents producing a system with desirable gelation times. A system having an average primary gelation time of 2 min. with a maximum variation of 2 min. and an average secondary gelation time of 20 min. with a maximum variation of 5 min. was made by diluting 200 ml. of the acidified bentonite suspension with 32.5 ml. of distilled water; then to 11.64 ml. of this suspension 0.37 ml. of 2.5 N potassium chloride solution was added drop by drop with shaking. The diluted acidified bentonite suspension used above contained 0.1233 g. of solid material per 10 ml. The pH meter was set to give correct readings by using a standard buffer, and the electrodes were thoroughly washed with distilled water. Immediately after the gel-forming mixture, prepared as outlined above, was made, 4 ml. of it was pipetted into a gelation vial. The glass and calomel electrodes were placed directly in the mixture and the pH readings were taken at specific times. After the material had been a gel for approximately 28 min., it was removed from the electrodes and liquefied by shaking. During the thixotropic liquefaction the electrodes were not treated in any way. They were replaced and the first reading during the secondary gelation wa9 taken 25 sec. after the gel was liquefied (the removing of the gel, shaking, replacing of the liquefied gel, and taking of the first reading required about 1 min.). The pH readings were continued until the material had undergone secondary gelation. The electrodes were not mashed off with distilled water nor the excess gel material removed in any mechanical way before a new sample was introduced. Contrary to the procedure used in investigating the other systems, the pH meter was not checked with the standard buffer to make sure that it had been giving correct readings until all the gelation experiments had been completed; it seemed that a treatment
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R. C. VOGEL AND hl. W. LISSE
similar to that used in the case of ferric hydroxide systems, xvhich was thought to remove the standard buffer adsorbed on the glass electrode, was not sufficient to give good results for the bentonite system. The data given in table 1 are representat’ive of three experiments performed on the bentonite system using the above method. The maximum variation in each group of three pH readings taken at a specific time after each experiment was started is 0.03 pH. DISCCSSION O F RESULTS
During the primary gelation the pH of a thorium molybdate system increased from 2.57 to 2.62, that of ferric hydroxide decreased from 4.50 to 4.40, and that of bentonite remained constant. During the thixotropic liquefactions and secondary gelations no changes in pH of any of the three systems were observed, using the glass electrode. Prakash and Dhar (8), using a thorium molybdate system also, obtained increases in pH (from 2.16 to 2.39) with a quinhydrone electrode during the preliminary processes of primary gelation x-hile the system still remained a sol. Their system was prepared by mixing 10 ml. of a thorium nitrate solution (48.1400 g. per liter) and 2 ml. of a potassium molybdate solution (12.0000 g. per liter). In the preparation of the authors’ system more potassium molybdate was used, so it seems reasonable that their system would be more alkaline than that of Prakash and Dhar (the final pH of the authors’ system vas 2.62, as compared with 2.39). However, Prakash and Dhar continued pH readings for only 80 niin., whereas the primary gelation time of their gel was 5 hr. and 30 min. Their last readings had been constant for 10 min. when they were discontinued. The data here reported, as vel1 as those of Prakash and Dhar, disagree nith those of Prasad and Desai (9) who, xith a quinhydrone elect,rode,also measured the pH changes of a thorium molybdate system during the early stages of primary gelation. They found the pH constant a t 2.96. Their system was prepared by mixing 5 ml. of a thorium nitrate solution (60 g. per liter), 1 ml. of potassium molybdate solution (100 g. per liter), and 4 ml. of 1%-ater. In the case of the ferric hydroxide system Prakash and Dhar (8) found an increase in pH during the early stages of primary gelation from 2.91 to 3.26. They prepared their system by mixing the following solutions: 6 ml. of M/’2 ferric chloride, 3.75 ml. of 2.84 N sodium acetate, 1.5 ml. of 4 N ammonium sulfate, and 0.45 ml. of 4.26 N ammonium hydroxide. The volume was made up to 30 ml. by adding water. Perhaps the difference in the direction of pH change during the primary gelations might be due to the fact that there were different ions in the two ferric hydroxide systems which were undergoing adsorption. SUMMARY
1. During the primary gelations ferric hydroxide and thorium molybdate gel-forming systems showed pH changes, but a bentonite system showed no pH change.
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2. During the thixotropic liquefaction there is no pH change within the limits of experimental error in the cases of a thorium molybdate, a ferric hydroxide, and a bentonite gel. 3. During the secondary gelation there is no change in pH within the limits of experimental error in the cases of a thorium molybdate, a ferric hydroxide, and R bentonite s>-steni. REFERESCES (1) BATCIIELOR, H. 5V.: J. Phys. Chcm. 42, 575 (1938). (2) BROUGISTON, G., A N D SQUIRES, L.: J. Phys. Chem. 40,1041 (1936). ( 3 ) FREUNDLICH, H . : Thirotropy, Actualit& scientifiques et industrielles, S o . 267 (1935). (4) G o o n E m , C. F.: Trans. Faraday SOC.36, 342 (1939). (5) HCRD,C. B., ASD GRIFFETH,R. L.: J. Phys. Chem. 39, 1155 (1935). (6) NCDOWELL, C. bl.,A N D USHER,F. L.: Proc. Roy. Soc. (London) A131, 564 (1931). (7) PRIKISH, s.: IColloid-2. 64, 293 (1933). (8) I’R.’IIL~SH, S.,.IXD DHAR,K. R.:J. Indian Chem. SOC.6, 391 (1929). (9) PRASAD, RI., AND DESAI,D. M.:J. Univ. Bombay 2, S O .7 , Part 3, 132 (1938), (10) P R ~ S A M., D , A N D HATTL~NGADI, R . R.: J. Indian Chem. SOC.6, 893 (1929) (11) USHER,F. L.: Proc. Roy. SOC.(London) A126, 113 (1929).
T H E SOLUBILITT OF SAPHTHALEKE IN AQUEOLS YOLUTIOSS OF NETHAXOI,, ETHAKOL, 1-PROPASOL, AND 1-BGTAKOL AT SEVERAL TEMPERATURES OTTO \Ir. MANNHARDT,‘ ROBERT E. D E RIGHT: WILFRID H. M14RTIN,J CHESTER F. BURMASTER,a AND WILLARD F. W.4DT4
Department of Chemistry, The University of Rochester, Rochester, New Y O T ~ Received April 14, 1948
In another research carried out in this laboratory (8) the solubility of naphthalene was determined in methanol, ethanol, 1-propanol, and 1-butanol (along with isomers of the last two) at several temperatures, and the effect of vvatcr on the solubility was determined for methanol and ethanol in the vicinity of 50’C. These last two experiments yielded results which indicated that further work was desirable; accordingly, very shortly thereafter about thirty runs were made on four different aqueous solutions of ethanol. -4n indication of a solubility gap was encountered with the solution containing about 82 per cent ethanol, and s.hundant evidence of such a gap was found for the 72 per cent ethanol solution. Further work seemed desirable uith ethanol, and it also appeared of interest to study the other aliphatic alcohols mentioned above from this and other points Present address: E. I. du Pont de Nemours and Company, Ino., Buffalo, New York. E. I. du Pont de Nemours and Company, Inc., Wilmington, Delaware. a Present address: Eastman Kodak Company, Rochester, New York. 4 Present address: Standard Oil Company of New Jersey, Bayonne, New Jersey. 1
* Present address:
