in this study. Further work on the identity of the colored species was not justified. The reproducibility of the oxidation product formation was sufficient for the analysis. The precision of the Ional determinations is shown in Table 111. Since polymers containing appreciable quantities of oxidized Ionol were not investigated, the effect of such products on the method is not known. ACKNOWLEDGMENT
The author gratefully acknowledges the helpful suggestions of Dean J.
Veal in the development of this analytical procedure. LITERATURE CITED
(1) Becconsall, J. K.; Clough, S., Scott, G., Proc. Chem. Soc. 1959,308. (2) Campbell, T. W., Coppinger, G. M., J . Am. Chem. Soc. 74,1469 (1952). (3) Cook, C. D., J . Org. Chem. 18, 261 (1953). (4) Cook, C. D., Nash, E.G., Flanagan, H. R., J . Am. Chem. SOC. 77, 1783 (1955). (5) \ - , Cook. C. D.. Woodworth. R. C.. Zbid,, 75, 6242 (1953). (6) Coppinger,. G. M., Campbell, T. W., Zbzd., 75, 734 (1953).
(7) Gersmann, H. R., Bickel, A. F., J. Chem. SOC.1959,2711. (8) Gersmann, H. R., Bickel, A. F., Proc. Chem. SOC.1957. 231. (9) Kharasch, M. S., Jdshi, B. S., J . Org. Chem. 22, 1439 (1957). (10) Spell, H. L., Eddy, H. L., AKAL. CHEM.32,1811 (1960). (11) Yohe, G. R., Dunbar, J. E., Pedrotti,
R. L., Scheidt, F. M., Lee, Fred G. H., Smith, E. C., J. Org. Chem. 21, 1289 (1956).
(12) Ibid., 24, 1251 (1959). (13) Yohe, G. R., Hill, D. R., Dunbar, J. E., Scheidt, F. M., J . Am. Chem. SOC. 75,2688 (1953).
RECEIVED for review December 15, 1961. Accepted April 4, 1962.
Potentiometric Studies of the Titration of Weak Acids with Tetra butylammonium Hydroxide LELAND W. MARPLE and JAMES S. FRITZ Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa ,The sources of amine, carbonate, and silver impurities in tetrabutylammonium hydroxide have been investigated, and techniques for their removal have been evaluated. The stability of the base in water, isopropanol, tert-butyl alcohol, and pyridine was determined. Salt bridge systems of the type
1
I
SCE aqueous phase MCL-HzO-organic organic phase MCL-HsO-organic have been devised for acetone, isopropanol, fert-butyl alcohol, and pyridine. Titrations of weak and very weak acids using the glass indicating electrode were reproducible to within 2 to 5 mv.
T
HERE are a t least two major advantages in using tetrabutylammonium hydroxide as a titrant in nonaqueous solvents. One is that the salts formed, in contrast to the salts of alkali hydroxide or alkoxides, are soluble in low dielectric media. The other advantage is that excellent potentiometric curves can be obtained with an ordinary glass electrode. Tetrabutylammonium hydroxide titrants can be prepared by two methods. Harlow, Noble, and Wyld (4) prepared the hydroxide in isopropanol by anion exchange starting with the iodide salt and obtained a 0.2N titrant with only 0.5% water. The titrant was not stable and decomposed to give a weak base at the rate of 1% in 6 weeks. Harlow aud Bruss (3) later prepared a 1.5.44 titrant by evaporation of iso-
796
ANALYTICAL CHEMISTRY
propanol from a 0.2M solution prepared by ion exchange. Their data do not show whether impurities were present in the titrant, or if the concentrated base solutions decomposed over a period of time. Cundiff and Markunas (1) were the first to prepare anhydrous tetrabutylammonium hydroxide in methanol by the silver oxide process. Starting with the iodide salt, they prepared an equimolar mixture of hydroxide and methoxide in methanol, and then diluted with benzene to give a titrant containing 10% methanol. The base in this solvent was stable for a t least 60 days. Because of the acidity of methanol, it is desirable to reduce the methanol content to 5% or lower. Pritchett (8) found that when this was done, the solution of base decomposed slightly within 1 month, even when stored under nitrogen and protected from light. Previous attempts to prepare anhydrous hydroxide solutions in isopropanol have been both successful and unsuccessful. Harlow and Bruss (3) reacted the iodide salt with silver oxide in isopropanol and prepared a 1.5M titrant. Malmstadt and Vassallo (7) prepared a similar hydroxide, methyltributylammonium hydroxide, in isopropanol without any apparent difficulty. However, Hummelstedt (6) found that when the oxide was added to an isopropanol solution of tetrabutylammonium iodide, a semicolloidal suspension formed that continued to settle for days despite repeated filtrations. I n spite of rigid exclusion of carbon dioxide during the preparation of
tetrabutylammonium hydroxide by the silver oxide process, preparations made in this laboratory frequently contained 1 to 2y0 impurity as a weak base. Cundiff and Markunas (I) noted that their preparations contained severe1 per cent impurities and suggested that the impurity might be tributylamine. However, addition of tributylamine to a strong acid solution did not alter their results for the titration of the acid with pure tetrabutylammonium hydroxide (prepared by ion exchange) in pyridine solvent. Because the impurity was removed by anion exchange, it now seems evident that it was indeed carbonate. However, the fact that tetrabutylammonium hydroxide can undergo Hofmann elimination to give tributylamine, 1-butene, and water should be taken into account when the stability of the base is considered. The instability of the base in 5% methanol-95yo benzene solution (8) and o.5y0water-99.5% isopropanol solution (4) was undoubtedly a result of the Hofmann elimination. The goals of this research were to avoid decomposition of the base when prepared by the silver oxide process, to evaluate methods for removing both carbonate and tributylamine, and to develop an electrode system that would give reproducible potentiometric titration curves in very weakly acidic solvents. Previously, the calomel electrode used was modified by replacing the aqueous saturated potassium chloride solution by methanolic saturated potaasium chloride. While good potentiometric curves for weak acids were
Figure 1 . Titration assembly for use in tert-butyl alcohol solvent a.
b. c.
Saturated potassium chloride in water Aqueous phase, 2-phase mixture HzOterf- butyl alcohol-KCI terf-Butyl alcohol saturated with tetrabutylammonium bromide
obtained, they were frequently displaced on the potential scale by as much as 50 mv. The main source of the discrepancy was believed due to a difference in junction potential between the solution and the modified calomel electrode. Since i t was impossible to use saturated solutions of tetrabutylor tetramethylammonium salts in the construction of a calomel electrode because of the decomposition of mercurous chloride, the only alternative was to develop a salt bridge system that avoided the problem of crystallization of salts at the junction of the consecutive salt solutions in the salt bridge.
general purpose glass electrode for the indicating electrode. Sleeve-type calomel electrodes were used in the construction of the reference electrodes. Titrations in tert-butyl alcohol were carried out in a 250-ml. electrolytic beaker equipped with a side arm attachment connecting the reference electrode. Figure 1 shows the titration vessel and reference electrode system. Titrations in pyridine, acetone, and isopropanol were also made in a 250-ml. electrolytic beaker, but the side arm attachment containing the reference electrode and salt bridge were modified as shown in Figure 2. Tetrabutylammonium hydroxide titrant was dispensed from a hlachlett type automatic buret with Teflon stopcocks. The procedure for preparing a pure 0.1M tetrabutylammonium hydroxide titrant is the following. Slurry 23 grams of purified silver oxide with 130 ml. of a 75% methanol-25'% water solution in an ice bath a t 0" C. Add, slowly, 32 grams of tetrabutylammonium bromide in 35 ml. of pure methanol, and stir for 10 to 15 minutes. Filter the base solution through a coarse sintered glass frit into a flask containing 0.5 gram of activated charcoal, mix well, and let settle for several hours. Filter the base solution through a fine sintered glass frit into a flask containing a magnetic stirring bar. Remove the methanol by evaporation a t a pressure of 25 mm. of Hg. Transfer the aqueous solutioa to a 250-ml. graduated addition funnel, and dilute to 200 ml. with boiled distilled water. Add 30 to 40 ml. of
i;;
i
I
W
C 0
Figure 2. Reference electrode and salt bridge system Pyridine solvent a. Sat'd N a C l b, c. 2-Phare,sat'd MerNCl Acetone solvent a. Sat'd NaCI-SCE b. 2-Phare,sat'd N a C l c. 2-Phase, rat'd MerNCl
nuoI~ TIM
Figure 3. Titration of tetrabutylammonium hydroxide solutions A.
E. C.
Solution plus added carbon dioxide Solution only Solution treated to remove carbonate
pure benzene, shake vigorously, and let the two phases separate completely. Pass the aqueous solution through a column containing 8 to 10 grams of strong base anion exchange resin in the hydroxide form, and wash with 20 ml. of boiled distilled water. Collect the eluate in a 500-ml. Erlenmeyer flask, protecting the eluate from carbon dioxide. Distill most of the water from the base a t a pressure of 20 mm. of Hg until the crystalline hydrate is formed. When titrant is needed, distill water from the crystalline hydrate until the vapor pressure is between 7 to 10 111111. of Hg. The resulting solution will be approximately 2.44. Dilute to 1 liter with a mixture of 20% isopropanol80% benzene. DISCUSSION
EXPERIMENTAL
All reagents, except silver oxide, were reagent grade or the equivalent. Silver oxide was obtained from both Fisher Scientific Co. and Baker and Adamson in purified form. Tetrabutylammonium bromide and iodide were obtained from both Eastman Organic Chemicals and Southwestern Analytical Chemicals. Activated charcoal was from Mallickrodt Chemical Works, and the ion exchange resin used was Rohm and Ham Amberlite IRA 401. Potentials were measured with a Beckman Model G p H meter using a
4
OF RESULTS
Determination of Impurities in Tetrabutylammonium Hydroxide. The silver oxide process was used exclusively to prepare the hydroxide titrant. Three impurities are likely to be present in any titrant prepared by this process-tetrabutylammonium carbonate, tri-n-butylamine, and silver in anionic complex form. When purified silver oxide was used as a starting material, carbonate was found in the titrant as a result of silver carbonate present in the oxide. The presence of silver carbonate in silver oxide was verified by x-ray powder diffraction analysis. Chemical analysis by decomposition of the carbonate with perchloric acid showed that the commercial oxides contain from 0.08% to 0.2195 by weight carbon dioxide, depending upon the source. Heating a t 230" C. for several hours failed to VOL 34, NO. 7, JUNE 1 9 6 2
797
remove the bulk of the carbonate. Since an excess of silver oxide is generally used in the silver oxide reaction, the amount of carbonate found in the titrant from this source alone may be as much as o.570. Added to what may be introduced from exposure of the titrant to air, as much as lYOcarbonate may be in the final titrant. Figure 3, Curve B, shows the titration of a sample of tetrabutylammonium hydroxide known to contain some carbonate with 0.14W perchloric acid in tert-butyl alcohol solvent. Potential sign convention is that recommended a t the Stockholm Convention of IUPAC. Evidence that the impurity found is indeed carbonate is shown by Curve A , where carbon dioxide was intentionally added to a sample before titration. The difference in total titer is not due to the addition of carbon dioxide, but to a different amount of sample taken. Curve C shows the potential break for a hydroxide solution containing less than 0.35% carbonate that was prepared by ion exchange treatment of material prepared by the silver oxide process. Tri-n-butylamine impurities may come from two sources. One is from impure tetrabutylammonium halides. One lot of the iodide salt contained 0.4% impurity that we suspect was tri-n-butylamine. The amine may also be introduced by decomposition of the hydroxide under certain conditions. Figure 4, curve A , shows the titration curve of a solution of tetrabutylammonium hydroxide, containing some carbonate, in tert-butyl alcohol. Curve B shows the titration curve for a solution of tetrabutylammonium hydroxide in tert-butyl alcohol that was allowed to stand 1 day a t room temperature. The extra inflection indicates the presence of the amine impurity. The other decomposition product, 1-butene, distills a t room temperatures, and is thus not found in the solution. The third common impurity, that seems to be minor in its effect on potentiometric titrations, is silver in the anionic complex form. These complexes originate from the reaction of tetrabutylammonium salts with the silver halide precipitated on the surface of the silver oxide particles. The extent of complex formatiox depends upon the method of addition of reagents, the solvent system employed, and the particular halide used. The formation of anionic complexes was confirmed by shaking approximately 0.5 gram of silver oxide with 15 grams of tetrabutylammonium iodide in absolute methanol and finding that the oxide completely dissolved. Since the reaction of the iodide with the oxide tQ form the base is not exceptionally fast, it is very easy to establish an excess of iodide in the preparation of 798
ANALYTICAL CHEMISTRY
I
f
I
I
+
I
-I-
i i
YlLYLIlERS nw, mRwT
Figure 4. Decomposition of tetrabutylammonium hydroxide solutions A. 6.
Solution known to contain some carbonate Solution after slight decomposition
the base unless the iodide salt is added very slowly to the reaction mixture. The removal of carbonate from the hydroxide titrant can be accomplished either by recrystallization from aqueous solution or by passing an aqueous solution of the base through a column of strong base anion exchange resin. If the titrant is in pure methanol, water can be added to the extent of approximately 20 times the base content, and the methanol can be pumped off at a pressure of 25 mm. Hg. Crystallization of solid, hydrated tetrabutylammonium hydroxide can then be carried out by pumping a t a pressure of 20.2 mm. The crystals are colorless, melt between 28' and 30' C., and contain considerably less carbonate than the mother liquor. This method is not recommended because .of the large loss of hydroxide accompanying the removal of the mother liquor from the crystals. Purification by ion exchange (3) consistently gives a solution with less than 0.3% carbonate, and is the most economical method of carbonate removal. The elution is carried out in a predominately aqueous solution to minimiie the possibility of decomposition of the exchange resin. The removal of amine impurities is more difficult. Ion exchange, as would be expected, is of no value. Extraction of the amine with benzene from a completely aqueous solution of the hydroxide gives the best results. Titrants with less than 0.1% amine impurity can be prepared in this way. Preparation of tetrabutylammonium hydroxide in methanol, starting with the bromide salt, yields a solution
containing significant amounts of complexed silver. When the preparation is carried out in methanol containing 25 to 50% water, there is little or no anionic silver present in the final base solution. What complexed silver remains may be removed by treatment with a small amount of activated charcoal. While it is known that only a small amount of complexed silver is removed by charcoal, the use of charcoal in the preparation of the base is warranted, especially if the procedure for the removal of carbonate by anion exchange is not performed. Stability of Titrants in Common Solvents. Methanol, isopropanol, methanolbenzene, or isopropanol-benzene mixtures are the solvents commonly used in preparing titrants. For this reason, the greatest amount of attention has been given to these solvents. To prepare a 0.1Jf solution of the base in 5% methanol-95% benzene solvent, it is necessary to prepare a 2M solution of the base in methanol. Unfortunately, tetrabutylammonium hydroxide is not stable a t this concentration a t room temperature for any length of time. In one case, a 2.2M solution decomposed 6% in less than 4 days. Thus, unless dilution with benzene is made immediately after preparation, significant decomposition will occur. The dilute 0.1M base is stable for 2 weeks, generally, but amine impurities do show up within 1 month. From this, it seems improbable that a dilute base solution with less than 5% methanol can be made and kept completely pure for any length of time. Water is the only solvent in which the hydroxide is completely stable. Dilute 0.1M solutions of the hydroxide in isopropanol-benzene solvent prepared by the addition of 2M base in water to 20% isopropano1-80% benzene mixture show no increase in amine impurity for approximately 2 weeks. These solutions are as good as 5% methanol-95% benzene solutions as far as the quality of titrations is concerned. Reduction of the water content of the 2M aqueous solution by the addition of isopropanol and codistillation of water and isopropanol a t a pressure of 14 mm. of Hg gives a solution that decomposes extensively within 1 day. Isopropanol solutions prepared by the addition of the solid hydrate to isopropanol show no signs of decomposition in a period of 1 month, but such solutions contain too large a percentage of water for the titration of very weak acids. Dilute 0.1M solutions of the base in n-propanol, dioxane, pyridine, ethylene glycol, cyclohexanol, and bis (2-ethoxyethyl) ether, prepared from 21M aqueous base all decompose significantly within 1 day. Solutions in dioxane and
pyridine decompose and turn redbrown n-ithin 10 minutes. Solid tetrabutylammonium hydroxide hydrate produced by evaporation of water a t 20 mm. of Hg has been kept for 3 months without the slightest indication of decomposition. Likewise, the 2M aqueous solution of tetrabutylammonium hydroxide, prepared by distillation of water a t a pressure of 7-10 mm. of Hg, was stable for over a month. For storage, we recommend that the hydroxide be kept in the solid hydrate form. The solid is very readily converted to the 2M base, so it is only a slight inconvenience to perform this step prior to dilution to the desired normality.
4 - p \
YM-
E ---
POTENTIOMETRIC TITRATIONS
tert-Butyl Alcohol Solvent. Titrations in tert-butyl alcohol were performed using the glass electrode and a calomel reference electrode with the following salt bridge: SCE aqueous phase HzO-tert-butanol-KC1 organic phase tert-butanol H20-teit-butanol-KCl sat'd Buc?r'Br
I
1
1
The aqueous layer analyzed 4.32M KCl. The lighter butanol layer analyzed 0.056M KC1 and contained approximately lOyo water. Ultrafine sintered glass frits were used to separate the butanol layer and the solution being titrated (see Figure 1). Except for occasional adjustment of the liquid levels, the reference electrode was virtually trouble free. The reproducibility of the glasselectrode-reference-electrode system in tert-butyl alcohol was determined in two ways. First, potential measurements were made as increments of a dilute perchloric acid solution were added to a measured volume of solution. Table I shows the results of three runs on three consecutive days, and a run 1 month later. On consecutive days, the reproducibility was within 1 mv., and over an extended period of time, within 2 mv. Results of similar type experiments using equimolar mixtures of benzoic acid-tetrabutylammonium benzoate, p-nitrophenol-tetrabutylammonium p-nitrophenolate, and phenoltetrabutylammoniumphenolate were as good as those for perchloric acid. Secondly, replicate titrations of ophenylphenol and 2,4dinitrophenol with tetrabutylammonium hydroxide were made, and the potentials were compared a t various stages of the titration. In the case of four replicate titrations of o-phenylphenol, the same curve was obtained each time with only slight differences in potential between runs in the presence of excess base. This difference was probably due to minor differences in the condition of the glass matrix of the indicating
I
0
I
I
2
I
3
I
4
I
5
MILLILITERS REAGENT A W E D
Figure 5. Effect of solvents on potentials in tert-butyl alcohol solution-0.0003M tetrabutylammonium hydroxide A.
B. C. D. E.
Ethylene glycol Water Methanol Isopropanol-0.005M Water
perchloric acid
electrode. Such deviations might make a considerable effect on the potential of the electrode in solutions where. the activity of hydrogen ion was so extremely 10-7. For 2,Pdinitrophenol, the titration curves were identical within 2 mv. over the entire curve. The addition of water to twt-butyl alcohol solutions decreases the available potential range for titration. Potentials in basic solution are affected much more than those in acidic solution. The effect of the addition of water ( B ) , ethylene glycol ( A ) , methanol (C), and isopropanol (D)on the potential of a glass electrode in 50 ml. of a 0.003M tetrabutylammonium hydroxide solution is shown in Figure 5 . The fact that water does not shift potentials much more positive than methanol explains why the titrant prepared from aqueous base solutions gives good titrations for very weakly acidic com-
Table 1.
I
1
Hg HgzClz, KCl aqueous phase
I
sat'd H20-Py-Me4NC1 organic phase pyridine H20-Py-Me4NC1 NH,SCN
1
Potentials of Glass Electrode in Perchloric Acid Solutions in tert-Butyl Alcohol
Milliliters 0.119M HCIOl in 50 Ml. Solvent
a
pounds. Since isopropanol has such a small effect on the potential of the glass electrode, the presence of a relatively large amount of isopropanol in the titrant has no detrimental effect on the titration of very weak acids. These results are in agreement with those reported by Harlow and Wyld (6), who studied the effect of alcohols on the titration curves of very weak acids. Water has a lesser effect on the potential of the glass electrode in a 0.005M perchloric acid solution as is shown in Figure 5. Although the potential change is not as large for acid solutions as for basic solutions, it still must be taken into account when performing potentiometric titrations in this solvent. Pyridine Solvent. It is possible t o obtain a two-phase system of pyridine and water by saturation of a mixture with potassium chloride. However, this system is unsatisfactory in that the pyridine phase contains a considerable amount of water and dissolved potassium chloride. When the pyridine comes in contact with the relatively water-free pyridine solution containing the sample titrated, potassium chloride precipitates out a t the junction. In addition, enough potassium chloride and water leak through the ultrafine frit in a period of 15 to 20 minutes to cause a positive shift of potentials in the titration of very weak acids (such as phenol). Although water was known to produce positive shifts, there is little information available on the effect of potassium (or sodium), other than the effect on the titration of phenol as reported by Harlow (2). Consequently, we attempted to determine how much potassium ion is needed to introduce a noticeable effect on the titration curves for phenol, o-bromophenol, and o-nitrophenol. Figure 6 shows the titration of phenol in pyridine using a saturated potassium chloride calomel reference electrode and the following salt bridge system :
Run 1
0.379 0.25 0 404 0.75 1.50 0 417 2:oo 0 420 4.00 0 430 0 434 6.00 10.00 0.440 Made after one month.
Electromotive Force Run 2 Run 3 0.379 0 403 0 416 0 420 0 429 0 432 0.431
0.384 0 405 0 416 0 420 0 428 0 432
RunT 0 380 0 402 0 415 0 419 0 426 0 430 0.433
VOL. 34, NO. 7,JUNE 1962
799
The pyridine phase of the two-phase mixture analyzed 0.014M MeaNC1, and the aqueous phase of 6.42M Me4NC1. For the aqueous phase, this is only a slight reduction in total salt content compared to a purely aqueous solution (6.771cl). The solution of ammonium thiocyanate in pyridine was included in the salt bridge to avoid the possibility of crystallization of potassium chloride at the junction as potassium ion was added to the sample solution. The curve shown by the open squares shows the effect produced by adding 1.0 ml. of a 0.66M solution of potassium hydroxide to 50 ml. of a pyridine solution containing 0.4 meq. of phenol, after approximately one-fourth of the phenol is titrated. This effect can be explained on the basis of the transference of potassium ion through the membrane of the glass electrode. The fact that the effect of added potassium ion is much less for a blue tip Beckman glass electrode than a general purpose electrode, when compared in the same solution, rules out the possibility of a change in junction potential at the ammonium thiocyanate-solution interface. Similar experiments with o-bromophenol and o-nitrophenol indicated that, as the level of acidity is increased, the effect of potassium ion is much less, For o-nitrophenol, the shift is only 20 mv. However, when sufficient base is added to establish an excess, there is still an apparent acidity, owing to the presence of potassium ion. The ordinary saturated potassium chloride calomel electrode can be used for short periods of time, but potassium chloride will slowly crystallize out where the saturated KCl solution comes in contact with the saturated tetramethylammonium chloride solution. Replicate titrations of phenol on a short term basis gave curves that differed by only 1 or 2 mv. If a salt bridge made from the aqueous layer of the two-phase system of water-pyridine-sodium chloride is inserted between the reference electrode and the aqueous saturated tetramethylammonium chloride solution, and the saturated potassium chloride solution in the reference electrode is replaced by a saturated solution of sodium chloride, then no crystallization occurs a t any junction. The following salt bridge system was satisfactory for over a period of 1 month:
I
Hg HgzClz, KC1 sat’di aqueous phase HzO-Py-NaCl
1
NaCl sat’d aqueous phase HnO-Py-MerNCII organic phase H~0-Py-Me4hTCl
I
I
The reproducibility of the above system was evaluated by replicate titrations of both phenol and benzoic 800
ANALYTICAL CHEMISTRY
-2001
I I I I I 2 3 4 MILLILITERS TITRANT, TETRABUTYLAHMONIUM HYDROYIOE IN BENZENE ISOPROPANOL SOLVENT
I
0
Figure 6. Titration of phenol in pyridine with tetrabutylamrnonium hydroxide in benzeneisopropanol 0
Complete titration of phenol Resumption of phenol titration with tetra butylammonium hydroxide Titration with tetrabutylammonium hydroxide up ‘/4 e.p., followed b y additjon of 0.66M potassium hydroxide solution
0
acid. Unfortunately, the results were not as good as expected, particularly in the case of benzoic acid, as there was generally a 3- to 5-mv. difference between potentials at the same stage of titration. Acetone Solvent. Titrations of acids in acetone were made n-ith the following reference electrode-salt bridge system:
I
Hg HgZC12, NaCl aqueous phase sat’d HzO-acetone-Mec”C1~ organic phase HzO-acetone-Me4XC1
!
Up to a period of 1 month, no precipitate formed at any of the junctions. The aqueous phase analyzed 5.84M tetra-
methylammonium chloride. The titration of p-toluenesulfonic acid was examined because of the exceptional behavior noted by Van der Heijde (9). We found, however, that titration curves were smooth and reproducible to within 2 to 3 mv. over almost all of the titration curve. Further, there were no spurious changes in potential during a potential measurement. The reproducibility was also checked by replicate titrations of formic acid and benzoic acid In each case, the titration curves n-ere reproduced to within 2 to 5 mv. Isopropanol Solvent. Titrations i n isopropanol were made with the
follolving reference bridge system:
electrode-salt
Hg HgLClz,XaC1 I aqueous phase sat’d I HzO-IPA-XaCl/ aqueous phase organic phase H20-IPA-Me4NC1 H20-IPA-Me4XCl
1
I
Duplicate titrations of phenol and benzoic acid were made with 0.146M tetrabutylammonium hydroxide, and, in each case, the curves were reproduced to within 2 mv. LITERATURE CITED
( 1 ) Cundiff, R. H., Markunas, P. C., ANAL.CHEM.30, 1450 (1958). (2) Harlow, G. A., Ibid., 34, 148 (1962). (3) Harlow, G. A., Bruss, D. B., Ibid., 30, 1833 (1958). (4) Harlow, G. A., Noble, C. M., Wyld, G. E.A., Zbid., 28,784 (1956). (5) Harlow. G. A.. Wvld. “ , G. E. A., Zbid., ‘ 30, 73 (1958). ( 6 ) Hummelstedt, L..E. I., Ph.D. thesis, I
Massachusetts Institute of Technology,
Cambridge, Mass., 1959. (7) Malmstadt, H. V., Vassallo, D. A., ANAL.CHEW31,862 (1959). (8) Pritchett, J. W., M.S. thesis, Iowa State Universitv of Science and Technology, Ames, Ibn-a, 1960. (9) Van der Heijde, H. B., Anal. Chim. Acta 16, 392 (1957). RECEIVEDfor review October 13, 1961. accepted A ril 2, 1962. Contribution S o . 1077. bork was performed in the
-4mes Laboratory of the U. S. Atomic Energy Commission.
