areas would be less than the geometric area. Although it might be supposed that the value of the effective electrode area approached would be 25% of the geometric area, there is no reason to expect that the carbon should be arranged a t the interface in a manner providing an electrochemical area equal to the weight fraction of carbon used in electrode construction. If a roughness factor of 2 (not a totally unrealistic value) was assumed, a value of 25% of the geometric area would be approached. However, it might \Tell be that the value of 5ooj, of the geometric area obtained with a smooth polish is fortuitous, and characteristic of the particular carbon-epoxy mixture. It seems obvious that the centrifugation procedure used in the fabrication of the electrode would serve to concentrate the carbon a t the bottom end of the tube. Hence, the fraction of carbon in the disk, which was taken from the lower end of the centrifuge tube, would undoubtedly be greater than that present in the initial 25 wt. % mixture. In practical application, the electrode should be calibrated. As is usually the case when working
with most solid indicating electrodes reproducing the character of the electrode surface from run to run can be a problem. The general shapes of both the voltammograms and chronopotentiograms can be distorted by subjecting the electrode to extremes of potential. However, the reversible appearance of the waves could usually be restored by lightly polishing the surface; a t times it was necessary to remove the electrode from solution, lathe off a few millimeters of surface, and then repolish.
greater than +0.80 volt, the scan direction and magnitude are important considerations. The reader should be cautioned about using this electrode in highly oxidizing or reducing media; it is only reasonable to assume that under appropriate conditions the bonds formed in the polymerization of the resin could be attacked and thus introduce anomalous electrode behavior.
CONCLUSIONS
(3) Delahay, P., J. Am. Chem. SOC.76,
Under the conditions of the present work, the carbon-epoxy indicating electrode performed in a manner consistent with what might reasonably be expected of any solid electrode. From all available data it appears that epoxy resin does serve satisfactorily as an inert binder for the conducting carbon, and in the practical application of the electrode does not produce anomalous behavior. Because of the previously discussed surface complication due to poising the electrode a t potentials
LITERATURE CITED
(1) Adams, R. N., Prater, K. B., ANAL. CHEM.38. 153 11966). (2) Beilby, ’A. L.; Mather, B. R., Ibid., 37, 766 (1965).
874 (1954). (4) Levich, V., “Physicochemical Hydrodvnamics.” Prentice-Hall. New York. 1562.
’
(5) Lingane, J. J., “Electroanalytical Chemistry,” Interscience, New York, 1958. (6) Miller, F., Zittle, H., Mamantov, G., Freeman, D., J . Electroanal. Chem. 9, 305 (1965). (7) Parsons, R., “Handbook of Electrochemical Constants,]’ New York Academic Press, 1959. (8) Reilley, C., Scribner, W., ANAL. CHEM.27, 1210 (1955). for review January 27, 1966. RECEIVED ACCEPTEDApril 15, 1966.
Acidity of Aromatic Sulfonic Acids and Their Use as Titrants in Nonaqueous Solvents DONALD J. PIETRZYK and JON BELISLE Deparfment o f Chemistry, University of lowo, Iowa City, Iowa Twelve aromatic sulfonic acids and several inorganic acids were titrated potentiometrically with basic titrants in nonaqueous media. The data were used to show the acid strength of the acids in methyl isobutyl ketone and glacial acetic acid. Titration characteristics in other solvents are also reported. A straight-line correlation between half-neutralization potential and the sigma value for p- and m-substituted benzenesulfonic acids was found. 2,4 Dinitrobenzenesulfonic acid, which is a solid, readily obtained, readily purified, and shown to be close in acid strength to HC104, was evaluated as a titrant for bases. Data for the titration of a wide variety of bases in glacial acetic acid, acetonitrile, and chloroform ore reported.
-
A
SUCCESSFUL nonaqueous
titration of bases will depend, in addition to selection of a suitable solvent, on the strength of the acidic titrant. Perchloric acid, one of the strongest acids
known, has been the one most frequently used. Although other common inorganic acids can be used as titrants, their acid strengths are less by varying degrees. An indication of their strength in comparison to HClOd in methyl isobutyl ketone is reported by Bruss and Wyld ( 1 ) . Of the acidic organic compounds, the sulfonic acids are strong acids in water. Most of the work with this type of acid in nonaqueous solvents has been with benzene, naphthalene, p-toluene, and simple aliphatic sulfonic acids. Caso and Cefola (3) studied the stoichiometry and general characteristics of the potentiometric titration of potassium acid phthalate with the above acids in glacial acetic acid. A variety of alkaloids in dichloromethane and chlorobenzene were determined by visual titration with p-toluenesulfonic acid ( 7 ) . Numerous other applications have been reported (6, 7, IS, 14). Acidity function, Ho, values for several other sulfonic acids were determined by Smith and Elliott ( l a ) . Other sulfonic acids studied as titrants were fluorosulfonic
acid in acetic acid (IO,11) and alcohols (9) and trifluoromethane sulfonic acid in acetic acid (8). This work was begun to evaluate a wide variety of substituted aromatic sulfonic acids. One of these, 2,4-dinitrobenzenesulfonic acid, which appeared to be close in acid strength to HC104, was studied as a strong acid titrant for the titration of bases. In the course of these potentiometric titration measurements, the acid strength of 12 aromatic sulfonic acids was determined and compared to several inorganic acids. To the author’s knowledge, measurements of this type of strong acids of similar structure have not been reported before. EXPERIMENTAL
The inorganic reagents and organic solvents were obtained in good grades from readily available sources. All solvents were used as received except glacial acetic acid, distilled from acetyl borate, and acetonitrile, distilled from PzOs.Methyl isobutyl ketone, Eastman Chemical Reagents.
VOL. 38, NO. 8, JULY 1966
969
Table 1.
Potential Break (f0.5-MI. End Point) for Titration of Amines in Several Organic Solvents with Sulfonic Acids
Solvent
Diphenylguanidine (10.1)"
Acetic acid Acetonitrile Nitrobenzene Chlorobenzene Methylene chloride Ethyl acetate Acetone Methyl isobutyl ketone lOyo2-propanolbenzene
160 510 500 * 460 540 350 430 360
Acetic acid Acetonitrile Nitrobenzene
150 490 490
370
Base pChloroImidazole aniline (7.0) (3.98) Potential break, mv.
p-Aminoacetophenone (2.70)
2-Naphthalenesulfonic Acid 160 145b 100; ... 31OC Insol. . . .d 340 * 15OC 200b 35c 230 30 300b 90
...
d
* , .d
pToluenesulfonic Acid 150 120 260
300
. . .d . . .d
2,4-Dinitrobenzenesulfonic Acid 210 170
210 Acetic acid 11OC 580 350 Acetonitrile 640 430 . . .d Nitrobenzene a pK, in water. b Precipitate formation with little effect on titration curve. c Preciaitate. when it forms, causes a shift in titration curve. d Preci'pitati formation prevents titration,
white label, was shown to be pure by gas chromatography and contained less than 0,02y0 water by Karl Fischer titration. Benzenesulfonic acid and p-hydroxy, p-methyl, p.-bromo, p-chloro, p-nitro, and 2,4,6-trinitro derivatives were obtained from Eastman Organic Chemicals. llesitylenesulfonic acid and m-nitrobenzenesulfonic acid were obtained from Aldrich and Antara Chemical Co., respectively. Pfister Chemical and Eastman were the sources of 2,4-dinitrobenzenesulfonic acid. p-illethoxybenzenesulfonic acid was made from the p-hydroxy derivative ( 2 ) . The p-cyano derivative was prepared by converting sulfanilic acid to the diazonium compound and then carrying out a Sandmeyer reaction. Elemental and infrared analyses were favorable. A detailed procedure will be published elsewhere. The procedure for purification depended on the quality of the sulfonjc acid. Since the main impurity is HzS04, initial tests included addition of Ba(I1) t o an aqueous solution of the sulfonic acid. The amount of HzS04 was determined by the difference in an aqueous titration of the sample, where HZS04titrates dibasically, and a nonaqueous titration in acetic acid, where it titrates monobasically. A general procedure for the purification of the sulfonic acids, which was determined by the qualitative tests, was first to treat an aqueous solution of the sulfonic acid with an excess of a Ba(0H)Z solution. The solution was filtered and passed through a column of Dowex 50W-X8, H-form, 100- to 200mesh resin. Water was then removed in a rotary evaporator under aspiration
970
ANALYTICAL CHEMISTRY
65 .
t
.
d
6OC ... 85~
100 10
20b
* . .d
50
. . .dd ... 90
.35. . d
and steam bath temperatures. The moist crystalline mass was transferred and dried in a vacuum desiccator containing PzO~ and was stored over Pz05. In a few instances the initial aqueous solution of the sulfonic acid was highly colored and was then treated with decolorizing charcoal. If a large concentration of HzSOp was present, the solution was initially treated with excess NaOH solution. Recrystallization of the sodium salt, which is much easier than the free acid, was carried out from alcohol-water, and the salt was dissolved and then treated with Ba(OH)z as described. The resin removes excess Ba(OH)z and converts the sodium salt of the sulfonic acid to the free acid. On the other hand, Eastman p-toluenesulfonic acid was found to be highly pure as the monohydrate, free of sulfate, and was used as received in many of the amine titrations. Tetrabutylammonium hydroxide was obtained from Eastman as a 25y0 solution in methanol. Tetramethylguanidine from American Cyanamid was distilled, and diphenylguanidine and imidazole from Eastman were used as received. All other amines and alkaloids were obtained from readily available sources and purified in some cases. Instruments. The potentiometric titrations were measured with the Precision Scientific Dual Titrometer. Preliminary data and data in Table I were collected by employing a glass electrode and a sleeve type calomel electrode containing aqueous saturated KC1. All other potentiometric data reported were collected with the Beckman combination electrode with the reference portion containing 2propanol saturated with KCI.
End points in Karl Fischer titration were detected amperometrically with dual platinum electrodes. Procedure. The purity and waters of hydration for the sulfonic acids were established by a combination of aqueous and nonaqueous potentiometric titration with a basic titrant and Karl Fischer titration. The procedure for determining halfneutralization potentials was to weigh out the sulfonic acid in a dry box, correct the weight for water content, and dissolve the acid in a tall-form electrolytic beaker or 50-ml. beaker with 30 ml. of the respective solvent. The basic titrant was made by dissolving the base (usually 0 . W ) in the solvent. Electrodes were inserted and the titrant w&s added from a 10-ml. buret under a blanket of Nz. The data were graphed, and the midpoint of the titration was determined from the graph. Equal equivalents of the sulfonic acids were weighed out in order to minimize the effects on HNP that would occur for different end point volumes (14). A similar procedure was used for the potentiometric titration of the amines and alkaloids with the sulfonic acids, Certain alkaloids were titrated visually with dimethyl yellow, p-dimethylaminobenzene, as indicator ( 7 ) . The acidic titrants were standardized against diphenylguanidine or potassium acid phthalate and the basic titrants were standardized against p-toluenesulfonic acid in their respective solvents. All data reported represent an average of a t least two individual measurements. DISCUSSION
Relative acidity (basicity) of a series of compounds in nonaqueous media can be determined by potentiometric titration with a basic (acidic) titrant. In this method the extent of the potential break for a series of acids (bases) is compared, or half-neutralization potentials, HNP, for the acids (bases) are plotted against pK, (water). The latter technique has been used to determine the acidity of substituted benzoic acids and phenols in pyridine and organic bases in glacial acetic acid, acetonitrile, nitromethane, and acetic anhydride (4). However, plots of HNP against pK, (water) do not always result in a linear relationship. Therefore, the assumption that aqueous dissociation constants are also a relative measure of the acidity of the acids in nonaqueous media is not always correct. In certain cases the structural effects become more or less pronounced when the compound is dissolved in nonaqueous media and consequently hinder the predictions. Examples of these are the ortho substituent, steric, and ion association effects. On the other hand, pK, (water) is not readily available for strong acids and plots of HNP-pK, (water) cannot be made. If the strong acid is such that its acidity is
ML BASE
Figure 1. Potentiometric titration with diphenylguanidine in methyl isobutyl ketone A. B. C.
D.
Mesitylenesulfonic acid p-Toluenesulfonic acid p-Nitrobenzenesulfonic acid 2,4-Dinitrobenzenerulfonic acid
affected by different substituents-for example, substituted benzenesulfonic acids-HSP can be plotted against substituent constant.;, such as Hammett's sigma values or potential breaks can be compared. Again a relative acidity scale is obtained. These acidity scales can also be used to suggest the possibility of differential titrations. The sulfonic acid group owes its strong acidity to the sulfonyl group, R-SO,-, since the negative charge is potentially distributed over three oxygen atoms. The acidity of the sulfonic acid can be further affected by changes in the R part of the molecule. This in part has already been shown in comparisons between methane, benzene, and naphthalenesulfonic acid. Additional differences can be brought about by substitution of electron-withdrawing or donating groups a t appropriate positions on the hydrocarbon. For example, p-nitrobenzenesulfonic acid would be predicted to be a stronger acid than benzenesulfonic acid because of the electron-withdrawing power of the nitro group. Therefore, halfneutralization potentials and potential breaks were determined for a series of substituted aromatic sulfonic acids and several inorganic acids. 2,4-Dinitrobenzenesulfonic acid was selected as a potential titrant and compared to HClO, for the titration of amines. Initial experiments were concerned with titrations in a variety of solvents such as 10% 2-propanol-benzene, acetic acid, acetonitrile, nitrobenzene, chlorobenzene, methylene chloride, ethyl acetate, acetone, and methyl isobutyl ketone. Bases of different strength were titrated with 2-naphthalenesulfonic acid, p-toluenesulfonic acid, and 2,4-dinitrobenzenesulfonic acid.
From these data, solubility characteristics, electrode response, solvent effects in a qualitative manner, and potential breaks were determined and are sumThe most marized in Table I. important observation, as seen in the table, is the increase in acidity of 2,4dinitrobenzenesulfonic acid, indicated by larger potential break, over the other acids, by about 30 to 40%. The solvents which appeared most useful for evaluating the acidity of the sulfonic acids were methyl isobutyl ketone and glacial acetic acid. RIethyl isobutyl ketone, an inert solvent, is an excellent differentiating solvent throughout the range of acid strengths and, therefore, provides a large potential scale for the determination of relative acidity ( I ) . Glacial acetic acid, an acidic solvent, which is lacking in basic properties and, therefore, nonleveling toward acids, is also useful as a differentiating solvent for strong acids. I t s potential range in terms of a relative acidity scale, however, is not as large as methyl isobutyl ketone. The potential range of the other solvents listed in Table I as suggested by the potential break data follow the order reported by van der Heijde and Dahmen (14).
Table II.
Potential Break for Titration of Aromatic Sulfonic Acids in Methyl Isobutyl Ketone (MIBK) and Glacial Acetic Acid (HAC)
Benzenesulfonic acid p-CK p-NO2
2,4-DiN02
IMD/ AIIBK
DPG/ MIBK
310 380 440
420 430 570 580
2,4,6-TriNO~ 310 "01 HCl 340 HClOa 400 570 a Imidazole (IhlD) pK, = 7 . guanidine (TblG) pK, = 13.6. Table 111.
Table I1 lists potential break data for the titration of several substituted aromatic sulfonic acids and several inorganic acids in methyl isobutyl ketone and glacial acetic acid with different bases in the respective solvent. Xormalized potentiometric curves for the titration of four of the sulfonic acids in methyl isobutyl ketone with diphenylguanidine are reported in Figure 1. The extent of the potential break and the excellent differentiating power of the ketone solvent are clearly defined. The bases for the titrants were selected not only because they are relatively strong bases but also because they do not form sulfonate precipitates in neutralization. Therefore, acid strengths of the different sulfonic acids can be compared with each other and with the inorganic acids. The potential break \vas defined as the difference between potentials recorded a t 0.5 nil. before and after the end point. As the electron-withdrawing power of the substituent and the number of such substituents are increased, the sulfonic acid becomes more acidic (greater potential break) regardless of the basic titrant. Qualitatively, it appears that 2,4dinitrobenzenesulfonic acid and 2,4,6trinitrobenzenesulfonic acid are close
Titrant" TMG/ TBAH/ RlIBK MIBK Potential break, mv.
DPG/ MIBK
TUG/ HAC
160 560 1000 150 610 190 220 710 1140 230 250 730 250 260 430 550 130 140 780 250 290 Diphenylguanidine (DPG) pK, = 10. TetramethylTetrabutylammonium hydroxide (TBAH).
AHNP for Series of Aromatic Sulfonic Acids in Methyl Isobutyl Ketone (MIBK) and Glacial Acetic Acid
Benzenesulfonic acid p-OH 2,4,6-TriCH3 V-OCH, &CH3 V-H
p-nr
p-c1
p-CN
m-xo2
DPG/MIBK - 54
- 41 -32
Titranta TBAH/ TRIG/AIIBK MIBK AHNP, mv.
- 48
DPG/HAc
TMG/HAc
-45 -35
- 32
- 36
-9 n
-8
- 21
- 38 - 13
n
n
37
22
35
0
42 83 83 81 106 126
37 72 94
~~
25 65 85
ai
n 26
41
43
10
40
11
44
90 39 43 104 p-NO? 2,4-D1N02 147 145 67 71 2,4,6-TriN02 197 165 70 79 -90 HCl -36 -43 150 HC104 73 100 a Diphenylguanidine (DPG), tetramethylguanidine (TLIG), tetrabutylammonium hydroxide (TBAH).
VOL. 38, NO. 8, JULY 1966
971
a
40
-
2
2 20-
ML ACID
Figure 3. Potentiometric titration of imidazole in glacial acetic acid
0-
A. B.
-20-40
1
-0.4 -0.2
1
0
0.2
1
0.4 SIGMA VALUE
1
0.6
Table IV.
972
isobutyl ketone or glacial acetic acid was employed, and no precipitation occurred. The data, AHSP, were made relative to the parent compound, benzenesulfonic acid, by subtraction of the HXP of benzenesulfonic acid from the HNP of each of the other acids. As the electron-withdrawing power of the substituent or the number of such substituents is increased, the acidity increases. The AHKP of the different singly substituted aromatic sulfonic acids were correlated to Hammett sigma values. These straight-line correlations are given in Figure 2 for methyl isobutyl ketone, glacial acetic acid, and what would be expected if water were the solvent. The AHNP used in Figure 2
Nonaqueous Titration of Bases with Perchloric Acid and 2,4-Dinitrobenzenesulfonic Acid (DNBS)
Base 1,3-Diphenylguanidine ImiGazole, Aminopyrine Strychnine m-timinoacetophenone m-8minobenzoic acid p-Bromoaniline p-Nitroaniline p-Aminoacetophenone Potassium chloridec 'Tetracaine hydrochloridec Benzocaine Strvchnine sulfate BrGcine Benzidine Ephedrine Aminopyrined Brucined a Acetic acid. b Precipitation takes place. c Mercuric acetate added. d Acetonitrile as solvent.
ANALYTICAL CHEMISTRY
HC104/HAca 100.5 98.8 100.0 99.9 76.2 97.8 94.0 96.8 99.2 99.9 98.5 99.7 100.0
Per cent purity DNBS/HAc 99.8 98.0 100.2 99.2 76.7 97.7b 93.gb 97.0b 98.8b 99.9 98.3 99.2b 100.3
DNBS/CCl,H 99.7 99.7
97.2 97.5 97.0 100.0 97.5
100.3 97.8
HC104
t
0.8
Figure 2. Correlation between sigma values of pand m-substituted aromatic sulfonic acids and A-halfneutralization potentials
to HC104 in acid strength. The differencea in the potential break for the different basic titrants are attributed to the fact that the titration curve past the end point is determined by the basic strength of the base and the resulting buffered mixture. As expected, the stronger the base, the greater the potential break. The use of tetrabutylammonium hydroxide was limited because of drifting in potential measurements, accompanied by excessive color formation. A more complete acidity scale is illustratcd in Table 111. I n this case, the H N P was determined from the potentiometric curve for each sulfonic acid titrated with three different bases. Again, a one-solvent system of methyl
2,4-Dinitrobenzenerulfonic acid
represents an average of the data in Table 111. The AHNP in Table 111 appears to be independent of the strength of the titrating base, which permits the use of the average. Hall (6) also observed in a similar manner that relative base strengths were independent of the titrating acid. The p-hydroxy derivative was not included because both the -OH and the -SOaH groups were titrated with the basic titrant. In addition to revealing an acidity scale, which is more quantitative in nature than the potential break comparisons, the differentiating power of the three solvents is illustrated. RIethyl isobutyl ketone, which has the greater slope, is the better differentiating solvent. It would appear that a measure of the differentiating power of a variety of solvents could be determined by comparing the slopes of AHNPsigma plots for each solvent. This would be a more fundamental measurement than the technique most frequently used, since the sigma value represents a structural property which is independent of solvent. The usual procedure is the comparison of slopes of HNP-pK, (HzO) curves (4). As pointed out previously, pK,(H20) is not always an indication of the compound's acidity in a nonaqueous solvent. Of the aromatic sulfonic acids studied, the nitro derivatives were shown to be the strongest and nearly as strong as HC1O4. Consequently, 2,4-dinitrobenzenesulfonic acid, which is readily available at modest cost, has good solubility characteristics, and is readily purified if needed, w'as selected for further study as an acidic titrant for the nonaqueous titration of amines. These data for the titration of a variety of bases with 2,4-dinitrobenzenesulfonic acid in acetic acid, acetonitrile, and chloroform are listed in Table IV. The titrations were performed potentiometrically, exczpt in the case of the alkaloids which were done visually in CHC13 with dimethyl yellow as indicator. The procedure is that of Safarik ( 7 ) , except that
2,4-dinitrobenzenesulEonicacid was used in place of p-toluenesulfonic acid. The potentiometric titration of imidazole in acetic acid with 2,4-dinitrobenzenesulfonic acid, which is typical of the compounds listed in Table IV, is illustrated in Figure 3 and compared to t’he HClO, titrant. The only difference observed between the two titrants is when the sulfonate salt precipitated. In these cases, usually a n aniline derivative, a larger potential break was found for the sulfonic acid titrant. The enhancement does not’ indicate the sulfonic acid to be a stronger acid than HClO, but) rather is a result of the added driving force of precipitation. This property has been observed before (5). The wide variety of compounds listed in Tahlc IV which can be routinely anal;\-zedby HC104titration were chosen in order to $how that 2,S-dinitrobenzenesulfonic arid and HCIOi are comparable in their applications. For esample, weak (p-nitroaniline) as n-ell as strong ( I ,3-dil,henyl~uanidi:ie)ba5ee and basic coml)ounds of a variety of structures can be titrated in a similar fashion with the sulfonic arid titrant. : I variety of solvent. can be u w l . The salt, KC1, which i. also characteristic of a n amine hydrochloride such a; tetracaine hydrochloride was analyzed by the addition
of mercuric acetate (5) and titration of the acetate ion which is formed. Interestingly, the titration of strychnine sulfate, which involves the conversion of sulfate to bisulfate, illustrates the strongly acidic nature of 2,4-dinitrobenzenesulfonic acid. In several instances amines as received were used, and the purity of the amines was determined by HC104 titration. As can be seen in Table IV, good stoichiometry was obtained. In addition to the properties already mentioned, 2,4-dinitrobenzenesulfonic acid is a solid and can be used in any solvent in which it is soluble, permitting a one-solvent system. Water, however, is not completely eliminated, since the sulfonic acid occurs under normal conditions as a dihydrate. Calculation shows that 0.36 and 0.25% water would be present for 0.1N solution if 2,4dinitrobenzenesulfonic acid dihydrate and 72% HC104,respectively, were used. The effect of adding increasing amounts of water to a n amine titration was the same for both acidic titrants. ACKNOWLEDGMENT
The authors thank American Cyanamid, Pfister Chemical, and Antara Chemicals for d a t a sheets and generous samples of several of the compounds.
LIlERAlURE CllED
(1) Bruss, D. B., Wyld, G. B. A., ANAL. CHEM.29, 232 (1957). (2) , , Carr. M. H.. Brown. H. P.. J . Am. Chem.’Soc. 69,’1170 (1947). ’ (3) Caso, M. hl., Cefola. hl., Anal. Chim. Acta 21, 374 (1959). (4) Critchfield, F. E., “Organic Func-
tional Group Analysis,” Macmillan, New York, 1963. ( 5 ) Fritz, J. S. Hammond, G. S., “Quantitative Orginic Analysis,” Wiley, New York, 1957. (6) Hall, H. K., Jr., J . Phys. Chem. 60, 63 (1956). (7) Kucharsky, J., Safarik, L., “Titrations in Nonaaueous Solvents.” Elsevier. New York, 19’65. (8) Lane, E. S., Talanta 8 , 849 (1961). (9) Paul, R. C., Pahil, S. S., Anal. Chim. Acta 30, 466 (1964). (10) Paul, R. C., Pahil, S. S., Malhotra, K. C.. T’ashisht. S. K.. J. Sci. Ind. Res. 21B, 41 (1962).‘ (11) Paul, R. C., Vashisht, S. K., Malhotra, K. C., Pahil, S. S., ANAL.CHEM.34, 820 (1962). (12) Smith, T. L., Elliott, J. H., J . Am. Chem. SOC.75. 3566 11953). (13) van der Heijde, H. B.,‘ Anal. Chim. Acta 17, 512 (1957). (14) van der Heijde, H. B., Dahmen, E. A. hl. F., Zbid., 16, 378 (1957). RECEIVEDfor review March 2, 1966. Accepted April 29, 1966. First Midwest Regional American Chemical Society Meeting, Kansas City, Mo., Xovember 4-5, 1965. Financial assistance came from a grant (GM 123106-01) from the National Institutes of Health and a Du Pont Fellowship (1965-1966) for one of the authors (JB).
Analytical Study of an Iodide-Sensitive Membrane Electrode G. A. RECHNITZ,’ M. R.
KRESZ, and S. B. ZAMOCHNICK2
Department o f Chemistry and Analytical Chemistry Center, University of Pennsylvania, Philadelphia, Pa.
b Evaluation of a precipitate-impregnated membrane electrode has revealed sensitivity, selectivity, and other response characteristics well suited to the analytical utilization of such electrodes. It has been demonstrated that iodide ion can be determined by direct potentiometry at concentration 1 O-’M in aqueous levels as low as 5 solutions with relatively little interference from chloride, bromide, and other common anions. In dilute solutions, the electrode response is independent of the nature of the cations present. Excellent results were obtained in potentiometric titrations of iodide with Ag+ using the membrane electrode as an indicator electrode. The response rates of such electrodes suggest that continuous monitoring of some changing systems may also b e feasible.
x
R
in the development of ion-selective electrodes for alkali metal and other cations (8) have created renewed interest in the possibility of devising electrode systems having selective response to other classes of common ions. Indeed, it has been suggested (1) that glass electrodes with response to anions should be feasible, but no concrete results along these lines have been obtained, as yet. Most promising among recent efforts directed toLvard the development of anion-selective electrodes has been the work of Pungor et al. (6, 7 ) , who described the preparation and some preliminary evaluation of several precipitate-impregnated membrane electrodes. These membranes consist of fine particles of a sparingly soluble precipitatee.g., BaS04 or AgI-immobilized in a polymerized silicone rubber matrix. ECEXT SUCCESSES
7 9 7 04
When fabricated into suitable form, the resulting electrodes display favorable mechanical properties and good chemical durability. The concepts underlying the design of such membrane electrodes are by no means new and have recently been comprehensively reviewed by Lakshminarayanaiha (4). I n 1958, Fischer and Babcock ( 2 ) published a n excellent analytical evaluation of electrodes consisting of BaSOAmpregnated paraffin and pointed out the possible application of such electrodes to analytical potentiometry, although the actual results obtained were not encouraging. The present study was undertaken in order t o provide some quantitative inAlfred P. Sloan Fellow. Present address: Department of Chemistry, Cornell University, Ithaca, N. Y. VOL. 38,
NO. 8,
JULY 1966
973
