LITERATURE CITED
(16)W. Horwitz, Ed., "Official Methods of Analysis," Eleventh ed., Association of Official Analytical Chemists, Washington, D.C., 1970,p 89. E. Barrett and A. Lapworth, J. Cbem. Soc., 93, 85 (1908). , A. Olander, Z.Physik. Cbem., 129, 1 (1927). J. G. Conant and P. D. Barllett. J. Amer. Cbem. SOC..54. 2881 11932). 6 . M. Anderson and W. P. Jencks, J. Amer. Chem. Soc., 8'2, 1773 (1960).
(1)L. Klein, "Aspects of Water Pollution,'' Academic Press, New York, N.Y., 1957. (2)6. K. Afghan, A. V. Kulkarni, et a/., fnviron. Led., 7, (l),53 (1974). (3)J. D. Roberts and M. C. Caserio. "Basic Principles of Organic Chemistry," W. J. Benjamin, New York, N.Y., 1964 p 451. (4)J. Heyrovsky and P. Zuman, "Practical Polarography," Academic Press, New York, N.Y., 1968,p 138. ( 5 ) J. Sourd, Mem. Poudres, 40, 453 (1958);Chem. Abstr., 55, 1267e (1961). (6) P. Zuman and L. Meites, "Progress in Polarography," Vol. 3,Wiley, New York, N.Y., p. 103 (1972). (7) B. Fleet and P. Zuman, Collect. Czech. Chem. Commun., 32, 2066 (1967). (8)H. Lund, Acta Chem. Scand. 13, 249 (1959). (9)6. Fleet, Anal. Cbem. Acta, 36, 304 (1966). (10)M. Pribyl and J. Nedbalkova, Fresenius' Z. Anal. Chem., 224, 244 (1969). (11) E. K. Afghan and P. D. Goulden, Environ. Sci. Techno/., 5,601 (1971). (12)6. K. Afghan, "Proceedings of International Symposium on Identification
P. Zumna, "Organic Polarographic Analysis, "The Macmillan Company, New York, N.Y., 1964,pp 19,121. R. Pasternack, Helv. Chim. Acta, 31, 753 (1948). M. Tokyoka. Collect. Czech. Chem. Commun., 7, 392 (1935). P. Zuman and 0. Exner, Collect. Czech. Chem. Commun., 30, 1832
(1965). I. M. Kolthoff and J. J. Lingane, ''Polarography,'' Vol. 11, lnterscience Publishers, New York, N.Y., 1952,pp 509,682-83,6-52-98, S. G. Mairanovskii, "Catalitic and Kinetic Waves in Polarography." Plenum Press, New York, N.Y.. 1968,Chapter 111. J. Heyrovsky and J. Kuta, "Principles of Polarography," Academic Press, New York, N.Y., 1966,p 314. P. Zuman, D. Barnes etal., Discussions Farraday Soc., 45,202 (1968). K. E. Oldham and E. P. Parry, Anal. Chem., 42, 229 (1970). E. P. Parry and K. B. Oldham, Anal. Cbem., 40, 1031 (1968).
and Measurement of Environmental Pollutants," Campbell Printing, Ottawa, Ontario, Canada, 1971,p 391. (13)6. K. Afghan, P. D. Goulden, and J. F. Ryan, Anal. Chem., 44, 354
(1972). (14)6. K. Afghan and I. Sekerka, Chem. Can., 26, 21 (1974). (15)B. K. Afghan and P. D. Goulden, Can. Res. Develop., 4 (4),21 (1972).
RECEIVEDfor review July 29, 1974. Accepted November 15. 1974.
Nonaqueous Potent iometric Titration of Pheno1s with PaIladiumHydrogen Electrodes Samuel Kaufman Naval Research Laboratory, Washington,D.C. 20375
Earlier electrochemical research has shown that palladium metal impregnated with hydrogen gas can be used as a reliable hydrogen-ion indicating electrode for aqueous solutions. However, this electrode has not enjoyed general use in potentiometric titrations. It has been found in this study that the palladium-hydrogen electrode is especially suited to potentiometric titration in a nonaqueous solvent because of its rapid response, sensitivity to hydrogen ions, low electrical resistance, stability, and compatibility with the nonaqueous system. Phenols are titrated in methyl ethyl ketone with an alcoholic potassium hydroxide titrant. Unusual and unstable reagents are avoided, and there is no requirement for the presence of water, as in many procedures which depend upon the glass electrode. The potential change observed in the end-point region of the titration is 250 to 500 millivolts, and in the system described, methyl ethyl ketone is a differentiating solvent.
The acid-base titration of phenols poses special problems because of the extremely weak acidic properties of phenols. In water, the titration is unsatisfactory and, in most anhydrous inert organic solvents, a glass electrode is slow to respond and therefore insensitive. Introduction of water or other protogenic solvent makes it difficult or impossible to distinguish the phenol electrometrically from the protogen in the solvent and, in the organic solvent alone, the glass electrode is very noisy because of its extremely high resistance. Use of a calomel reference electrode in nonaqueous titrations is not recommended because it can introduce water and potassium chloride into the solution to be titrated, and because the potential a t the liquid junction between the aqueous and nonaqueous phases may become irregular or slow to stabilize (1-3). 494
The problems of nonaqueous titrations in general have been discussed in the literature (1, 4-6), and the nonaqueous titrations of weak acids, including phenols in particular, have been studied in considerable detail (2, 3, 7-10). The potentiometric titration of phenols has been investigated in numerous solvents with various electrode combinations and with different titrating reagents. Solvents have included tertiary butanol (7), ethylenediamine (2, 3), dimethylformamide (3, 8 ) , pyridine (8, g), benzene-isopropano1 ( B ) , acetonitrile ( 8 ) ,dimethyl ether (9), chloroform (9), acetone (9),methyl ethyl ketone (9),piperidine (9),methyl, ethyl, and propyl alcohols (9), and butylamine (10). Electrode combinations studied have included glass-calomel (2, 3, 7-9), glass-modified calomel ( 8 ) ,glass-platinum (2, 9), platinum (anodically and cathodically polarized)-platinum (2, 9), and glass-antimony (10).Among the titrants investigated have been tetrabutylammonium hydroxide in water (B), in benzene-methanol (8), in isopropanol ( 9 ) ,in isopropanol-water (3, 8 ) , and in benzene-isopropanol-water (7); potassium hydroxide in isopropanol (3, 9); and sodium methylate in benzene-methanol (10). The potentiometric systems described in the literature all suffer in varying degree from one or more inconveniences and difficulties associated with the acquisition, preparation and stability of the solvents and titrants, the sensitivity and stability of the electrode pair, and the compatibility of the electrodes with the solvent and titrant. Under circumstances of repetitious analysis of phenols, perhaps the reagents, solvents, electrodes, and equipment can be maintained routinely. However, when the analysis is performed infrequently and a t unpredictable intervals, the effort can be excessive. In a study requiring the preparation of several metallic salts of phenols ( I I ) , it was required that the preparations be stoichiometrically neutral, because excesses of either the
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
metallic ion or the phenol were known to adversely affect the experimental results. An assessment of the available methods for determining the replaceable hydrogen content of the phenols from which the salts were prepared motivated the pursuit of a simple and reliable analytical procedure which could be used intermittently with conveniently prepared reagents of satisfactory quality. It has been demonstrated by Nylen (12, 13) that palladium and platinum, if palladized and charged with hydrogen, are stable, reversible indicating electrodes for hydrogen ions in aqueous solutions. Schuldiner and coworkers (14) studied bare palladium and palladized platinum and palladium extensively, and have determined the relationship between potential and the hydrogen content of the palladium. There is a phase of low hydrogen content (a-phase) and one of high hydrogen content (&phase). These two phases (a and p ) coexist between H/Pd atomic ratios of 0.03 and 0.6. In this region, the potential is dependent upon the hydrogen ion activity of the solution in which the electrode is immersed, and is independent of the amount of hydrogen dissolved in the palladium. The preferred electrode pair described herein consists of two bare palladium wires that have been similarly charged with hydrogen. One is used as the indicating electrode for hydrogen ions, and the other as a reference electrode immersed in a solution whose composition and hydrogen ion concentration approximates those expected a t the end point of the titration. This electrode pair proved to be eminently suitable in the nonaqueous system described here. An alternate reference electrode is a bare platinum wire in the titrant, as used by Harlow (2). Isopropanolic potassium hydroxide was the titrant of choice because of its suitability and ease of preparation and storage. Methyl ethyl ketone, which readily dissolves the phenols, was chosen as the solvent on the basis of its availability, chemical stability, and its previously demonstrated suitability ( 9 ) for nonaqueous acid-base titrations.
EXPERIMENTAL Apparatus. Figure l a shows the constructional details of the indicating electrode. Although simpler designs may be used, the illustrated one is durable, well shielded from electrical disturbances, and easy to manipulate and clean. The platinum wire should be sealed t o soft glass in a graded seal. If the palladium wire is sealed through glass, the seal will crack under the pressure developed by the volume increase attending the dissolution of hydrogen in the metal ( 1 5 ) .Therefore, the palladium wire is fused to the supporting platinum wire. The shield of the connecting cable to the p H meter terminates within the high impedance input plug, where it is joined to a lead which is the low impedance input. The shield is common with the reference electrode, and is connected to it by a clip lead which is captive a t the indicating electrode. The preferred reference electrode (Figure IC) is a removable palladium wire immersed in a soiution (described in the materials section) of fixed hydrogen ion concentration contained in a crackedjunction miniature electrode tube, available from Leeds and Northrup Company. The alternate reference electrode (Figure l b ) is a platinum wire sealed through soft glass in a graded seal forming the side arm on the Pyrex reservoir of the microsyringe. A heavy gauge platinum wire will avoid troublesome bending and breakage. The reference potential at this alternate electrode is determined by the titrant, and is communicated to the titration cell through the liquid junction at the tip of the hypodermic needle. A Beckman Model 76 continuously indicating p H meter was used in the millivolt mode for detection of the electrode potential difference. The meter output was connected t o the y input of a Varian Model F80AM x-y recorder whose x input was fed by an external manual potentiometer. After each addition of titrant, the x input was adjusted and a point was printed. Hand tabulation of potential and volume readings may be used with perhaps a tolerable loss of precision. A 2-nil Gilmont Model S 1200 microsyringe fitted with a 24gauge Teflon hypodermic needle was used to dispense the titrant.
C L I P TO R E F ELECTRODE Pd P t
GRADED SEAL
BRAIDED TEFLON SHIELD INSULATION
EPOXY CEMENT
I PYREX
SOFT GLASS
SIDE ARM A F T E R DESICCATION PLUG TO HIGH IMPEDANCE TERMINAL
/
/
PHONE TIP TO LOW IMPEDANCE T E R M I N A L (a)
;I;
’Pd I
y-:oFTuc
GRADED SEAL Pt
MINIATURE REF
‘PYREX
GLASS
-
CRACKED JUNCTION
(cl
(b)
C L I P LEAD
,h ,
MICROSYRINGE
-INDICATING ELECTRODE
-
&A~;NETIC
TEFLON NEEDLE
STIRRER
, (d)
Figure 1. Electrode details and
titration assembly
( a )Constructional details of indicating electrode and cable, (b)alternate reference electrode incorporated into syringe reservoir; (c)reference electrode assembly, (d) titration assembly
The syringe, whose micrometer head is graduated in 0.002-ml divisions may be read easily to 0.001 ml. The design permits the titrant to contact only the glass reservoir, the Teflon piston, and a Viton or Teflon “0” ring seal, the latter of which is preferred for the caustic titrant. Reagents. Methyl Ethyl Ketone, Reagent Grade. The supply needed should be passed through a column of activated alumina to remove any moisture and acidity, and should be tightly capped until used. Isopropanolic KOH, I N . Reagent grade isopropanol, preferably containing less than 0.2% water, is used to dissolve the reagent grade KOH. Any carbonate is eliminated by sedimentation. Filling Solution for the Reference Electrode Tube. A binary solvent of methyl ethyl ketone and isopropanol is used to dissolve a sodium or potassium salt of a phenol. The solvent composition and molar concentration of the phenolate are estimated from the predicted composition of those at the end point of the proposed titration. The author has used specially synthesized phenolates which were freeze-dried and retained in sealed containers. Other fillings were prepared directly as solutions derived from preliminary titrations of phenols, using the alternate platinum reference electrode and back titration to the end point. The quantity of phenolate required is miniscule. Procedure. The palladium reference and indicating electrodes are charged by immersing both in a 1%solution of aqueous sulfuric acid and connecting them in parallel as cathodes to a 1.5 to 3 volt dc source with a platinum anode. Gas is evolved a t the anode when electrolysis commences, but not a t the cathode until the requisite ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
495
0
I
200 [
*. b
'I'
.
200
'.,
05
IO
- 300
1
200
0
*
w
0
*
*&
100
0
**
0 0
4-100
0
-900
05
0
IO
15
20
-20011 , ; ; ;n;j
ml
Figure 2. Titration curves for two separate lots of m-(mphenoxyphenoxy)phenol using the alternate reference electrode (platinum)
charge is approached or exceeded, whereupon the electrolysis is terminated. The duration of the charging process depends upon the current density and the history of the electrode, especially the recency of previous charging. If a completely discharged electrode gases at the outset, the current density should be reduced by diminishine the amlied Dotential. A convkient sample is 0.0015 mole of the phenol dissolved in 15 ml of methyl ethyl ketone. The palladium electrodes are rinsed thoroughly with distilled water and dried with a tissue to exclude moisture in the analysis. The reference electrode assembly and indicating electrode are immersed in the solution of the sample contained in a small beaker as in Figure I d . If the alternate reference electrode is used, the specially adapted microsyringe reservoir is integral with it, and of course substitutes for the palladium reference. The potassium hydroxide titrant is introduced below the surface of the solution which is magnetically stirred by a miniature stirring bar. The magnetic stirrer chassis should be grounded. Examination of the potential-volume data, which should extend through and beyond the end point, will give a clear determination of the latter, especially if the differential data are derived and examined. The titration blank is not measurable if the methyl ethyl ketone is prepared as directed.
,
I
-300
,
I
***+ *
OOO
-400
I
05
0
b
&ELOH
mi
IO
-500
o o
15
d ~ O ~ ) O + $ O HM e
Figure 3. Titration curves using the preferred reference electrode (palladium-hydrogen)
(a) o-[p-(mphenoxyphenoxy)phenyl]phenol.(b) pphenylphenol. ( c ) m j m phenoxyphenoxy)phenoi, (a) 4-(4-phenoxyphenoxy~2,6dimethylphenol. Coordinates displaced to avoid overlap of curves
RESULTS AND DISCUSSION The ideal filling solution for the reference electrode tube would be one of composition exactly identical to the analyzed solution a t its precise end point. With this composition, the liquid junction potential a t the end point of the titration should be zero, and the observed potential difference between the electrodes a t the end point should be zero. In practice, however, this exact condition is difficult to achieve. A reasonable approximation, however, is obtained by preparing a filling solution as directed above. I t is not mandatory that the phenolate in the filling solution be the same species as that formed by titration; another species resembling it is satisfactory. A fundamental objection to the alternate platinum reference electrode would be that in the course of titration, the potential difference due to the liquid junction a t the titrant-phenol solution interface may be changing considerably. However, a t this Laboratory the magnitude of this effect was not found to be great, but the objection deserves earnest consideration. A more important problem is the minor oscillatory potential derived from the motion of the stirring - bar as it alternatelv obstructs and clears the conducting path between the palladium electrode in the titrated solution of high resistance and the platinum electrode in the highly conductive alcoholic potassium hydroxide. Figure 2 shows curves representing the titrations for two different lots of commercially purchased rn-(m-phenoxyphenoxy)phenol. The reference electrode used for both these titrations was platinum immersed in the KOH titrant. A comparison of the two curves illustrates the differ496
,
I
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
ence between the relative purities of the two lots. Curve a exhibits the single inflection point expected of a pure substance whereas curve b shows two inflections, characterizing the sample as a mixture. Curve b also verifies the differentiating (16) capacity of the solvent. The curves of Figure 3 were obtained with the palladium-hydrogen reference electrode immersed in potassium m- (rn-phenoxyphen0xy)phenolate in isopropanol-methyl ethyl ketone solution. Curve a represents the titration of a specially synthesized sample of 0-[p-(m-phenoxyphenoxy)phenyl]phenol. Again this curve shows the presence of one or more weakly acidic impurities, probably other similar phenols generated in the synthesis and not separated by distillation because of their similar physical constants. Curves b, c, and d, respectively, represent titrations of p - phenylphenol, m- (mphenoxyphenoxy)phenol, and 4-(4-phenoxyphenoxy)-2,6dimethylphenol. The last was specially synthesized for this Laboratory, whereas the other two were purchased commercially. These three curves are clear examples of sharp end points attainable with this electrode combination. A rise in the indicating electrode potential is observed after completion of the titration in curve a of Figure 2 , and to a lesser extent in curve d of Finure ., 3. This behavior is noticed occasionally, and is attributed to the low solubility of potassium hydroxide in methyl ethyl ketone. As long as free phenol is present, the hydroxide is consumed in the neutralization. Subsequently, only a limited concentration of the hydroxide can remain stably dissolved in the solvent mixture in the titration cell. I t is speculated that additional hydroxide can supersaturate the solution temporarily. The electrode follows these concentrations and, when the excess
the solution would be isolated around one of the electrodes, could present advantageous electrometric comparison of the isolated solution with the main solution after an increment of titrant is added to the latter, thus eliminating the need for preparation of a filling solution for a reference electrode tube. Further experimentation in this direction might result in even more sensitivity at the end point and greater precision of the determination.
hydroxide crystallizes out (as observed by development of turbidity), the potential rises. As more titrant is added, the alcohol content rises, contributing more solubility to the hydroxide, whereupon the potential may decrease again. Since these events occur considerably beyond the end point, they do not interfere with the analysis. The drift of the palladium-hydrogen electrode during a titration is essentially negligible during the period of a titration. For example, in an aqueous buffer solution unprotected from the atmosphere, the electrode was observed to drift LO millivolts over a period of 18 hours. If the maximum duration of a titration is arbitrarily considered one hour, the drift should be less than one millivolt during the titration. In most cases of routine titrations, the duration should be considerably less than one hour, since only the data in the region of the end point need be collected carefully. With rapid titrations. it is believed that the titration cell may be an open beaker, unprotected from the atmosphere. Observation of recorded data which were collected carefully for single titrations over periods of approximately one hour showed potential-volume data equivalent to rapid titrations. The palladium-hydrogen electrode responds rapidly to changes of acidity in the titration cell as titrant is metered in. If the solution is stirred continuously, equilibrium of the system is attained within a few seconds following the addition of an increment of the titrant. The negligible resistance of the electrode pair contributes materially to this rapid response. After sustained use of the electrode, the palladium may become poisoned, but it is easily restored to its originally useful condition by immersing it briefly in hot nitric acid, rinsing, and gently heating to redness in a hydrogen flame. No attempt has been made to use two palladium-hydrogen electrodes in a direct differential titration system. It is considered that such a system, wherein a small portion of
ACKNOWLEDGMENT The suggestion of using the palladium-hydrogen electrode for the application described here came from Sigmund Schuldiner of the Naval Research Laboratory. The author is indebted to him for his suggestion, subsequent advice, and criticism of this manuscript.
LITERATURE CITED J. 1.Stock and W. C. Purdy, Chem. Rev., 37, 1159 (1957).
G. A. Harlow, C. M. Noble, and G. E. A. Wyld, Anal. Chem., 28, 784 (1956). V. 2. Deal and G. E.A. Wyld, Anal. Chem., 27, 47 (1955). J. S. Fritz, "Acid-Base Titrations in Nonaqueous Solvents," G. Frederick Smith Chemical Co., Columbus, Ohio, 1952. C. W. Pifer. E. G. Wollish, and M. Schmall, Anal. Chem., 25, 310 (1953). J. A. Riddick, Anal. Chem., 28, 679 (1956). J. S. Fritz and L. W. Marple, Anal. Chem., 34, 921 (1962). R. H. Cundiff and P. C. Markunas, Anal. Chem., 28, 792 (1956). G. A. Harlow, C. M. Noble, and G. E. A. Wyld, Anal. Chem., 28, 787 (1956). J. S. Fritz and N. M. Lisicki, Anal. Chem., 23, 589 (1951). H. Ravner and S. Kaufman, ASLE Trans., in press. P. Nylen, 2.Nectrochem.. 43, (12), 915 (1937). P. Nylen, Z.Electrochem., 43, (12), 921 (1937). S. Schuldiner. G. W. Castellan, and J. P. Hoare, J. Chem. Phys., 28, 16 ( 1958). D. P. Smith, "Hydrogen in Metals," University of Chicago Press, Chicago, Iil., 1946. R. G. Bates, "Electrometric pH Determinations," John Wiley and Sons, New York, N.Y.. 1954.
RECEIVEDfor. review September 11, 1974. Accepted November 14,1974.
Evaluation of Unique Directly Digital Computer-Controlled and Hardware-ControlledAutomatic Titrators T. W. Hunter, J. T. Sinnamon, and G. M. Hieftje Department of Chemistry, Indiana University, /3Ioomington, Ind. 4740 1
Several automatic titrator configurations, all of which incorporate a unique digital titrant delivery system, are compared and the performance of each is evaluated. The configurations differ mainly in the method used to control the delivery of titrant (either computer, hardware, or manual) and the end-point detection technique (either fixed-level or derivative). The best performance, 0.16% relative standard deviation, was obtained with a configuration that was computer-controlled and utilized a first or second derivative end-point method. The computer-controlled titrator was also superior in its versatility, in the choice of end-point detection, and its adaptability to non-routine analysis using operator interaction. The hardware-controlled titrator, utilizing a fixed-level end-point technique, produced relative standard deviations of only 0.4%; however, it was less expensive than the computerized system and was well suited to rou-
tine applications. In all titrator configurations, the precisionlimiting factor appeared to be the end-point detection process rather'than the titrant delivery system.
Titration has been a standard analytical procedure for many years. In applications where large numbers of routine samples are analyzed, however, manual titration becomes tedious and time consuming. To solve this problem, automatic titration procedures have been developed, and many systems are in wide use today. These systems range from simple operator-controlled titrant delivery devices to complex assemblies that handle titrant delivery, titration curve measurement, end-point detection, sample changing, and readout of final concentration values. Most of these automated systems control titrant delivery by means of a mechanical displacement as in the case of a motor driven buret. ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
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