Potentiometric Titration of Very Weak Acids Titration in Ethylenediamine Solution Using Platinum Electrodes G. A. HARLOW, C. M. NOBLE,' and GARRARD E. A. WYLD Shell Development Co., Emeryville, Calif. salicylic acid (4). The second inflection is not obtained using the system described by Deal and Kyld ( 2 ) . The present investigation was undertaken to find an indicating electrode which produces the larger inflections without sacrificing the reproducibility of the glass electrode. Preliminary tests of antimony and platinum electrodes indicated that platinum Fas more promising. The use of platinized platinum in the hydrogen electrode is, of course, well known. Platinized platinum has also been used as an oxygen or air electrode in acid-base titrations. I n all of these platinum-gas electrodes, however, it is customary to pass a stream of the gas over the electrode surface during the titration. Utilization of the bare metal has not been thoroughly investigated Popoff and McHenry (8) have used shiny platinum for the titration of alkaloids. Brunnich ( 1 ) reported the use of the platinnnigraphite system but stated that small inflections were obtained. These and other applications have been discussed by Kolthoff and Furman ( 5 ) . The use of the platinum indicator electrode in the titration of very I\ eak acids in nonaqueous solvents has been mentioned in several papers. Higuchi and others (6) used a platinum electrode for titrations in tetrahydrofuran with lithium aluminum hydride. 9 platinum electrode was also used by Gran and Althin ( 3 ) and by Katz and Glenn (4)for titrations in ethylenediamine. Very little detailed information is available from these papers on the characteristics of platinum as an indicator electrode for acid-base titrations in nonaqueous solvents. S o data have been presented, for example, on the sensitivity of the platinum electrode compared with other electrodes or on the reproduciblity of the titration curves obtained.
;inodically polarized platinum wire acquires characteristics which greatly enhance its usefulness as an indicating electrode for the titration of very weak acids in ethylenediamine. Titration curves are obtained which span a potential range two to three times as great as those obtained with the glass electrode, and w-hich are more reproducible than those obtained w-ith the antimony electrode. Platinum wire inserted into the titrant stream serves as a satisfactory reference electrode.
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HE use of alcoholic potassium hydroxide for the titration of very weak acids in ethylenediamine solvent has been described by Deal and Wyld ( 2 ) . This titrant is referred to as a solution of potassium hydroxide, although it probably contains both hydroxyl and isopropylate ions in equilibrium TI ith each other. Deal and Wyld also shox that the glass electrode can serve as an indicating electrode in ethylenediamine if sodium ion is absent. This technique has the advantage of using a titrant (potassium hydroxide in isopropyl alcohol) which is already available in most petroleum laboratories for the deterniination of acidity of lubricating oils (ASTM D-664), and anelectrodesystem which is commercially available and relatively stable.
To Solvent Reservoir
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APPARATUS
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Titrator Feed Mechanism
-4 Precision-Dow Recordomatic Titrator was employed in conjunction with the specially designed cell assembly shorn in
Platinum Wire Reference Electrode
Magnetic Stirrer
Figure 1. Titration cell assembly I 0
Other workers, however, have reported larger inflections for the titration of phenols using an antimony or platinum indicating electrode ( 4 , 7 ) . I n addition, they have found two inflections for I
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Figure 2.
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0.6 0.8 1.0 Volume of Titrant. ml.
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Comparison of electrode systems
0.24 meq. of phenol Electrodes: A . Platinum-calomel; B. Glass-calomel; C. Antimony-calomel
Present address, Shell Chemical Corp., Houston. Tex.
784
V O L U M E 28, N O . 5, M A Y 1 9 . 5 6 rnents prior to its use in a titration results in titration curves quite different in character. Figure 3 shows curves for the titration of phenol using platinum electrodes which were previouel? treated in several different wags. The small first inflection is due t o carbonate in the solvent. The treatment referred to as polarization consisted of applying a potential of 3 volts to a pair of platinum electrodes immersed in dilute (1 to 100) sulfuric acid solution and allowing electrolysis to proceed for about 1 minute. The platinum \$ire which served as anode is referred t o as being anodically polarized; similarly the one which served as cathode is cathodically polarized. It is not certain exactly n h a t occurs on the surface of the platinum during this treatment, but apparently the surface is charged nith hydrogen or oxygen, depending on whether the platinum was polarized a t the cathode or anode.
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Figure 3.
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Effect of treatment of platinunl indicating electrode
0.24 meo. of Dhenol Electrod: treatment: 4 . Anodically polarized B. Treated with cerium sulfate C . Cathodically polarized D. Treated x i t h nitric acid E. Untreated F . Treated n i t h hydrogen peroxide
Figure 1. The titration asEembly was equipped \\ith a 5-ml syringe which permits the titrant to be delivered a t one fifth of the usual rate. The scale of the titration was thus similar t o the manual titrations previously described ( 2 ) . The apparatus, except for the titrator, is contained in afume hood This arrangement protects both operator and instrument from toxir or corrosive fumes, such as those of ethylenediamine. The electrode holder consists of a rubber stopper drilled with holes of appropriate sizes. This permits edective sealing against atmospheric carbon dioxide and makes interchange of electrodes convenient. Solvent is added to the sample after the cell is in place by means of the solvent delivery tube. Agitation is accomplished nith a magnetic stirrer, and the nitrogen inlet tube allows the sample t o be covered with a blanket of inert gas just before the solvent is added and before the system is closed to the atmosphere. PROCEDURE
The titration procedure employed n'as similar to that previously described (,37j,except that in this case an automatic titrator was used. The solvent was ethylenediamine (95 to loo%), Eastman Organic Chemicals. The titrant was 0 . 2 5 potassium hydroxide in isopropyl alcohol solution, except TThere otherwise indicated. PRELIAIINARY TESTS OF ELECTRODES
Antimony and platinum electrodes were tested as possible substitutes for the glass electrode. A comparison of their behavior with that of the glass electrode is given in Figure 2, nhich shows curves for the titration of phenol in ethylenediamine. The small first inflection is due to carbonate in the solvent, and the second inflection is due to the phenol. The curves produced by the antimony and the glass electrodes \Yere similar. It was difficult, however, to obtain smooth ciirves JT-ith the antimony electrode, because the potentials tended to be erratic. I n vielT of the more promising results obtained with commercially available platinum, the reasons for the instability of the antimony electrode were not investigated CHARACTERISTICS OF PLATINVAT INDICATING ELECTRODE
Effects of Pretreatment. The behavior of the platinum electrode is very much dependent upon the condition of its surface. This is evidenced by the fact that eubjecting it to various treat-
Volume of T i t r a n t , rnl.
Figure 1. Titration of resins in ethylenediamine
Anodically polarized platinum produced curves having the greatest potential range and showing the longest and sharpest inflections The treatment is not lasting, holsever, and must be repeated bcfoie every titration to gain any degree of reproducibility. After prolonged use platinum develops an insensitivity to the polaiizatiori process and the curves cover a progressiveljsmaller potential range; hoi$ever, the platinum may be rejuvenated I-,?- letting it stand in hot dilute (1 to 1)hydrochloric acid for 3 or 1hours. The mechanism of operation of the electrode TTas not investigated, but it may be operating as a n oxygen electrode of the type discussed by Kolthoff and Furman ( 6 ) . +4110dkpolarization largely erases the effects of other previous treatments. K h e n electrodes which have been treated in various wags are anodically polarized, they yield titration curve3 which are very similar to one another. The platinum electrode has from tTTo to three times the voltage response of the glass electrode in ethylenediamine. The greater sensitivity is not limited to the most alkaline area of a titration but holds throughout the range of basicity encountered in thiq solvent. Stability of Platinum Electrode Potentials. The anodic polarization of the platinum electrode, vihile causing it to span a large potential range throughout a titration, also causes it t o exhibit an unstable initial potential. If the freshly polarized electrode is alloned t o stand in a solution of an acid in ethylenediamine a few moments before any titrant is added, the initial potential may be observed to become slowly more positive. It was found necessary, therefore, to wait 2 to 3 minutes before starting the titration. When this was done, the potentials ob-
ANALYTICAL CHEMISTRY tained with the platinum electrode were comparable in reproducibility to those obtained with the glass electrode. Applicability. The anodically polarized platinum electrode has proved applicable to the titration of a variety of acids including phenol, resorcinol, catechol, substituted catechols, and 4,4’-isopropylidenediphenol,both alone and in mixtures with stronger acids. I n all of these cases larger inflections were obtained than in the corresponding titration using a glass electrode. Curves for the titration of resinous materials are also improved by using the treated platinum electrode. Figure 4 shows comparative curves for the titration of two resins containing phenolic hydroxyl groups. The phenolic (second) inflections are particularly enhanced. The first inflections are due to unidentified acidic components which are stronger than phenol. The undesirable effect of sodium ion on the behavior of the glass electrode, which is n-ell known in water, is particularly marked in ethylenediamine. Figure 5 shows curves for the titration of salicylic acid with sodium aminoethoxide, using both the glass and the anodically polarized platinum electrodes. As can be seen, sodium has no apparent effect upon the potential range of the platinum electrode. Sodium aminoethoxide gives an inflection for the hydroxyl group of salicylic arid, whereas potassium hydroxide (not shown) does not.
comparable regarding durability, availability, and reproducibility of potentials. There seems to be no ready explanation for the difficulty which was experienced with the stability of the antimony electrode. It is possible that impurities either in the ethylenediamine or the antimony may have been the cause of the instability,
Electrodes
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PLATINUM REFERESCE ELECTRODE
Space is very limited in the special reduced scale titration assembly used for ethylenediamine titrations; therefore, it was desirable to remove the bulky calomel electrode from the titration cell if possible. I n addition, the aqueous salt bridge used with the calomel cell is quickly contaminated with ethylenediamine, and what is more serious, it in turn contaminates the ethylenediamine with water. Moss and others ( 7 ) reported that a satisfactory reference electrode was obtained by inserting an antimony rod into the titrant stream. When this was tried, however, stable potentials could not be obtained. The substitution of a short length of platinum wire into the titrant stream (9) was found to serve very satisfactorily. This yielded stable potentials for long periods of time and necessitated only infrequent cleaning. The removal of the reference electrode from the titration cell in this way allows a greater degree of freedom in the design of titration assemblies; it permits the apparatus to be scaled down, if desired, for titrations on a microscale. DISCUSSION AND CONCLUSIONS
The anodically polarized platinum electrode is a more sensitive indicator electrode than is the glass electrode when potassium hydroxide is used as the titrant in ethylenediamine titrations. When sodium aminoethoxide is used as the titrant, the platinum electrode retains its sensitivity, but the glass electrode becomes practically useless as an acidity indicator, although it may be useful as a reference electrode. The two electrodes are roughly
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Figure 5.
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I I I I 0.6 0.8 1.0 1.2 Volume of T i t r a n t , ml.
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Comparison of electrode behavior with sodium aminoethoxide
0.30 meq. of salicylic acid in 2 rnl. of isopropyl slcohol, titrated with 0.2N sodium aminoethoxide in isopropyl alcohol
Because the main advantage of the platinum electrode over the glass electrode is its freedom from alkali ion errors, the two electrodes should be more comparable in sensitivity when alkali ions are absent. A study of quaternary ammonium titrants is now in progress. LITERATURE CITED
Brunnich, J. C., Ind. Eng. Chem. 17, 631 (1923). Deal, V. Z., Wyld, G. E. A., A s . 4 ~ .CHEM.27, 47 (1955). Gran, G., Althin, B., Acta Chem. Scand. 4 , 967 (1950). Katz, M., Glenn, R. A., ANAL.CHEM.24, 1157 (1952). Kolthoff, I. M., Furman, N. H., “Potentiometric Titrations,” p. 215, Wiley, Kew York, 1931. (6) Lintner, C. J., Schleif, R. H., Higuchi, T., ANAL. CHEM.22, 534 (1) (2) (3) (4) (5)
(1950). (7) Moss, M. L., Elliott, J. H., Hall, R. T., Ibid., 20, 784 (1948). (8) Popoff, S., hfcHenry, M. J., J . Am. Pharm. Assoc. 14,473 (1925). (9) Willard, H. H., Boldyreff, A. W., J . Am. Chem. SOC.51, 471 (1929) I
RECEIVED for review September 6. 1955. Accepted February 17, 1956.
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