LITERATURE CITED
W.J., Meloche, V. W., “Elementary Quantitative Analysis,” Row, Peterson and Co., Evanston, Ill., 1957. (2) Booman, G. L., ANAL. CHEM. 29, 213 (1957). (3) Brdicka, R., Hanus, V., Koutecky, J., in “Progress in Polarography,” P. Zuman, ed., with the collaboration of I. M. Kolthoff, Vol. 1, Chapt. 7, Interscience, New York, 1962. (4) DeFord, D. D., Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, April 1958. (5) Delahay, P., “New Instrumental Methods in Electrochemistry,” pp. 166-7, Interscience, New York, 1954. (6) Delahay, P., in “Advances in Electrochemistry and Electrochemical Engi(1) Blaedel,
neering,” P. Delahay and C. W. Tobias, eds., Vol. 1, Chapt. 5, Interscience, New York, 1961. (7) Gerischer, H , 2. Elektrochem. 5 5 , 98 (1951). (8) Gerischer, H., 2. Physik. Chem. 198, 286 (1951). (9) Kelly, M. T., Fisher, D. J., Jones, H. C., ANAL. CHEM. 31, 1475 (1959); 32,1262 (1960). (10) Koryta, J., Tenygl, J., Collection Czechoslov. Chem. Communs. 19, 839 (1954). (11) Koutecky, J., Ibid., 18, 311 (1953). (12) ,Koutecky, J., Koryta, J., Electrochzmica Acta 3, 318 (1961). (13) Lingane, J. J., Loveridge, B. A., J . Am. Chem. SOC.72, 438 (1950). (14) Matsuda, H., Delahay, P., Kleinerman, H., Zbid., 81, 6379 (1959).
(15) Randles, J. E. B., Dascussions Faraday SOC.1, 11 (1947). (16) Smith, D. E., Division of Analytical Chemistry, 140th hleeting, ACP, Chicago, September, 1961, and D. E. Smith, Thesis, Columbia University, New York, 1961. Also D. E. Smith and W. H. Reinmuth, ANAL. CHEv., submitted for publication. (17) Smith, D. E., . ~ . I L . CHE\f. 35, 602 (1963). (18) Smith, D. E , unpublished work. (19) Smith, D. E., Reinmuth, W. H., ANAL.CHEM. 33, 482 (1961). RECEIVEDfor review Sovember 26, 1962. Accepted March 1, 1963. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, April 1963. Work supported by the National Science Foundation.
Determination of Tech neti um by Cont roIled-PotentiaI Coulometric Titration in Buffered Sodium T ripoIy p hosphate M edium A. A. TERRY’ and H. E. ZITTEL Analytical Chemisfry Division, Oak Ridge Nafional Laboratory, Oak Ridge, Tenn.
b A controlled-potential coulometric method for the determination of technetium has been developed whereby Tc(VII) i s titrated in an acetatebuffered (pH 4.7) solution of sodium tripolyphosphate at a potential of -0.70 volt vs. S.C.E. In the range from about 0.5 to 5 mg. of Tc(VII) titrated, the relative error of the method i s about = t l % and the relative standard deviation about 0.5%. Various possible interferences were studied; Tc(VII) can be quantitatively removed from most interfering ions by distillation from sulfuric acid solution. The method i s relatively simple and rapid. HE
COULOMETRIC
REDUCTION Of
T T c ( V I 1 ) in phosphate medium was reported by Thomason ( I d ) , who studied the reduction in order t o interpret polarographic data rather than t o determine the usefulness of the coulometric reduction for quantitative analysis. The suitability of an aqueous solution of sodium tripolyphosphate as a supporting medium for the coulometric reduction of Tc(VI1) was indicated by preliminary polarographic studies. When Tc(VI1) is coulometrically reduced at a controlled potential of -0.70 volt vs. S.C.E. in Present address, Department of Chemistry, Texas Woman’s University, Denton, Texas.
614
ANALYTICAL CHEMISTRY
acetate-buffered sodium tripolyphosphate medium of p H 4.7, Tc(II1) is formed quantitatively. Better accuracy and precision are attainable with this method than with the reported radiochemical ( I ) , spectrophotometric (3, 7, 8), and polarographic (6, 9) methods for technetium.
All other reagents were ACS approved grade and, when necessary, were standardized by conventional means. Instrumentation and Apparatus. Coulometer, ORKL Model Q-2005, electronic controlled-potential (4). Potentiometer. Rubicon 0- to 1.6volt range. Polarograph, ORKL Model Q-1160 I -\
EXPERIMENTAL
Reagents. Standard Solutions of Technetium(VII), approximately 1 mg. per ml. Ammonium pertechnetate, NHhTc04, of better t h a n 99% purity (obtained from t h e Isotope Division of Oak Ridge Kational Laboratory) was dissolved i n water. T h e resulting solutions were standardized spectrophotometrically (7, 8) and coulometrically. The coulometric standardization was done by using the theoretical electrical factor for the coulometer; this factor converts the read-out voltage to coulombs used in the reaction, thereby giving the number of equivalents of reducible ion titrated (2)* Sodium Tripolyphosphate Solution, 6% {w./v.). Fifteen grams of commercial grade sodium tripolyphosphate, Na5P3OI0 (available from Monsanto Chemical Go.), was dissolved in 250 ml. of mater. Ammonium Acetate-Acetic Acid Buffer Solution, 351 NH4C2Ha02:5.5M HC2H302of p H 4.7. To a n aqueous solution of 116 grams of ammonium acetate, NH4C2H302, was added 140 grams of glacial acetic acid; the resulting solution was diluted t o 500 ml. with water.
(6)
Titration cell, conventional, with a mechanically stirred mercury electrode and a reference S.C.E.(11). RECOMMENDED PROCEDURE
The technetium in the test solution must all be present as Tc(VI1) before the coulometric titration is begun. If there is any possibility that the technetium is not in the Tc(VI1) state, i t can be oxidized Rith concentrated perchloric acid under conditions of total reflux. Add, in order, to the titration cell 4 to 5 ml. of mercury. 10.0 ml. of 6% (w./v.) Na5P3010solution, and 2.0 ml. of 3.0X NHdC2H30 : f1.5~l4 CH3COOH buffer solution. Turn on the stirrer. Deaerate the supporting medium for 1 minute with an inert gas. The atmosphere in the cell must be a nonoxidizing one during the complete titration. Therefore. allow the inert gas to flow throughout the complete titration. Reduce (titrate) the supporting medium at -0.70 volt us. S.C.E. to a background current of 100 Ma. Transfer a test aliquot of the sample solution to the titration cell. Prereduce the test solution a t -0.20 volt us. S.C.E. to a
I
I
I
I
I
f
ipa.
1.
0
3
i
z W
LT
0
- 0.2
- 0.4 - 0.6 -0.8 APPLIED POTENTIAL, volts vs. S.C.E. Figure 1.
-1.0
B’
A
Polarogram of Tc(VII)
Composition of test solution: [Tc(VIIl], 3 8 pg./ml.; [Na6P3010], 5% w./v.; [NHaCzHaOz], 0.3M; [CI-l3COOH], 0.6M. pH, 4.7. Volume of test solution, 10 ml. T, 25’ C. m2/af’/s, 2.10 mg.2/asec.-1’2 Scan rate, 0.1 volt/ min. Damping, 9
background current of 100 pa. Reduce the test solution a t -0.70 volt us. S.C.E. t o a background current of 100 pa.; record the time, t, required for this reduction at -0.70 volt vs. S.C.E. Stop the titration and measure the readout voltage (&). From the time required for the titration at -0.70 volt us. S.C.E., calculate the total current (100 consumed as background-Le., pa.) ( t second) = 100 t Fa. second = 1.0 X 2 coulomb. Convert t h e background correction to milligrams of Tc(VI1). From the read-out voltage, calculate the amount of Tc(VI1) either by using the Coulometric factor for Tc(VI1) or by means of a standard curve. Correct this cuantity of Tc(VI1) by subtracting the q x m t i t y of Tc(VI1) equivalent to the brtckground current. T h e derivation of the coulometric factor is given by Varrar, Thomason, and Kelley ( 2 ) . RESULTS AND CllSCUSSlON
Before any coulometric research was done, the polarographic behaviors of the supporting electrolyte and the substance of interest, ‘I’c(VII), were investigated. -4polarogram of Tc(VI1) in sodium triployphosphate solution is shown in Figure 1. The data of Table I show that a shift ir half-wave potential occurs with a change in pH; as the solution becomes more basic, the half-ware potential becomes more negative. I n addition t c the shift in halfwave potential, the wave form of the polarogram varies with pH. The plateau region becoines more definite as the pH decreases. The best defined waves occur at pH about 5 in sodium tripolyphosphate medium. .4t this pH the diffusion current varies linearly with the concentration of Tc(VI1). This variation is ind cated by the data of Table 11. Tc(VI1) Reduction-Oxidation Couples. F a r a d a y s laws of electrolysis are the basis for all of coulom-
I
I
-02
L
I
-04
-06 -08 A P P L I E D POTENTIAL, F , volts vs S C E
Figure 2. Coulograms of Tc(VII) reduction at two pH values Composition of test solutions: Tc(VIIJ,O.94 mg.; [NaaPaOlo], Buffer: A, acetate; E, phosphate. Volume of test solution, 1 3 ml. T, 2 5 ’ C. Controlled potential, -0.70 volt vs. S.C.E.
5% w./v.
etry. When t h e weight of material electrolyzed a n d t h e current used in t h e electrolysis are known, t h e electron change t h a t occurs in t h e reaction can be calculated. I n a controlled-potential coulometric titration, the plot of applied potential, E , us. read-out voltage, Q (a measure of the current consumed), shows the potentials necessary for reaction. I n the discussion t h a t follows, the plots of Q us. E are designated “coulograms.” Data for this type of curve are obtained by periodically stopping the current during the electrolysis by adjustment of the controlled potential and recording the values of Q and E at that time. I n the following section, the various oxidation states obtained in the controlled-potential coulometric titration of Tc(VI1) in sodium tripolyphosphate medium are discussed. I n Figure 2 are given two coulograms of the reduction of Tc(VI1). Coulogram A shows the current consumed in the reduction of 0.942 mg. of Tc(VI1) in buffered (pH 4.7) sodium tripolyphosphate medium. The first step corresponds t o a four-electron reduction. The electron change is measured by determining the number of equivalents per volt of read-out voltage ( 2 ) . The second step apparently is associated with the reduction of Tc(II1). A second plateau is not reached, but it is assumed that the
Table 1. Effect of pH on Half-Wave Potential of the Reduction of Tc(VII) in Sodium Tripolyphosphate Medium
Composition of test solution: [Tc(VII)], 7.4 pg./ml.; [pYTabP~O~~], 5% w./v.; [HzS04],necessary to adjust pH. Volume of test solution, 10 mi. T , 25” C. m2’3 t1’6, 2.10 mg.2’3 sec.-1’* Scan rate, 0.1 volt/min. Damping, 9 PH Eiiz (Volt US. S.C.E.) 4.0
5.5 6.4 7.2 9.0 9.0-9.5
-0.44
-0.66 -0.67 -0.68 -0.79 -0.81
Table II. Relation between Diffusion Current and Concentration of Tc(VII) in Sodium Tripolyphosphate Medium
Composition of test solution: [Tc(VII)], as indicated; [I\jaJ’3010], 5% w./v.; [ H z S 0 4 ] , necessary to adjust pH. pH, 5.5 to 6.0 Em, -0.65 volt US. S.C.E. Other conditions as given in Table I Concn. ilv. difof Tc(VII), fusion curi d / c , pa./ c, pg./ml. rent, i d , pa. pg./ml. 100.0 11.90 0.119 50.0 6.10 0.121 20.0 2.50 0.125 10.0 1.36 0.136 5.0 0.70 0.141 2.5 0.37 0.149
VOL. 35, NO.
6, MAY 1 9 6 3
615
second step is due to Tc(II1) + Tc(I1) reduction. This reduction is not completed because Tc(I1) reacts with the medium. The lack of completion of the reduction of Tc(II1) is confirmed b y the lack of a plateau after the second steep portion in coulograms A and B. Coulogram B is for a solution that also contained 0.942 mg. of Tc(VI1) in sodium tripolyphosphate medium t h a t was buffered a t pH 7.0. I n the more basic medium, the reduction of Tc(VI1) to Tc(II1) occurs at a more negative applied potential, and the break between the Tc(VI1) + Tc(II1) and Tc(II1) + Tc(I1) reductions is not so pronounced. The coulogranis of Figure 2 also show how a change in pH will cause a shift in the potential required for reduction. As the p H decreases, the potential required for reduction becomes less negative. The Tc(VI1) -+ Tc(II1) reduction is accompanied by several color changes; Tc(VI1) is colorless. -4s reduction proceeds in p H 4.7 acetate-buffered sodium tripolyphosphate medium, the color is first colorless, Tc(VI1) ; then pink, Tc(1V); then orange, Tc(1V and 111); and finally yellow, Tc(II1). I n p H 7.0 phosphate-buffered sodium tripolyphosphate medium, the colors are slightly different; first the solution is colorless, Tc(VI1); then pink, Tc(1V); then gray, Tc(1V) and Tc(TI1); and finally green, Tc(II1). The colors in the pH 7.0 medium correspond to those reported by Thomason (12). The color changes in each of these media indicate that the reduction may be sequential. The sequential nature of the reaction is also indicated by the fact that when the solution of Tc(II1) is allowed to stand in the presence of air, the color becomes pink. When Tc(II1) mas coulometrically oxidized a t -0.10 volt us. S.C.E., the read-out voltage was one fourth of that obtained when Tc(VI1) was reduced t o Tc(II1). This result indicated that the Tc(II1)Tc(1V) couple is reversible. Further evidence of the reveraibility is that the same read-out voltage was obtained when Tc(1V) was reduced at -0.70 volt us. S.C.E. and when Tc(II1) was oxidized at -0.10 volt us. S.C.E. The Tc(1V) + Tc(II1) reduction was also studied polarographically. h solution of Tc(1V) was prepared in buffered sodium tripolyphosphate medium of p H 4.7 b y reduction of Tc(VI1) to Tc(II1) at -0.70 volt us. S.C.E. and then by oxidation of Tc(II1) to Tc(1V) a t -0.10 volt us. S.C.E. Curve A of Figure 3 is a polarogram of this coulometrically prepared Tc(1V) solution. The Tc(1V) reduction polarogram shows that the half-ware potential of the reduction occurs a t -0.48 volt us. S.C.E. A sample of Tc(II1) was prepared by reduction of Tc(VI1) t o
616
rn
ANALYTICAL CHEMISTRY
0
-0.2
-0.4
-0.6
-0.8
-1.0
APPLIED POTENTIAL, volts vs. S.C.E.
Figure 3. Polarograms of Tc(lV) and Tc(lll) showing reversibility Composition of test solutions; [Tc(VII)], 94 pg./ml.; [NajPsO~o],5% w./v.; [NHaC2H302],0.3M; [CH&OOH], 0.6M. pH, 4.7. Volume of test solution, 10 ml. T, 25' C. m2/at1/8, 2.1 0 mg.'/3 sec.-'/Z Scan rote, 0.1 volt/min. Damping, 9. Oxidation states of Tc:
A, IV; B , 111 and I V
Tc(II1) in an acetate-buffered sodium tripolyphosphate medium of pH 4.7 a t a potential of -0.70 volt us. S.C.E. This sample of Tc(II1) was transferred quickly to the polarographic cell to minimize air oxidation. Curve B of Figure 3 is the polarogram of the Tc(II1). As indicated, some of the Tc(II1) was oxidized t o Tc(1T') in the transfer to the cell. Curve B shows a polarographic reaction having a halfwave potential of -0.48 volt us. S.C.E. Curve B has both anodic and cathodic properties, thus the presence of both Tc(II1) and Tc(1V) is indicated. The fact that both these reactions-that is, Tc(II1) + Tc(1V) and Tc(1V) + Tc(II1)-have the same half-rrave potential proves that the couple is reversible. As a further study of the chemical behavior of Tc(1V) and of the reversibility of the Tc(VI1) + Tc(1V) reduction, coulometrically reduced Tc (117) was titrated with strong oxidants. No distinct end point was obtained with ceric sulfate (with and n-ithout osmium catalyst) , potassium dichromate, or alkaline permanganate. Optimum Conditions for t h e Coulometric Reduction of Tc(VI1). T h e following factors were studied t o establish optimum analytical conditions for t h e coulometric reduction of Tc(VI1) : the controlled potentials for t h e pretitration and t h e titration, pH of the test solution, concentration of supporting electrolyte, a n d range of concentration of ion being determined. The polarographic study yielded
data from which it could be predicted that Tc(VII) would be reduced quantitatively a t a potential more negative than -0.60 volt US. S.C.E. The coulogram that is curve A of Figure 2 indicates that at a controlled-potential of -0.50 to -0.75 volt US. S.C.E. Tc(VI1) can be reduced to Tc(II1) in buffered sodium tripolyphosphate medium of p H 4.7. Coillogram d of Figure 2, also indicates that pretitration a t a voltage from -0.10 to -0.20 volt US. S.C.E. is feasiblc. The pretitration is desirable, as it can be used to eliminate possible interfering ions that react a t potentials more positive than -0.2 volt us. S.C.E. The effect of pH on the reduction of Tc(VI1) in an unbuffered medium was studied. The time required for the reduction of Tc(VI1) in unbuffered medium is considerably longer than that required in buffered medium. Instead of 10 minutes as required for a buffered titration of 0.5 mg. of Tc(VII), a t least 30 niinuteq was required for an unbuffered titration of tlie same quantity of Tc(T'I1). Also, for the unbuffered titration the final pH of the solution is less than the initial pH; this is indicated in Table 111. As the pH decreases, the potential required for the Tc(VII) + Tc(II1) reduction becomes less negative. The decrease in p H makes difficult the quantitative determination of Tc(VI1) in unbuffered sodium tripolyphosphate medium. When the potential is controlled a t one value and the pH decreases, then as the potential required for the Tc(VII) + Tc(II1) reduction becomes less nega-
tive the controlled :?otential may no longer be on the plateau. If the applied potential is sufficiently in excess of t h a t required for the reaction, the next step in the reduction-i.e., Tc(II1) + Tc(I1)-will occur. Because the Tc(II1) +. Tc(I1) 1,eaction does not reach completion, it it; necessary t,o control the pH b y m:ans of a buffer throughout the titration. The effect of t’he concentration of sodium tripolyphosphate was studied. The most precise anlS accurate results are obtained when sodium tripolyphosphate is present in concentration from 3 t o 570 (w./v.). The coulometric reduction of Tc(VI1) is made with best accuracy and precision if the quantity of Tc(VI1) titrated is in the range from about 0.5 to 5 mg. Within this rmge, the method is accurate to =klY0 :is shown in Table IV. The minimum amount, determinable is limited by the relatively large error in the read-out voltage measurement for small quantities and by the large background current. Both of these factors cause high results. The contribut,ion from the total background current is of the order of 0.06 coulomb for a 10-minute reduction. When less than 0.5 mg. of Tc(V.[I) is reduced, the maximum current consumed in the reduction mill be 11.56 amp.-second (coulombs). The contribution from the total background cuyrent will be 4y0 of the total current used in the reduction. T h e time required for the reduct,ion is roughly proportional to the weight of Tc(VI1) present, and all determinations must be corrected for background current. The long time required for the titration limits the maximum amount of Tc(VI1) determinable with convenience to about 5 mg., for which quantity E,bout 0.5 hour is required for the reducl’ion. The precision of this method is also indicated by the data in Table IV. The relative stanclnd deviation is ahout 0.5%. Interferences. Solutions were prepared t h a t contained both Tc(VI1) and one of several 3ossible interfering ions. T h e results of t h e coulometric reduction of Tc(VI1) i n t h e presence of t h e po:;sible interfering ions are shown in Table V. KOinterference is found with Ce(IV), Co(II), Cr(VI), Cu(lI), Eu(III), M n (VII), RelVII), Zn([I), C1-, Clod-, and S04-2 a t the levels tested. All metal ions were added as the sulfate or chloride or as the potassium salt of an anion that contained the polyvalent metal ion. The anions were added as either the acid or the sodium salt. To check for possible interferences by interaction among the potential interfering ions, a solution was prepared t h a t contained 0.590 mg. of Tc(VI1) and about 0.05 mg. of each of
the followine ions: CelIV). Co(I1). Cr(VI), c ~ T I I ) , E ~ ( I I I ) , ’bIn($IIji ~ ~ ( ~ 1~1l 1- clod-, , and so4+. When this solution was analyzed, 0.589 mg. of T c was found. This satisfactorv result indicated that the noninterfering ions can be present either independently or together. The following ions interfere strongly: Fe(III), Mo(VI), Ru(IV), U(VI), V(IV), F-, and SOs-. To eliminate these interferences. TclVIII can be removed from the solution b$ distillation from “pure H2S04” (sic) (10). If i t is desired t o separate Tc(VI1) from Re(VII), the Tc(VI1) must be distilled from perchloric acid. ACKNOWLEDGMENT
The authors acknoTvledge the assistance received from P. F. Thoniason throughout this research.
Table IV.
Table (11. Change in pH in Unbuffered Controlled-Potential Coulometric Titrations of Tc(VII)
Composition of test solution: Tc(VII), 0.590 mg.; [I;aaP301~], 57” w./v. (added as 10 ml. of 6% w./v. KasPaOio); [HzS041, necessary to adjust initial pH to indicated yalue. Volume of test solution, 11 ml. f. 25’ C. Controlled ootential. -9.70 volt us. s.c.l3. Initial
pH
4 5 7.0 9.0
Terminal 2.5
5.5 8.0
Tc(VI1) found,a mg.
1.00 0.90 0.48
a Values calculated from read-out voltage after 1 hour of titration; background current of 100 pa. was never reached.
Precision and Accuracy of the Determination of Tc(VII)
Composition of test solution: Tc(VII), mg. as indicated: ITasP3010, 10 ml. of 6fj, w./v. SasP3010. Buffer, 2 ml. of 311 iXH4C2H302:5.5kfCHBCOOH buffer solution. pH, 4.7. T , 25” C. Controlled potential, -0.70 volt us. S.C.E. Tc(VII), mg. No. of Relative std. Relative dev., 70 error, 7; titrations Prebent Found, av. 0,472 0.470 10 0.5 -0.5 0.590 0.592 8 0.2 +0.3 0.944 0.944 10 0.2 0.0 -0.1 7 0.1 1.414 1.416 -0.4 0.1 10 1.880 1.888 -0.1 ... 2 4.717 4.720 -4.4 ... 1 9.440 9.027
Table V. Results of Check of Interferences in the Determination of Tc(VII) Composition of test solutions: Tc(VII), 0.590 mg.; [Na5P30m],5% w./v. (added as 10 ml. of 6% w./v. r\’aaPaOlo); [KH4C2H302],0.5M (added as 2 ml.. of 3111 NHdC2Hdh); ICHXOOHl. 0.9M (added as 2 ml. of 5.5M CHnCOOH). Foreign ion, as indicated. volume of iest solution, 13 ml. T , 25’ C. Controlled potential, 10.70 volt vs. S.C.E. Foreign ion ______ .%mount present, Tc(VI1) found, mg. Recovery, yo Identity mg.
Ce( IV) Co(I1) Cr(V1) Cr(V1) Cu(I1)
Eu(II1)
Mn(VII) Re( VII) Zn(I1) Mixture“ C1c104-
so,-2
Fe(II1)
Mo(V1) Ru(IV) U( VI 1 V(1V) F-
a
Soninterfering Ions 0.590 0.70 0.590 0.25 0.594 0.26 0,603 0.52 0,590 0.64 0.590 0.60 0.590 0.54 0.590 0.76 0.590 0.36 0.589 0 . 470b 216 0.590 0.70 0.471b 49 Interfering Ions 0.28 0.46 0.50 0.60 0.50 0.50 62
0.693 0.890 0.663 1.484 1.583 5.445 1.200b
100 100
101 102
100 100 100 100 100 100 100 100
100
117 151 112 252 268 923 255
See text for composition. Only 0.471 mg. of Tc(VI1) present.
VOL. 35, NO. 6, MAY 1 9 6 3
617
(6) Magee, R. J., Scott, I. A. P., Wilson, C. L., Talanta 2, 376 (1959).
LITERATURE CITED
( 1 ) Boyd, G. E., J. Chem. Educ. 36, 3 (1959). (2) Farrar, L. G., Thomason, P. F., Kelle,y, M. T., ANAL.CHEM.30, 1511 (1958). (3) Howard, 0. H., Weber, C. W., Zbid., 34,530 (1962). (4) Kelley, M. T., Jones, H. C., Fisher, D. J., Ibid., 31,488,956 (1959).
( 5 ) Kelley, M. T., Miller, H. H., Ibid., 24, 1895 (1952).
(7) Miller, F. J., Thomason, P. F., ANAL. CHEM.33,404 (1961). (8) Zbid., 32, 1429 (1960). (9) Miller, H. H., Kelley, M.T., Thomason, P. F., in I. S. Longmuir, “Ad-
vances in Polarography,” Vol. 1, pp. 716-26, Pergamon Press, New York,
1960. (10) Parker, G. W., Reed, James, Ruch, J. W., U. S. Atomic Energy Commission Rept. AECD-2043, Jan. 9, 1948.
(11) Shults, W. D., Thomason, P. F.; ANAL.CHEM.31,492 (1959). (12) Thomason, P. F., U. S. Atomic Energy Commission Rept. ORNL-2453, Dec. 31, 1957.
RECEIVED for review September 20, 1962. Accepted November 21, 1962. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. The Oak Ridge National Laboratory is operated by the Union Carbide Corp. for the U. S. Atomic Energy Commission.
Some Applications of Ion Exchange Membranes in Automatic Coulometric Aqueous Acid-Base Titrations PAULINE P. L. HO and MAX M. MARSH Analytical Research Department, Eli Lilly and
b Anion and cation exchange membranes, combined in a single compartment, have been applied to the separation of the anode and cathode compartments in coulometric acid-base titration. In the range of 0.200 to 1 .OOO meq. of acid or base, the standard deviations were found to b e less than f0.004. An automatic coulometric titration setup i s described.
I
N THE usual coulometric procedure.
particularly for internal electrolytic generation of reagent, a salt bridge or a sintered-glass disk is commonly employed to separate the cathode and anode compartments. However, in many cases-e.g., the electrolysis of water in acid-base titrations-a clearcut separation is difficult because of the high mobilities of the hydronium and hydroxyl ions. Feldberg and Bricker ( I ) found that a synthetic cation exchange membrane \$ as satisfactory as a cathode isolator in coulonietric titration of bases. Although these membranes were not 100% permselective, they were aufficiently selective to prevent the mixing of anodic and cathodic oyidation reduction products. Hydrostatic pressure exerts almost negligible effect on these membranes; this, combined with inertness and ion selective properties, provides a distinct advantage over the sintered-glass disk or other barrier for electrode isolation. T o facilitate the coulometric titration and overcome the tedious procedure in ordinary potentiometric titrations, in which a preliminary sample must be run in order to preset the pH change at the end point, an automatic titration apparatus has been set up. It consists of a coulometer (3) for current supply, a titration compartment, and a simple
618
ANALYTICAL CHEMISTRY
Co., Indianapolis, Ind.
end point-detecting circuit. The end point of a titration can be detected either spectrophotometrically or potentiometrically with the XIalmstadt second derivative end point termination method (4, 5 ) . The acid-base titration procedure is further simplified by mounting cation and anion exchange membranes together inside the electrode isolation cell, so t h a t the same cell can serve for both purposes. Only the total resistance across the membranes increases. The perniselective capacity of each type of membrane is not altered. EXPERIMENTAL
Apparatus. T h e basic components of t h e automatic spectrophotometric titrator are illustrated in Figure 1. An integrating motor type of coulometer (5’) was used for the electrolytic generation of H f and OH- ions. The quantity of electricity passed through the titration cell is directly proportional to the voltage applied to the terminals of a special low inertia motor on which a mechanical counter is geared to the motor shaft; thus, the integrated current is directly registered as number of counts instead of involving exact measurement of time for current flox at constant current. The coulometer has been calibrated in terms of counts per milliequivalent, using the silver deposit method. The titration compartment lvas made by modifying a n obsolete spectrophotometer. The light source consists of a coiled filament incandescent lamp, connected to the line through a voltageregulating transformer. Wavelength, in the region of 380 to 700 mp, can be easily selected by turning the grating of the monochromator via a wavelength dial on top of the box. The sample holder used for this experimental work was a 100-ml. borosilicate glass
beaker, but beakers ranging from 50 to 150 ml. can be used. For the maximum efficiency of mixing, a motordriven glass stirrer was mounted on the cover of the titration compartment; the speed could be adjusted from 0 to 2000 r.p.m. by turning the knob of a potentiometer mounted on the box. The titration cell assembly is shown in Figure 2,.4. Both the coulometer and the stirring motor were connected to the relay of the control system (4). They can be started simultaneously when the automatic button is pushed, and stopped when the inflection point of the titration is reached. The light detector is an inexpensive CdS photoconductive cell. The output signal is converted to voltage through a simple electric circuit and fed into the input of the control unit, similar to the one in the commercially available Sargent-hlalmstadt Spectro-Electro titrator (6). The setup is suitable for both spectrophotometric and potentiometric titrations. For potentiometry, the voltage signal between the two electrodes is fed into the control unit directly. Adaptors for the mounting of electrodes have been made on the cover of the titration compartment. A positive voltage swing is required for proper operation of the commercial control unit at the end point of the titration. At a potentiometric end point, electrode reactions employing a change in electromotive force in either a positive or negative direction (with respect to a reference electrode) were expected to be encountered; a t the spectrophotometric end point a n increase or a decrease in absorption mas anticipated. T o make these reactions or absorption changes compatible with the control unit, a reversible switch was connected to the output terminals of the light detector and the electrodes, so that proper signal could always be applied to the input of that unit. The cation exchange membrane, Nep-
