Teflon Dropping-Mercury Electrode for Polarography in Hydrofluoric Acid and Other Glass-Corroding Media Evaluation with Thallium(1) e Thallium Reaction HELEN
P.
RAAEN
Analytical Chemistry Division, Oak Ridge National Iaborafory, Oak Ridge, Tenn.
b The Teflon D.M.E., which is intended for polarography in media that corrode glass, was evaluated further. To learn whether data taken with glass and Teflon D.M.E.'s are interchangeable, the polarographic charTIo reaction acteristics of the TI+ in 0.1M KCI-1mM HCI were determined with a Teflon D.M.E. The results are in agreement with known polarographic characteristics of the TI+ S TIo reaction established with glass D.M.E.'s. The agreement indicates that polarograms obtained with Teflon D.M.E.'s have the same meaning as those taken with glass D.M.E.'s and can be analyzed in the same way. Data taken with the two types of electrodes are, indeed, interchangea ble. Furthermore, the results ensure confidence in the certainty of measurements made in glass-corroding media with the Teflon D.M.E.
*
D
fabrication, and initial work with the Teflon dropping-mercury electrode (D.M.E.) were described earlier ( I S ) . Additional experiments have now been made to show more exactly that polarograms from the Teflon D.M.E. have the same meaning and can be analyzed in the same way as those from glass D.M.E.'s and that data, taken with the two types of electrodes are interchangeable. Thereby, confidence in the certainty of measurements made in glass-corroding media with the Teflon D.M.E. is ensured. Fundamental polarographic data were taken with a Teflon D.M.E. for the sysT1° in 0.1JI KCl-lmM tem T1+ HCl. Experimental conditions other than the supporting medium were made to be identical with those under which the Teflon D.M.E. is intended for use in glass-corroding media. The data obtained include an electrocapillary curve of mercury in the supporting medium; undamped regular, average-currents regular, and first-derivative polarograms of the supporting medium and of 0.02 to 1.OmJf solutions of TI+; values for the half-aave potential (E14, the potential at which the height of the first-derivative ESIGN,
2420
ANALYTICAL CHEMISTRY
wave is maximum ( E p )and , the electron change (n) for the T1+ +. TI0 reduction; data that show the reversibility and diffusion control of the T l + S T1° reaction; and data that show a linear relationship between T1+ concentration (C) and firstderivative peak height, The experimental results are in agreement with the polarographic characteristics of the T1+ e T1° redox reaction that have been established with glass D.M.E.'s. EXPERIMENTAL
Reagents. All the solutions were prepared from ACS reagent grade chemicals and triple distilled water. Test solutions were deaerated with nitrogen or argon t h a t had been passed through a wash of the supporting medium. The gas was passed over the test solution during the analysis. SUPPORTING LfEDIUM, 0.1-Jf KC11mM HC1. STANDARD SOLUTION5 O F Tl+, 0.02 to 1.Om.l.I. A 1.Om.M stock standard solution of T1+ was prepared by dissolving 60 & 0.1 mg. of anhydrous thallous chloride, TlC1, in 250 ml of the supporting medium. More dilute solutions of T1+ were prepared by diluting suitable aliquots of the stock solution with the supporting medium. Instrumentation
and
Apparatus.
POLAROGRAPH. T h e ORNL Model Q-1988h controlled-potential and derivative polarograph ( 4 ) was used in its three-electrode function both as a source of potential and for obtaining the polarograms. X-Y RECORDER.An Electro Instruments, Inc. (8611 Balboa, San Diego 12, Calif.), Model 500 X-Y recorder was used to record the polarograms. ELECTRODES. Indicator. The indicator electrode was a Teflon D.11 .E. ( I S ) that consisted of a 21-em. long glass capillary segment of 70-micron orifice diameter and a Teflon segment of 72-micron orifice diameter. Reference. A saturated calomel electrode (S.C.E.), usually a t 25" f 0.2" C., was the reference electrode. Counter. The counter electrode was a length of 16-inch diameter graphite drawing lead (9030-1 P, HB, A. \V. Faber-Castell Pencil Co., Inc.,
Sewark 3, K. J.). Except for the conical end, the part of the electrode exposed in the electrolysis cell was rubbed with Kel-F grease or coated with beeswax to protect it from the test, solution. SALTBRIDGES. For all work except the accurate measurement of values, the S.C.E. was connected to 0.1M KC1 solution via a bridge of saturated KCl solution contained in unfired Vycor tubes. .I XaF-agar agar salt bridge joined the KC1 solution to the test solution. The total resistance through the S.C.E., bridge system, and a simulated test solution was about 15,000 ohms, a resistance well within the capabilities of the Q-1988.4 po1arograi)h when used as a three-electrode instrument. The KaF-agar agar gel was prepared as follows. A 3-gram quantity of agar agar (Sargent I-.S.P. granular) was dispersed in 100 ml. of ivater. The mixture was heated in a boiling- water bath to give a clear solution. .4 n-gram quantity of NaF was dissolved in the hot solution. (The order of addition of agar agar and NaF is significant because it is not possible to obtain a rigid gel if the X a F is dissolved in the water before the agar agar is added.) With a large syringe, the solution was drawn into a Tygon tubing-Kel-F tubing bridge, which was plugged at the Kel-F end with a disk of l'a-inch thick porous Teflon (Pall Corporation, Glen Cove, N. Y.). \Vhen the solution in the tubing had set t'o a gel, the bridge was ready for use. The Teflon-plugged end is lilaced in the test salution. For the accurate measurement of values, a bridge of saturated KCl solution was used. INSTRUMENTATIOK F O R .\UTOMATICALLY ~ I E A S U R I m N GA N D t . Thi. instrumentation has been described previously ( 1 4 ) . POTENTIOMETER. -1 Rubicon potentiometer of 0- to 1.61-volt range was used to determine accurately the potentials a t the D . M . E . RESULTS A N D DISCUSSION
The TI+ .+ T1° reaction in chloridecontaining medium selected for this study is a simple reaction often uqed a< a reference in polarographic woi k. For this reaction, the literature contains
I
I
I
1
4.0
0
% 3.c
--
.?
W
I Ia 0 LT
0
2.c
1.0 P O T E N T I A L , volts vs. S.C.E. Figure 1 . Electrocapillary curve for mercury in 0.1M KCI-1 rnM HCI determined with the Teflon D.M.E. a t 25" C.
fundamental polarographic data, taken with glass D.M.E.'s, that can be compared with the Teflon D.M.E. data. Certain conditions apply to all the data presented here for the Teflon DJ1.E. The data were taken a t controlled potential with reference to a S.C.E., which was usually a t 25" i 0.2" C. In no case was a maximum suppressor present in the test solution. The id and (di/dt),,, values were all corrected for the contribution from the supporting medium. The X-Y recorder has the advantage that one polarogram can be recorded on top of another, thus permitting small differences among polarograms to be detected readily. The precision of the recording of polarograms was checked from time to time by using this feature of the X-Y recorder. The negligible differences among the polarograms indicated excellent precision. Electrocapillary Curve in Supporting Medium, 0.1M KC1-lmJf HCl. A plot of drop time ( t ) as ordinate against potential of the D . M . E . (Edr) us. S.C.E. was determined with t h e Teflon D . M . E . in the supporting medium, 0.1M KC1-lmM HC1, at
25' C. The plot is shown in Figure 1. The shape of the curve and the electrocapillary maximum (-0.55 volt) are consistent with those reported for mercury in 0 . M KC1 with a glass D.1I.E. (7). Polarograms of Reducible Ion, TI +. With the O R S L 1Iorlel Q-l988.-1 polarograph, it id possible to take average-currents regular and firstderivative polarograms, as well as undamped regular polarogranis. Polarograms of each of these types were recorded for both the supporting medium and solutions of T1-. The polarograriis prepented here are typical of a large number rerorded with the Teflon D.11I.E.during the study. ~ N D A M P E L ) REGLLARPOLAROGEAMS. Figure 2 shows an undamped regular polarogram. The uniformity of the recordings of the oscillations indicates that drop formation at the Teflon D.1I.E. is precise. Polarograms of this type taken with the Teflon D.1I.E. have the same appearance as those of the same type obtained with glass D.AI.E.'s. Because average-currents regular and derivative polarogranis can be analyzed more easily than undamped regular polarograms, they were used more often than undamped regular polarogranis to obtain the polarographic data. of the average-currents regular type are shown. The data obtained in the test of the equation of the wave arc' also shown superimposed on the polarographic wave for T1+ and are discussed separately below. With the X-Y recorder, it is possible to record, on the same chart and from the same starting point, both the polarogram of the supporting medium and the polarogram of the reducible ion in that medium. The value for the diffusion current (id) is then measured
-020
-1.0
-040 -060 -0.80 POTENTIAL, volt vs. S.C.E.
Figure 2. Undamped regular polarogram for the TI+ + TIa reduction taken with a Teflon D.M.E. Test solution: 1.OmM TIC1 in 0.1M KCI-lmM HCI. Voltage scan rate: 0.1 volt per minute. Voltage scan direction: positive to negative. h: 1 1 3 cm.
directly as the difference between the heights of the t x o polarograms a t some point in the voltage region over which the polarograms are parallel. The forms of the polarograms of Figure 3 are excellent: the shape of the T1+ wave is very similar to that determined with a glass D.1I.E. for l m J l TlCl in 0.9JI KCl (8). .\lthough the test solutions did not contain a maximum suppressor, the average-currents regular polarographic waves for T l + in the concentration range studied give no evidence of maxima. FIRST-DERIVATIVE POLAROGRAMS. Typical first-derivative polarograms obtained for the supporting medium and for two concentrations (0.02 and l.Oni_M) of Tlf in that medium are shoxn in
6
0.0-
d
3.
I-W IL: IL: 3 0
z
W
U
W
a CT
W
>
a
Figure 3. Average-currents regular polarogram for the TI+ + TIo reduction taken with a Teflon D.M.E., and test of the equation of the wave Test solutions: polarogram A, 0.1M KCI-1mM HCI; polarogram B, 1.OmM TIC1 in 0.1M KCI-1rnM HCI. Voltage scan rate: 0.1 volt per minute. Voltage scan direction: positive to negative. h : 1 1 3 cm.
A
V
-0.20
-0.40
-0.60 -0.00 P O T E N T I A L , v o l t vs. S.C.E.
-!.O
VOL. 36, NO. 13, DECEMBER 1964
2421
0.1 volt /min.
7-
I/ L
3 pa. mi". 0.05 volt / min.
0.02 volt / min.
A
v
0.20
-0.40
-0.20
, e-
-
-0.60
-0
P O T E N T i A L , v o l t v5. S.G.E.
Figure 4. First-derivative polarograms for the TI+ reduction, taken with a Teflon D.M.E.
-+
TIa
Test solutions: polarograms A, 0.1 M KCI-I m M HCl; polarograms E: frame I, 0.02mM TIC1 in 0.1M KCI-ImM HCI: frome 11, O . l m M TlCl in 0.1 M KCI-1 m M HCI. Voltage scan rate: 0.1 volt per minute. Voltage scan direction: positive to negative. h: 1 1 3 cm.
Figure 4. The forms of these polarograms are the same as those of the derivative polarograms recorded by Belew ( 1 ) with a Sargent 2- to 5-second glass capillary for a system which differed only in that it contained O . O l ~ ogelatin as a maximum suppressor. Belew also used an ORSL Model Q-1988h polarograph. The excellent sensitivity possible with the derivative technique is evident from the height of the firstderivative polarogram of the 0.02mM T1+ solution (Figure 4, frame I ) . This polarogram is similar in every respect to that shown by Schaap and AlcKinney (15) for O.OlmM TI+ in 0.lW KC1, which they recorded with a glass D.M.E. and an ORNL Model Q-1988.l controlled-potential derivative polarograph. The effects of the damping circuits
-0.30
0 02 0 05 0 10 b
Test solution: 1 .OmM TlCl in 0.1 M KCI-1 m M HCI. Voltage scon rote: as shown. Voltage scan direction: porititre to negative. h: 1 13 cm.
in the polarograph as a function of voltage iLan rate on the characteristics of the first-derivative polarograms taken with a Teflon D.>l.E. are shown in Figure 5 . D a t a from that figure are presented in Table I, where they are conipared with some of the same types of data determined by Fisher, Belew, and Kelley (3) with a glass D.;LI.E. in the presence of O . O l ~ o gelatin. The similarity in the performances of the Teflon and glass D.M.E.'s is again evident. Polarographic Characteristics of Redox Reaction, T1+ s TlO. For the T1+ T1° reaction, the polarographic characteristics determined with Teflon and glass D.lI.E.'s are qummarized in Table I1 for ease cf comparison. T h e d a t a taken with the Teflon D . M . E . are discussed below.
e
2422
-
Data taken from Figure 5 Test solution: l.OmM TlCl in 0.1.11 KCI-lmM HC1 Voltage scan direction: positive to negative Current range: 30 pa. h : 113 cm. n Calculated E,, volt us. S.C.E. from W1/zn ________Teflon Glass W1,2, Teflon Glass (di/dt)max, D.1I.E.b mv. D.M.E. D.>Z.E.b D , M E. pa./min. 6 45 -0 458 -0 463 88 1 03 1 00 15 6 -0 468 -0 476 95 0 954 0 964 29 9 -0 480 -0 490 98 0 924 0 906
n = 90 6/W1/*,where W l i zis expressed in millivolts ( 6 ) From Fisher, Belew, and Kelley (3)
0
ANALYTICAL CHEMISTRY
-0.70
Figure 5. Effects of damping circuits in the polarograph as a function of voltage scan rate on first-derivative polarograms of TI+ taken with the Teflon D.M.E. (Table I)
Table 1. Effects of Damping Circuits in Polarogroph as a Function of Voltage Scan Rate on First-Derivative Polarograms Taken with Teflon D.M.E.
Voltage scan rate, volt/min.
-0.40 -0.50 -0.60 POTENTIAL, volt vs. S.C.E
HALF-WAVE POTENTIAL (Eljz) AND POTENTIAL AT WHICH HEIGHT OF FIRSTDERIVATIVE W A V E IS M A X I M U M (E,). For the Teflon D.M.E., the approximate observed half-wave potential, measured gTaphically from an undamped regular polarogram that was recorded during a positive-to-negat,ive voltage (Figure 2 ) . scan is -0.45 volt us. S.C.E. The average of the (Eli2)oba values obtained graphically from seven averagecurrents regular polarograms -e.g., Figure 3-also recorded during a positiveto-negative voltage scan is -0.477 volt us. S.C.E. (range, -0.473 to -0.480 volt). The Ellz value was also measured accurately. For these measurements, a Table II. Comparison of Polarographic Characteristics of TI+ 5 TIo Reaction Determined with Teflon and Glass D.M.E.'s
Polarographic characteristic ~. El1*,volt V S . ~~
S.C.E.'
E,, volt u s . 8.C.E.b nb
Reciprocal slope of
Type______ of D.M.E. Teflon Glass. -0 460
-0.459 (f2) -0.460 ( 1 0 , f l )
-0.460
-0.463 1.00
1.05
f3j
(3,9 )
log [ i / ( i d - i)] L's.
E d e ,volt 0.0608 0 0.i9Ic f9j a T-alues ta,ken from the references
indicated. Determined a t a scan rate of 0.02 volt per minute. Theoretical value. ,
bridge of saturated KC1 solution was used between the test solution and the S.C.E. instead cf the NaF-agar agar/ 0.LU KC1 bridge. Average-currents regular and first-derivative polarograms were recorded for voltage scans made in both t h e positive-to-negative and negative-t'o-positive directions, and the averof the two recordings age of the (El,P)oba's was taken as the El,*value. The following Eli2values (volt us. S.C.E.) were obtained: -0.461 (average-currents, 0.02 volt per minute) and -0.460 (first-derivative, 0.02 volt per minute). These values are in agreement with the value -0.459 ,volt us. S.C.E. reported by Lingane (12) for the T l + + T1° reduction in 0.1JI KCl that contained 0.01% gelatin and also with the \-slue -0.460 volt us. S.C.E. reported by Kolthoff and Lingane (10, 1 1 ) for t,he reduction in 0.1M KC1 and in O.lAIIHCI. The values determined with the Teflon D.hl.E. are also consistent with the presumably average value -0.50 volt reported for the TI+ -t. T10 reduction in 0.1 to 1X chloride medium ( 2 ) . The potential a t which the height of the first-derivative wave is maxiniuin (E,) is a function of the voltage scan rate (Figure 5 and Table I) because the time lag of the parallel-T filter becomes significant at faster scan rates. Only the value for E , that was determined at' a scan rate of 0.02 volt per minute is included in Table 11, where it is favorably compared with the value measured by Fisher, Belew, and Kelley (3). For each type of D.M.E. a t the slowest scan rate, E , is the thermodynamic Eli*. ELECTRON CHANGE(n). Froin data taken with the Teflon D.M.E., n for the TI+ 4 TI0 reduction was determined with a plot of log [il(id- i)] versus Edc (Figure 3). For the reduction of a unipositive metal ion, t,he theoretical value for the reciprocal of the slope of the straight-line portion of such a plot is 0.059 volt (9). The reciprocal of the slope calculated from the polarograin of Figure 3 is 0.0603-i.e., 0.210 volt/ 3.48 --which is in excellent agreement with the theoretical value and thus indicates that n = 1. The value for n was also determined from the relationship n = 90.6/T5'l,~( 5 ) , where TT-,,, is the width of the firstderivative polarographic wave at halfpeak height for a polarogram recorded at sufficiently slow voltage scan rate (0.02 volt per minute). The average value of n for the T l + + T1° reduction determined from five such polarogram is 1.05. The Teflon D.X.E. is thus shown to be suitable for establishing the n for the reduction of a unipositive nietal ion. REVERSIBILITT. With the Teflon D.M.E., data were taken that show the reversibility of the T l + f TI0 redox reaction. Some of these data are given in Figure 3, where the test for the equation of the wave
Table 111.
Data That Indicate Suitability of Teflon D.M.E. for Showing Diffusion Control of TI+ 3 TI0 Reduction Test solution: l.0mM TlCl in 0.1M KCl-lrnlll HC1 Yoltage scan rate: 0.1 volt per minute Voltage scan direction: positive t o negative T: 25O 0.2" c. t at h = 113 em.: 3.58 seconds m at h = 113 cm.: 2.28 rng. per second P b a & at h = 113 em.: 1.54 c m . From firstderivative uolaroarams From averagecurrents polarogranis ____
*
i d / h i 2)
h,a cm.
hIt2,cni.1/2
78.5 87 0
8 86
9 29 9 75 10 18 10 56
95 0 103 5 111 5 121 5
11 03
pa./cm. ' I 2
i d , pa.
5 5 5 6 6 6
27
0 59 0 60 0 61
55 92 22 34 50
8 53 8 9 9 10
0 61
0 60 0 59
90 30 80 20
0 0 0 0 0
92 91 91 93 92
Corrected for
is shown together with the averagecurrents regular polarograms froin which the values needed to test the equation were taken. The data for the test of the equation of the wave are comparable with those reported for 1m.U TlCl in 0.9JI KC1 (8). The plot of log [i ( i d i)] meets the three criteria of a reversible electrode reaction: the plot has a straight-line region; the reciprocal of the slope of the straight-line region is the theoretical slope for a one-electron reduction; and the Ede a t which log [iI'(-a d i)] is zero is the Ell2. An additional, although not infallible, criterion of reversibility of the electrode reaction is the constancy of El!z with change in C. By deduction, E , should also remain constant as C is changed. This criterion was likewise met in a test made with first-derivative polarograms. For eight polarograms representative of T1+ concentrations in the range from 0.02 to 1.0ni31, the values for (Ep)Ob8 were constant (-0.477 to -0.480 volt us. S.C.E.) ; the polarogranis were recorded at a scan rate of 0.1 volt per minute. DIFFUSIONCOXTROL.The data of Table I11 indicate the suitability of the Teflon D.X.E. for showing diffusion control of the T l + + TI0 reduction. Over a range of values for mercury height ( h ) , the ratios id, h1'2 and (di 'dt),,, -___ hl,z are given. The h values were all corrected for back pressure, P b a & = 1.54 cm., calculated from the equation Pback =
3.1/ (mt)1'3
(2)
which is the equation used to determine this correction for a glass capillary (6). Relation of Concentration (C) of Reducible Ion, T1+,to P e a k Height, (di/dt)m*x. The relation between C for T1+ and (di/dt),,, over the con-
centration range fi,oni 0.02 to 1.0mJI is linear; the line passes through the oiigin. The derivative polarograms for the solutions a t the extremes of this concentration range are shown in Figure 4. The data indicate that a polarographic calibration curve for a reducible unipositive nietal ion can be established satisfactorily with a Teflon D.1I.E. ACKNOWLEDGMENT
P. F. Thomayon supervised this work and advised throughout the course of it. In frequent discuysions, D. J . Fisher, IT. L. Belew, and R. W. Stelzner contributed much information that was needed in the work. LITERATURE CITED
W. L., ORXL Analytical Chemistry IXvision, unpublished data. ( 2 ) Brezina, M.,Zuman, P., "Polarography in lledicine, Biochemistry, and Pharmacj-," p. 735, Interscience, New York. 1938. ( 3 ) Fisher, D. J., Belew, \V. L., Kellev, Kelley, M.T., ASAL. CHEX.,in press. (4) Kelley, 31. T., Fisher, 11. J., Cooke, W. I)., I).> Jones, Jones,,,H. H. C., in "ildvances in Polarography, I. S. Longmuir, E Polarography," Ed., d, Yol. 1, pp. 158-82, Pergamon Press, New York. 1960. ( 5 ) Kelley, 11. T., Jones, H. C., Fisher, D. J., 4 s ~CHEM. ~ . 31, 1475 (1959). (6)Kolthoff, Kolthoff, I. M., Lingane, J. J., "Polarograph,~,"2nd ed., 1-111.I , pD.. 86, "Polarogra~)hv," Interscience, Yew York. York, 1952. . ( 7 ) Zbzd., Zbid., I). 88 88. (8) Zbid., p. 193 ( 9 ) Zbid., p. 194. (1) Belew,
(10) Zhld..> D 198 ~ - ( l l j I b i d . , 5'01. 2 , p. 520. I
(12) Lingane, J. J., J . B m . Chem. SOC. 61, 2099 11939). (13) Raaen, H. P., ANAL.CHEM.34, 1714 (1962). (14) Raaen, H. P., Jones, H. C Zbcd., p. 1594. (15) Schaap, W. B., NcKinne?, P. S., Zbid.,36, 29 (1964). RECEIVEDfor review June 16, 1964. Accepted September 14, 1964. Research sponsored by the C . S. Atomic Energy Commission under contract with the Union Carbide Corporation. ~
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