of the complex with ~ 0 . 4and ~ a theoretical slope equal to 0.05919. I n Figure 2 there are three distinctly linear regions. One extends from 0 to 1.5 p 0 . k units and displays a slope of 0.12. I n this region it is concluded that two acetate groups are associated with the europium(II1) ion. h second linear region extending from 1.5 to 2.5 pOAc units with a slope of 0.062 suggests that the europium(II1) here is complexed with only one acetate. Finally a third region with zero slopf3 is observed for p 0 A c values greater than 2.5. Here no complexation of the europium with acetate occurs. This conclusion is supported by the fact that values observed for Eu(II1) in 1JI XaC104 solution, where complex formation definitely does not occur, are in good agreement with the value of -0.59 volt us. S.C.E. found for this zero slope region (1).
Least square analysis of the data depirted in Figure 2 yielded values of -0.125 + 0.004 and .-0.0617 0.003 volt for the slopes and -0.828 + 0.004, -0.745 0.006, and -0.590 0.005 volt us. S.C.E. for the intercepts of the various linear segments. From these
*
* *
the following values were calculated for the stability constants of the acetate complexes a t a temperature of 25" C. and an ionic strength of 1 : Eu3+
+ OXc- : E u ( O d c j 2 + ; K1 = 324
Eu(OAcj2+
+OhEu(OAc)Z+; KP
=
20.4
These values compare well with the values of 202 and 39.7 for the corresponding formation constants of acetate ion with europium(II1j reported by Kolat and Powel (7). These latter values were determined by potentiometric measurement of the hydrogen ion concentration a t 20" C. in an ionic strength of 0.1. LITERATURE CITED
(1) Anderson, L., Macero, D. J., J . Phys. Chem. 67, 1942 (1963).
( 2 ) Delahay, P., "Yew Instrumental Methods-in Electrochemistry", p. 195, Interscience, Kew York, 1954. (3) Delahay, P., Mattax, C. C., J . Am. Chem. SOC.76, 874 (1954). (4) Furlani, C., hlorpurgo, G., J . Electroanal. Chem. 1, 351 (1959/60). (5) Gierst, L., Cornelissen, P., Collection
Czech. Chem. Communs. 25, 3004 (1960). ( 6 ) Holleck, L., Z. Electrochem. 45, 249 i1939).
( 7 j Kc&, R. S., Powel, J. E., Inorg. Chem.
1, 293 (1962). (8) Laitinen, H. A., Taebel, W. A , , I N D . ENG.CHEM.,ANAL.Eo. 13, 825 (1941). (9) Macero, D. J., Rulfs, C. E,., J . Am. Chem. SOC.81. 10.64) --, -2942 " - - i,-IVY,. (10) hfisumi, S., Ide, Y., Bull. Chem. SOC. Japan 32, 1139 ( 1959). (11) Noddack. W. , Bruckl, A., Angew. Chem. 50. 362 i l v d i I (12) Onstott, E. I., J . rim. Cliem. ~ o c . 74, 3773 (1952). (13) Palke, IT. F., Russell, C. D., Anson, F. C., ANAL.CHEM.34, 1171 (1*62). (14) Rulfs, C. L., J . Am. Chem. SOC.76, 2071 (1954). (15) Sand, H. J. S., Phil. Mag., 1, 45 i1901). (16) Shain, I., Llartin, K. J., J . Phys. Chem. 65, 254 (1961). (17) Relcher, F. J., "The Analytical Uses of EDTA", p. 181, Van Nostrand, Princeton, K. J., 1958. ,>"m\
RECEIVEDfor review April 17, 1962. Resubmitted June 18, 1964. Accepted February 17, 1965. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962. Research supported in part by a NSF Cooperative Fellowship awarded to one of us (HBH) by the National Science Foundation.
Dropping-Mercury Electrode of Teflon for Polarography in Hydrofluoric Acid and Other Glass-Corroding Media Evaluation with the Lead(I1)
Lead Reaction
HELEN P. RAAEN Analytical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn.
b The D.M.E. of Teflon, which was TI" studied in detail with the TI+ reaction in 0.1M KCI-1mM HCI, has now been evaluated in the same way with the Pb+2 $ Pb" reaction in 1M HCI. The polarographic characteristics of the reaction were determined with the D.M.E. of Teflon; they are in agreement with those established from studies done with glass D.M.E.'s. Polarograms for the Pb+2+ Pb" reaction taken with the two types of electrodes have the same meaning and can be analyzed in the same way. The results provide additional experimental evidence that data taken with D.M.E.'s of Teflon and of glass can b e interchanged and that data taken with the D.M.E. of Teflon in glass-corroding media will be reliable.
e
of the dropping-mercury electrode (D.M.E.) of Teflon (Du Pont trade-mark) by means of the T1+ T1" reaction was described earVALUATIOX
lier (9). A similar study has now been made with the P b + 2 ePb" reaction to evaluate the performance of the electrode with respect to a two-electrontransfer reaction. The pattern of experimental work was the same in the two studies. ,4s for the TI+ T1" reaction, so for the Pb+2 e Pb" reaction, the polarographic data obtained with the D.-M.E. of Teflon are in agreement with those reported from studies with glass D .M ,E,' s . EXPERIMENTAL
All the solutions were prepared from triple-distilled water and ACS reagent-grade chemicals. Yitrogen or argon, passed through a wash of the supporting medium, was used to deaerate the test solutions a n d was passed over them during the analyses. Supporting medium, 1;M HC1. Standard solutions of Pb+2, 0.02 to 1.OmiM. h 1.OmM stock standard soluReagents.
tion of Pb+2was prepared by dissolving 69 0.1 mg. of dry lead chloride, PbC12, in 250 ml. of 1M HC1. More dilute solutions of P b + 2 were prepared by diluting suitable aliquots of the stock solution with 1M HC1.
*
Instrumentation
and
Apparatus.
The instrumentation and apparatus were as described earlier (9). Unless indicated otherwise, the instrumental conditions were: voltage scan rate, 0.1 volt per minute; voltage scan direction, positive-to-negative; h , 113 cm.; and t a t Ell2, -4 seconds. RESULTS A N D DISCUSSION
+
The P b + 2 Pb" reaction was selected, because it is a well-understood reaction for which a good amount of polarographic reference data exist, Since P b + 2 is more stable in acid than in neutral salt solution and since polarographic maxima are not expected to occur for the P b f 2 + Ph" reduction a t concentrations of supporting electrolyte VOL. 37, NO. 6, M A Y 1965
677
sentially one polarogram and thus indicat'e the excellent precision of the recordings. The derivative polarograms indicat,e that measurable waves would be obtained with t'he D.M.E. of Teflon for much more dilute solut,ions. The form of first-derivative polarograms taken with t'he D.11.E. of Teflon, as with glass D.M,E,'s, is a function of the voltage scan rate. Derivat'ive polarograms for 1.Om31 PbC1, recorded a t t'hree scan rates are shomn in Figure 3. The values determined from these polarograms for (di/ldt)maxlfor potent'ial at' which the observed first-derivative wave height is and for wave maximum [(Epjobsd]> width at' half-peak height, TT71/,, are given in Table I as a function of scan rat'e. The values for the electron change, n, calculated from Tf'i,,, are also given. The effects of the damping circuits in the polarograph as a function of scan rate on the characteristics of these first-derivative polarogranis for P b + 2 taken with t'he D.1I.E. of Teflon are consistent with those found under the same condit,ions for Tlf (9) and also for the same types of polarograms taken with glass D.N.E.'s. Polarographic Characteristics of Redox Reaction, P b f 2 Pbo. The polarographic characteristics of the P b +* Pb" reaction det'ernlined with D.M.E.'s of Teflon and of glass are given t'ogether in Table 11 for ease of comparison. The characterist'ics determined from data taken with the D.1I.E. of Teflon are discussed below. H.4LF-WAvE POTEKTIAL (Ell,) AXD POTENTIAL AT WHICH THE FIRSTDERIVATIVE WAVEHEIGHT Is 1\lAXIhKM ( E p ) . The numerical value of the
1.6 -
-
!.2
$ +* 0
0.4-
-------- 3 2 a
2 -0.4-
w
(3
s
P
-0.8-4.2
-
-1.6-
-0.20
Figure 1.
-0.30
-0.40 -0.50 -0.60 POTENTIAL, v o l t vs. S.C.E.
- 0.80
-0.70
Average-currents regular polarograms for Pb+2 -+ Pb" reduction and test of equation of wave Taken with D.M.E. of Teflon
as high as lM, the supporting medium chosen was llzl HC1. Experimental conditions were as described previously (9). Forms of Polarograms. Polarograms of three types were recorded for the Pb+2 + Pb" reduction in 1M HC1: undamped regular, averagecurrents regular, and first-derivative. For the undamped regular condition the polarograms show a reduct'ion wave for Pb+2 that is of excellent form; it is in no way different from a polarogram of the same type recorded with glass D.M.E.'s. The uniformit'y of the drop oscillations indicates excellent precision of drop formation at the D.M.E. of Teflon. Undamped regular polarogranis are not so easy to analyze as are averagecurrents regular and first-derivat'ive polarograms; therefore, the latter types were used t o obtain most of the polarographic data given here. Figure 1 shows polarograms of the average-currents regular type for 1M HC1 and for l.OmX P b + 2 in 1M HC1. Pb" The polarogram for the P b + Z reduction was recorded in duplicat'e; the fact that the two recordings are indistinguishable from each ot'her indicates the excellent, precision attainable in recording polarograms with the D.M.E. of Teflon. The data used in the test of the equation of t'he wave are shown superimposed on the polarographic waves of Figure 1; t'he test of the wave is discussed below. Figure 2 shows polarograms recorded a t higher sensitivity; the concentrat'ion of Pb+2 that produced t'he wave was 0.02mJf. These polarograms indicate that with the D.1I.E. of Teflon it would be possible to obtain measur-
-
678
ANALYTICAL CHEMISTRY
able Pb+2 waves for solutions considerably more dilute than 0.02niM. There is no evidence of the presence of maxima in any of the averagecurrents regular polarograms recorded in this study. The form of first-derivatihre polarograms for 1.U HC1 and for 0.02 and l.OmAlfpbClz in 1*lf HC1 is in no way different from that of first-derivative polarograms recorded under similar conditions with glass D.M.E.'s. The polarograms for this condition were recorded in duplicate; they are esI
I
I
I
I
e
I
1
I
I
I
I
I
i :I. Y
c
z
W
a a
3 V
W
u a a W
3
I
-0.20
Figure 2.
I -0.30
I
I -0.50 POTENTIAL, volt
I
I
,
I
- 0.40
- 0.60 VS.
I
I
- 0.70
I
I
-0.80
S.C.E.
Average-currents regular polarograms for Pb+2 + Pb" reduction Taken with D.M.E. of Teflon
Table I. Effects of Damping Circuits in Polarograph as a Function of Voltage Scan Rate on First-Derivative Polarograms of Pb+* Taken with D.M.E. of Teflon
Voltage scan rate, volt/min.
(d?/dt)nmx, pa./min.
0.02 0.05 0.10
18.0 41.4 69.0
Q
( E ,l o b s d l
WI/~ mv. , 44 48 57
volt us. S.C.E. -0.457 -0.465 -0.477
n calcd. from W l / z a
2.06 1.89 1.59
n = 90.6/W7i/1, where Vi/, is expressed in millivolts.
Table II. Comparison of Polarographic Characteristics of the Pb+* E Pb" Reaction Determined with D.M.E.'s of Glass and Teflon
Polarographic characteristic volt GS SCEb
E,, volt us. S.C.E.6 nc
Reciurocal slope of'log [ i / (2,1 -
Ed
e
?)I
T y p e of U.1I.E. Teflon Glassa - 0 434 -0 -0 -0 -0 435 - 0
2 06 0 035
435(7,8) 44 (4) 49 ( 3 ) 434 ( 1 )
2 (6) 0 O3Oc ( 6 )
us.
, volt
a Yalues taken from references indicated. Determined at a scan rate of 0.02 volt/min. c Theoretical value.
the (El/r)obsdOr (Ep)obsdO f the tW0 recordings mas taken as the El/, value (Table 11). X bridge of saturated KCl solution was used. The E > / ,values determined with the D.N.E. of Teflon are in very satisfactory agreement nith the most reliable of the reported values-that is, -0.435 volt us. S.C.E. in 1JI HC1 (or KC1) via a bridge of saturated KC1 solution ( 7 , 8). The E , of the deiivative wave for the P b f 2 + Pb" reduction becomes less negative as the scan rate decreases, because the time lag of the parallel-T filter becomes significant a t faster scan rates. -kt a scan rate of 0.02 volt per minute, the E , is the thermodynamic El/*. This same relation between E, and El/, exists for the T1+ T1° reaction a t D.lI.E.'s of both Teflon and glass (9). The therniodynainic El,, for the Pb+* Pb" reaction was found to be -0.435 volt us. S.C.E. via a bridge of saturated KCl solution. ELECTROK CHASGE:( n ) . By means of the methods previously described (9), the D.PI1.E. of Teflon was shown to be satisfactory for establishing n for AILz+ N o . The reciprocal of a straight-line section of log [ i : ( i d - i)] us. E,, e was found to be 0.035 (cf. 0.030 theoretical).
e
El,, us. S.C.E. varies with the type of bridge used between the test solution and the S.C.E. ( 2 ) . I n the work described here a KaF-agar agar/O.lM KC1 bridge was used except in the measurement of E1 values that were to be compared with values reported in the literature, in which case a bridge of saturated KC1 solution !\as used. For the latter bridge, the El,, values are about 20 to 30 mv. niore positive than for the SaF-agar agar/O.lJZ KC1 bridge. However, when the supporting medium is 1JI KC1, the value for the El,, of the Pb+* + Pb" reduction does not differ significantly for the two types of bridges and is the same as that obtained in 1JI HC1 with the bridge of saturated KC1 solution. This difference between 1JZ HC1 and 1M KC1 with respect to effect of bridge type on the Ellp of the Pb+*+ Pb" reduction may result from the difference between the mobilities of the hydrogen and potassium ions. Thus, it appears that for some supporting electrolyte> it is essential to specify the type of bridge used in measuring a reported El/, value with reference t o a calomel electrode. For the purpose of comparing El,* values \\ith those reported in the literature, average-currents regular and first-derivative polarograms were recorded for voltage scans made in both the positive-to-negative and negativeto-positive directions; the average of
I
1
I
Also, the average of the values for n determined from the W1/z's of 12 firstderivative polarograms is 1.98. RWERSIBILITY.Data taken with the D.1I.E. of Teflon demonstrate the reversibility of the Pb+* Pb" redox reaction. From Figure 1, the log [ i / ( i d- i)] us. E d e plot meets the following criteria for a reversible reaction. The plot has a straight-line section, the reciprocal of the slope of the straight-line portion (0.035 volt) is in agreement with the theoretical value 0.030 volt, and the value of log [i/(id - i)] is zero a t Ellz. Constancy of the E l / , with change in concentration of the reducible ion, C, can also indicate reversibility of the electrode reaction. The values for average-currents regular polarograms of 0.02 to 1,OmA' Pb+* solutions are in the range from -0.474 to -0.477 volt us. S.C.E. (average -0.476 volt). Similarly, values determined from eight first-derivative polarograms are from -0.476 to -0.480 volt vs. S.C.E. (average -0.478 volt). DIFFUSIOSCONTROL.The suitability of the D.M.E. of Teflon for showing diffusion control of a two-electrontransfer electrode process is demonstrated by the data of Table 111. (dildt),,, The values €or the ratio h1' 1 over a range of mercury heights, h , are given. The back pressure correction (Pback = 1.54 cm.) was applied to all the h values. The Pba& was calculated from the equation Pbsck
=
3.1/(mt)"8
(1)
where m is the rate of flow of mercury from the capillary and t is the drop 1
I
POTENTIAL, volt v5.
I
I
S.C.E.
Figure 3. Effects of damping circuits in polarograph as a function of voltage scan rate on first-derivative polarograms of Pb + * Taken with D.M.E. of Teflon See Table I 1 .OmM PbC12 in 1 M HCI Bridge. NaF-agar agar/O.l M KCI
Test solution.
VOL. 37, NO. 6, M A Y 1965
679
Table 111. Suitability of D.M.E. of Teflon for Showing Diffusion Control of Pb" Reaction PbfZ
e
Test solution. 1.0ni.If PbC1, in 1M HCI t , 3.24 seca m, 2.20 mg./sec.a P b n c k , 1.54 CITI." (di/dt)m,x h l / l j (di/dt),,,,, cm.'/2 pa./nun.
h,* em.
X 86
78 5
9 9 10 10 11
87 0 95 0 103 5 111 5 121 5 a
b
29 75 18 56 03
h'/z pa./min.
cm.-'/z
.i7.0
60 0
63 5 66 5 69 5 73 8
6.43 6 46 6 51 6 53 6 58 6 69
113 em. Corrected for P b a c k .
At h
=
time. This equation is the one used to correct for the back pressure of a glass capillary (5). Relation of Concentration, C, of Reducible Ion, Pb+*, to Diffusion Current, (id), and to Peak Height, (dildt) max, of First-Derivative Wave.
Plots were made of C us.
i d
and of C
us. (di/dt)max. In each case, the relation
is linear and the line passes through the origin. The data thus shon that I t is possible with the D.M.E. of Teflon to make quantitative polarographic nieasurements of C from plots of C us. either i d or (di,'dt),,,ax. ACKNOWLEDGMENT
Grateful acknowledgment is made to
P. F. Thomason, who supervised this study, and to I). J. Fisher, IT.L. I M e a , and R.Jf-. Stelzner for the advice they generously gave during the course of it.
LITERATURE CITED
(1) Belew, W. L., Analyt,ical Chemistry IXvision, Oak Ridge Kational Laboratory, Oak Ridge, Tenn., unpublished data. (2) Belew, W.L., Itaaen, H. P., J . Electroanal,. ('hem., 8, 4 i 5 (1964). (3) Brezina, LI., Zurnan, P., "Polarography in LIedicine, Biochemistry, and Pharmacy," p. 11, Interscience, Kew York, 1938. (4) Ibid., p. 735. ( 5 ) Kolthoff, I. M., Lingane, J. J., "Polarography," 2nd ed., 1-01, I, p. 86, Interscience, S e w York, 1952. (6) Ibid., p. 194. i i ' i Ibid.. 1-01. 11. D. 529. ( S j Lingane, J. i.,J . Am. Chem. SOC. 61, 2099 (1939). (9) Raaen, H. P., AXAL. CHEM. 3 6 , 2120 (1964). RECEIVED for revieF September 23, 1964. Accepted February 17, 1965. Research sponsored by the L-. S. Atomic Energy Commission under contract with the Union Carbide Corp.
High-Sensitivity Controlled-Potential Coulometric Titrator Controlled-Potential Coulometric Determination of Milli- a n d Microgram Quantities of Uranium a n d Iron H. C. JONES, W. D. SHULTS, and J. M. DALE Analyfical Chemisfry Division, Oak Ridge National Laborafory, Oak Ridge, Tenn.
b An instrument has been built specifically for the potentiostatic coulometric determination of small amounts (-0.010 to TOO peq.) of materials. Emphasis was placed on careful construction using unmodified amplifiers and conventional circuitry. Precise calibration circuitry i s incorporated into the instrument for convenience and also to provide a source of easily adjusted constant current for other electroanalytical applications. The instrument has been tested with and used for the determination of milli- and microgram quantities of iron and uranium over a period of several years. The procedures used for these determinations involve both reversible and irreversible systems, the use of either mercury or platinum as the controlled electrode, and both reduction and oxidation electrolyses. They are therefore representative of the procedures that are most often encountered in controlled - potential coulometry. This paper describes the instrument and procedures, and it presents typical analytical results.
I
i w m t year+. several electronic in\tiuiiicnt. that can be used for conti olled-l)otential coulometi I C analy-1. hnvc h e n de-crihed (1-6, 9, flN
680
ANALYTICAL CHEMISTRY
I S ) . Generally, these instruments have been designed and used for the determination of quantities of about 500 l e q . and up. The instrument described in this paper, designated ORKL Model Q-2564, was built specifically for the determination of sniall amounts of materials having large equivalent wightsthat is, for quantities of about 0.01 to about 100 l e q . The instrument is designed to be simple, stable, and easily maintained. Accordingly, t,he chassis layout and construction are inade carefully, with particular emphasis on the elimination of ground loops and switching transients. Provision is inade for internal absolute calibration of the integrator in coulombs per readout volt. (The internal calibration current may be used externally for other electroanalytical applications.) The principle of control of potential and of integration of cell current used in this instrument is essentially the same as that' described by 13ooman (1) and was selected for thip application because of its simplicity, espected ease of maintenance, and because the three G.IP/R ( I O ) US.1-3 chopper-stabilized operational amplifiers do not have to be modified in any way. Current amplifying stages are omitted. One G.IP/R R-10013 power supply furnishes the
regulated *300 volts d.c. necessary for the amplifiers. .I block diagram of the instrument is presented in Figure 1; a complete circuit diagram is presented in Figure 2 . DESCRIPTION OF INSTRUMENT
Current Amplifier. .-is seen in the block diagram (Figure l ) , one of the amplifiers ( S o . 2 ) i q used both as a control amplifier and current amplifier. .-implifier No. 2 maintains the controlled electrode, which is connected directly to the input of the amplifier, a t virtual ground potential. Because the total cell current flows through one of the microequivalent range resistors in the feedback path of amplifier KO. 2 , the voltage developed at the output of this amplifier is proportional to the cell current. This voltage is used as an input signal for amplifier No. 3, the integrating amplifier. Control Amplifier. The input of amplifier KO. 1 sees the difference between the control potential and the potential of the controlled electrode with respect to the reference electrode. .implifier To. 1 forces this difference t o be essentially zero by maintaining the counter electrode a t such a potential t h a t , by current
