AIDS FOR ANALYTICAL CHEMISTS Teflon Modification of a Hanging Mercury Drop Electrode for Stripping Voltammetry in Glass-Corrosive Media D. R. Canterford and A. 6. Waugh Department of Chemistry, University of Melbourne, Parkvllle 3052, Melbourne, Australia
Anodic stripping yoltammetry is a convenient and highly sensitive technique suitable for the determination of many metals ( I ) . Probably the most common working electrode used is the hanging mercury drop electrode (HMDE), which consists of a mercury drop suspended from a thin mercury thread in a glass capillary. New drops are formed by means of a displacement piston actuated by a precision micrometer screw. Some of the other electrodes that have been used for stripping analysis possess advantages over the HMDE, particularly with regard to sensitivity and resolution ( I , 2 ) . However, the HMDE is extremely simple in operation and is adequate for many practical applications. If stripping voltammetry is to be carried out in glasscorrosive media, such as hydrofluoric acid, a normal HMDE with a glass capillary cannot be used. This article describes a simple modification, with Teflon (Du Pont), of a commercially available HMDE, which will enable stripping voltammetry to be carried out in such media.
Table I. Reproducibility of Mercury Drops with Modified HM DE Weight of 5 drops, mg Average weight per drop, mg 52.5 53.4 52.8 52.9 52.6 52.2 53.0 53.0 52.6 52.7 Av 52.8 f 0.3
MODIFICATION OF HMDE
Av 10.6 f 0.1
Table II. Reproducibility of Peak Current with Modified HMDE Drop numbera Peak current, pA
EXPERIMENTAL Solutions of Cd(I1) in 1M HCl were prepared from reagentgrade chemicals and doubly distilled water. Voltammograms were recorded with a Metrohm Polarecord E261. A Metrohm HMDE (BM 5-03) and a Ag/AgCl (1M NaC1) reference electrode were used. After solutions were deaerated with nitrogen and were thermostated at 25.0 & 0.1 "C, a potential of -1.0 V was applied t o the HMDE for exactly 3 min. The solution was stirred for the first 2 min of this electrodeposition period. An anodic scan, a t the rate of 1V/min, was then commenced.
10.5 10.7 10.6 10.6 10.5 10.4 10.6 10.6 10.5 10.5
1 2 3 4 5 6 7 8
59.8 59.3 56.0 59.3 59.0 57.5 58.8 57.0 Av 58.3 i 1.2
aSuccessive depositions and scans on same solution Cd(1l) in 1M HCI).
( 1 X 10-4M
Modification of the HMDE involved the construction of a Teflon extension to the glass capillary. In glass-corrosive media, only the Teflon portion would be immersed. Recently, one of us has developed an extremely simple electrical-discharge method of piercing small holes in Teflon (3, 4). This method has been successfully applied to the fabrication of a gas flowmeter (3) and dropping mercury electrode (DME) (4). However, when Teflon capillaries prepared by this technique were connected directly to the end of the HMDE glass capillary, it was not possible to obtain reproducible mercury drops by rotation of the micrometer screw. Apparently, this difficulty was due to the diameter of the hole being too small. The hole was therefore enlarged with a tapered needle [similar to that employed by Raaen ( 5 ) for the construction of a Teflon DME] until satisfactory operation of the HMDE was obtained. A hole of about 400-1 diameter gave very good response to rotation of the micrometer screw. (1) E. Barendrecht, in "Electroanalytical Chemistry, Vol. 2," A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1967, p. 53. ( 2 ) T. M . Florence, J. Electroanal. Chem., 27, 273 (1970). (3) A . 6.Waugh and P. W . Wilson, Anal. Chem., 44, 2118 (1972). (4) A. M . Bond, T. A. O'Donnell. and A. 6 . Waugh, J. Elecfroanab Chem., 39,137 (1972). (5) H. P. Raaen, Anal. Chem., 34, 1714 i1962).
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Figure 1. Modified HMDE showing connection between Teflon and glass capillaries
Figure 1 shows the method of connecting the Teflon capillary to the glass capillary of the HMDE. The upper
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
end of the Teflon rod was drilled out to a depth of about 6 mm with a flat-bottomed bit. The diameter of this hole was slightly smaller than the outside diameter of the glass capillary to ensure a tight seal. Mercury drops adhered well to the orifice of the Teflon capillary and were not dislodged by vigorous stirring of the solution.
EVALUATION OF MODIFIED HMDE If the HMDE is to be used for analytical purposes, the surface area ( A ) of the mercury drop must be reproduced exactly since the peak current (Ip)is proportional to A ( I ) . A convenient method for estimating the reproducibility of drop size is to measure drop weight. Table I shows the results obtained with the modified HMDE. The average deviation in the total weight of 5 successive drops was about 0.6%. A similar experiment with the unmodified HMDE gave an almost identical result and indicates that the proposed modification does not affect the reproducibility of the HMDE. A stripping voltammogram recorded with the modified HMDE is shown in Figure 2. The peak current obtained with 8 successive drops for 1 X 10-4M Cd(I1) showed a mean deviation of just under 2% (Table 11). It is anticipated that the modified HMDE can be used for the direct analysis of samples that are readily soluble
-0 4
-06
-0 8 Volt
vs
Ag/AgCI
Electrode
Figure 2. Stripping voltammogram of 1 HCI recorded with the modified H M D E
X 1OW6MC d ( l l )
in 1 M
in 50% HF, for example, the determination of various metals in geological specimens or the defermination of lead in glass. It could also be used in strongly caustic solutions that etch glass on prolonged contact. Received for review April 9, 1973. Accepted June 4, 1973.
Simple Means of Semiautomation of Kinetic Studies as Performed by Conduction Calorimetry W. J. Evans,
E. J. McCourtney, and V. L. Frampton
Southern Regional Research Laboratory, Southern Region, Agricultural Research Service, P. 0. Box 19687, New Orleans, La. 701 79
The heat conduction type calorimeter ( I ) is particularly suited for kinetic analysis. However, it is necessary to apply certain corrections to the time-temperature curve before this analysis is possible. Several means exist for doing this. One, particularly suited to modern instrumentation, is that of Lueck et ul.'(Z). In essence, the method requires the evaluation of areas under the time-temperature curves a t various time intervals along the curves. Planimetric or weighing methods are tedious and time consuming a t best. A quite simple way of evaluating the requisite areas involves the use of a digital readout system such as the Infotronics electronic integrator ( I ) , or a similar device, to which is attached a simple program cycle timer such as the Cramer Model 540. In normal operation, the Infotronics device. yields a number which is proportional to the total area under a given experimental curve. However, as mentioned above, in addition to the total area, it is the areas at various times along the curve which are required to complete the data needed for calculation of the rate constants. To this end the timer serves the purpose of causing the digital readout system to print out at predetermined time intervals with the resulting numbers being directly proportional to the integral under the curve at that time. Simultaneously, a built-in event marker triggers the recorder pen making a reference mark on the curve. Thus, it becomes a simple
Table I.Calorimetric Data for the Alkaline Hydrolysis of Ethyl Acetate (25 "C) Time, min
Integrator reading
1.15 2.40 3.65 4.90 6.1 5 7.40 8.65 9.90
391 5 7040 9045 10482 11490 12253 12858 13341 15921
Final
k (I. mole-' sec-1)
0.091 0.098 0.010 0.103 0.106 0.105 0.106 0.106 Mean = 0.102=
"To be compared with a reported value of 0.107 ( 3 ) .
matter to collect automatically most of the data required for obtaining the rate constant. The only other factor needed is the vertical height at each of the points indicated by the event marker. This is a very simple procedure and we normally use the number of chart divisions, and fractions thereof, on the recording potentiometer as a measure of the vertical heights. Some data obtained by this means on the alkaline hydrolysis of ethyl acetate are shown in Table I. Received for review February 12, 1973. Accepted May 24, 1973.
(1) W. J. Evans, E. J. McCourtney. and W. B. Carney, Chem. Instrum., 2, 249 (1969). (2) C. H . Lueck, L. F. Beste, and H . K. Hall, Jr., J. Phys. Chem.. 67, 972 (1963).
(3) F. Daniels, "Outlines of Physical Chemistry," 7th printing, John Wiiey and Son, New York, N.Y., 1952, p 352.
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