Construction and Evaluation of Planar Mercury Pool Electrodes John
R. Kuempel and
Ward B. Schaap, Department of Chemistry, Indiana University, Bloomington, Ind.
has been widely T used for many years as a working electrode for chronopotentiometric and HE MERCURY POOL
other voltammetric studies ( I , 2, 6, 8). I t s main advantages are that it possesses a smooth, reproducible surface that is relatively free of films and that it has a high overpotential for hydrogen. Some minor disadvantages are its sensitivity to mechanical vibrations and its limited anodic range. The most serious disadvantages of the mercury pool electrode, however, arise as a result of its curved meniscus when in contact with most surfaces--e.g., glass. This curvature makes it impossible to determine exactly the surface area of a mercury pool by geometric nieasurements, so that one must resort to indirect calibration methods. hIoreover, around the edges of the mercury pool, where the mercury comes in contact with its container, there exists the possibility of thin layer electrolysis effects which are hard to reproduce or control and which may be expected to change as the surface tension and contact angle of the mercury surface change with potential (2,8). We have recently succeeded in constructing strictly planar mercury pool electrodes, thereby eliminating problems arising from surface curvature and enabling the surface area of the pool to be calculated directIy from its geometric
Figure 1. Cell with platinum-tipped tube allowing formation of mercury pool electrode with planar surface
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dimensions. The construction and quantitative evaluation of such planar mercury electrodes are described in this communication. CELL DESIGN
Three different arrangements were devised which may he used to obtain planar mercury surfaces. ,ill of them function in a similar manner.-Le., the mercury meniscus, 1%hich is normally convex downward, becomes flat if the perimeter of the pool sticks to a nietsl ring or washer nhich is wet from the mercury. Each cell will be discussed in turn to explain how this is achieved. The cell shown in Figure 1 utilizes a platinum washer carefully cemented onto the end of a glass tube with epoxy resin (Carter's Ink Co., Cambridge, hlass.). The tip of the glass is ground flat before sealing. The diameter of the hole drilled in the washer is made to correspond exactly to the inside dianieter of the glass tubing. The washer is constructed from 0.0015-inch-thick platinum foil. After the nasher is cemented onto the end of the glass tube, it is cleaned and cathodized folloning the procedure of Ramaley, Brubalrer, and Enke ( 7 ) . I n this case this was done by inserting the tube into a dish containing a mercury pool cathode in contart nith a 1.1.1 perchloric acid solution. The tube was held a t an angle and inserted to a depth such that a portion of the washer was in contact with the mercury while the rest contacted the solution. The tube was then rotated while the pool was maintained a t a cathodic potential for about 1 minute. For use as a cell, the tip is inserted into a pool of mercury and solution is slowly added above it in the solution compartment until the hydrostatic pressure of the solution forces the meniscus to be Aat, as shown in Figure 1. The solution is unable to escape through the bottom because the mercury is wetting and sticking to the platinum surface which, in turn, is sealed to the glass. The diameter of this flat mercury pool is equal to the inside diameter of the glass tubing, which is easily measured. The cell shown in Figure 2 utilizes a platinum wire ring sealed onto the inside wall of a glass tube such that about half of its area remains exposed. This
exposed ring of platinum is cleaned and amalgamated iyith mercury by cathodizing and rotating as described above. When the mercury level in the tube is raised above the wire ring and then slowly lowered, the edge of the meniscus sticks to the wire and the mercury level can be adjusted so that' the surface is flat. If the mercury wets the platinum ring, a well defined, circular, planar mercury surface is obt,ained which has a diameter equal to the inside diameter of the glass tubing. t-nfortunately, it is difficult to achieve a tight, longlasting seal between borosilicate glass and platinum. I n the above application, if a crack develops between the platinum ring and the glass, there will be a thinlayer, high-resistance current' path around the wire, causing the chronopotent,iogram to be drawn out and distorted in the region of the transition time. I t \vas found that, if the ring is constructed from wire of 0.005-inch diameter, the platinum-to-borosilicate glass seal could be made and would last a reasonable length of time. .I smaller diameter wire could not be used because insufficient platinum surface is then exposed to the mercury and the meniscus cannot become flat before the mercury breaks away. A much better seal is obtained using uranium glass tubing instead of borosilicate glass. Uranium glass-to-plat'inum seals were
Figure 2. Glass-platinum ring cell for obtaining planar mercury pool electrode
satisfactory with wires as large as 0.013inch diameter. Such wires present enough surface area to the mercury so that the meniscus can become flat and even concave upward with no difficulty. The cell shown in Figure 3 works on the same principle as that in Figure 2, except that the exposed platinum ring is obtained by pressing a platinum u-ssher between two pieces of Teflon which have the screw arrangement shown. The assembly of Teflon is machined to fit snugly int'o a 40/50 female standard-t'aper joint at t'he bottom of a glass solution compartment. The diameter of the hole drilled in the platinum washer is equal to, or very slightly less than, the diameter of the hole drilled through the pieces made of Teflon. K h e n the assembly is screwed t,ightly together, the inside edge of the washer serves as a ring to hold the mercury meniscus flat. The diameter of this flat mercury surface is equal to the diameter of the hole through the Teflon. (llecau*e of the effects of compression, the diameter of the hole through the Teflon should be measured with the assembly screwed together tightly.) Kasher thicknesses ranging from 0.005 to 0.02 inch lvere sufficient to maintain flat mercury surfaces and gave identical chronopotent,ionietric results. In each of the cell arrangements, the auxiliary electrode (a heavy platinum wire), the reference electrode (a saturated calomel electrode), and t,he nitrogen inlets and outlets are introduced through a tightly fitting stopper as shown in Figure 1. I n the cells shown in Figures 2 and 3, the mercury level is adjusted by means of a moveable rubber bulb a t the bottom of the cell through which electrical contact is made with a platinum wire. Other means of adjusting the mercury level are also possible-e.g., with a mercury-filled leveling bulb connected to t'he cell via plastic tubing, or with an adjustable screw arrangement at the bot'toiii of the cell, similar to that used in the cell of Bruckenstein and Rouse
RESULTS A N D DISCUSSION
COMPARTMENT
TEF
f l J-2 w
PA R T M E NT
Figure 3. Teflon-platinum washer cell assembly for obtaining planar mercury pool electrode
experiments was the same as that of Lingane (S). A relay-operated electric stop clock and a dual-beam cathode-ray oscilloscope were used to measure transition times (4). The transition potential was taken to be - 1.0 volt us. the S.C.E. in all experiments. Prepurified nitrogen, presaturated with water, was passed through the test solutions between runs and over the solutions during runs. The temperature was controlled at 25.00 + 0.05' C. by circulating thermostated water around the solution compartments. All experiments were carried out on a 12- x 16-inch wooden platform suspended from a solid frame by means of gum-rubber tubing. This hammock-like arrangement was far superior to other types of vibrationdamping devices and eliminated mechanical vibrations almost entirely. The diameters of the circular mercury electrodes were measured with a smallhole gauge set and micrometers.
Two criteria are used to check the reliability of the application of planar mercury electrodes to quantitative voltainmetric experiments. The first is to determine if the values of the chronopotentiometric transition time constant, ~ T " ~ / Aare C , constant over a wide range of transition times. The second is to show that the values of the diffusion coefficient, D , calculated from the transition time constant and the geometric area of the electrodes are comparable with each other and with established values from the literature. The first criterion demonstrates the absence of thin layer effects (around the edges of the electrode and in cracks) and also the absence of the effects of curvature, which give rise to nonlinear diffusion. The second criterion demonstrates that the geometric c r o w sectional area of the electrode is equal to the real area of the electrode, a relationship which previously has not been realized for mercury pool electrodeq. Table I summarizes the results for 2 X 10-3AlI Pb'2 in 0.1X K S O l solutions. The transition time constants are shown with their sample standard deviation. The areas used in the calculations are the cross-sectional areas of the tubing or holes measured just above the exposed mercury burfaces. The values of D calculated n-ith these areas can be compared with values obtained by other workers: 0.914 X 10-6 em.* second-' in I J f K?;Os 15), 0.98 X 10-5 cma2second-1 in 0 . l X KxO3 (S), 1.00 X 10-5 cm.2 second-' in 0.1Jf K?JO3 ( I ) , 0.828 X second-' in 0.1.11 k'n'O3 (IO), and 1.06 X 10-5 cm.2 second-' in 0.01JI K S O a (9). With the exception of the one low result, the values of these workers are in good agreement with the results of this study, considering the variation in concentration of the potassium nitrate.
(0.
Temperature control is achieved in the cell in Figure 1 by placing t,he air jacket down into a water bath. The ot'her two cells are equipped with water jackets through which water from the bath is circulated.
EXPERIMENTAL
The lead nitrate solutions were prepared b y neighing requisite amounts of reagent grade anhydrous Pb(S03)2 into 0.121 KNOI solutions. The water used to prepare these solutions was distilled and then passed through a n ion exchange column. All other chemicals were of reagent grade. Except for slight modifications, the circuitry for the chronopotentiometric
Table 1.
Transition Time Constants and Calculated Diffusion Coefficients for Planar Mercury Pool Electrodes
Transition time constant Diffusion coeff. (amp. cm. mole-') (cm.* second-') Cell in Figure 1 533.5 f 2.46" 0.973 X 10-5 1531.5 f L 7 1 b 0.966 X Cell in Figure 2 535.0 f 2.1iC 0.979 x 10-5 535.7 f 3.0gd 0.981 X 10-5 Cell in Figure 3 535.4z!= 3.32e 0.980X 10-6 Electrode area = 0.2827 cm.2; 21 experiments at 7 currents with 7 ranging from 2 to 28 seconds Electrode area = 0.5205 crn.l; 17 experiments at 8 currents with T ranging from 2 to 19 seconds c Electrode area = 0.4745 cm.2; 21 experiments at 10 currents Kith 7 ranging from 1 to 22 seconds Electrode area = 0.2288 cm.2; 16 experiments a t 8 currents with T ranging - - from 2 to 23 seconds e Electrode area = 0.4395 cm.2; 18 experiments at 9 currents with ranging from 2 to 19 seconds
VOL. 38, NO. 4, APRIL 1966
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According to the results shown in Table I, any of the three types of cell could be used with equal reliability in electroanalytical experiments. There are, however, some practical advantages and disadvantages of each type. There are two methods by which the mercury level can be adjusted to give a planar configuration. One is simply a visual observation, from the side or from above. The other is to measure transition times a t a given current and to adjust the mercury level until a minimum transition time is obtained, which should correspond to the minimum, uncurved area. The first method is very easy to apply using the cell shown in Figure 2, where the )observation can be made from the side. With the other two cells, the observation must be made from above and can be complicated by refraction in the solu-
( 2 ) Delahay, P., LIattax, C. C., J . A m . Chem. SOC.7 6 , 874 (1954). (3) Lingane, J. J., J . Eleclroanal. Chem. 1, 379 (1960). (?) Lingane, J. J., Ibzd., 2, 46 (1961). ( a ) hleites, L., “Polarographic Tech-
tion. The second method can be applied in all three cells, but is more time consuming, more tedious, and less precise. From this standpoint, then, the cell shown in Figure 2 is the easiest to use. The cells shown in Figures 2 and 3 allow linear diffusion in the solution for cathodic processes and in the mercury for anodic processes. The cell in Figure 1 does not allow linear diffusion in the mercury for anodic processes. The cells in Figures 1 and 3 are relatively easy to fabricate while the fabrication of the cell shown in Figure 2 is more difficult because of the platinum-to-glass seal.
niques,” 1st ed., p. 270, Interscience, New York, 1955. (6) Nicholson, AI. M., Karchmer, J. H., ANAL.CHEM.27, 1095 (1955). (7) Ramaley, L., Brubaker, R. L., Enke, C. G., Ibzd., 35, 1068 (1963). (8) Reilley, C. N., Everett, G. W., Johns, R. H.. Ibid.. 27. 483 (1955). (9) Strehlt, C’. A:, Cooke, JIr. D., Ibid., 2 5 . 1691 (19j3). (10) ‘Yon Stackelberg, AI., Pilgram, M., Toome, T.’., Z. Electrochem. 57, 342 (1953).
Investigation supported in part by Public Health Service Predoctoral Fellonship (No. 1-F1-GLI-28, 705-01) from the National Institute of General RIedical Sciences and in part by the U. S. Atomic Energy Commission (A. E. C. document NO. COO-256-53).
LITERATURE CITED
(1) Bruckenstein, S., Rouse, T. O., ANAL. CHEM.36, 2039 (1964).
A KWIC Index to X-Ray Diffraction Powder Data F. W.
Matthews and L. Thomson, Canadian Industries Limited, Central Research Laboratory, McMasterville, Quebec, Canada
the use of a T permuted title program (KWIC), which is available in the standard “softHIS PAPER DESCRIBES
ware” package for most data processing computers. The work described made use of a KWIC program written for an 11311 1401, 8K, 4 tape machine. It is available from the authors. The ICKIC program is a development of Luhn ( 2 ) and has been widely used for information retrieval purposese.g., the *\merican Chemical Society publication, Chemical Titles. Through these programs orderly arranged lists of diffraction data for each of the five strongest lines of the X-ray diffraction patterns can be provided, as ne11 as lists by chemical name, empirical chemical formula, and structural fragment. These lists are generated from
I
two single line entries for each compound. A KWIC program usually provides a field of six or more characters (alphabetic or numeric) to be used for a serial number for identification of an item and a field of 60 t o 70 characters for the “title” entry to be permuted. See Figure 1. The program recognizes each entry between blank spaces in the title as a “word.” Words may contain any combination of letters, numbers or punctuation marks. A record is made for each word in the “title” and the resulting records are sorted bringing each “word” to the center column of the page. A list of words for which a record should not be made, whenever they appear in a title, may be appended to the program as a ‘ h o n significant word
6
Figure 1 .
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’\
list.” The computer will omit entries under these words. The sort sequence in the example shown is alphabetic followed by numeric characters. Punctuation marks precede both. “Words” longer than twenty characters are not indexed in the program used. As much of the title is included after the word as can be printed on the same line. The remainder is printed a t the beginning of the line. This is referred to as the “wrap-around” feature. An asterisk denotes the beginning of a title. In adapting the KWIC program for listing x-ray diffraction data, a number of conventions were adopted to improve the usefulness and completeness of the entries. The first line of the title consists of the chemical name and the five strongest “d” spacings of the diffraction
Sample input form for key punching
