V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3
1393 other workers. Consequently, all measurements should be corrected for temperature unless it is possible to control the temperature of both cup and bob, when the mean temperature should be kept constant. If the temperature of the bob cannot be controlled, it is essential to run the viscometer sufficiently long a t each rate of shear for thermal equilibrium to be established. With the suspensions examined a hysteresis loop was found a t low rates of shear, but once the structure in the material was completely broken down a linear flow curve resulted. The mineral and vegetable oils examined over rates of shear range from 0 to 10’ sec-1, for which all available data have not been presented, were found to be Newtonian. It is possible that the thixotropic nature of these oils reported by others at similar shear rates was a manifestation of temperature rise. ACKNOWLEDGMENT
The authors wish to thank the director and council of the Printing, Packaging, and Allied Trades Research Association for permission to publish results of this investigation. Figure 6. Hysteresis Loops for 15% Carbon Black in Mineral Oil
LITERATURE CITED
(1) Bruijn, H. de, Doh, H., and Kuyper, C., Rec. trav. chim., 63, 6 9 6 7 0 4 (1943).
curve wasobtained. Fromtheresults (Figure6)it can be seen that both the last two up curves fell between the up and down branches of the first complete experiment and that the down curves of the two complete flow curves were coincident. Experimental conditions in all cases were identical, and the hysteresis loop was probably caused by a difference in the rates of build-up and breakdown of the structure of the material, the rate of build-up being very much slower than that of breakdown. CONCLUSION
The temperature rise of a sample during measurement in a rotation viscometer may be substantial and may account for a considerable part of the hysteresis loop found in flow curves by
(2)
Buchdahl, R., Curado, J. G., and Braddicks, R., Jr., Rez. Sci.
(3)
Goodeve, C. F., and Whitfield, G. W., Trans. Faraday SOC.,
I&T., 18, 168-72 (1947). 35, 511-20 (1938). (4) Green, Henry, IND.ENG.CHEM., AXAL.ED., 14, 576-85 (1942). (5) Green, Henry, and Weltmann, R. N., Ibid., 15, 201-6 (1943). 16) Lower, G., Walker, A. C., and Zettlemoyer, A. C., paper pre-
sented before the Division of Paint, Varnish, and Plastics Chemistry, 120th Meeting of the AMERICAN CHEMIC \L SOCIETY, September 1951. (7) Mill, C. C., Chemistry &. Industry, 1952, KO.8 , 1 5 6 9 . (8) Weltmann, R. N.,I n d . Eng. Chem., 40, 272-80 (1948). (9) Weltmann, R. N., IND.ESG. CHEM.,ANAL.ED., 15, 424-9 (1943).
(10)
U7eltmann, R. N., and Kuhns, P. W., J . Colloid Sci., 7, 218-26
(1952). RECEIVED for review January 24, 1953.
Accepted June 16, 1953.
Coulometric Determinations of Submicrogram Amounts of Cadmium and Zinc With Stationary, Mercury-Plated Platinum Electrodes K. W. GAKDINER AND L. B. ROGERS Department of Chemistry, Massachusetts Institute of Technology, Cambridge 39, Mass.
THE
coulometric determination of microgram quantities of reducible metals by electrolytic deposition into mercury, or onto stationary electrodes, followed by quantitative dissolution has been used for a number of years. A modification of the stationary electrode method used by Zbinden (8) and Zakharevski1 ( 7 ) has recently been applied to the determination of submicrogram amounts of silver in solution by Lord, O’Neill, and Rogers (5), using both stationary and rotating platinum electrodes. Unfortunately, platinum electrodes are more limited in their application than mercury, whose higher hydrogen overvoltage permits less noble elements to be deposited. For this reason, the usefulness of mercury films supported on platinum has been studied. Platinum is an obvious element to select, because it does not amalgamate appreciably and hence provides a pure layer of mercury. Silver was suggested independently by Cooke (2) and by Perley (6) as the best metal to use among those which
amalgamate. However, the behavior of such an electrode changes with time. APPARATUSANDPROCEDURES
The mercury-plated platinum electrodes were prepared by first fusing short lengths of platinum wire 0.025 cm. in diameter in 3inch lengths of soft glass tubing, so that a portion of the wire, about 0.5 to 1.0 mm., was exposed. Mercury mas deposited onto this portion by electrolysis from a 0.01 or 0.02 M mercuric nitrate solution. A typical electrode gave a diffusion current of 30 pa. a t the -0.5-volt plating potential. In general, a t least a 15- to 20-minute plate from a 0.01 1M solution of mercury was required to give satisfactory coverage of the platinum electrode. After 15 minutes, the mercury layer if uniformly distributed over the platinum, was approximatefy 8400 A,, or about 1 micron thick. However, as a thicker film was considered desirable, for most of the work reported below, an electrode covered with approximately a 4-micron layer of mercury was employed. Consequently, the danger of saturating the
ANALYTICAL CHEMISTRY
1394
The characteristics of mercury-plated electrodes were investigated as a means of extending the range of usefulness of coulometric dissolution techniques. The mercury plating of platinum provided a surface having favorable hydrogen overvoltage characteristics otherwise afforded only by the dropping mercury electrode. Its successful application is dependent upon factors which vary with the particular metal ion undergoing electrolysis. The substitution of silver for platinum produced a more adherent coating of mercury, such that the electrode could be rotated and used to analyze more dilute solutions. The resulting amalgam had some undesirable properties which will limit its applicability.
mercury layer with the deposited element during a determination c\ as never realized. Silver electrodes were prepared in essentially the same way as the platinum electrodes. Although mercury does adhere if silver is simply dipped into a pool oi mercury, a knowledge of the exact amount of mercury on the electrode seemed desirable and this Was best attained by deposition. The electrolytic cell was a plain 4-ounce bottle fitted with a three-hole stopper to accommodate the cathode, the salt bridge from the reference electrode, and a fritted-glass dispersion tube for nitrogen. By using 60 ml. of solution, a change in concentration of the bulk of the solution was minimized. As a result, calibration curves could be prepared by plating for different lengths of time from a given (unstirred) solution without introducing an appreciable error from depletion of the ion throughout the solution. Some calibration curves were also obtained by plating for a fixed length of time from solutions of different concentrations. ‘‘Purified’] nitrogen was used directly from the tank without further purification. A period of 5 minutes Was employed for deaeration. The recording coulometer described by Lord, O’Neill, and Rogers ( E ) was used throughout this work. For comparative purposes, several plating-dissolution cycles were also followed with the Sargent Model XXI polarograph. I n general, recording of the current was begun only a few seconds before starting the dissolution cycle. The potential selected for deposition, as well as the initial potential for the dissolution c cle, was usually several tenths of a volt more cathodic than t i e polarographic half-wave potential of the metal under study. Thus, for cadmium, dissolution was accomplished by uniformly changing the potential from -1.0 x to -0.3 volt us. the saturated calomel electrode (S.C.E.); dissolution of zinc, by covering the span -1.3 to -0.7 volt; and mixtures of cadmium and zinc, by employing the span -1.3 to -0.3 volt. A polarization rate of 0.020 volt per second w a ~ usually employed. The areas of the recorded dissolution peaks were determined by counting squares, a method found to be as satisfactory as using a planimeter.
proximately steady value only after the minimum depth of 1 micron of mercurv had been deposited. The slight increase in the amount of cadmium recovered from layers of mercury thicker than the minimum may be attributed to the slight increase in surface area of the electrode. TWOadvantages of ubing mercury-plated platinum instead of bare platinum electrodes boon became apparent. First, the background curves for both deposition and dissolution cycles of the 0.1 J I sodium perchlorate solution were flat from 0.0 to - 1.0 volt, the background of 0.06 pa. for mercury being one third to one half that for the same bare platinum electrode. The value of a flat background curve is that a base line under the stripping area can be drawn with much less ambiguity. Secondly, an electrode containing the optimum deposit of mercury could be used for as long as 2 months with no detectable permanent changes in its residual current or sensitivity, providing potentials were never used which allowed the mercury layer to dissolve. There is no reason to believe the electrode could not be used indefinitely. If a film is inadvertently removed in part or in whole by anodizing or by chemical attack, it can be readily replaced without the need of a tedious recalibration.
SOLUTIONS AND CHEMICALS
Approximately millimolar stock solutions of both cadmium and zinc nitrates were made by dissolving the required weight of reagent grade salts in 0.1 144 sodium perchlorate. The solutions were stored in clean borosilicate glass volumetric flasks. Dilutions to 10-4 and 10-6 M were made with a solution of 0.1 M sodium perchlorate. Known mixtures of zinc and cadmium were prepared in the same way. Hydrated salts of cadmium and zinc were used, so it was necessary to establish the exact concentrations of the stock solutions. Standard solutions of knonm concentration were made by dissolving known amounts of pure cadmium oxide or zinc metal in a very slight escess of nitric acid. Then the relative heights of the polarographic diffusion currents were determined for the standards and the test solutions a t a dropping mercury electrode. From the ratios of the diffusion currents, the eyact concentrations of the stock solutions were found to be 1.00 X ilf for the Jr for the zinc. cadmium and 0.93 X RESULTS
Operating Characteristics of Mercury-Plated Platinum Electrodes. The dependence of electrode “sensitivity”upon the amount of mercury present is shown by the plot of Figure 1. The curves represent the dissolution areas for amounts of cadmium plated in 5 minutes from a 10-6 .lI solution as a function of the amount of mercury present on the electrode. It can be seen from these curves that the amount of recovered cadmium reached an a p
5’ RESIDUAL WRRENT
40”
0
I
I
I
10
20
30
-0
“
40
TIME IN SECONDS
Figure 1. Dissolution Areas for Cadmium Plated in 5 minutes from 10-6 M cadmium solution as a function of amouut of mercury on electrode. Amount of mercury present indicated by minutes o plating from 0.02 M mercuric nitrate oolution
The need for pretreatment of mercury-plated platinum electrodes is an undesirable feature of minor importance, which a p pears to be shared by similar mercury electrodes. Cooke (2), using mercury-plated silver wires, and Arthur et al. (I) using stationary mercury drops on an inverted capillary, have found, as the authors did, that an electrode had to be pretreated in order to obtain maximum sensitivity and reliability. Thus immediately following the preparation of an electrode or upon several minutes’ contact of a reliable electrode with air (or a solution which has not been deaerated) the surface apparently becomes contaminated, as evidenced by excessively large residual currents and a reduced sensitivity toward the element being determined,
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3 These effects were encountered even in solutions that had been deaerated for long periods. The entrapment of water or air on the platinum surface by mercury during its deposition could account for such behavior of a freshly prepared electrode. Formation of a film, possibly of absorbed oxygen or an oside of mercury, could account for the behavior after contact with osygen. A series of curves similar to those in Figure 1 showed that the first plating-stripping cycle produced a much smaller value of the dissolution area for cadmium than was observed for the succeeding runs. If the datum was omitted from the initial run, the succecding runs showed excellent reproducibility.
8
I
I
INITIAL MOLAR CONC CADMIUM
Figure 2. Apparent Dissolution Efficienq and Grams of Cadniiuni Heco\ered
1395 to errors in the calibration of the coulometer and, a t the same time, to study the effect of using a slorer rate of polarization, :I Sargent Model X X I polarograph was substituted for the coulomet,er. Deposition and dissolution cycles with the same electrode and solutions gave excellent agreement with those in Figure 2. Figures for the apparent efficiencies were slightly better i n these runs, being 83 to 86% for the I O W 3Jf solution and 16% for the Jf solution. However, the actual grams of cadmium recovered at each level were nearly the same. The recovery of cadmium was also proportional to the time of deposition, even though the apparent efficiency decreased as the time increased. Furthermore, the efficiency for a particular set of conditions could be improved noticeably by extended deaeration with "purified" nitrogen. These ohservations demonstrated that reduction of oxygen was the major reason for nppa1'ent efficiencies of less than 100%. Evolution of hydrogen undoubtedly contributed to some extent, especially in the runs i l l volving longer times of deposition. The fact that simultaneow reductions of oxygen and of hydrogen ions did not seriously illterfere with the determination of cadmium is of great practic:il importance. Effect of Rate of Polarization. It, was desirable to know thr effect of an increase in the polarization rate on the apparent dissolution efficiency and sensitivity. Typical data are given in Figure 3, which show the decrease in apparent efficiency and the corresponding reduction in the amount of cadmium recovered following a %minute deposition from a 10-5 d l cadmium solution. At a don. rate of polarization-i.e., 0.0035 to 0.0040 volt per second, 1.28 X lo-* gram of cadmium was recovered with an efficiency of 55%. When the polarization rate was i n c r d to 0.013 volt per second, the amount of cadmium recovered was 1.15 X IOe8 gram and the apparent efficiency was 38%. Thus, when the rate of polarization was increased threefold, all other esperiniental conditions being rigorously constant, approximately 10% less of the deposited cadmium was recovered. .4lthougli maximum recovery is not a requirement, maximum precision will result only i f a constnnt rate of polarization is maintained for the dissolution.
5-minute electrolysis as function of initial cadmium concentration
I 56-
Efficiency Studies. -4study of the completeness of stripping a t about 0.02 volt per second has shown that a 4-micron layer of mercury does not appear to retain a measurable amount of the deposited metal a t the end of a dissolution cycle. 9 second dissolution run, started a t -0.8 volt immediately following the initial strip, showed no measurable amount of cadmium remaining in the electrode. The relative efficiencv of the plating and dissolution cycles has been studied as a function of the initial concentration of the reducible ion, length of plating time a t a given potential, and rate of change of potential during the dissolution cycle. Figure 2 represents a plot for cadmium of the ratio, Grams dissolved Apparent grams deposited
x
100
as a function of the initial concentration of cadmium. I t can be seen that the apparent efficiency of the process fell off very to 10-5 M. rapidly as the conrentration decreased from Although an apparent efficiency of only about 5 to 10% was ob-11solution, the actual metal recovered was served for the the amount to be esprcted, from a calculation based upon the Jf solution (for which 80% efficiency amount found for a was found). The linearity of the plot of the amounts of cadmium recovered in duplicate iuns a t three concentration levels is indicative of the precision and accuracy that can be attained in spite of the low efficiencies of deposition. I n order to make certain that the low efficiencies were not due
52-
F
2
-- w
u
48-k
-
w
e
44- K
-
40-
2 69
36 I '3
Figure 3.
l
2
l
l 4
i l ! ! I I 6 8 10 POLARIZATION RATE VOLTSlSEC x IO'
!
I
I2
Effect of Polarization Kate upon Apparent 1)issolution Efficiency and Sensitivity Cadmium plated from 10-5 .Vf solution
Determinations of Cadmium. Of the various esperimental conditions that may he used in ohtaining the calibration curve, perhaps the most important is the necessary compromise between length of plat,ing time and the sensitivity of the recorder. While a very high sensitivity will permit a much shorter plating time to be used, the residual current and hence the signal-to-noise ratio in the actual determination become less favorable. For the initial concentrations of the older of 10-5 -If cadmium, plating times of 5 to 60 niinutes a r e useful.
ANALYTICAL CHEMISTRY
1396 The precision obtainable with mercury-plated platinum electrodes under optimum experimental conditions is indicated by the following data. Five successive deposition (5 minutes) and dissolution (40 seconds) cycles were made a t 10-3 M and -41 concentrations of cadmium. .For the millimolar solution, a mean value of 1.21 X lo-’ gram of cadmium was recovered with a coefficient of variation of 3.0%; for the 10-6&1 solution, 1.35 X gram mas recovered with a coefficient of 6.7%. The coefficients of variation show that comparable precision is attainable a t the two levels of concentration. As the signal-to-noise ratio decreases noticeably in the region of gram, one would expect the precision to drop off sharply as it did in the similar study on silver using platinum electrodes (6).
AGED 5 DAYS
y-
I
POTENTIAL IN VOLTS
Figure 4.
AGE0 4 W t ~ 6 5
for the dissolution peak. For example, there was evidence in more than one case that dissolution of cadmium started before the dissolution of zinc was completed. In any event, interaction of amalgamated elements undergoing dissolution is not unexpected in the light of reports such as that of Furman and Cooper ( 3 ) on dropping amalgam electrodes. The data in Table I have an additional uncertainty in that, in some cases, each element had to be determined in a separate run a t a different sensitivity; in others, both were determined in one run a t a sensitivity very unfavorable for one element. Hence, the large differences between values for an element alone and in the mixture reflect at least two major sources of error. I t is obvious that quantitative data on mixtures must be interpreted with caution. Characteristics of Mercury-Plated Silver Electrodes. Rotation of an electrode is desirable because it permits more dilute solutions to be analyzed. However, Hershenson (4) had found that platinum, even if heated and then quenched in mercury before being plated in the mercury in the usual way, was apparently unable to retain consistently an unbroken film of mercury on its surface as evidenced by irregular and higher residual currents and lower hydrogen overvoltage. Later, Cooke ( 2 ) and Perley (6) suggested that silver be substituted for the platinum, so this was investigated thoroughly in the present study. Mercury de-
-
POTENTIAL I N VOLTS
Cadmium Dissolution Curves
Obtained with amalgamated silver electrode at various aging times
Determinations of Zinc. To show further the utility of the mercury-plated platinum electrode, deposition and stripping experiments were carried out with IO-5Af solutions of zinc. Using a platinum electrode covered with a 4 micron layer of mercury, a calibration curve for submicrogram amounts of zinc was obtained in the usual manner by quantitative dissolution of amounts deposited a t various time intervals. The lower sensitivity of the electrode for zinc is immediately evident, since a 12-minute d e p -41 cadmium solution gave a recovery of osition from a gram of cadmium, while a similar electrolysis about 2.5 X with a 10-4 M solution of zinc gave a recovery of only about 4.6 X gram of zinc. Furthermore, the comparative broadness of the zinc dissolution peak as contrasted with that for cadmium is evident from the plot of Figure 5 . Thus in the determination of different metals by this method, consideration must be given to the relative rates of both deposition and dissolution. In addition t o the above differences, an accurate determination of zinc required a longer period of conditioning of the electrode than was required for cadmium. A fresh electrode, when used for cadmium, usually required only two dissolution cycles, of several minutes’ duration, before the optimum response of the electrode tvas obtained. However, when IO-t ’If zinc solutions were employed, the minimum time required for conditioning was extended to about 1.5 hours. Codeterminations of Zinc and Cadmium. As indicated by the data in Table I, there was a distinctly unfavorable interaction when these two elements were present in a mixture. The recovery of zinc was severely decreased by the presence of cadmium. There were also large errors in the values for cadmium which, though real, may have been apparent in the sense that the presence of zinc made it extremely difficult to draw a valid base line
./ I
I
.0.93
-0.50
POTENTIAL IN VOLTS
Figure 5. Dissolution Curve for 5Minute Electrolysis of Equimolar (10-6 M‘) Mixture of Zinc and Cadmium Polarization rate of 20 mv. per second
Table I.
Determination of Zinc and Cadmium in Mixtures
Zinc Molarity
____
10-1
1 . 5 7 X 10-aCd 1 . 4 7 x 10-aCd
a
10 - 6
10 - 0
1.13 X 10-9
x 10-9zn 1 . 3 7 x 10-8Cd
10 -3
3.60
10-4a
0 20 X 13-8Zn
10 - 6 10 - 6
Cadmium Molarity
off-scale Cd
1.36
x
io-aCd
1 26
x x
10-aCd
1 83
0 . 3 9 x lO-gZn 0.70 X 1 0 - Q Z n 0.69 X Cd 0.49 X 10-QZn 0.69 x 1 0 - 9 c d 0 36 X 1 0 - Q Z n 1 . 0 8 X lO-’Cd
10-nCd
Value for 10-4 .M zinc with zero cadmium present
IS
1.55 X 10-8 gram.
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3
i41 I
’P
1397
I
ELECTRODE AGE IN DAYS
Figure 6. Change in Sensitivity for Cadmium of Amalgamated Silver Electrode on Aging
posits from 2 to 4 microns thick produced an erratic electrode sensitivity typified by the plot of Figure 6. I n this particular case, the amount of cadmium deposited from a lobs M solution in 5 minutes and then recovered by dissolution from a 2-micron layer of mercury changed drastically as the electrode aged. The a p pearance of a maximum in sensitivity was typical, but the time of its appearance was retarded as greater thicknesses of mercury were employed. Cooke ( 2 ) recently reported work based upon his suggestion to the authors, which indicated satisfactory application of such electrodes. The authors, meanwhile, had abandoned silver as a supporting metal because calibration curves obtained with such electrodes did not remain constant over long periods of time. The apparently contradictory conclusions can be reconciled by observations that calibration curves remained more nearly constant for longer periods with thicker layers of mercury. Significantly, the decrease in sensitivity noted after 5 days of aging of the electrode used in Figure 6 was accompanied by a characteristic change in the dissolution curve. This is shown in Figure 4,which contained curves representing the typical dissolution behavior of a “normal” case, that associated with the maximum in sensitivity, and that after the electrode had aged for
approximately 4 weeks. Simultaneously with the attainment of the maximum in sensitivity, the dissolution curve exhibited a second dissolution peak at a slightly more positive potential than that characteristic of cadmium. The smaller peak was larger and more clearly resolved after 4 weeks of aging. After much longer periods of time, it was impossible to detect a dissolution peak of any kind for cadmium. Normal behavior could, however, be restored to this aged electrode by replating with mercury. iibnormal behavior was observed with several such electrodes and cannot be explained with certainty. However, one may speculate that, a t the time the second wave appeared, the estent of amalgamation of the silver had progressed to the point where, effectively, two electrode surfaces were competing with each other: one, pure mercury; the other, silver-amalgam either underneath the remaining pure mercury or directly exposed to the solution. after amalgamation became complete, deposition of cadmium probably could not he effected because of too lorn an overvoltage of hydrogen. In view of the erratic response obrerved with these electrodes it is felt that their application to microcoulometric measurements, particularly to mixtures of elements, is of limited value, even though the advantage of increased sensitivity through rotation is enjoyed by such a system. ACKNOWLEDGMENT
The authors are indebted to the Stomic Energy Commission for partial support. LITERATURE CITED (1)
(2) (3) (4)
(5)
(6) (7) (8)
Arthur, P., Naness, R. F., Komyathy, J., and Vaughn, H., presented at Southwest Regional RIeeting, A i f E R I C A N CHElrIIcAL SOCIETY, December 1951. Cooke, W. D., private communication, August 1951. Furman, N. H., and Cooper, W. C., d . A m . Chem. Soc., 72, 5667 (1950). Hershenson, H. AI., and Rogers, L. B., unpublished studies, 1951. Lord, 6. S., Jr., O’Neill,It. C., and Rogers, L. B., .XN.AL. CHEM., 24, 209 (1952). Perley, G. A,, private communication, August 1951. Zakharevskii, 11.S., Khim. Referat. Zhur., 2, (4),84 (1939). Zbinden, C., Bd1. soc. chim. biol., 13, 35 (1931).
RECEIVED for review February 2 , 1953. Accepted July 3, 1953.
Microdetermination of N-1-Naphthylphthalamic Acid Residues in Plant Tissues ALLEN E. SMITH Naugatuck Chemical Dimision,
T
GRACIE M. STONE U. S. Rubber Co., Naugatuck, Conn.
AND
HE selective pre-emergence herbicides, N-l-naphthyl-
phthalamic acid and its functional derivatives, are useful in growing a number of food crops ( 2 ) . Since pre-emergence herbicides are applied to the soil before weed emergence and before or shortly after crop emergence, surface residues on the crop a t harvest are not likely to be a problem. However, arylphthalamic acids may be absorbed and translocated by plants under some conditions. It was therefore necessary to determine if traces of the herbicide were present in the crop a t harvest. An analytical method capable of detecting and determining microgram quantities of the herbicide in plant tissues was required. I n the following discussion, N-1-naphthylphthalamic acid is used as the specific example, but the same analytical method can be used for its derivatives such as salts, esters, or the imide. The
method involves basic hydrolysis of the amide linkage to give 1naphthylamine and sodium phthalate, as shown in Reaction 1.
0, /’
- C O N0 H+ D 2 S a O H - + COzH
