Table II. Results of Analysis of 0.5 mg/ml Sodium Ampicillin Solution Using the Enzyme Electrode Found. rnolml Day
1
2
3
Average Rei std. dev
Electrode 1
0.48 0.46 0.46 0.66 0.40 0.55 0.48 0.50 16.7
Electrode 2
Electrode 3
0.52 0.46
0.63 0.55
0.44
0.48
0.47
0.55
8.9
13.6
Application to Penicillin Analysis. This enzyme electrode has a linear response down to a penicillin concentration of approximately lO-4M. Thus, it is competitive in terms of its sensitivity capabilities with the two most widely used chemical methods for penicillin analysis, namely the iodometric titration method and the hydroxamic acid colorimetric procedure. An attempt was made to evaluate the general utility of this electrode for penicillin analysis.
This work was done over a period of three days using three different enzyme electrodes and sodium ampicillin as the model compound. Calibration curves for all three electrodes were obtained on a daily basis. Analysis of a known, namely a 0.5 mg/ml sodium ampicillin solution, was attempted several times during the day using the three electrodes. The results of this work are summarized in Table 11. As can be seen, the reproducibility is poor. This is, of course, in part a reflection of the manner in which the data is plotted, that is, log [penicillin] us. observed potential. Thus, any small change in potential has a tremendous effect on the concentration value recorded. A possible source of' error is the contamination of the electrode by retention of part of the previous sample in the membrane. However, careful water washing and soaking in water for several minutes in between each measurement was performed, which should have been sufficient to overcome this problem. Further development is under way in these laboratories to improve the reproducibility of the penicillin electrode. In particular, a study of the geometry and configuration of the electrode is being pursued. Received for review November 9, 1972. Accepted December 21, 1972. This work was presented a t the Eastern Analytical Symposium, Atlantic City, N. J., November 2, 1972.
X-Ray Microdetermination of Chromium, Cobalt, Copper, Mercury, Nickel, and Zinc in Water Using Electrochemical Preconcentration B. H. Vassos,' R . F. Hirsch, and H. Letterman2 Department of Chemisfry. Seton Hall University, South Orange, N.J. 07079
X-Ray fluorescence exhibits good specificity and reasonable freedom from interferences, but its sensitivity in dealing with aqueous solutions is limited. A vast extension of the range for analysis a t trace levels can be obtained by preconcentration ( 1 -5). This paper describes a method in which the preconcentration step consists of electrodeposition of the metals to be determined onto a pyrolytic graphite electrode. In this way, small amounts of reducible metal ions can be separated from large volumes of dilute solutions. The desired metals are isolated in a form particularly suitable for analysis by X-ray fluorescence. After the deposition step, a thin disk is cleaved from the electrode surface (an operation possible only with pyrolytic graphite) and analyzed by X-ray spectrometry. The electrodeposited film behaves analytically as if it were infinitely thin. (The thickness of the deposit ranges 'Present address, Department of Chemistry, Colorado State University, Fort Collins. Colo. 80521. 'Present address, Bristol-Myers Products, Hillside. N.J. 07207. ( 1 ) W. T. Grubband P. D . Zemany, Nature. 76, 221 (1955). (2) C. L. Luke, Anal. Chim Acta. 41, 237 (1968). (3) K . Beyerman, H. J. Rose, J r . , and R . P. Christian, Anal. Chim. Acta. 45,51 (1969). (4) T. E. Green, S . L. Law, and W. J. Campbell, Ana/. Chem.. 42, 1749 (1970). (5) A . T. Kashuba'and C R . llines, Anal. Chern.. 43, 1758 (1971).
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from 10 to 20 A per pg of metal.) The graphite disks are durable and easy to store. Because of their small thickness and great purity, they generate a minimum of background interference (ti, 7). Previous attempts to electrodeposit on metal electrodes (8,. 9) have been seriously handicapped in sensitivity by the background interference from the electrode material. By utilizing our approach, however, the method is in principle limited in its over-all sensitivity only by the electrolysis time, if large volumes of sample are available. (This trade-off between sensitivity and electrolysis duration was shown to be true for a t least one order of magnitude beyond the standard conditions described below.) We have evaluated the method for solutions of low electrolyte concentration (approximating fresh water) and for samples with conventional levels of added supporting electrolyte.
EXPERIMENTAL Apparatus. T h e electrolysis cell uses I-cm diameter graphite rods of l-cm length(union Carbide, carbon products~ i ~ i ~ i N e w York, N.Y.), c u t along the crystallographic C axis f r o m plate ( 6 ) 6. H. Vassos. F. J. Berlandi, T. E. Neal. and H. B. Mark, Jr., Anai. Chem.. 37, 1653 (1965). (7) B. H. Vassos. R. F. Hirsch, and D. G. Pachuta, Ana/ Chern.. 43, 1503 (1971). (8) J. Natelson and P. K. De, Microchem. J , 7 , 448 (1963) (9) I . W. Mitchell. N. M. S a m and C. L. Hiltrop, Noreico R e p , 11, 39 (1964).
stock by means of a hollow mill, and surrounded tightly by a heat shrunk plastic tube (Birnbach SK-l05J/4 inch). This tube ensures a watertight fit and prevents seepage of electrolyte along the sides of the cylinder. Stirring is done by means of a magnetic stirring bar driven by a synchronous motor (1000 rpm). The solutions are maintained within one degree of 45 or 75 "C by circulating thermostated water. Constant current regulation is preferable to potentiostatic control since several cells can easily be wired in series and operated with identical currents. Since X-ray fluorescence permits easy discrimination between all the metals studied, the separation ability of constant voltage electrolysis is not needed for the method described here. In addition, the galvanostatic technique allows the electrolysis to occur beyond the plateaus of all metals studied and well into the HZ wave while maintaining constant at all times the rate of hydrogen evolution. The galvanostat designed for this technique is shown in Figure 1; it provides currents between 0.25 and 8 mA with 10-V compliance. Four to five cells can normally be operated in series. Reagents. The supporting electrolyte was preelectrolyzed in concentrated form for four days at 20 mA between graphite electrodes. The metal ion solutions were made by dilution from reagent grade chemicals. Doubly-distilled water was used throughout. Procedure. Various current density levels were investigated. (For large currents, the rapid gas evolution would require an inverted arrangement with the electrode at the bottom of the cell or a rotating electrode system.) As long as the current is sufficient t o guarantee operation well into the el&trodeposition plateau for all metals to be analyzed, there is no significant advantage in using still larger currents. A current of 0.5 mA was satisfactory for all subsequent studies. After the electrolysis was completed, the surface of the electrode was sprayed with Krylon 1301 (Borden Inc.) acrylic lacquer, and a 0.2-mm thick disk was cleaved with a razor blade. In some cases, depending upon the degree of crystalline perfection of the pyrolytic graphite, only thicker disks can be safely cleaved (-0.5 mm). The X-ray instrument used for counting the disks is a General Electric XRD-5 spectrometer equipped with an SPG-6 Eas proportional counter or a SPG-4 sciniillation detector, and a tungsten tube. The normal operating conditions are 50 mA at 50-kV ueak. For kach sample, measurements were made at the peak of the selected line (usually KO, but L d for Hg) as well as adjacent to the peak on each side for base-line correction. The difference between the peak value and the corresponding interpolated base line was taken as the corrected sample value CS A similar operation was carried out with a blank graphite disk, leading to a corrected blank count C E The value ( C S - C E ) was utilized for all analyses. R E S U L T S AND DISCUSSION In order to achieve good accuracy and precision, all sources of error must be identified and controlled. T h e counting process is not a n important source of error except a t the lowest concentrations, where it constitutes the limiting factor. For example, the random counting error for a 5-pg sample of Cu was only about 4%. T h e precision of the electrolysis is much more difficult t o optimize. Variations in the rate of stirring, pH, current density, and the presence of foreign ions, affect considerably the rate of electrodeposition. Considerable care must be exercised to maintain all these factors constant, or else special techniques must be used for minimizing their effects. Of couse, if one deposits all the metal in the sample, the end result does not depend on the rate of deposition; but this approach is time consuming. As shown below, however, it is not necessary t o use exhaustive electrodeposition to minimize the effect of kinetic factors. This permits a considerable saving in time with no sacrifice in precision or in experimental simplicity. Assume first order electrodeposition kinetics, and a n initial concentration M,. The quantity of interest is the variation, (ly),in the amount deposited, Y. This depends on the constancy of the rate of electrolysis, which
el Is
+ Figure 1. Galvanostat design
can be expressed as the variation, AN, in the number of electrodeposition half lives N for a given total time, or
100(AN/N). T h e kinetic equation governing the process is:
M
=
M , e x p (-0.693 N)
where 0.693 is In 2 and M is the amount of metal left after N half-lives. At this stage the amount deposited is
Y
=
M,
-
M , exp (-0.693 N )
(2)
The relation between A Y and AN is approximated by the for the propagation Of error ( l o ) : =
(%Y(AN)'
(3)
After performing the differentiation, one obtains
AY
= [0.693
M , e x p (-0.693 N ) ] A N
(4)
From here, it is possible to calculate the per cent error in the amount deposited AY as a fraction of the total metal present M,: A V
Y1
100-
M,
= 0.693 N e x p (-0.693 N ) X
100-A N N
=
K
X
100-A N N
(5)
This equation holds exactly only for small values of AN and for electrodepositions approaching completion, but can be used as an approximate guide outside this region. Note t h a t the value of K decreases exponentially a s the number of half-lives increases. For a short electrolysis time, where the depletion of the solution is negligible, the amount deposited is in direct proportion t o t h e rate of electrolysis or
AY AN 100= 1 0 0 7 Y
(6)
corresponding to a value of K = 1 in Equation 5. K is thus the residual fraction of the error as the number of halflives increases (Figure 2). In order to choose optimum experimental conditions, the actual value of N need be known only approximately. (It may vary from run t o run as kinetic factors or total electrolysis times vary.) If, for example, the half-life is estimated (from previous experiments) t o be 30 minutes, then a 90-minute electrolysis time corresponds to three half-lives (87% deposition). Assuming a 25% uncertainty in N , the error in the amount deposited, Y, is 6.5%. D Young, Statistical Treatment of Experimental McGraw-Hill Company, N e w Y o r k N Y , 1962 p 96
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ANALYTICAL CHEMISTRY, VOL. 45,
NO. 4,
APRIL 1973
Data
0
793
~
~~
Table I. 90-Minute Electrodeposition at 45 “C (75 “C for Zinc) in 0.033M K2S04-15-ml Total Volume Metal
Copper 0. (L
0 K K
Mercury
20
Zinc
40 60 80 10 20 30 40
W
LL 0
z
0 + Y
O
LL K 2
a 3 P
Present, k g 6.6 11.7 16.7 21.8 26.9
50
Nickel
v)
W (L
2
5 10 15 20
0
Cobalt
5 10 20 30 40
Chromium
I
0
I
2
3
1,
5
N
Figure 2. Improvement
in precision as the number of half-lives
increases It appears that even if total electrodeposition is in principle the most precise, runs of only two or three half-lives should be almost as satisfactory, even for variations in electrolysis parameters as large as 25%. The experimental conditions, chosen based on the above criterion, called for three half-lives. In order to achieve this within a reasonable period of 90 min, the sample volume was chosen to be 15 ml. It was also necessary to add supporting electrolyte in concentration of 0.033M NaZS04. For trace analysis, it is considered undesirable to add reagents for fear of contamination, but lower concentrations of supporting electrolyte produced a several-fold decrease in the deposition rate. The temperature was maintained within a degree of 45 “C (75 “C for Zn). A set of typical results is shown in Table I. By the criterion of “total error” (ZZ), essentially all the data fall into the “excellent” category except for the chromium results which are “acceptable.”
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E. F. McFarren, J. Lishka, and J. H. Parker, Ana/. Chem.. 42, 358 (1 970).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
5 10 20 30 40
6
na 3 3 3
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 5
5 5 5
5
Average found, r g b 6.0 12.7 16.5 21.4 27.1 21.1 38.1 60.6 80.3 8.8 21.1 30.7 39.8 49.5 2.0 5.2 9.8 14.8 20.2 6.1 9.7 17.7 30.6 40.6 5.7 9.2 20.3 29.3 40.5
Std deb pg
0.2 1.2 1.2 0.6 2.9 4.0 3.0 6.3 1.6 1.3 0.3 1.6 1.7 2.5 0.2 0.4 0.1 9.8 1.4
0.5 1.1 1.5 2.5 3.9 1.3 3.2 8.0 8.9 4.2
Std dev, % 3.3 9.4 12.0 2.8 10.7 18.9 7.8 10.4 2.0 14.7 1.4 5.2 4.2 5.1 10.0 7.7 1.0 5.4 6.9 8.2 11.3 8.5 8.2 9.6 22.8 34.8 39.4 30.4 10.4
is the number of runs bThe samples were used as their own standards
O n
CONCLUSIONS The combination of electrodeposition on graphite with X-ray fluorescence has proved to be fruitful for ppm determination of several metals. The data show also that quasi-exhaustive electrolysis permits satisfactory analysis without recourse to either special control of experimental conditions or to long duration runs. We are confident that the method can be easily extended to a number of other metals and also that more modern X-ray equipment can increase several-fold the sensitivity of the analysis. The fact that the graphite disks are physically strong and easy to store could be important if, for legal or other purposes, the sample must be kept for future reference. Received for review June 13, 1972. Accepted December 11, 1972. Portions of this work were presented (by B.H.V.) a t the 163rd National Meeting of the American Chemical Society, Boston, April, 1972.
