835
Anal. Chem. 1982, 5 4 , 835-836
transistor with the light path blocked and clear. The voltage at the noninverting input (pin 3) of the operational amplifier is then adjusted to be midlway between these two voltages. Resistors R5 and R6 are necessary to adjust the output of the comparator such that its voltage swing provides a corresponding switching at the output of nand gate 1. The values given for these two resistors in Figure 1 have been found to be adequate for this purpose. However, if these rihould not work, one only needs to adjust them so that the voltage level between them, leading to the input of nand gate 1, falls below 0.8 V when the light path of the diode/transistor is blocked. The light emitting diode, as shown in Figure 2, is mounted directly on the faceplate of the pressure gauge. The phototransistor is positioned directly opposite the LED. Approximately 3-4 mm of separation, for passage of the indicator needle, is left between the diode and the phototransistor. The diode, phototransistor, and associated wires are then secured with silicone rubber cement.
RESULTS AND DISCUSSION Two separate electrical shutdown devices, identical with that described in this report, have been in continuous operation in our laboratory for the past year. Neither has improperly shut off the system when operating in a pressure range below the critical value. And, both have a perfect record in eliminating high-pressure conditions. LITERATURE CITED (1) Nikelly, John C.; Ventura, Dominic A. Anal. Ch8m. 1979, 51. 1585-1 588. (2) Mefford, Ivan N.; Barchas, Jack D. J . Chromatogr. 1980, 181, 187-193.
RECEIVED for review September 15,1981. Accepted December 24, 1981. The support provided by NIMH/DHEW/PHS through Grant No. MH-26866-05 and NINCDS/DHEW/PHS through Grant No. K\;18-16887-01is gratefully acknowledged.
Cadmium Ion Selective Electrode and Ethylenedinitrilotetraarcetatocadmium(II)for End Point Detection in Chelatometric Titration of Nickel( I I)Ion Isamu Uemasu and Koschltake Iwamoto" Department of Ch@misfty,College of General Educatlon, The Unlversiky of Tokyo, Komaba, Megoro, Tokyo 153, Japan
In the course of the preparative investigation of Hofmann-type and the relahed clathrate compounds with a general formula Cd(diam)Ni(CN)4mG (diam = (NH3)2 or diamine; G = aromatic guest molecule) ( I ) , we were in need of developing a rapid procedure to determine both the Cd and Ni contents in the products. Using a cadmium ion selective electrode (CdISE) as an indicator electrode, we succeeded in determining both suecessively in a sample solution by EDTA (ethylenedinitrilotetraacetate) titration with acceptable accuracy and preciidon. The mechanism of end point detection is, however, not simple but dependent on the fact that the rate of formation reaction of Ni(I1)-EDTA is considerably smaller than that of Cd(I1)-EDTA at room temperature. Although several metal ion selective electrodes and metalEDTA complexes have heen applied to the end point detection in chelatometry (2-6), the mechanism described here is different. In the presence of a small amount of Cd(I1)-EDTA, Ni(I1) ion alone can allso be titrated. Since no Ni(I1) ion selective electrodes are commercially available, we report on the end point detection in EDTA titration of Ni(I1) ion using CdISE in the presence of Cd(I1)-EDTA. The method does not suffer any interference by redox phenomena and by chloride and acetate anions.
EXPERIMENTAL SECTION Apparatus. The CdISE used was a DKK Type-7120electrode (Denki Kagaku Keiki & Co., Ltd., Tokyo) with the solid membrane of cadmium sulfide-silver sulfide; the reference electrode was an Orion Model 90-02-00 (A.g-AgC1) double junction one with 10% KNOBin the outer compartment. An Orion Model 801 digital pH/mV meter W,BS used to measure the electrode potential, its output being recorded on a Hitachi Model 056 recorder as the potential-time curve. The output of the CdISE potential was M CdNernstian in the concentration range from 10" to (NO,), with the slope of 29.0 mV/decade at 25 "C (theoretical: 29.58 mV), the correlation coefficient being 0.9999. The time of response was ca. 5 s for IO-fold and ca. 10 s for 100-fold change in the concentration, respectively. Chemicals. All the reagents used were of analytical grade; deionized water iwas used throughout the experiments. 0003-2700/82/0354-0835$0 1.25/0
RESULTS AND DISCUSSION Titration of Ni(1I) in the Presence of Cd(I1)-EDTA. When a mixed solution of Cd(I1) and Ni(I1) was titrated with a standard solution of EDTA, the CdISE potential dropped sharply a t first to a certain extent by adding a small portion of the EDTA solution but was recovered to the value a little less than the previous one within a few minutes. This dropand-recovery of potential was observed repeatedly until the total amount of Ni(1I) was titrated. The phenomenon can be interpreted in terms of the slower formation reaction of Ni(I1)-EDTA than that of Cd(I1)-EDTA in spite of the higher stability of the formler than that of the latter. At first the EDTA added is chelated with Cd(I1) leading to the decrease of the activity of Cd(I1) ion in the solution. Meantime a replacement reaction proceeds between Cd(I1)-EDTA and Ni(I1) to form Ni(I1)-EDTA as follows: Cd(I1) EDTA Cd(I1)-EDTA
+
Ni(I1) + Cd(I1)-EDTA
fast
slow
Ni(I1)-EDTA
+ Cd(I1)
The Cd(I1) liberated in the slow reaction recovers the potential. Thus, a saw-tooth-shaped potential-time curve is obtained for the titration of Ni(I1) with the end point at the last edge of the curve aa shown in Figure 1. After the end point for Ni(I1) the curve shows a shape similar to that obtained for the titration of Cd(I1) alone. When an appropriate amount of Cd(I1)-EDTA is added to a Ni(I1) solution, the Ni(I1) can be titrated with an EDTA solution using the CdISE as the indicator electrode. Since the difference in the stability constants between Ni(I1)-EDTA (log K = 18.56) and Cd(I1)-EDTA (log K = 16.59) is ca. 2 in the logarithmic scale, the percentage accuracy of end point detection should be less than 100 owing to partial formation of Cd(I1)-EDTA before the equivalence point. In the titration of 50 mL of 4 X M Ni(N03)2 with a 0.01 M EDTA standard solution, up to 4 X M Cd(I1)-EDTA does not affect the end point. By adding 1mL of 0.01 M Cd(I1)-EDTA to 25 mL of 4 X M NiC12 prepared from bis(dimethy10 1982 American Chemical Society
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Anal. Chem. 1982. 54,836-837
d
I
* F o
0
a
1nin
~
-233.0nV
End Point
Time
Figure 1. CdISE potential-time curve for the titration of Ni(I1) in the
presence of
Cd(I1)-EDTA.
10-4 M; NI(NOJ,, 4
x
Initial concentrations:
Cd(I1)-EDTA, 4 X
10-3 M; 30 o c , p~ 5.
glyoximato)nickel(II) the Ni(I1) content was determined with the EDTA solution 20.32% (calcd 20.32%) with the standard deviation 0.02%. Effects of pH, Coexisting Anions, and Temperature. The titration procedure was examined in the pH range 3-11; sodium acetate-acetic acid (acetate buffer) and ammonium chloride-ammonia (ammonium buffer) were used to adjust the pH value. In the acetate buffer region so far examined, pH 3-6, the higher the pH was, the sharper and greater decrease of the potential was observed owing to the increase of the conditional stability of Cd(I1)-EDTA a t higher pH. The use of ammonium buffer is not recommendable because of the narrower potential gap and of the delay of the response time at the CdISE in the vicinity of the end point owing to the formation of considerably stable ammine complexes of Cd(I1). At least
1000-fold molar amounts of C1- added as KC1 against Cd(I1) gave no interference in the titration of the acetate buffer solution. The effect of temperature on the potential drop at the end point was examined from 20 to 40 "C. No significant improvement was observed above 25 "C. It should be noted that the titration a t 20 "C gave a poorer end point presumably owing to a critical decrease of reaction rate below 25 "C. Recommendable Procedure. Into a 100 mL beaker with the sample solution of Ni(I1) containing ca. 0.1 mmol of Ni(1I) ion, add 1mL of 0.01 M Cd(I1)-EDTA solution and 5 mL of sodium acetate-acetic acid (2 M-1 M) buffer. Dilute the solution to 25-50 mL by adding deionized water. Set the beaker in a constant temperature bath at 30 "C. Titrate the solution with a 0.01 M EDTA standard solution with constant stirring of the solution by a magnetic stirrer. Monitor the potential of a cadmium ion selective electrode inserted in the solution on a recorder. Wait for a t least 30 s in the vicinity of the end point. The last edge of the saw-tooth-shaped potential-time curve gives the end point.
LITERATURE CITED (1) Iwamoto, Toschitake J. Mol. Struct. 1981, 75, 51-65. (2) Rellley, Charles N.; Schmidt, R. W. Anal. Chern 1958, 30, 947-953. (3) Fritz, James S.; Garralda, Barbara B. Anal. Chem. 1964, 36, 737-741. (4) Ross, James W., Jr.; Frant, Martin S. Anal. Chern. 1969, 4 7 , 1900-1902. (5) Baumann, Elizabeth W.; Wallace, Richard M. Anal. Chern. 1969, 47, 2072-2074. (6) Napoli, A.; Masclnl, M. Anal. Chirn. Acta 1977, 89, 209-21 1.
RECEIVEDfor review October 16, 1981. Accepted December 22, 1981.
Rotating Voltammetric Membrane Electrode James W. Freese and Ronald B. Smart" Department of Chemistty, West Virginia University, Morgantown, West Virginia 26506
Conventional electrochemical instrumentation with the three-electrode arrangement may provide unreliable measurements when used with membrane-covered working electrodes because membranes with high resistivity could cause substantial iR drop. Gough and Leypoldt (I) have described a membrane-covered rotating electrode where the working and reference electrode were isolated behind the membrane. The counterelectrode was located in the test solution which permitted measurements using membranes of low or moderate resistivity. Electrical connection was made through Hg-pool contact. Smart et al. (2) have prevously described a membranecovered probe for in situ ozone measurement, where all three electrodes were isolated behind the membrane. The membrane used, silicone rubber, was a high resistivity homogeneous material. Before the analyte could undergo reduction, it had to dissolve in the membrane, diffuse through the membrane, and finally emerge at the electrode surface. This electrode system has now been modified to permit the entire electrode to be rotated using a commercial rotator with carbon brush contacts. Rotation of this electrode will permit a more reproducible and well-defined transport regime to be established. The rotating electrode has been used to measure chlorine dioxide in aqueous solutions (3).
ELECTRODE DESIGN The working disk electrode (lI2in. 0.d. X lIs in.) was made
from glassy carbon, GC (Vitrecarb, Fluorocarbon Process Systems Division, Anaheim, CA). A brass tube of the same diameter was cemented to the GC disk using a heat curing, silver-filled conductive epoxy cement (E-Solder 3012, Acme Chemicals, New Haven, CT). The brass tube was dipped in a nonconductive varnish (Maraset BV790, Acme Chemicals, New Haven, CT) to coat the inside and outside as well as the inside surface and edge of the GC disk. After the electrode was cured, the resistance between the GC disk and brass tube was less than 0.5 Q . The outer body of the electrode was machined from Teflon. The inside diameter of the body was only slightly larger than the brass tube to ensure a very tight fit. The body was heated to 100 "C for 15 min and the brass tube/GC disk was gently pushed through until the GC disk slightly protruded from the end. The disk was then sanded flush to the Teflon and polished in the usual manner (4). A lIs in. hole was drilled diagonally through the electrode face next to the disk and into the brass tube. A lI8in. 0.d. X in. porous Vycor glass rod was inserted into the hole to allow electrical contact between the polished GC disk and the Ag/AgCl reference and Pt counterelectrodes. Wires were soldered to the respective electrodes and the female three-plug connector. The top half of the electrode was machined from aluminum. An insulating sheath of Teflon was placed over the shaft, and three brass rings were fitted over this sheath. Wire connec-
0003-2700/62/0354-0636$01.25/00 1982 American Chemical Society
