836
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
1
of C102. The membrane was a 2.54 X lo-, cm thick silicone rubber polycarbonate composite (MEM-213, General Electric Co., Schenectady, NY). At an applied potential of +0.700 V vs. Ag/AgCl, the steady-state residual current was low and noise-free a t rotation rates from 400 to 2500 rpm, using pH 4 acetate buffer electrolyte. Calibration curves run a t 12-h intervals for a period of 72 hr gave a combined correlation coefficient of 0.996. These data show the rotating electrode stability and capability for part-per-billion meawrement. The time to attain 90% steady-state response was 57 s at 400 rpm. With established experimental procedures ( 5 ), the membrane diffusion coefficient for MEM-213 was determined to be (3.28 f 0.21) X cm2 s-l. We are currently investigating the use of this electrode for in situ trace metal analysis in natural water systems. Use of the proper’ membrane would isolate the free metal from possible interference in typical mercury electrode ASV measurements (6, 7).
t o PIR R o t a t o r
SIDE VIEW
Set Screw
3-
k% -Membrane L e n s Tissue
~TOP Figure 1.
VIEW
a37
Anal, Chem. 1982, 5 4 , 837-838
ACKNOWLEDGMENT We thank Carl Wise for assistance with electrode design and construction.
Glassy Carbon
LITERATURE CITED
Schematic of the rotating voltammetric membrane electrode.
Gough, D. A.; Leypoldt, J. K. Anal. Chem. 1979, 57,439. Smart, R. B.; Dormond-Herrera, R.; Mancy, K. H. Anal. Chem. 1979, 57,2315. Freese, J. W. M.S. Thesls, West Virginia University, Morgantown, W.V.. 1981. Sawyer, D. T.; Roberts, J. L., Jr. “Experimental Electrochemistry of Chemists”; Wlley: New York, 1974. Hitchman, M. L. “Measurement of Dissolved Oxygen”; Wlley: New York, 1978. Brezonlk, P. L.; Brauner, P. A.; Stumm, W. Water Res. 1976, 70, 605. Buffle, J.; Comlnoll, A.; Greter, F. L.; Haerdl, W. Roc. Anal. Div. Chem. SOC.1878. 15.
tions were made to the brass rings and a male three-plug connector. The chuck end of the shaft was machined to fit the analytical rotator (Pine Instrument Co., Grove City, PA). Contact to the pollarogratph (PARC Model 174A, Princeton, NJ) was made via carbon brushes. A lens paper disk is saturated with the electrolyte and placed over the GC disk and then the membrane is placed over the electrode face and secured with a silicone rubber O-ring. The brass tube is filled with electrolyte and the reference and counterelectrodes are inserted. The schematic of this electrode is shown in Figure 1.
RECEIVEDfor review September 14,1981. Accepted January 15, 1982. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
RESULTS AND DISCUSSION This electrode has been initially used for the measurement
Aqueous Potentiometric Titration of Silicate and Hydroxide in Alkali Silicate Solutions Victor L. Grosvtmor ARC0 Solar, Inc., ,20554 F’lummer Street, Chatsworth, California 9131 1
The semiconductor industry commonly etches silicon in sodium hydroxidle by the following reaction:
-
+ 20H- + Si + H20
+ SiO2- + 2Hz
(1) We required a rapid and simple technique to determine simultaneously the concenltrations of hydroxide and silicate in caustic silicon etch solutions. The method we developed involves titrating the sample with standard HC1 in the presence of excess1 sodium citrate. The sample to be titrated must have its pH[ adjusted above 11 with alkali to put the silicate in monomeric fo’rm. We sought this new technique because of important practical drawbacks associated with established methods: most approaches do not permit simultaneous measurement of silicate and hydroxide. Further, techniques such as atomic absorption ( I ) require a large investment in equipment. A variety of methods analyzing silicomolybdic complexes, surveyed by Morozyuk (2), are time-consuming and limited to silicon assay. A simultaneous titration of hydroxide and silica 2Na+
2Na+
0003-2700/82/0354-0837$01.25/0
content, such as can be done with the fluorosilicate ion ( 3 ) , requires use of H.F, a hazardous reagent we preferred to avoid. Sodium silicate is the basic form of the weak acid Si(OH)4. Until now, silicate ions in aqueous alkali solutions could not be determined by a chemical measurement that involved any changes in the concentration of hydroxide (4-6). If the concentration is changed, the equilibrium between monomeric and various polymeric species will shift and the result is poorly defined titration end points. However, when a dilute solution of monomeric sodium silicate is titrated with acid in the presence of a large excess of sodium citrate, two sufficiently distinct end points are observed that correspond to
nSi032-+ mOH- -I- ( n + m)H+ nHSi0,-
-
+ nH+ + nH20
nHSi0,-
+ mH20 (2)
nSi(OH)4
(3)
where m and n are not necessarily integral numbers but signify quantity of compound. 0 1982 American Chemical Society
