508
Anal. Chem. 1981, 53, 508-512
b
6 -1.0
-o9
- 0.7
- 0.e
E
.0.6
- 0.5
CONCLUSIONS The normal-pulse pseudoderivative technique, as performed with the inexpensive microprocessor-based instrumentation described here, appears to be a most useful technique. For reversible processes, it is identical with differential pulse polarography without the dc distortion terms. It has the advantage over dp polarography in that several experiments may be effectively performed at once. The ability to decrease background currents appears to be closely related to that of differential-pulsepolarography. Data on other than reversible systems indicate that in these cases dp and pdnp polarography are also closely related. However, for systems exhibiting product adsorption or film formation, the pdnp polarography appears to offer a distinct advantage.
.O.L
vs Ag/AgCI
Figure 5. Differential-pulse (top curve) and variable-ampliiude pdnp (bottom)polarograms of 5 X lo4 M Na2S in 0.1 M NaOH: 0 5 s drop time (area 0.009 cm2) with A€ = 40 mV (20 ms) and 5 mV ramp steps.
return to the initial potential appears to have a "cleaning" effect on the surface. Interestingly, a review of the literature reveals that variable-increment pdnp as described here has been effectively accomplished previously as differential double-pu]se voltammetry (ddpv) (16). Although the waveform used in ddpv is somewhat different, the results are clearly the same' The use Of ddpv has been limited to 'lid but the advantages over dp voltammetry with respect to adsorption were clearly shown. At that time the close relabetween ddpv (Or pdnp) and dp voltammetry was not realized. Unlike ddpv, variable-increment pdnp polarography as performed by using microprocessor-based instrumentation may provide a number of values of AE from one experiment. This may prove even more useful when solid electrodes are used since a number of experiments are not required for various values of hE.
LITERATURE CITED (1) Bond, A. M. "Modern Polarographic Techniques in Analytical Chemistry"; Marcel Dekker: New Ywk, 1980; Chapter 6. (2) Christie, J. H.; Jackson, L. L.; Osteryoung, R. A. AMI. Chem. 1978, 48, 242-247. (3) Klein, N.; Yarnitzky, J. J . €lecffoana/. Chem. 1975, 67, 1-9. (4) Van Bennekm, W. P.; Schute, J. B. AM/. Chlm. Acta 1977, 89, 71-83. (5) Peterson, W. M. Am. Lab. (FalrfleM, Conn.) 1979, 7 7 (Dec), 69-78. (6) Bond, A. M.; Jones, R. D. AM/. Chim. Acta, in press. (7) Anderson, J. E.; Bond, A. M.; Jones, R. D., submitted for publication h Anal. Chem. ( 8 ) Chrlstie, J. J.; Osteryoung. R. A. J . €/ecfroana/. Chem. 1974, 49, 301-31 1. (9) Parry, E. P.; Osteryoung, R. A. AM/. Chem. 1965. 3 7 , 1634-1637. (10) Auerbach, c.; Finston, H. L.; Kissel, G.; Glicksteln, J. AMI. them. 1981, 33, 1480-1484. (11) Bond, A. M.; O'Halloran, R. J. J . Elecfroanal. Chem. 1978. 68, 257-272. (12) Bond, A. M.; Grabaric, B. S. Anal. Chlm. Acta 1978, 101, 309-318. (13) Evans, 0.M.; Hanck, K. W. Anal. Chlm. Acta, in press. (14) Anderson, J. E.; Bagchi, R. N.; Bond, A. M.; (3eenhiii, H. B.; Henderson, T. L.; Walter, F. L. Am. Lab. (FalrfkM, Conn.), in press. (15) Turner, J. A,; Osteryoung, R. A. AM/. Chem. 1978, 50, 1496-1500. (16) Lane, R. F.; Hubbard, A. T. AM/. Chem. 1978, 48, 1287-1293.
RECEIVED for review August 21,1980. Accepted November 3,1980. Financial assistance in support of this work from the Australian Research Grants Committee is gratefully acknowledged.
Diethyldithiocarbamate-SensitiveElectrode for the Simultaneous Determination of Metals Saad S. M. Hasan'' and M. M. Habib Department of Chemlsfty, Faculty of Science, Ain Shams University, Cairo, Egypt
An electrode sensitive for diethyldithiocarbamate ions (DDC) down to M is prepared by precipitation of copper diethyldlthiocarbamate and silver sulfide within a graphite rod. The electrode is used as a monitor for slmuitaneous tnration of binary, tertiary, and quaternary metank mixtures of Cu, Cd, Ni, Pb, V, Th, and Zn in 50-75% ethanol at pH 4-6 using NaDDC as a titrant. Stepwlse titration curves with sharp end point breaks and results with an average recovery of 96.5 % (mean standard deviation f0.4%) are obtained with as little as 50 pg/mL of ail these metal ions. Application of the method to slmultaneous determination of some malor elements in five different copper-base alloys gives results whlch compare favorably with certified values. Present address: Department of Chemistry, University of Delaware, Newark, DE 19711. 0003-2700/81/0353-0508$01 .OO/O
Considerable effort has recently been devoted to the development of rapid and reliable methods for the simultaneous determination of elements in various combinations without prior separation. Polarography (I),amperometry (2),coulometry (3),potentiometric stripping (41, and anodic stripping voltammetry (5) have been used for trace determinations of elements in some mixtures. Sequential potentiometric titrations have also been reported for determination of major elements in a few mixtures by use of metal and solid-state indicator electrodes (6-9). However, the wide application of this instrumental technique for various metallic mixtures has been hampered by the lack of suitable electrodes that can detect successive reactions of the same titrant with various metal ions. We felt it was worthwhile to prepare and study an indicator electrode sensitive for a reagent that is capable of forming 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
complexes or precipitants with a varieqy of metals. Graphite electrodes with surface-bound organic compounds (IO),coated with polyvinyl chloride membrane (11),saturated with organic liquid ion exchanger (12),and impregnated in wax (13) or metal sulfides (14) have previously been used for many analytical purposes. In the present work, however, electrodes prepared by precipitation of various organic chelating agents and their metal complexes within a graphite rod as a conducting support were investigated. A graphite-copper diethyldithiocarbamate electrode proved to be highly sensitive for diethyldithiocarbamate, a water soluble and stable reagent known to complex, precipitate, and extract a large host of metal ions differentially (15,16). This electrode presents a convenient approach for the simultaneous titration of up to four metals in concentration levels (down to 50 pg/mL. Moreover, the absence of an inner reference electrode and internal filling solution results in simplie construction and a sturdy, easily used electrode. EXPERIMENTAL SECTION Apparatus. ,411 potentiometric measurements were made with an Orion Microprocessor Ionalyzer, Model 901 (Orion Research Inc., Cambridge, MA), using graphite-copper diethyldithiocarbamate or graphite-(copper diethyldithiocarbamate-silver sulfide) electrodes in conjunction with a double junction reference electrode (Orion 90-02) containing 10% potassium nitrate in the outer compartment. Reagents. All reagents used were of analytical reagent grade, and deionized water was used throughout. Sodium diethyldithiocarbamate (0.01M) in 50% and 75% ethanol solutions were prepared and standardized by titration with standard 0.01 M copper sulfate by use of graphite-copper diethyldithiocarbamate as indicator electrode. Solutions of copper, cadmium, lead, nickel, thorium, vanadium, and zinc nitrates (0.01 M) were prepared and standardized with EDTA by using visual titrimetric procedures (17). Spectroscopic graphite rods ( - 5 mm diameter) of the type used for spark source mass spectrometry wiere employed (Varian, Palo Alto, CA). Preparation of the Graphite-Copper Diethyldithiocarbamate Electrode. A 3 cm piece of spectral pure graphite rod is placed in a clean test tube and 5 mL of a saturated 96% ethanolic solution of sodium diethyldithiocarbamate (NaDDC) added. The rod is allowed to stand in the solution for 1 h, removed, resoaked in 5 mL of saturated aqueous copper sulfate solution for another 1h, washed several times with deionized water and ethanol till no copper or DDC ions can be detected in the washing solutions, and then dried with a filter paper. Electrical contact is ensured by wrapping one end of the rod with a small sheet of pure copper metal (-3 cm X 0.5 cm) attached to a copper wire. The rod is inserted in a polyvinyl chloride (PVC) sleeve in which about 2 cm of the other end of the rod is protruding as a measuring surface. When the electrode is not in use, it is stored in deionized water. The graphite-(copper diethyldithiocarbamate-silver sulfide) electrode is prepared by successive soaking of the graphite rod in saturated aqueous solutions of sodium sulfide and silver nitrate for 1h. The rod is then washed thoroughly with deionized water till free from silver and sulfide ions, and the above steps are followed to precipitate copper diethyldithiocarbamate within the same graphite rod. Simultaneous Determination of Metals in Copper-Base Alloys. A 0.05-0.1-g sample of the alloy is weighed out and transferred to a 50-mL beaker and 5 mL of 10 M HN03 added. The mixture is heated on a sand-bath (-,250 "C) and slowly evaporated to complete dryness. Addition of acid and evaporation are repeated until the complete dissolution of the alloy sample. Then 1mL of 1M HNO, and 20 mL of deionized water are added, and the beaker heated gently to dissolve the salt residue. The pH is adjusted to 4-6 by addition of O.OEi M NaOH and the solution transferred to a 100-mL volumetric flask and completed to the mark with deionized water. A 5-mL aliquot of the solution is transferred to H 100-mL beaker and 15 mL of 96% ethanol added. The graphite-(copper diethyldithiocarbamate-silver
509
sulfide) electrode is inserted in conjunction with a double-junction reference electrode in the solution and the titration is conducted with 0.01 M sodium diethyldithiocarbamate in 75% ethanol. Alternatively, a known addition titration technique is used: A 5-mL aliquot of the alloy solution is transferred to a 100-mL beaker, a 1.00-mL aliquot of a standard synthetic mixture of Cu, Ni, and Zn (0.5 mg each) is added to the brass, constantan, and die-casting alloy solution, and a 1.00-mL aliquot of a standard synthetic mixture of Cu, Pb, and Zn (0.5 mg each) is added to the bronze and motor brass alloy solution. Fifteen milliliters of 96% ethanol is then added to the alloy solution and the titration is conducted with 0.01 M NaDDC as above. One milliliter of the standard solutions is similarly titrated in the absence of the alloy sample. The titers of the standard and the sample plus the same standard are compared at each inflection, and the differences are due to the metals of the alloy in the alloy sample solution. The 5-10-mL sample of binary, tertiary, and quaternary metallic mixtures containing 50-700 pg/mL of each metal are similarly determined. RESULTS AND DISCUSSION Diethyldithiocarbamate Electrode. The electrode shows Nernstian response toward DDC ions with an average anionic slope of 55 mvlconcentration decade a t 25 "C over the concentration range of 10-1-10-5 M in 50-75% ethanol background. No linear response, however, was noticed for copper ions. The response time of the electrode for DDC ions was tested by measuring the time required for the electrode to attain a steady potential after successive immersion in different DDC solutions each having a 10-fold difference in concentration. The results indicate fast and stable response (15-25 s for solutions M and 30-40 s for solutions M). The concentration of copper diethyldithiocarbamate in the graphite electrode has no significant influence on the electrochemical behavior of the electrode. No measurement of selectivity coefficients has been made as the electrode is only designed for use as a sensor for multielement titration. Titration of Metal Ions w i t h NaDDC. A preliminary evaluation of the electrode was made by potentiometric titration of Cu(II), Ni(II), Pb(II), Cd(II), Zn(II), Th(IV), and V(1V) ions in concentrations down to 50 pg/mL, a t different pHs in aqueous solutions and in the presence of water-miscible organic solvents (dioxane, dimethyl sulfoxide, acetonitrile, methanol, ethanol, and 2-propanol). Quantitative recovery and maximum potential break (not less than 150 mV) a t the equivalence points (metal:DDC, 1:2) were obtained at pH 4-6 in 50-75% ethanol. With the exception of Pb(II), the presence of 1000-fold molar excess of NO
