never more than 4% of the corresponding peak of the previously injected sample. Contamination of a component by a ghost peak was completely eliminated by “washing out” the system prior to each injection as mentioned in the experimental. The method was rapid and simple. The fore-columns were easily packed, required little or no conditioning, and had good stability and reproducibility. There was no evidence of deterioration of the basic fore-column after fifty 2 4 injections of aqueous solutions at pH 2. The chief advantages of the method appeared to be simple sample preparation without loss of components to be analyzed, the use of water as a solvent for the analysis of water insoluble acids and amines, and the stability of standard chromatographic solutions. Although the main limitations of the method will undoubtedly be decomposi-
tions and undesirable side reactions on the fore-columns, the products from side reactions may serve equally well for the analysis of the reactants. For example, Peterson and Tao (4) analyzed trifluoroacetate esters in trifluoroacetic acid using a base-packed forecolumn which trapped out the solvent and converted the esters entirely to their corresponding alcohols. Corrosion of the metal parts of the instrument which came into contact with the acids and bases appeared to be nonexistent or negligible. Although only the qualitative aspects of the method have been studied, we are currently investigating its quantitative potential. ACKNOWLEDGMENT
The authors express appreciation to Harry D. Koster for help with the ex-
Lauric Acid-Diethanolamine Maximum Suppressor SIR: A number of communications (4-7) have discussed the importance and utility of soaps (sulfonated phenyl, xylyl, and tolyl stearic acids, dodecyl and dodecyl benzene sulfonates, dodecyl pyridinium bromide, and isothiourea dodecyl ether hydrobromide) in suppressing the polarographic maxima of simple and complex metal ions which are not ordinarily suppressed by the usual maximum suppressors. I n addition to the anionic and cationic soaps, nonionic soaps may also be used for this purpose. However, very few references describe this approach and those available refer to commercial products. Therefore, investigations on the use of these compounds in polarographic work was considered worth undertaking. This Table I.
Ions or complexes Pb+2 in KNOs Ni+2 in KCl Co+2 in KC1 Co+*-Ni +z in pyridine CdI,-KI complex (%-biuret
communication describes the results from the use of a simple nonionic soap, lauric acid-diethanolamine condensate (LDC), in suppressing the polarographic maxima of Pb+2,Ni+2,C O + ~N,i + W o + 2 mixture, iodide cadmium complex, copper biuret, copper succinimide, and copper glycine complexes. Data on the influence of LDC on the electrocapillary curves of the dropping mercury electrode (d.m.e.) in suitable supporting electrolytes have been included to demonstrate the superiority of nonionic soaps in polargraphy. EXPERIMENTAL
Reagents. L D C (8) was prepared by condensing pure lauric acid (BDH) with diethanolamine. Biuret (2) and
STSA
SXSA
10-5~ 2.0 7.0 9.0
10-5M 23.5 43.9 24.0
10-6M 23.8 56.01 107.8
10-6M 12.1 56.01 56.01
200.0 10.0 6.0 9.0 12.5
555.0 238.0 83.3 16.9
23.8 23.8 60.5 60.05
32.71 23.8 4.9 6.05
SPSA
Ci-gly cine Cu-succinimide ... ... LDC = Lauric acid-diethanolamine condensate SPSA Sulfonated phenyl stearic acid STSA = = Sulfonated tolyl stearic acid SXSA DPB IDEH 1592
= = =
Sulfonated xylyl stearic acid Dodecyl pyridinium bromide Isothiourea dodecyl ether hydrobromide ANALYTICAL CHEMISTRY
LITERATURE CITED
(1) Brivin, E. S., Marco, G. J., Emery, E. M., J . Dairy Sci. 44,1768 (1961). (2) Brochmann-Hannsen, E., Svendsen, A. B., J . Pharm. Sci. 51, 1095 (1962). (3) Decora, A. W., Dinnean, G. U., ANAL.CHEM.32, 164 (1960). ( 4 ) Peterson, P. E., Tao, E. V. P., J. Org. Chem. 29, 2322 (1964). (5) Swoboda, P. A. T., Chem. d% Ind. (London) 1960,1262.
GEOFFREY F.THOMPSON KATHLEEN SMITH Department of Psychiatry Washington University School of Medicine and the Research Laboratories Malcolm Bliss Mental Health Center St. Louis, >lo. 63104 WORK supported by Grants MH 05415 and MH 5804 from the National Institutes of Mental Health, United States Public Health Service.
Condensate as a Polarographic
Relative Effectiveness of Ionic and Nonionic Soaps as Maximum Suppressors
LDC
perimental work and to the Smith Kline & French Laboratories, Philadelphia, Pa., for a supply of amphetamine.
...
DPB i o - 5 ~
6.17 7.38 23.8 4.95 14.55 4.95 6.172 ...
IDEH 10-5~ 7.38 7’38 7.38 8.599 34.88 7.38
...
succinimide (9) were prepared in the laboratory. Analytical reagents and chemically pure reagents werk used in all the experiments. Double distilled water (all glass) was used in making the solutions. Triple distilled mercury was used for the dropping mercury electrode. Apparatus. The polarographic measurements were made using a Heyrovsky polarograph (No. L p 55A) in conjunction with a Pye scalamp galvanometer (No. 7903/5). All measurements u-ere carried out at 25’ =k 0.1’ C. in the water thermostat. Procedure. Polarographic measurements were made after adding a known volume of the metal salt or complex metal ion solution t o the polarographic cell, adding the supporting electrolyte, and making u p the total volume t o 20 ml. The solutions were deaerated bv bubbling purified nitrogen gas through them for about 20 t o 30 minutes. The nolarouam was taken. and the nrocess ;vas thYn repeated in’ the presence of the nonionic soap. An increasing amount of soap was added until the maxima were completely eliminated. For studying the effect of soap on the electrocapillary curves, a known volume of the ion to be reduced was added to the polarographic cell and mixed with suitable supporting electrolyte. The drop time with and without the soap was measured between the potential range -0.0 volt to - 1.0 volt. RESULTS AND DISCUSSION
Lauric acid-diethanolamine condensate is a useful maximum suppressor for a number of metal and complex ions
loo 1
a
APPLIED POTENTIAL, VOLTS I DIVISION = 0.2 VOLT
Figure 1.
Effect of LDC concentration on suppression of maxima
A: 2.5 X 1 O - W CuSOn in 2.5 X 10-2M biuret at Concn. of LDC in a. None b. 2.0 X 1 O"M
3.0 X 10-5M d. 5.0 X 1 O-5M e. 6.0 X 10-6M Each curve starts at -0.0 volt c.
6: 2.5 X IO-SM Cdlz in 0.1M KI at Concn. of LDC in a. None b. 2.5 X 10-6M c. 5.0 X 1 O-sM
d. 8.75 X 10-6M e. 10.0 X 1 0-6M Each curve starts at 0.3 volt
-
benzene sulfonates causes increased whose maxima could not be suppressed repulsion a t the d.m.e., with the result by the surfactants usually employed. that a large quantity of the soap is reFor example, Kolthoff and Lingane (3) quired. No such effect is observed with found that the iodide-cadmium complex gave a pronounced maximum which LDC which is nonionic and is easily adsorbed a t the mercury drop-solution was difficult to suppress. Addition of interface as is evident from the electroLDC of concentration as low as 10.0 X capillary curves studied in different supl O - 5 M could suppress the wave (El,z = - 0.7 volt) without changing its reversible nature (Figure 1). I n our early investigations (d-?'), it 4.8 was found that the cationic soaps generally suppress the negative maxima and the anionic soaps eliminate the n 4.6 positive ones. LDC has an extra advantage over these suppressors be- Y cause it can suppress both the positive (Pbfz, copper biuret, and copper glycine F 4.4 complexes) and the negative (Ni+2, C O + ~ Ni+'Co+z , mixture, iodide cad- 8 mium complex, and copper succinimide 0" 4.2 complex) maxima although it appears to be more sensitive toward the positive 4.0 1 maxima, For example, the amount of 0 0.P 0.4 0.6 0.8 1.0 APPLIED POTENTIAL, VOLTS LDC (2.0 X 10-6X) required to suppress the positive maxima (Pb+2) is Figure 2. Electrocopillory curve quite small in comparision to the A. Without soap amount of LDC (9.0 X 10-5M) re6. With soap quired to suppress the negative maxima (CO+~). Similar behavior has been observed in the case of copper glycine porting electrolytes. Figure 2 illustrates and copper biuret complexes (Figure 1). a typical case of the electrocapillary Moreover, a much smaller amount of curve in 0.1M KN03, with and without the reagent is required for suppression the addition of LDC. The soap deas compared to the anionic and cationic creases the drop time to a great extent soaps (Table I). and a maximum decrease is observed as From Table I it is seen that a sufthe potential reaches the electrocapillary ficiently large amount of anionic soap is maximum potential (-0.55 volt). required to eliminate the maxima, in Moreover, one observes a fairly large comparison to LDC. It appears that in flat portion of the curve, which is charthe higher p H range, the sulfonate ion acteristic of nonelectrolytes (1). Both of alkyl-aryl , dodecyl , and dodecyl
I
v)
5 v)
g
-
Table 11. Drop ~i~~ at Different potential in Copper Glycinate LDC Mixture in supporting ~ l
+
(KNOJ Copper glycinate Potential volt -0.0 -0.1
-0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0
+0.1M
KNOs t, sec.
Copper glycinate
+ +
0.1M KNOa soap (9.0
x
4.28 4.43 4.55 4.65 4.67 4.7 4.65 4.60 4.58 4.52 4.40
10-6M)
4.14 4.22 4.38 4.42 4.46 4.46 4.46 4.45 4.46 4.44 4.40
observations- i.e. , decrease in drop time and the appearance of a flat portion in the electrocapillary curve-indicate large adsorption of the soap. Ionic soaps do not behave in this manner. The effect of the soap concentration on the diffusion current also supports the above views. Just a slight excess of the soap considerably decreases the diffusion current without affecting Eliz, and beyond a concentration of 5.0 x 10-4M the polarographic wave vanishes althogether. It may, therefore, be concluded that LDC is strongly adsorbed a t the mercury drop-solution interface.
V O L 37,
NO. 12, NOVEMBER 1 9 6 5
1593
~
LITERATURE CITED
(1) Frumkin,
A., “Polarography,” Kolthoff, I. M., ed., Vol. I, p. 143, Interscience, New York, 1952. (2) Howorth, R. C., Mann, F. G., J. Chem. Soc. (London) 1943, 603. (3) Kolthoff, I. M., Lingene, J. J., “Polarography,” 2nd ed. p. 509, Interscience, New York, 1952.
(4) Malik, W. U., Haque, R., ANAL. CHEM.32, 1528 (1960). (5) Malik, W. U., Ha ue, R., 2. Anal. Chem. 180, 425 (19SI’f. (6) Malik, W. U., Haque, R., Naturwiss. 49, 346 (1963). (7) Malik, W. U.,Kafoor Khan, H. A., Ind. J. Chem. 2 (11) 455 (1964).
(8) Schwartz, A. M., Perry, J. M., “Surface Active Agents and Detergents,” p. 212, Interscience, New York, 1949.
(9) Vogal, A. I., “Practical Organic Chemistry,” p. 790, Longmans, Green, London, 1947.
WAHIDU. MALIK PURAN CHAND Department of Chemistry Roorkee University Roorkee, India The authors are grateful to C.S.I.R. (India) for the award of a fellowship t o one of us (P. C.) to carry out %is work.
Composite Graphite-Mercury Electrode for Anodic Stripping Voltammetry SIR: The technique of anodic stripping voltammetry is well suited, in terms of sensitivity and convenience, to determinations of trace quantities of electro-reducible metal ions in natural media. Total deposition of the metal ions of interest is advantageous where unknown complexing agents are present and may cause a very slow attainment of equilibrium. A wide variety of experimental procedures and electrode materials have been used (4) in anodic stripping methods. However, the full potentialities of the technique are not easily realized because the shape of the anodic current peak is greatly dependent upon the electrode. Thus, bulk mercury electrodes, usually in the form of a hanging drop or a pool, cause excessive tailing and poor rezovery after long deposition times. Graphite or carbon electrodes have the disadvantage of relatively low hydrogen overvoltage and the peaks are broad. We report a variation of the graphite electrode which gives very sharp current peaks, allowing high sensitivity and resolution. This electrode consists of mercury deposited on graphite. The deposited metals are then in a mercury solution with a high surface-to-volume ratio and as such are not subject to variable activity effects, as are solid deposits. The advantages of using mercury plated platinum electrodes (2) appear to be increased with this composite mercury-graphite electrode.
Table 1.
-1.0
-0.8
-0.6
-0.4 -02 E vs. S.C.E.
0
0.2
Figure 1. Anodic stripping curve of aerosol sample in 0.1M KCI, pH = 5.0 Plating time 70 minutes a t Sweep rate 40 mv./recond
- 1 .O
volt vs. S.C.E.
EXPERIMENTAL
The unit used for the stripping analysis was built around Heathkit operational amplifiers after the circuit of Enke ( 1 ) . A synchronous motor with a magnetic Teflon-covered stirbar was used for stirring. Saturated calomel reference and platinum counter electrodes were separated from the solution to be analyzed by porous Vycor plugs (Corning Glass Co.) in tubing made of Teflon (Du Pont). The cell was made of Vycor to prevent contamination by lead from borosilicate glass, and nitrogen was introduced Apparatus.
Determinations of Lead by Anodic Stripping Voltammetry
14ml. sample volume Coulombs X 106 Amount Pb added Coulombs Amount Pb Moles x 106 Peak area Blank found 1 . 0 0 x 10-10 1 . 0 0 x 10-10 9 . 0 x 10-10 a
19.3 19.3 174
21.5“ =IC 0.5 20.8 178
Average of three successive determinations on one sample.
1594
ANALYTICAL CHEMISTRY
2.0 2.0 2.0
19.5 18.8 176
through a fine-tipped needle made of Teflon. Reagents. All water used was double-distilled under nitrogen first from alkaline K M n 0 4 and finally from H3P04. Reagent grade mercury was dissolved in double-distilled nitric acid for the mercury additions. Lead standards more dilute than 10-4M were made up every few days and always assigned to the same flask. Nitrogen was prepurified and passed through a distilled water and a glass wool trap before being introduced to.the cell. Procedure. A spectroscopic grade graphite electrode (Fisher Scientific Co.) was impregnated with a good grade of molten paraffin wax under vacuum until the evolution of gas from the electrode ceased. Approximately 1 sq. cm. of the electrode was then scraped clear of wax and polished with 4/0 emery paper. The electrode was then placed in the solution to be analyzed and between 10-7 to 5 X lo-* mole of mercury was added to the cell. To plate out the mercury the electrode was held a t -200 mv. us. S.C.E. for 10 minutes with vigorous stirring, during which time the oxygen was also flushed from the cell. The electrode was then regulated to the desired plating potential. RESULTS AND DISCUSSION
The electrode exhibits a hydrogen overvoltage of 550-600 mv. and in media of pH 4-5, can be used up to -1.3 volts vs. S.C.E. I n similar media, waxed graphite alone can be used up to about -0.8 volt vs. S.C.E. The stripping peaks are quite sharp (100-150 mv., Figure 1) and for lead, the only metal investigated thus far, show total recovery of plated material (Table I) with very little tailing. The cell can accommodate samples of 4-20 ml. and the half time for total plating is 6-13 minutes, depending on electrode area, stirring rate, and volume. For a 1 sq. cm. area electrode, 15 ml. of solution, and a stirring rate of 120-160 r.p.m., the half time is 10 j=1 minutes. Under these conditions, the sensitivity
