smoke the ratio of indole (13.9 gg/cig.) to 1-methylindole (0.42 p g ) is 100:3 and that of 3-methylindole (14.0 pg) to 1,3dimethylindole (0.28 pg) is 100:2. In the pyrolyzate, the ratios are 100 :2.25 and 100 :1.6. Despite the use of different analytical methods the indole value for the tryptophan pyrolysis (880 “C) is comparable to the 3 x yield reported by Patterson et al. from the pyrolysis of tryptophan at 850 “C (14). This suggests that free and/or protein bound tryptophan in
(14) J. M. Patterson, M. LaFuse Baedecker, R. Musick, and W. T. Smith, Jr., Tobacco Sci., 13,26 (1969).
tobacco is an important precursor for indoles and N-alkylindoles in cigarette smoke. Biological data are required in order to verify whether 1-alkylindoles contribute to the carcinogenicity of the particulate matter of cigarette smoke and its most active subfraction BI. Bioassays o n mouse skin are being conducted to determine the tumorigenicity of 1-methylindoles.
RECEIVED for review September 29, 1969. Accepted December 24, 1969. This study was supported by American Cancer Society Grant E-231, and in part by National Cancer Institute Grant CA-08748.
Square-Wave Polarographic Determination of Lead as a Pollutant in River Water E. B. Buchanan, Jr., Thomas D. Schroeder,’ and Bozena Novosel* Department of Chemistry, The Unioersity of Iowa, Iowa City, Iowa 52240 A square-wave polarograph has been used to determine trace amounts of lead in potable water as part of a pollution study in the vicinity of Iowa City, Iowa. By using a combination of sodium perchlorate and sodium fluoride as a supporting electrolyte under acidic conditions, as little as 2 & 0.32 ppb lead could be determined without preconcentration of the samples. The sensitivity of this technique i s roughly 100 times that of similar methods by atomic absorption.
ALTHOUGHSINE-WAVE polarography has been much more widely investigated as a means of chemical analysis, squarewave polarography is potentially more sensitive to trace quantities of reversibly reduced species in solution. The greater separation of faradaic and capacitance currents possible with the electronic gating of square-wave signals provides this improved sensitivity. Despite this, few reports have been published where square-wave polarography has been used as a n analytical tool for trace analysis. Barker and Jenkins ( I ) , who first reported the use of a square-wave polarograph, predicted a sensitivity to reducible species as low as 2 X 10-SM. Since then Ferrett, Milner, and Smales have used this technique to analyze for less than 1 ppm lead in cocoa ( 2 ) , Niki, Sirai, and Kyoya have measured 2 X lOU7Mlead and cadmium in phosphoric acid (3),Goode and Campbell ( 4 ) have analyzed uranium metal for less than 5 ppm each of copper, lead, cadmium, and zinc and Kashiki and Oshima determined 0.01 ppm free sulfur in petroleum ( 5 ) . Square-wave polarography in combination with anodic Present address, Shippensburg State College, Shippensburg, Pa. 17257 * Present address, Department of Chemistry, Faculty of Science, University of Zagreb, Zagreb, Yugoslavia
stripping has been used by von Strum and Ressel to determine copper, antimony, lead, tin, thallium, cadmium, indium, zinc, manganese, and barium in concentrations ranging down to 1 X 10-9M (6). We have employed a squarewave polarograph to analyze potable water as part of a study of lead pollution in the rural Iowa City, Iowa, area. Concentrations ranging down to 2 X 10-*M have been measured without preconcentration of samples. INSTRUMENTAL
Briefly, the instrument used in these investigations is a controlled potential device employing a variable frequency, variable amplitude square-wave signal to modulate a dc ramp voltage applied to a polarographic cell. The ac signal resulting from the oxidation-reduction process occurring at the surface of the D M E is directed to a gating circuit which passes current during a preselected portion of each half square-wave period. Since capacitance current drops off rapidly after each transition and faradaic current does not, separation of the two is achieved o n the basis of time by allowing the gate to pass current only during a later portion of each half-period. This current is amplified, rectified, and sent to a recorder. The polarogram is a symmetrical peak whose height is proportional to the concentration of the electroactive species. F o r a more detailed discussion of the instrumentation and theory, references (7) and (8) should be consulted. EXPERIMENTAL
During the course of trace analyses performed with the instrument, the square-wave frequency was maintained at a value of 500 Hz. The gate was set such that it was open for of each half-period just before transition. The dc ramp voltage was scanned over a range of -0.1 to -0.4 volt US. a saturated silver/silver chloride electrode.
(1) G. C. Barker and I. L. Jenkins, Analyst, 77,685 (1952).
(2) D. J. Ferrett, G. W. C. Milner, and A. A. Smales, ibid., 731 (1954).
(3) E. Niki, H. Sirai, and T. Kyoya, Japan Analyst, 15, 257 (1966). (4) G. C. Goode and M. C. Campbell, Anal. Chinz. Acta., 27, 442 (1 962). (5) M. Kashiki and S. Oshima, Bull. Chem. Soc. Japan, 40, 1630 (1967); Electroanal. Abstr., 6, 2180 (1968).
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(6) F. von Strum and M. Ressel, “Proc. 1st Australian Conf. Electrochem.,” Pergamon Press, Long Island City, N.Y., 310 (1965). (7) E. B. Buchanan, Jr., and J. B. McCarten, ANAL.CHEM., 37, 29 (1965). (8) J. R. Bacon, Ph.D. Thesis, The University of Iowa, Iowa City, Iowa, 1968.
A stock solution of the supporting electrolyte was prepared from reagent grade sodium perchlorate, sodium fluoride, and distilled water which had been passed through a monobed ion-exchange resin. The concentration of the salts in this stock solution were 2.5M sodium perchlorate and 0.025M sodium fluoride. This stock solution was electrolyzed a t - 1.O V over a stirred mercury pool at room temperature for a period of two hours t o remove any lead contaminate. A covered 2-1. beaker containing a mercury pool cathode (1 cm of mercury in the bottom) and a platinum wire anode served as the electrolysis vessel. F o r the purpose of constructing a calibration curve, polarograms were obtained on standard solutions ranging in concentrations from 1 X 10-8Mto 1 x 10-6Min lead nitrate. Each solution was made 0.5M in sodium perchlorate and 0.5 X 10-2Min sodium fluoride by the addition of a n appr0priat.e quantity of the stock solution of supporting electrolyte. The pH of the standard solution was adjusted to a value of 3 with nitric acid. Each solution was deaerated for a period of five minutes and a waiting period of five minutes was observed between the end of deaeration and the time a polarogram was taken. A blank solution was analyzed first, followed by the stock solutions in order of increasing concentration. After each solution had been analyzed, the cell and electrode assembly, less the reference electrode, was washed in distilled water several times. River water samples were treated in one of two ways before analysis, depending upon the amount of organic surfactant contained in it. Those containing little o r no organic matter were analyzed directly after the addition of electrolyte and adjustment of p H to 3. Those samples containing large amounts of the organic material were treated with concentrated nitric and perchloric acid and heated to fumes of perchloric acid. After cooling, the samples were diluted with the electrolyte solution and p H was adjusted with sodium hydroxide. Polarograms were obtained o n the river water samples in a manner similar to that of the standard solutions. The washing procedure was repeated between each sample and often polarograms of more than one portion of a sample were taken as a check of reproducibility of the procedure. I n other cases, small volumes of standard solution were added to river water samples after a polarogram had been obtained and a second one recorded. This was done as a check for possible interferences that might be part of the original sample or introduced in the sample treatment procedure. A blank solution was analyzed before each series of samples and the peak heights due to lead in the blank solutions were subtracted from those measured in the samples. RESULTS AND DISCUSSION
I n initial attempts at determiriing trace lead with the squarewave polarograph, a 1M potassium chloride solution was used as a supporting electrolyte. Few problems were encountered with these solutions until polarograms were recorded at concentration levels in the range of 5 X 10-eM lead ion. At this point it was very difficult t o obtain a relatively constant base line current. Polarograms obtalned at the high sensitivity levels necessary for trace analysis showed a rather broad peak occurring a t a reduction potential 80 mV more negative than that of the lead-chloride complex. The peak was present in polarograms of dilute as well as concentrated solutions of both lead and chloride. Further investigation into the nature of the peak showed that the size of the peak varied as a function of the frequency of the applied square-wave signal. The size of the peak grew with increased frequency to a point where it was much larger and broadcr than the peak because of the lead reduction. At a frequency of 3 KHz, the lead peak appeared simply as a shoulder o n the side of the spurious peak. This phenomena has been discussed else-
where by Jennings (9). H e found that lead, tin, and thallium ions could not be determined in trace concentrations using 1M hydrochloric acid as a supporting electrolyte because of the existence of a “hump.” The author concluded that it resulted from a variation in the capacity of the mercury drop as a function of applied dc potential. Bromide and iodide ions show similar behavior when used as complexing agents for lead by adsorbing o n the surface of the mercury drop. Sodium fluoride proved to be a much better supporting electrolyte. I n replacing chloride with fluoride as the electrolyte, it was hoped t o retain the inherent electrochemical reversibility that lead is known to have in the presence of halide ions but still avoid the apparent adsorption problem associated with the larger chloride ion. Initial experiments with the fluoride ion in an acidic p H range were very encouraging. The “hump” associated with the lead-chloride solution was absent and the sensitivity to smaller concentrations of lead was improved. Attack by hydrofluoric acid o n the glassware of the polarographic cell was evident, and it was necessary to reduce the concentration of the fluoride ion t o a value where attack o n the glassware was not serious. Sodium perchlorate was added to the solutions as the major constituent to prevent migration currents and fluoride was present in a concentration sufficient t o complex any lead in the samples. A study of the square-wave polarographic peak height due to lead us. p H was conducted by recording polarograms on solutions containing identical concentrations of lead and supporting electrolyte but varying in hydrogen ion concentration. The p H range of 2 t o 5 was examined. The greatest sensitivity was obtained a t a p H of 3. A half of a p H unit on either side of the maximum decreases the sensitivity to threefourths of that obtained at a p H of 3 while a full p H unit difference reduced the sensitivity to one-half that of the maximum value. The cause of this change in sensitivity is the subject of further investigation. Jnitial attempts to establish a calibration curve for lead concentrations less than 5 x 10-6M were hampered by the amount of lead introduced with the sodium perchlorate and sodium fluoride reagents. To reduce the blank t o a level less than 1 X 1O-aM lead, the standard solution containing 2.5M sodium perchlorate and 2.5 X 10-2M sodium fluoride was electrolyzed over a mercury pool. This procedure reduced the lead content to a value of approximately 1 X 10-*M in lead, a value low enough for these studies. It was necessary to perform the same operation on the saturated sodium chloride solution used in the silver/silver chloride reference electrode. Studies had shown that significant amounts of lead were introduced into the polarographic cell from the reference electrode if the sodium chloride had not been previously purified by electrolysis. The reference electrode was constructed in such a manner that the sodium chloride contained in it would have a high rate of effusion to the sample solution in the polarographic cell. When the polarograph was operated a t the high sensitivity necessary for trace analysis and a glass frit electrode was used in the cell, large noise currents were recorded. These noise signals caused very erratic behavior of the chart recorder, making it impossible t o record polarograms of trace lead samples. The relatively larger effective contact area of a cotton fibertipped electrode had the effect of significantly reducing the resistance between the electrode and the solution in the polarographic cell. This resulted in very low background current and a steady base line (9) V . J. Jennings, Analyst, 87 (1036), 548 (1962). ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
371
Table I. Data for Lead(11) Calibration Curve Molar Peak height in concentration chart divisions Blank 33 1 x 10-8 30 2 x 10-8 38 50 4 x 10-8 90 6x 8 x lo-@ 123 I x 10-7 131 2 x 10-7 253 4 x 10-7 347 5 x 10-7 390 6X 578 8x 629 1 x 10-6 749
recorded o n polarograms. The steady base line made it easy t o measure even small peaks resulting from oxidation and reduction of solutions as low as 1 X 10-*Min lead. Washing the polarographic cell and electrodes with concentrated nitric acid and distilled water proved to be very effective in removing possible adsorbed lead salts from the glassware. With this procedure it was possible to obtain the polarogram of a dilute solution after a more concentrated solution had been analyzed. The reference electrode was not washed in this manner because of probable attack on the cotton tip by the acid, but separately washed with distilled de-ionized water and dried on paper toweling. In all cases there was no evidence of lead contamination from one solution to the next after cleaning in this manner. The frequency of the square-wave signal applied t o the cell was maintained at 500 H z throughout the course of these investigations even though a study of frequency us. peak height for lead seemed to indicate more sensitivity resulting from higher frequencies. Increasing the frequency of the square-wave signal results in a n undesirable increase in capacitance current. Since the RC time constant for the capacitance current remains unchanged, a higher frequency means a shorter period of time for this discharge to take place before the transition t o next half-period of the signal. This in turn means that capacitance current is present t o a greater extent at a latter part of a half-period than it is at a lower frequency. If the frequency is high enough, it becomes impossible to separate faradaic and capacitance current on a time basis through electronic gating. The value of 500 Hz was a compromise between highest possible sensitivity and the undesirable capacitance current. After each solution was deaerated for a period of five minutes, it was necessary t o wait a n additional five minutes before a polarogram was recorded. In early work, difficulty was encountered in obtaining reproducible peak heights on polarograms recorded successively on a single solution directly after deaeration had ceased. If polarograms were taken one after another immediately after the nitrogen stream was shut off, the resulting peaks appeared to grow as a function of time. To observe this phenomenon more directly, a lead sample was introduced into the cell and deaerated for five minutes. The dc ramp voltage was adjusted manually to the summit potential for lead and held there. The recorded current increased as a function of time u p to about five minutes, after which it remained steady. This phenomenon repeated itself for every solution investigated. In all cases it was necessary t o observe the five-minute waiting period after deaeration before polarograms were taken. No com372
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Table 11. Water Pollution Data Date (1969) 1-6 1-21 1-27 1-30 2-3 2-7 2-10 2-14 2-19 2-21 2-25 2-25
Lead above blank (ppb) 18.1 15.3 31.9 52.2 71.9 Suppression 0
Suppression 71.9" 36. ga 6.4a 10.3 (After addition of standard) 3-3 5.e 3-7 10.25 3-27 11.5a 3-3 Tap water 5. Sa After treatment with perchloric acid.
Conditions River frozen Snow melting Snow melting Snow melting Slight freezing Snow melting Cold (no run-off) Freezing Snow melting Snow melting Snow melting Melting High run-off Flooding
prehensive explanation of this event will be attempted here, but observations throughout the course of these investigations seem to indicate that agitation of the solution had a direct effect on the peak height obtained for lead. An increase in agitation seemed to diminish the peak in size. Table I lists the data recorded for the lead(I1) calibration curve. Polarograms taken on both the blank solution and 1 x 10-*M( 2 ppb) solutions of lead showed nearly identical peak heights. A polarogram recorded for a blank solution that had not been previously purified by electrolysis showed a lead peak corresponding t o roughly 30 ppb. Electrolytic purification reduced the lead blank to a value of 2 ppb which was low enough not to interfere with the lead analyses performed in the pollution studies. There was an average recorded deflection of 3.93 chart units per ppb of lead at maximum sensitivity. The standard deviation over the entire range of concentrations studied was 1.31 units per ppb which corresponds to a maximum error of 33 %. The pollution data compiled in Table 11, although by no means extensive, begin to show possible trends in lead content of the river as a function of weather conditions. It appears that when conditions favor melting of ice and snow, which is likely followed by run-off into the river, the lead concentration increases. This is possibly due to lead salts from auto exhausts, collected in snow along roadways, being drained into the river. I t appears from the first part of the Table that this lead concentration increase continues as long as the melting conditions last. The presence of organic material in the water samples was detected by the severe suppression of both the small amplitude charging current associated with the growth of the mercury drop and the recorded peaks due to lead reduction. After samples were treated with nitric and perchloric acids, normal polarographic currents were recorded from them. This does not constitute a sufficient criterion for the necessity of the acid pretreatment. Therefore, it is recommended that the pretreatment be considered as routine unless the analyst has complete confidence that no surfactant is present. SUMMARY
Square-wave polarography has proved to be particularly useful in a pollution study of lead in drinking water supply.
I n the absence of interfering organic materials in the water, sample preparation time is at a minimum. When interfering materials are present, they first must be destroyed by an oxidation treatment. Regardless of the nature of the samples two or more samples can be analyzed hourly with good repro-
ducibility and at sensitivities 100 times greater than that obtainable with atomic absorption procedures.
for review August 11, 1969. Accepted December RECEIVED 29,1969.
Coulometric Titration of Acid Hydrazides with Pressuremetric End Point Detection D. J. Curran and James E. Curley Department of Chemistry, Unicersity of Massachusetts, Amherst, Mass. 01002
The application of pressure measurements to end point detection for coulometric titrations is demonstrated by the oxidation of acid hydrazides with electrogenerated bromine. Approximately 6- to 10-mg samples of isonicotinic-, p-toluic-, or phenyl acetic acid hydrazides and adipic dihydrazide, all favorable cases, have been titrated with an accuracy of a few parts per thousand and a precision of about five parts per thousand. Linear-segmented type titration curves result when the transducer output voltage is plotted against microequivalents of titrant generated. The slopes of the lines of the titration curves yield useful information concerning stoichiometric or kinetic complications in the titration reaction. An extreme example is the case of a compound believed to be succinic dihydrazide which apparently reacts with bromine but does not produce any nitrogen. Other cases where complications were found were: carbohydrazide, 1,2diacetyl hydrazine, isobutyric acid hydrazide, and malonic dihydrazide.
methyl red end point detection techniques for the coulometric determination of isonicotinic acid hydrazide in pharmaceutical preparations. This paper deals with the titration of various acid hydrazides with electrogenerated bromine using the pressuremetric end point method of Curran and Driscoll (9). Acid hydrazides are oxidized by electrogenerated bromine in the anode compartment of a modified H-cell to yield nitrogen according t o (IO):
2Br,
0
0
II
I1
+ R C N H N H L + HsO -,RCOH + Ns(g) + 4HBr (1 )
Simultaneously, hydrogen gas is generated in the cathode compartment : 2H+
THE COULOMETRIC DETERMINATION of acid hydrazides with electrogenerated chlorine or bromine has been investigated by a number of workers. Several end point detection methods were employed. Kalinowski (1-3) titrated isonicotinic acid hydrazide with chlorine liberated by the electrolysis of hqdi ochloric acid. Decolorization of methyl red placed in the anode compartment of the titration cell signaled the end point. Krivis et al. (4, Kawamura and coworkers (9,and Olson (6) titrated various hydrazides using amperometric end point detection techniques. Wijnne and coworkers (7) used a dead stop technique for the coulometric titration of isonicotinic acid hydrazide which had been separated from other components by thin-layer chromatography. Stoicescu and coworkers (8) employed both the amperometric and the
+ 2e-
+
Hp(g)
(2)
Thus, before the equivalence point of the titration, nitrogen and hydrogen are both evolved, but after the equivalence point only hydrogen is evolved. EXPERIMENTAL
(1) K. Kalinowski, Acra. Polon. Pliarm., 11, 113 (1954); Chem. Abstr., 48, 14123 b (1954). (2) K. Kalinowski, Przem. Cliem., 10, 73 (1954); Anal. Abstr., 3, 528 (1956). (3) K. Kalinowski, and Z . Zwierzchowski, Acta. Polorz. Pharm., 20, 309 (1963); Anal. Abstr., 11, 1470 (1964). (4) A. F. Krivis, E. S. Gazda, G . R. Supp, and P. Kippur, ANAL. CHEM.,35, 1955 (1963). (5) F. Kawamura, K. Momoki, and S. Suzuki, Bull. Fac. Eng., Yokohama Nut. Unio., 4, 123 (1955). (6) E. C. Olson, ANAL.CHEM., 32, 1545 (1960). (7) H. J. A. Wijnne, E. Bletz, and J. M. Frijns, Plzarm. Weekbl., 102 (38), 959 (1967); Chem. Abstr., 67, 102844d(1967). (8) V. Stoicescu, C . Ivand, and H. Beral, Reo. Chirn. (Bucharest), 1968, 19 (8), 484; Chem. Abstr., 70, 6568h (1969).
Reagents. Solutions were prepared using laboratory distilled water that had been redistilled from alkaline permanganate. All chemicals, except acid hydrazides, were reagent grade. Stock anolyte solution was 0.75M in HCl and 1 M in KBr. A 3M HCI solution was used as catholyte. Isonicotinic acid-, p-toluic acid-, and phenyl acetic hydrazides were obtained from Aldrich Chemical Co., Inc., Milwaukee, Wis. Carbohydrazide, 1,2-diacetyl hydrazine, adipic- and malonic dihydrazides were obtained from Olin Mathieson Chemical Corp., New Haven, Conn. Samples of isobutyric acid hydrazide, carbohydrazide, malonic dihydrazide hydrochloride, and succinic dihydrazide were supplied by L. A. Carpino, University of Massachusetts at Amherst. All acid hydrazides were purified by successive recrystallization from ethanol except malonic dihydrazide which was precipitated from ethanol with concentrated HCl and isobutyric acid hydrazide which was recrystallized from a 1 :1 ethyl ether-water mixture. Solutions were made by dissolving accurately weighed samples in 3M HC1. Aliquots
(9) D. J. Curran and J. L. Driscoll, ANAL.CHEM.,38, 1746 (1966). (10) W. A. Waters, “Mechanisms of Oxidation of Organic Compounds,” John Wiley and Sons, New York, N. Y., 1964, p 84.
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