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V O L U M E 22, NO. 10, O C T O B E R 1 9 5 0 An application of a Cellosolve solvent was found in the direct polarographic determination of tetraethyllead in gasoline (2). The gasoline samples were dissolved in Cellosolve containing a sufficient concgntration of hydrogen chloride to decompose the tetraethyllead; after heating a t steam temperature, the resultant solution was electrolyzed a t the dropping mercury electrode for lead content. As the half-wave potential for lead ion is 0.4 volt (os. the mercury pool anode), it was not necessary to purify the Cellosolve (Figure 2). The success of this determination suggests the possible analysis of other metallo-organic compounds in gasoline or similar solvents.
fusion current of lead in Cellosolve-hydrogen chloride electrolyte was found to be approximately one eighth of that obtained from the same concentration of lead in an aqueous hydrochloric acid electrolyte. Muller has pointed out that the necessity for using wkll buffered solutions for the analysis of reducible organic compounds (5) and the change of half-wave potentials with a change in hydrogen ion concentration. Well buffered solutions were obtained in Cellosolve using acetic anhydride-odium acetate and ammonium hydroxide-tetraethylammonium bromide mixtures. DISCUSSION
A brief study of the use of the glycol ethers as solvents for polarographic analysis showed they have promise in the analysis of organic compounds. Cellosolve dissolves a wide variety of polar and nonpolar organic compounds, organometallic a m pounds, inorganic acids, and salts. I t generally allows a choice of supporting electrolyte and dissolves sufficient salts for buffer systems. While the present investigation of organic compounds in the Cellosolve medium was confined to naphthalene, 1- and %methylnaphthalene, and tetraethyllead, it is believed that the Cellosolves make possible the polarographic study of a wide variety of compounds. ACKNOWLEDGMENT
The authors wish to thank C. W. Smith for his suggestion of the use of the glycol ethers for this purpose. LITERATURE CITED APPLIED VOLTAGE
F i g u r e 2.
After
Typical Lead Polarogram decomposition of tetraethyllead in 1 N hydrogen
chloride
in Cellosolve
S o maxima were observed in the application of Cellosolve electrolytes to either the tetraethyllead or the naphthalene determinations. However, maxima have been encountered in lead reduction waves where the lead was added to the Cellosolvehydrogen chloride electrolyte as lead acetate; in this case, methyl red was found to be an efficient suppressor. The dif-
(1) Burdett, R. A., and Gordon, B. E., ANAL. CHEM.,19, 843-6 (1947). (2) Hansen, K. A,, Parks, T. D., and Lykken, Louis, Zbid., 22, 1232 (1950). (3) Kolthoff, I. M., and Lingane, J. J., “Polarography.” p. 344, New York, Interscience Publishers, 1941. (4) Laitinen, H.A.,and Wawzonek, S., J . Am. Chem. Soc.. 64, 17658 (1942). ( 5 ) Muller, 0. H., Chem. Rev., 24, 95-124 (1939). (6) Wawzonek, S.,and Laitinen, H. .I.,J. Am. Chem. Soc., 64, 2365-8 (1942). (7) Wawzonek, S., Laitinen, H. A., and Kwiatkowski, S. J., Ibid., 66. 830-3 (1944). RECEIVED April 17, 1950.
Manual Polarograph for Rapid Determinations of lead and Cadmium in Zinc L. C. COPELAXD
THE
AND
F. S. G R I F F I T H , The New Jersey Zinc Company (of Pa.), Palmerton, Pa.
spectroscope has been generally used by The New Jersey Zinc Company (of Pa.) for rapid routine determinations of cadmium and sometimes of lead. Lead was most frequently determined by electrolysis. The desirability of providing better analytical service for one plant, for which no spectroscope was available, prompted this investigation of the use of the polarogrsph, The literature contains numerous references-many of which are given by Kolthoff and Lingane-to the successful application of the polarographic method to this analysis. The more recent work of Nickelson and Randles (6),Hawkings and Thode (e), and Ford ( I ) among others present thorough studies of this method. This paper describes a simplified modification of this method, particularly the use of a relatively inexpensive manually operated
polarograph and spot readings a t specific voltages for rapid routine determinations. In this laboratory the new method, in t h e hands of operators of limited experience, has successfully replaced spectroscopic and electrolytic determination of impurities in zinc. Figure 1 shows a typical polarographic curve for the determination of lead and cadmium in acid zinc chloride solutions as obtained by conventional procedures using a recording polarograph. The abscissa indicates the voltage applied between t h e dropping mercury cathode and the mercury pool anode and t h e ordinates show the diffusion current obtained a t each potential. The “wave height” is proportional to the concentration of the ion being reduced, and in this example the solution being analymd contained 0.237% lead and 0.071% cadmium. The usual pre-
A N A L Y T I C A L CHEMISTRY
1270
An inexpensive manual polarograph is described with which it is possible to determine lead and cadniium in zinc rapidly by Obtaining the diffusion currents at only three potentials. Inexperienced personnel can easily handle four samples an hour, including solution of the metal, with an accuracy comparable to the more conventional spectroscopic and chemical methods. Other applications of this machine and method are mentioned.
cautions were followed of degassing the solution and maintaining the same temperature, drop rate, and supporting electrolyte as in the calibration of the capillary. Xo interfering ions were encountered in the determinations on high purity zinc samples but in some of the cases cited, procedures were developed to eliminate interference from tin and indium. PROCEDURE
T\\-enty grams of zinc were weighed into a 600-ml. beaker and 10 to 15 ml. of mater were added, folloued by 70 ml. of hydrochloric acid (specific gravity 1.19). .4s the initial vigorous reaction subsided, two drops of a 3% cobalt chloride solution were added as catalyst. The solution was completed on the hot plate (undissolved copper may be left as a ivashed decant residue with no detectable loss in lead or cadmium), cooled, and diluted t o 100-nil. volume.
The voltages chosen to test the method were 0.30, 0.50, and 0.70 volt against a silver-silver chloride anode (lead half-wave a t 0.42 volt, cadniiuni half-ware a t 0.59 j , LIexmenieiits were made at the three voltages mentioned on the photographic records previously obtained in t,he preliminary investigation, and the Tesults were calculated and compared with the results obtained by the more usual procedure. Excellent agreement was obtuiiied by the two methods. In addition, the three-point method x a ? ujed for four t o ten daily samples for a 25-day period and the results were compared with the spectroscopic results on cadmium aiid the electrolytic results on lead. These samples ranged from 0.0002 t o 0.001i70 lead and from 0.0001 to 0.004370 cadmium. Over two hundred such comparison? were made with no significant discrepancy appearing. IbEXPENSIVE MACHINE FOR ROUTISE WORK
THREE-POINT DETERRIINATION
Lingane (4)and o t h m have suggested that, if sufficiently routine procedures are developed, it is only necessary to measure the difference in the residual current betwecn fixed potentials on each side of the appropriate “half-wave potential” to determine the concentration of a given ion. Figure 1 also illustrates the application of this method t o the lead aiid cadmium waves in zinc. Here it is obvious that the vertical distance between A and B and B and C is proportional to the wave heights of lead and cadmium, provided that: 1. -4correction is made for the vertical component of the b from the vertical charging current-that is, by subtracting a distance, A B , in Figure 1. I n general, this correction was needed only when using shunts with multiplication factors less than 10. 2. The position of -4,B , and C is definitely fixed in the straight-line portion of the residual current.
+
Because of the apparent reliability and speed of this simplified procedure, an inexpensive machine was assembled to free the recording machine for other exploratory work and special nnalysis
n
I 1
R2 0A
, , IO00 MV I
I
t RI
I
f
b+ Figure 2.
VOLTS Figure 1.
Polarographic Curve
Lead and cadmium in acid zinc chloride solution
’
The photographic machine requires 6 minutes to run through the 0.5-volt range, a little time to develop the paper, and finally about 3.5 minutes to measure the heights of the two waveB. I n contrast, the measurement of the diffusion currents a t the three potentials can be completed in 3.5 minutes. The saving in time prompted a comparison of the accuracy of the two methods.
Wiring Diagram of lfanual Polarograph
Figure 2 shows the wiring diagram for the polarograph which was assembled from standard equipment; this is siniiler to the simple circuit given by Kolthoff and Lingane (3). The galvanometer, G, serves a dual purpose, acting as an accurate voltmeter in the standardization circuit and as a microammeter in the polarographic circuit. The potentiometer, R1,is a ten-turn helical coil with a total resistance of 10 ohms. The knob of this unit is graduated from 0 to 100, and a second concentric dial indicates the turn; consequently, with a potential of 1 volt established across the resistance, a direct-reading slide wire graduated in divisions of 1 mv. results. Figure 3 is a photograph of this machine. Similar manual machines are commercially available. Routine procedures have been facilitated by a cell design permitting shifting of the mercury capillary and silver reference electrodes to the various therniostated and outgassed solutions.
1271
V O L U M E 22, N O . 10, O C T O B E R 1950 T a b l e 1. Standard Sample of Z i n c S u b m i t t e d as Unknown
. -.
Lead. % 263
__
hi" A*+"*
Cadmium. % 263
0.00142
The cells consisted of the lower half of test tubes with the rim flared so aa to support the tubes in holes cut in a thick aluminum plate which covered the constant temperature bath, The inert gas was passed in series through a group of the cells, each cell having a rubber stopper carrying inlet and outlet tubes. At the start of the determination, the degassing head of a cell was replaced by a stopper carrying gas inlet and outlet, the dropping mercury capillary, and a silver chloride rrference electrode. The silver anode is in the form of a helix passing around the gaa inlet tube and capillary; it terminstes nbove the end of the capillary in order t o avoid amdgamation of the silver. After transferring the cell heads the cell is deg&?sed for an additional 0.5 minute before taking the readings.
tion has bcen obtained by this method 011 samples known to contain tin and indium and analyzing as high BS 0.82% lead and 0.18% cadmium. Some success has also been obtained in determining tin by the difference between the lead waves in acid solutions after pH adjustment. It appears likely t h a t indium can be determined from the cadmium r a v e in za similar manner. As mentioned previously, copper may be filtered off after solution of the sample in hydrochloric and boiling for a few minutes. Generally, some copper is dissolved but not enough to cause trouble. No appreciable loss has been found in lead or cadmium. Many other uses are being found for this machine such as the determination of zinc in soils: This method is superior to the dithiozone method in sensitivity and accuracy although it is not &s readily applicable to field work. Recently 21 soil samples were analyzed for zinc using the manual polsrograpb and two-pcint readings. The zinc content varied from 0.02 to 0.4%. After solution and dehydration of the samples (1 gram), less than half a day was required by one mnn.to obtain the results. This work would have required about B, m e k if dctelmined by the dithiozone method.
As a check on the accuracy of this method of analysis, a standard sample of high purity zinc wa8 submitted frequently as an unknown over a period of 8.5 months. Four different operators working a t different times totaled 263 separate solutions of the zinc and determined both cadmium and lead. Table I shows the results of this study and Table I1 shows the distribution of the polarographic results. The accuracy of this polarographic method
of Polarographic R e s u l t s known: four analusta) Cadmium value. % NO. 0.0006 0.0007 0.0008 0.0009 0.0010 0.0011
0.001s
1 -
263
1
4 26 205 26 1
2 x
This method has made possible considerable saving in man, hours, elapsed time, and chemicals. Where previously 5 hours were required to determine lead and 3 hours to determine cadmium on four samples per shift of high purity zinc, results are DON reported within 1hour. The saving in elapsed time is often much greater; samples received rtt 3:30 P.M. are now reported a t 4:30 P.M. whereas previously they were not reported until 8:OO A.M. the fallowing morning. A two-man night shift formerly occupied mainly with these determinations has been eliminated. I n general an experienced man requires approximately 30 minutcs far
.-.... deter] mine( fully satisf:sctory.
Figure 3.
Manual Polamgraph
As B further example of the reliability of the sy.stem a second instrument was constructed and shipped to mother plant where analysts with no previous polarographic experience quickly mastered the technique on the basis of a few typewritten instructions. LJTERATURE CITED (1) ('2)
JNTERFERING ELEMENTS
wtLen analyzing impure metal samples containing tin and indiuim, the results for lead and cadmium are high. Since tin is campletely precipitated above pH 3 and indium above p H 5.5, n h e n:as lead and cadmium do not precipitate below p H 6 when prese nt. in moderate amounts, the tin and indium may be removed b y adljusting t.he pH of the solut.ion with ammonia to a p H of 5.6 at ro(,m temperature. Good agreement with electrolytic deposi-
Ford, E. G.. Cnn. Chcn. P r u c m Inrls.. 33, 1051 (IS49). Hawkings, R.C., and Thode, H. G., IN". ENO.CHEM., ANAL.ED..
16.71 (1944). (3) Kolthoff, I. M., and Lingane, J. L.. "Polarography," New York, Intrrsoicnae Publishers, 1941. (4) Linssne. .I.L.. private communication. (5) Nickelsoon and Randles, "Polarographic and Spectrographic Andysis of High Purity Zinc and Zinc Alloys for Die Casting," London, H.M. Stationery Office. 1945. RECEIVED Marah 1. 1950. Presented at Coderenee on Anslytical and ~ Applied Spectroscopy, Pittsburgh Section, A\(ERICAN C h e x r o ~SOCIETY, Pittsburgh. Pa.,February 17, lS60.
