(18) Gauglitz, E. J., Jr., Gruger, E. H., Jr., J . Amer. Oil Chem. SOC.,42,561-3 (1965). (19) Hanson, S . W. F., Olley, J., Biochem. J., 89 (3), 101-2r (1963). (20) Reinert, R . E.. Pest. Monit. J., 3,233-40 (1970). (21) Renold, A. E., Cahill, G. F., Jr., Handbook of Physiology, Sec. 5 , Adipose Tissue, American Physiological SOC., Washington, D.C., 1965. (22) Hansen, D. J . , Parrish, P . R., Lowe, J. I., Wilson, A . J., Jr., Wilson, P . D., Bull. Enuiron. Contam. Toxicol., 6 (2), 113-19 (1971). (23) Jensen, S., Johnels, A,, Olsson, M., Otterlind, G., Nature, 224,247-50 (1969).
(24) Hutzinger, O., Safe, S., Zitko, V., The Chemistry of PCB’s, CRC Press, Cleveland, Ohio, 1974. (25) Adamson. R. H.. Fed. Proc.. 26 (4). 1047-55 (1967). (26) Gillette, J. R., ibid., pp 1040-3. (27) LaDu, B. N.. Mandel. H . G.. Wav, E. L.. “Fundamentals of Drug Metabolism and Drug Disposition,” Williams & Wilkins Co., Baltimore, 1971. Received for review December 3, 1973. Accepted October 21, 1974. Paper presented at 29th Northwest Regional Meeting, American Chemical Society, Cheney, Wash., June 13-14, 1974. Mention of commercial products is for identification only and does not constitute endorsement by the U S . Department of Commerce.
Development of a Portable Polarograph for Determination of Aldehydes in Automotive Exhaust and Production Plant Samples James D. McLean” Dow Chemical Co., Michigan Division, Midland, Mich. 48640 John F. Holland Dept. of Biochemistry, Michigan State University, East Lansing, Mich. 48824
While commercial polarographic instruments are useful and sensitive laboratory analytical tools, they have found extremely limited applications in production facilities and on-stream analysis due to the complex and fragile nature of the accompanying equipment. A truly portable polarograph has been developed which overcomes these difficulties with no sacrifice in sensitivity and accuracy. The instrument, with solid state electronics, consists of a small package of about 10 lb. The strip chart recorder of the commercial version has been replaced by direct digital readout. Scan time has been reduced to 1 min. The dropping mercury electrode, with its cumbersome reservoir and stand tube, has been replaced with a small hanging mercury drop electrode with a self-contained mercury supply. The instrument is especially useful for monitoring aldehydes in both aqueous and nonaqueous chemical plant process streams, automotive exhaust, and in air samples where aldehyde pollution is suspected. The method of Lupton and Lynch ( 1 ) has been successfully employed for the polarographic determination of various aldehydes in a wide variety of samples. In 1970, the Automotive Laboratory of The Dow Chemical Company received several federal government contracts which required the determination of aldehydes as part of an automotive emissions survey involving a variety of engine load, speed, and fuel conditions. Several analytical techniques were tested for determination of aldehyde species in both particulate and condensate samples. Colorimetric and mass spectrometric techniques often demonstrated large interferences due to the large number of organic species present in the samples. When the polarographic method was applied, no interferences were observed and sensitivity down to 1 ppm was achieved. Several hundred samples were determined employing this technique, and it was established by differential pulse polarography that the predominant aldehyde species in most samples was formaldehyde. When catalytic mufflers were tested, the aldehyde content of the exhaust was significantly lower. Thus, it would be possible to monitor exhaust condensate for formaldehyde as a measure of the condition of proposed anti-
pollution devices (i.e., is it time to replace the catalytic muffler system?). Since an instrumental technique was desired which would be reasonably portable for use in various testing stations and garages, the standard polarographic system was not acceptable. Thus, work began on a truly portable polarographic system.
Procedure Equipment. A Sargent Model XXI polarograph monitored the hanging mercury drop and mercury pool electrodes when recorded datc was necessary for method development. A Princeton Applied Research Model 170 electrochemistry system equipped with a Princeton Applied Research Model 172 Droptimer recorded all differential and derivative pulse polarograms. A Metrohm Model E-410 hanging mercury drop electrode was employed for much of the work with the portable polarograph. The equipment used in the construction of the portable polarograph is described in the Discussion section. A Heath-Schlumberger Model SR-255B Recorder recorded polarograms with the portable polarograph. Reagents. Acetate buffer, approximately pH4, prepared as an equimolar mixture of acetic acid and sodium acetate, 0.1M in water. Hydrazine reagent, prepared as a 2% by weight aqueous solution of hydrazine sulfate. Formaldehyde stock solution, prepared as an aqueous dilution of reagent grade material to approximately 100 ppm. This solution is stable for several weeks. Analytical Procedures for Automotive Exhaust Sampling Samples were obtained according to government contract specifications by pulling a portion of the exhaust stream through a gas scrubbing tower containing a known volume of water. The tower is immersed in an ice bath during sampling (at a known flow rate and for the desired time period). Once obtained, these samples have demonstrated shelf-lives (from a aldehyde stability point of view) of several months under ordinary laboratory conditions. Volume 9, Number 2, February 1975
127
Table I. Polarographic Parameters for FormaldehydeHydrazone at a Mercury Pool HCHO concn, fig
11 22
Peak current, fi a m p
0.77 1.40
Peak potential,
v -1.07 -1.06
Cell volume, 10 ml; pool diameter, 3 mrn. reference electrode, Satur a t e d Calomel; Polarograph-Sargent Modi1 XXI.
Sample preparation consists of placing an aliquot of the desired size into a 10-ml volumetric flask. Five milliliters of buffer and 1 ml of hydrazine reagent are added, and the solution is diluted to volume with water. This solution is placed in the polarographic cell and purged for 5 min with prepurified nitrogen, prior to the polarographic scan.
Amplifier C serves as a voltage follower, detecting the voltage displayed by the peak-seeking circuit and driving voltmeter H employed as the readout device. Other components present in the circuit allow adjustment of the initial and final potentials selected for the scan, the time-rate of the scan, balance to null of the various electronic components, and zero suppression for use with samples giving a high background reading. Switch I, when closed, shorts out the potential scan and returns the potential applied to the cell to its initial value. When the switch is opened, the scan is initiated. Switch J is operated simultaneously with switch I . In the
Development of a Portable System-Electrodes It was necessary to replace the dropping mercury electrode (DME), since the accompanying stand, tubes, and reservoirs are not conducive to portability. Figure 1 demonstrates an ordinary D-C polarogram of a formaldehyde standard. The deviation of this curve from the theoretical S-shape (the broad hump) is due to the presence of a polarographic maximum. This is characteristic of hydrazone systems. The wave is still useful since a linear response between current and concentration is obtained. Figure 2 demonstrates the response of the same chemical system, but with the DME replaced by a small quiescent mercury pool. The increase in the current magnitude is due to the increased surface area of the pool vs. the drop. The accentuated peak is caused by depletion in the electrochemical double-layer, since no stirring is present. With the DME, stirring is caused by the actual drop fall. Table I shows the peak location and current vs. concentration response of several aldehyde standards. These data, as well as Figure 2, demonstrate the feasibility of using a portable mercury pool electrode for the purpose of measuring aldehydes a t the 1-ppm level. Instrument Design A small solid state circuit that could perform the functions of a laboratory polarograph was designed. The most critical part of the instrument is the current measuring device, since current sensitivities of 1-2 pamp full scale are necessary. Microammeters with this sensitivity are both fragile and expensive, and therefore it is preferable to convert the current to a potential and monitor the cell current with a voltmeter. The basic components of the circuit for the portable polarograph are shown in block diagram in Figure 3. All solid state components were employed. Amplifier A is used to apply a voltage scan between the electrodes in the polarographic cell. This scan is a linearly increasing negative voltage derived from the charging of capacitor D. In actual practice, a scan of -0.6 to -1.2 V vs. a saturated calomel reference electrode (SCE) is employed for aldehyde determinations. Amplifier B is a current follower to monitor the current that flows between the cell electrodes as the potential is applied to the cell, thus converting this current to a potential. The use of a l-megohm resistor in the feedback loop results in 1V of response from each pamp of current. Circuit E is designed as a peak-seeking device, for ease of operation. Diode F will only pass current in the forward direction (as long as the signal in Figure 2 is increasing). After the current passes the peak and begins to decrease, the diode blocks the decrease and continues to display the maximum value until capacitor G exhibits leakage. 128
Environmental Science & Technology
Dropping Mercury Electrode pH 4 Acetate Buffer HCHO Concentration 21.6 Microgramdl0 ml
Potential, Volts
Figure 1. D-C polarogram
I -0.8
VI
SCE
of formaldehyde-hydrazone
I
I
1
-1.0
-1.2
-1.4
Potential, Volts VI SCE
Figure 2. pool
Polarogram of formaldehyde-hydrazone at a mercury
Table II. Repetitive Scans on a Mercury Pool Electrode Peak current, mmp
Scan
2.6
1 2 3
1.3 1.0
HCHO concentration, 23.8 *g/lO ml
Table 111. Polarographic Response of ForrnaldehydeHydrazone at a Hanging Mercury Drop Electrode HCHO, r9
Drop
47.6 47.6 47.6 47.6
New Rescan New Rescan
Current,
0.55 0.46 0.56 0.46
Cell volume, 10 ml; drop size, 0.76 rnrn; initial potential, -0.60 V vs. SCE; scan t i m e , 60 sec.
SCE; final potential, -1.00
V vs.
closed position, switch J returns the readout device to its initial value (usually zero). An exact description of the electrical components is included in the patent disclosure of McLean and Holland (2). A response curve for formaldehyde standards from the portable polarograph connected to a small (2-3-mm diameter) mercury pool is shown in Figure 4. Linearity is good up to 40 wg of formaldehyde, and sufficient sensitivity is present to allow the determination of as little as 1 ppm. An optimum scan rate of 1 min is employed. Reagent blanks demonstrated no response with this system. Thus, the initial trial of the complete portable polarographic system was highly successful. Further experiments with the new system established that the mercury pool did not give reproducible results in some systems. Also, as shown in Table 11, repetitive runs on the same standard solution demonstrated a decrease in the peak current value. This indicates coating of the mercury surface by products of the electrochemical reduction. Thus, only one polarographic scan can be made with a given mercury pool. The difficulty encountered in reproducing the exact area of the pool makes this electrode difficult to use. A micrometer-controlled hanging mercury drop electrode (HMDE) was also tested, since its area should be ex-
tremely reproducible. The drop is formed by a small piston acting on a self-contained mercury reservoir. The piston is controlled by the micrometer and can be positioned very accurately. With this device, drops of various sizes can be employed. A 0.76-mm diameter drop with a surface area of 1.80 f 0.05 mm2 was selected as the best combination of sensitivity and reproducibility. Extensive testing demonstrated the excellent reproducibility of this electrode. Figure 5 shows a polarographic scan of a formaldehyde standard employing the HMDE. In Table 111,the response of this system is demonstrated. Table I11 shows that while the reproducibility with new drops is excellent, repetitive scans of the same drop demonstrate a significant decrease in peak current caused by coating of the drop surface. Addition of maximum suppressors, such as gelatin or Triton X-100, does not eliminate this effect. Thus, a new drop must be employed for each experiment. Figure 6 shows the calibration curve obtained from a series of formaldehyde standards run by the portable polarograph equipped with a HMDE.
Application and Validation This portable system was used to determine aldehydes in automobile -exhaust samples obtained from The Dow
.
Mercury Pool Working Electrode Saturated Calomel Reference Electrode
10
M 30 40 HCHO Concentration, MicrogramrllO ml
50
I
Figure 4. Response curve from first portable polarograph
0.76 mm Drop Diameter pH 4 Acetate Buffer HCHO Concentration 22.8 MicrogramsllO ml E ‘j,
b
4
r-l
-1
v-
f1
Figure 3.
I
B
-
9
s
~~
-0.6
V+
ograph
:: e
Block diagram of basic components of portable polar-
-0.8 Potential. Volts
YS
-1.0 SCE
-1.2
Figure 5 . Polarogram of formaldehyde-hydrazone at a hanging mercury drop electrode
Volume 9, Number 2, February 1975
129
Samples were run in an independent comparison with two different analysts. The results were identical within experimental error in all 'cases, as shown in Table V.
Improued Version A digital version of the portable polarograph has been successfully constructed and evaluated. An Analog Devices digital voltmeter is employed to display the results directly, and meter reading errors have been eliminated. A recorder jack was added to the portable polarograph to allow a permanent record to be made, if desired. The
$/
I
I
I
70
80
90
,
0.2
0.1 0 I)
10
20
30
4.3
60
60
HCHO Concentration. Micm~ramrilOml
Figure 6. Calibration curve for portable polarograph Chemical Co. Automotive Laboratory. An independent comparison between the portable polarograph operated by automotive laboratory personnel and the standard commerical differential pulse polarograph operated by analytical laboratory personnel was made on these sam-
130
Environmental Science & Technology
0.10
c
Table IV. Comparison of Results from Differential Pulse Laboratory Polarograph and Portable Polarograph Sample
Laboratory, p p m as HCHOc
191x 192X 193X 194X 195X 196X CD600-2-50L CD600-2.54L C D600-2-56L CD600-2-57H CD600-2.60L
17 20 24 71 26 170 170 12 22 93 7.3
Portable,b p p m as HCHO
17 22 25 77 29 140 180 15 24 100
7.8
Single determination, pp,m in aqueous condensate samples, not Pas Dhase exhaust levels. It is not t h e DurDOSe of this DaDer to discuss gldehyde levels in exhaust streams, 'but' rather to demonstrate t h e capability of such measurement. Gas phase levels can be directly calculated from the listed values if desired. b Average of duplicate determiAations. a
recorded scans also show multiple reducible components when present, while the digital readout displays only the total current for all reducible species present between the initial and final potentials. A few automotive exhaust samples demonstrated the presence of aromatic and higher aliphatic aldehydes, as well as formaldehyde, when examined by differential pulse polarography. The response of the portable polarograph to mixtures of aldehydes is shown in Figure 7. Aromatic aldehyde hydrazones respond prior to formaldehyde hydrazone, while acetaldehyde hydrazone responds a t a more negative potential. Propionaldehyde and butyraldehyde hydrazones respond a t the same potential as acetaldehyde-hydrazone. While ketone hydrazones demonstrate polarographic response, investigation of that phase a t this time is beyond the potential scan selected for automotive exhaust analysis because of government contract requests. Figure 7 demonstrates that the total current is observed as a digital or recorded readout when several aldehydes are present. It should be mentioned, however, that due to the different diffusion coefficients of the various aldehyde hydrazones, use of a single aldehyde standard would give different results, depending on the ratio of the aldehydes present in the sample. Some drop coating by the reduction products of the first component is observed in a decreased sensitivity to subsequent components. If samples contain primarily one aldehyde component,
Table V. Comparison of MBTH Colorimetric Method and Polarographic Method for Aldehyde Levels in Automotive Exhaust Condensate Samples Sample
92-E 94- L 90.0
MBTH method p p m aldehyde; as HCHO
340 1500 430
Polarographic method, p p m aldehydes as HCHO
300 1530 480
then the portable polarograph (when calibrated with the same aldehyde standard) will give excellent results. If identification of various aldehydes is desired, either differential pulse polarography or the liquid chromatographic method of Papa and Turner ( 4 ) is recommended. The portable polarograph is 10 x 10 x 4 in. Figure 8 shows the instrument connected to a typical cell assembly, including the HMDE. This instrument has been applied to a large variety of aqueous and nonaqueous samples from chemical plant processes for the determination of various aldehyde species. Successful tests have also been made on other reducible species such as Zn2+, Cd2+, Pb*+, nitro compounds, ketones, and some anions (e.g., iodate and bromate). Only minimal maintenance has been required after several hundred determinations.
Conclusion The portable polarograph is a new instrument for the on-site determination of aldehydes in both gaseous and liquid phases. Many other polarographically reducible species can also be determined. Because of its sensitivity, the portable polarograph can be applied to a number of environmental pollution problems. Acknowledgment
The authors thank Vernon Stenger for suggesting the need for a portable aldehyde monitor. L i t e r a t u r e Cited (1) Lupton, J . M., Lych, C . C., J . Amer. Chem. Soc.. 66, 697
(1944). ( 2 ) McLean, J . D., Holland, J . F., patent application, pending, US.Patent Office. (3) Sawicki, E., Hauser, T. R.; Stanley, T. W., Elbert, W., .Ana/. Chem., 33,93 (1961). (4) Papa, L. J., Turner, L. P., J . Chromatogr. Sci., 10,,747 (1972). Received for reuieu .April 18, 1971 Accepted October 21, 1974
Volume 9, Number 2, February 1975
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