of very high ionization energies (just below the photon energy) where the discrimination from small escape width of electrons having low kinetic energy also introduces a serious problem of vertical sample inhomogeneity.
(8) C. K. Jgrgensen, L. Bakenc, and H. Berthou, Chimia, 27, 384 (1973). (9) V. I. Nefedov, N. P. Sergushin, J. M. Band, and M. B. Trzhaskovskaya, J. Nectron Spectrosc., 2, 383 (1973), (for Z = 1 to 20). (10) J. J. h a n g and F. 0. Ellison, Chem. Phys. Lett., 25, 43 (1974). (for 2 = 1 to 10). (11) W. J. Carter, G. K. Schweitzer, and T. A. Carlson. Proc. lnte. Conf. Electron Spectrosc., Namur (1974), in press (for Z = 3 to 92). (12) C. K. Jqkgensen and H. Berthou, Chem. Soc. Faraday Discuss., 54, 269 (1973). (13) C. D. Wagner, private communication. (14) C. S.Fadiey, Chem. Phys. Lett., 25, 225 (1974). (15) T. A. Carlson, J. C. Carver, L. J. Saethre. F. Garcia Santibanez, and G. A. Vernon, Proc. lnt. Conf. Nectron Spectrosc., Namur (1974) in press.
LITERATURE CITED C. K. J0rgensen and H. Berthou, Mar. Fys. Medd. Danske Vid. Selskab, 38, NO. 15 (1972). K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin.. U. Gelius. T. Beromark. L. 0.Werme. R. Manne. and Y. Baer. “ESCA Applied Free M%iecules,” North-Holiand Publishing Company; Amsterdam, 1969. C. D. Wagner, Anal. Chem., 44, 1050 (1972). P. E. Larson, Anal. Chem., 44, 1678 (1972). R. M. Friedman, J. Hudis, M. L. Perlman. and R. E. Watson, Phys. Rev. B 8, 2433 (1973). L. E. Cox and D. M. Hercules, J. Nectron Spectrosc.. 1, 193 (1973). R. M. Friedman, R. I. Declerk-Grimee, and J. J. Fripiat, Proc. lnt. Conf. Nectron Spectrosc. Namur (1974), in press. ~
~~
to
RECEIVEDfor review July 1, 1974. Accepted October 3, 1974. Grant (2-323-70) from the Swiss National Science Foundation permitted the purchase of the photoelectron instrument.
Determination of Nanogram Quantities of Carbonyl Compounds Using Twin Cell Potential Sweep Voltammetry Badar K. Afghan, Achut
V. Kulkarni,’
and James F. Ryan
Analytical Methods Research Section, Canada Centre for Inland Waters, P.O. Box 5050, Burlington, Ontario, Canada L7R 4A6
Twin cell potential sweep voltammetry is used to determine and differentiate various classes of carbonyl compounds in natural waters and industrial effluents. A systematic study of polarographic behavior of these compounds in various media, such as alkaline medium, citrate buffer, and in the presence of various amines, is reported. It is possible to detect and distinguish various classes of carbonyl compounds using the above media. Individual carbonyl compounds can also be determined down to 0.25 pg/liter without any separation or preconcentration of the sample. The above method is applied to determine various carbonyl compounds in natural waters and industrial effluents.
The determination of “carbonyl compounds” in natural waters and industrial effluents is of considerable interest ( I ). In this paper, the term “carbonyl compounds” is used in a collective sense to describe those aldehydes and ketones which are known for their strong taste, odor, corrosive and related problems. In our laboratory a preliminary survey of various samples ranging from drinking waters to industrial effluents was carried out to obtain information concerning the nature and levels of these compounds. This survey indicates that formaldehyde and related compounds d o occur in considerable quantities in some pharmaceutical, chemical, petroleum, and industrial effluents. Levels as high as 50 mg/ liter, in terms of formaldehyde equivalent concentrations, are found in som-e industrial waters ( 2 ) . Although the methods used in our preliminary investigations give reasonably accurate results for relatively clean waters, they are not suitable for industrial effluents and receiving waters ( 2 ) .The fluorometric procedure was too specific for formaldehyde alone, and the colorimetric method Permanent address, Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay, 400085, India. 488
.
gave very high molar extinction coefficients for the unknown compounds present in these effluents. Although the colorimetric procedure was relatively more reactive to carbonyl compounds, it did not react with most common aldehydes such as benzaldehyde, vanillin, furfural, etc. Therefore, work was initiated to develop a method which would react with as many commonly occurring carbonyl compounds as possible and which would more accurately determine the total carbonyl content of all samples. The majority of carbonyl compounds condense with a wide variety of amines to produce >C=N- compounds and water ( 3 ) . Resultant products such as imines, substituted hydrazone, oximes, and semicarbazones are also known to reduce at the dropping mercury electrodes ( 4 , 5 ) . Furthermore, these addition products reduce a t a dropping mercury electrode (DME) involving the same number of electrons regardless of the nature of the carbonyl compounds (6-8). Similar values for the diffusion current constants ( I ) for some aldehydes with semicarbazide have already been reported (9). Therefore, it should be possible to obtain the same order of current for similar concentrations for different carbonyl compounds and, hence, use this approach for the determination of total carbonyl content in a sample. T h e use of semicarbazone for the determination of carbonyl compounds has been reported in the literature (9, IO);however, earlier workers used conventional d c polarography in conjunction with preconcentration and separation steps to determine these compounds. It is possible to increase the sensitivity by at least 2-3 orders of magnitude if the reduction of semicarbazones or other azomethine derivatives is monitored by twin cell potential sweep voltammetry. In our laboratories, higher sensitivities can be achieved using a twin cell set-up since it eliminates all of the nonfaradaic and faradaic currents not relevant t o analysis (11-15). In addition to the increase in sensitivity, this tech-
ANALYTiCAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
nique is also the fastest polarographic technique and is suitable for routine work where a large number of samples are analyzed daily. This paper reports a systematic study of the formation of azomethine derivatives by the reaction of various carbonyl compounds with a variety of amines and the utilization of the most favorable azomethine derivative to determine formaldehyde and related compounds in water and industrial effluents. I t is possible to differentiate and determine these compounds down to 1 ,ug/liter without any separation or preconcentration.
EXPERIMENTAL Apparatus. T h e twin cell Potential Sweep Voltammeter used was manufactured by Southern Analytical Ltd., Surrey, England, under the name “Davis Differential Cathode Ray Polarograph T y p e A-1660.” T h e set-up was complete with a n electrode stand type A-1661 a n d a Southern Analytical Ultraviolet Oscillographic Recorder T y p e F-10-300. T h e capillaries used in each of t h e two cells were matched so t h a t when mercury drop rates were adjusted t o 10.5 f 0.2 seconds (open circuit), t h e signals from the two cells containing t h e same solutions were effectively identical. Other polarographic d a t a was obtained with the Polarographic Analyzer Model 174 manufactured by Princeton Applied Research Corporation, Princeton, N.J. T h e dropping mercury electrode was operated a t 41.2-cm pressure with a natural drop time of 2.3 seconds in 0.1M lithium chloride a t -1.0 V us. silver-silver chloride reference electrode. Two Fisher “Electro-hosecock” with selenoid controlled hose clamps were used for purging nitrogen through t h e cells and for filling and emptying new polarographic cells, respectively. Three pendant drop cord switches, which are remote electrical cord switches, were used t o control the operation of t h e electrohosecocks and Southern recorder. One Lustar gas manifold, with two independent rate-controlling valves for each cell, was used t o control t h e flow of nitrogen through t h e polarographic cells. Two new cells were constructed t o replace those cells supplied with t h e polarograph, which proved t o have some disadvantages, particularly when they were used for analyzing very low concentrations in routine situations. T h e new cells were designed so t h a t a n external reference electrode replaced the mercury pool. They consisted of a central compartment for the sample, a nitrogen inlet for deoxygenation of t h e sample, and a n external silver-silver chloride electrode. T h e reference electrode and t h e sample compartment were separated by a n ultra-fine scintered disk 1 cm in diameter and 1 m m thick. T h e physical dimensions and porosity of the disk were matched t o ensure easy and accurate balancing of t h e cells. T h e maximum capacity of t h e sample compartment was four milliliters, and t h e volume of t h e sample compartment was maintained by a small diameter overflow tube set to regulate the height and volume of t h e solution inside. T h e overflow outlets in both cells were arranged so as t o maintain an equal volume of solution in both. These new cells proved easy t o fill, to empty, and to purge with nitrogen. T h e essential features of these cells are shown in Figure 1. T h e two new cells were fitted into t h e existing electrode stand by drilling four holes into t h e side of t h e stand near the base of t h e water bath. T h e holes were fitted with unions which could accommodate the tubing from the overflow waste and cell drain outlets of each cell. T h e cells were aligned with t h e DME and suspended in t h e bath by clamping t h e overflow section of the cells t o the external ring stands. Reagents. Stock solution: 1000 mg/liter of formaldehyde was prepared from concentrated formaldehyde solution obtained from Fisher Scientific Company. T h e stock solution was standardized by t h e method proposed in AOAC Methods (16). Standard formaldehyde solution: 1.0 mg/liter of t h e formaldehyde solution was prepared daily from the above standardized stock solution. Citric acid (1M): 48.03 grams of anhydrous citric acid were dissolved in distilled deionized water and diluted t o 250 ml. Sodium citrate solution (1M): 73.53 grams of trisodium citrate was dissolved in distilled deionized water and diluted t o 250 ml. Semicarbazide solution (0.1M ): 2.7885 grams of semicarbazide hydrochloride were dissolved in distilled and deionized water and diluted to 250 ml. Solutions of other amines were prepared in a
ECOCK
Figure 1. Details of construction of the polarographic cell used for semiautomated system for twin cell potential sweep voltammetry 105
80 -
1
C
20
40
pg/l
60
82
1oc
FORMALDEHYDE
Figure 2. Typical calibration for formaldehyde 20- 100 pg/liter using proposed method similiar way by dissolving t h e appropriate weights in a known volume. Special reagent for low levels of formaldehyde: a 250-ml aliquot of solution containing 4 X 10-2M semicarbazide, 1M citric acid, and 1M sodium citrate was electrolyzed a t -1.0 \’, using stirred mercury pool, platinum, and Ag/AgCl as working, counter, and reference electrodes, respectively, t o remove any aldehydic contamination. A 2.5-ml aliquot of this solution was used as a buffer and reagent during the determination of low concentrations (below 25 pg/liter) of aldehydic content of samples. EDTA solution (0.1M): 37.22 grams of disodium ethylenediamine tetraacetic acid dihydrate were dissolved in distilled deionized water and diluted t o 1 liter. Lithium chloride ( 1 M ) ; 42.39 grams of lithium chloride (purified) were dissolved in distilled deionized water and diluted to 1 liter. Procedure. General Operating Procedure for the Polarographic Setl‘p. T h e selenoid controlled hose clamp (1) is closed and the central compartments of t h e two cells are filled, one with a blank solution and t h e other with t h e sample solution. T h e flow of nitrogen for deoxygenating t h e solution is adjusted and matched by gas manifold and the flow is controlled by electrohosecock (2). T h e cells are then drained by opening t h e selenoid valve (1).After t h e cells have been washed this way two or three times with their respective solutions, during which time an inflow of nitrogen is kept running through them, they are filled with t h e appropriate solutions and are deoxygenated for 15 minutes. After t h e deoxygenation procedure, a polarograph is taken by selecting a suitable scan range and suitable sensitivity of t h e polarograph. Recommended Procedure for the Determination. Pipet 50 ml of deionized water into 100-ml volumetric flasks. Pipet 0, 2 i - , 5.0-, 7.5-, and 10.0-ml aliquots of the dilute formaldehyde standard into these flasks. Add 1 ml each of t h e EDTA, citric acid, sodium citrate, semicarbazide solutions, and 10 ml of lithium chloride solution. For low levels, add 2.5 ml of t h e special reagent instead of buffering components and semicarbazide. Dilute t h e flasks t o the mark with distilled deionized waters. Transfer the blank solution
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
489
Table I. Reduction Characteristics of 2.5 X lO-SM Formaldehyde in Various Media Polarographic characteristics Medium f
-!
POTENTIAL
!
0.01M LiCl + 0.1M LiOH 2 M Glycine + 1M NaOH + 0.l.U LiCl Acetate buffer pH 4 . 5 + 0.1M LiCl(A) 0.01M Glycine +A 0.01M hydroxylamine hydrochloride + A 0.01ilZ Phenylsem ic arbaz id e +A 0.01M Semicarbazide + A
Figure 3. Polarograms of various azomethine derivatives
+
+
hydrazine, sens. = 4 X 1 X 1. (6) formaldehyde 4phenylsemicarbazide, sens. = 1 X 10 X 10; (c)formaldehyde semicarbazide, sens. = 1 X 10 X 10 ( a ) formaldehyde
+
containing no formaldehyde to cell 2 and an equivalent amount of lowest standard to cell 1. Polarograph the solution between -0.75 and -1.25 V us silver-silver chloride reference electrode. The applied potential affects the surface tension of the dropping mercury electrode and, hence, alters the area of the electrode. For quantitative work, it is essential that the start potential be kept the same for the samples and their corresponding standard solutions. Measure the peak height (-1.00 f 0.02 V) using a suitable shunt scale and amplification scale. Prepare a calibration curve by plotting relative peak height us. concentration. A typical calibration curve is shown in Figure 2. During the determination of natural water samples and industrial effluents, prepare a sample solution containing suitable aliquots of sample as described above and corresponding blank without semicarbazide, and analyze as described above.
Possible product
Imine
Imine
Ep[w
i p(1~-7~)
-1.65
0.32
-1.20
0.05
NW“
...
ma
...
Hydrazone or azine
-0.95b
0.18
Semicarbazone
-0.91b
5.3
Semicarbazone
-l.Olb
6.3
NW = KO wave obtained. Available potential range (-1.4 V before the appearance of catalytic hydrogen wave.
(‘
u s . Ag/AgCI)
R E S U L T S A N D DISCUSSION Choice of Suitable Amine. It, is a well established fact that many compounds related to ammonia, containing NH2 group, condense with carbonyl compounds to give addition products with a carbon-nitrogen double bond resulting from the elimination of water from the initial addition product, as seen in Equation 3.
+
H’ ?t RR‘C-OH
(1)
R”NH2 + H’ Z=? R’’NH3
(2)
RR’C-0
OH RR’C-OH
+
I
R”NH, TZRR’C--”,R
+
RR’C-N-R In a number of cases, the condensation of the carbonyl group with amines is acid-catalyzed (17-19). T o obtain maximum formation and reaction rate, it is necessary t o select the proper p H of the medium t o ensure that the significant fraction of carbonyl compound is in the protonated form, as seen in Equation 2, and, at the same time, the concentration of protonated amine remains low, as seen in Equation 3. Therefore, initial studies were carried out with formaldehyde, benzaldehyde, and 2,4-dimethoxybenzaldehyde in the presence of various amines using acetate buffer, p H 4.5. Hydrazine dihydrochloride, 4-phenylsemicarbazide, and semicarbazide produced polarographic waves in the presence of the above carbonyl compounds. Semicarbazones gave maximum response and the polarographic waves were well defined. In the case of formaldehyde hydrazone, a relatively broader peak was obtained and benzaldehyde hydrazone produced four reduction waves in the range of -0.75 to -1.3 V (us. Ag/AgCl). Acetone hydrazone did not give any polarographic wave in the electrochemical domain of the medium. Formaldehyde in the presence of 4-phenylsemicarbazide produced a shoulder in addition to the major peak. No waves were obtained in the case of imines and ox490
ANALYTICAL CHEMISTRY, V O L . 4 7 , NO. 3, MARCH
imes during our initial studies using p H 4.5, probably because either (i) the resultant compounds reduce beyond -1.4 V (us. Ag/AgCl) and the waves are masked by the sudden rise in current due to the catalytic hydrogen wave, or (ii), the optimum p H for the condensation of carbonyl compounds with glycine and hydroxylamine is not p H 4.5, which was used for our initial studies. It has been reported in the literature t h a t it may be possible to increase the reaction rates of the above condensation reactions in basic solutions (17, 20). In fact, acetone and its related compounds are determined as ketimines in the presence of glycine using a strong alkaline medium (21 ). Therefore, it was decided to carry out additional work to establish the possibility of obtaining polarographic waves for various imines and oximes by changing the p H of the medium. Oximes did not produce any polarographic waves in the p H range between 1-10.5. It was possible to obtain the reduction waves for various imines using a large excess of glycine a t p H 10.5. However, the relative peak heights for all imines were 28-50 times smaller than the peak heights obtained for corresponding semicarbazones. In addition to the above, ketimine with acetone was found to be volatile and was very difficult to measure accurately during the determination. Typical results of the reaction of formaldehyde with various amines are shown in Table I. In this table, polarographic characteristics of formaldehyde in 0.1M of lithium hydroxide and 0.01M of lithium chloride are also given for comparison purposes, since they have been used as base electrolyte for the routine determination of various carbonyl compounds (21 1. Therefore, the reaction of semicarbazide with different carbonyl compounds was chosen for further optimization since semicarbazones gave the maximum response, and the waves were well defined as shown in Figure 3. Effect of pH. The behavior of organic compounds at the dropping mercury electrode is strongly dependent upon the p H of the medium and the base electrolyte (22). In the majority of cases, the half-wave potential and limiting current of organic electrode process are reported t o be affected by
1975
Table 11. Reduction Characteristics of Various Semicarbazones in Citrate Buffer Using 0.1M Lithium Chloride Composition of solution 0.1 M . LiOH
+
0.01 LiClb 1
Nature of carbonyl
6 21
33
4c
1
50
comwund ( 2 . 5 x
MIa
Cit. Buff.(r) + Semicarbazide? 1
0.1 M LiCl
-Ep
ip
-Ep
(V)
(IO-*A)
(V)
i p
iP
-E P A)
A)
(V)
PH
OH
Figure 4. Effect of pH on E , and i, on t h e wave due to formaldehyde semicarbazide
+
p H (22-24). Therefore, the effect of p H of the medium on the polarographic behavior of various semicarbazones was studied in some detail. T h e typical effect of the dependence of peak potential (E,) and peak current (i,) on pH, for formaldehyde semicarbazone, is shown in Figure 4. Hydrochloric acid-lithium chloride and citrate buffers were used to cover a pH range between 1.4-6.0.The E of semicarbazone with formaldehyde moved towards increasingly negative potential with p H until p H 3.75 was reached and then remained constant between p H 3.75-5.0. However, i during the reduction increased as p H of the medium moved from 1.4-3.0, remained constant between 3.0-4.2, and then decreased markedly with increasing pH. Other semicarbazones gave similar E , and i, plots with pH; however, the potential region where E , and i, remained constant varied slightly with the type of carbonyl compound used during the formation of semicarbazone. These results agree with the conclusions of earlier workers (17-19) who suggested that the condensation reaction between various amines and carbonyl compounds require acid catalyst. In the majority of cases, the rate of these reactions passes through a maximum with p H and these maxima generally lie in weakly acidic regions. The exact position of the maximum depends upon the basicity of the amine and the nature of the carbonyl compound. During our further studies p H 4 f 0.2 was selected because the majority of semicarbazones gave maximum response a t this pH, and it was anticipated t h a t the electroactive species taking part in the electrode process and the species that predominate in bulk solution would be the same a t this pH.
,
Reactivity of Semicarbazide with Various Carbonyl Compounds. Table I1 shows the reduction characteristics of various semicarbazones in citrate buffer using 0.1M lithium chloride. In this table, the reduction characteristics of these compounds in alkaline medium are also included since this medium is most widely used for the determination of these compounds. Reduction characteristics of cadmium and benzophenone a t p H 1.4 are also included for comparison purposes because of their well-established reversible reduction nature, involving a two-electron transfer a t the dropping mercury electrode ( 1 7 ) . Aldoses, disaccharides, and aliphatic dicarbonyls did not give polarographic waves in all three media described above. Aliphatic aldehydes gave polarographic waves only in an alkaline medium and in solutions containing semicarbazide. Aromatic aldehydes produced polarographic waves in all three media, and the peak potentials shifted toward a more positive direction as the medium was changed from one of alkaline to one containing semicarbazide. Aliphatic and aromatic ketones were less reactive than the corresponding aldehydes. Benzophenone did not give polarographic waves with semicarbazide; however, acetophenone
Aliphatic aldehydes Formaldehyde Propionaldehyde Acrolein Aliphatic ketones Acetone Acetyl Acetone Sugars Glucose Xylose Galactose Succrose Maltose Lactose Aromatic aldehydes Benzaldehyde Salicyldehyde p - Anis aldehyde o-Vanillin 2-Hydroxy- 5methoxy benzaldehyde 2,4-Dimethoxybenzaldehyde Furfural Aromatic ketones Acetophenone
. . . NR
... .. .
NR NR
.. .
...
NR NR NR NR NR NR NR NR NR NR
. . . 1.25 0.96 . . . 1.33 0.84 ... ... . . .
0.6 1.6 1.5 1.6 1.3
1.15 1.17 1.20 1.13 1.10
1.4 1.6 2.2 3.4 3.5
0.93 12.2 0.94 15.5 0.95 21.6 0.94 23.2 0.92 23.2
1.58 1.8
1.17
2.4
0.91 28 .O
1.45
1.5
1.15
2.2
0.94
1.62 1.2
1.31
1.66 0.3
NR
1.82 1.6
NR NR NR NR NR NR NR NR NR 1.48 1.71 1.58 1.73 1.58
... ... ...
NR
1.01 1.13 0'.94
1.09
7.1 3 .O 0.4 1.88
...
... NR
NR NR NR NR NR NR
7.6
1.8 1.03 1.38 1.30 1.56 Benzophenone 1.45 1.7 0.98 5.6 0.98 5.6 a Similar concentrations of cadmium and benzophenone at pH 1.4 gave peak currents corresponding to 0.42 and 0.34 PA, respectively. Drop rate = 7.8 seconds with scan voltage between 1.5-2.0 V w.r.t. silver-silver chloride reference electrode. 80
I POTENTIAL Figure 5. Typical polarograms of various semicarbazones ( a ) With formaldehyde, sens. = 1 X 1 X 10; ( b ) with benzaldehyde, sens. = 2.5 X 1 X 10; ( c )with 2,4dirnethoxybenzaldehyde, sens. = 4 X 1 X 10
showed some reactivity with semicarbazide and the semicarbazone peak increased wih time. The E , and the shape of the curves varied with the nature of carbonyl compound used during the formation of semicarbazones. In addition, i for various semicarbazones also changed considerably. In some instances, an increase of up to 15-fold in peak currents was obtained when similar concentrations of various carbonyl compounds were polarographed under identical conditions in the presence of semicarbazide. 2,4-Dimethoxybenzaldehyde produced the maxi-
,
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
*
491
mum response with i approximately 10 times higher than with similar concentrations of cadmium. Figure 5 shows typical polarograms obtained when 2.5 X 10-6M solutions of formaldehyde, benzaldehyde, and 2,4-dimethoxybenzaldehydes were polarographed as described above. These results are in disagreement with the work already reported in the literature (6-8) where it is generally believed that the majority of semicarbazones give similar values for diffusion current constant ( I ) irrespective of the nature of the carbonyl compound. Such great differences in the peak currents for various semicarbazones may be due to one or more of the following reasons: (1) the formation of some semicarbazones is relatively slow under the experimental conditions and is not quantitative; (2) that in some instances the other type of polarographic currents such as adsorption currents, kinetic currents, or catalytic currents may also be observed in addition to conventional diffusion currents; (3) that different semicarbazones may be reduced at the dropping of mercury electrode with varying degrees of reversibility; or (4) these compounds may involve different numbers of electrons during the reduction a t the dropping mercury electrode. In order to establish which of the above reasons contribute to high peak currents in some cases, it was decided to carry out additional work on various semicarbazones. Effect of T e m p e r a t u r e a n d Time. The formation of various semicarbazones, listed in Table 11, was complete within a few minutes a t room temperature except acetophenone. Acetophenone semicarbazone peak ( E P = -1.03 V) increased with time when the solution was kept for a longer period while the peak with acetophenone alone ( E P = -1.3V) decreased constantly over the same period. These results indicate that the majority of semicarbazones are formed quantitatively and the different peak currents for these semicarbazones may be due to points (2) or (3)-i.e., other factors such as adsorption, catalytic current, kinetic currents, or reversibility of various semicarbazones. N a t u r e of P e a k C u r r e n t s f o r Various Semicarbazones. There are various types of currents which may contribute to total i during the reduction of any electroactive substance at the dropping mercury electrode. The role of adsorption in the organic electrode process has been well documented (25, 26 ). Although lower concentrations of these compounds (1.0-2.5 X 10-6M) were used in our experiments to minimize any increase in peak currents due to adsorption, it was decided to confirm this effect. The addition of gelatin or Triton X-100 did not significantly alter the nature of the reduction. This was further confirmed by recording any half-wave potential ( E 1 / 2 ) shift of oxygen wave (27). There was no significant shift of the oxygen wave (E112 = -0.15 V us. silver-silver chloride electrode) when a blank with semicarbazide was compared to the solution containing the above semicarbazones. The effect of the supporting electrolyte cation nature on the electrode kinetic, particularly in linear sweep voltammetry, is known. The marked effect of the nature of the support electrolyte on i p, E p, and other characteristics of the waves, depending upon the double layer structure, has been r e ~ o r t e d ~ ( 2 8No ) . significant effect of alkali metals on the i or E current voltage curves was experienced during the reduction of various semicarbazones. This indicates that the kinetics of these reactions is relatively fast. Increased concentrations of buffer solutions did not alter the E or i of the semicarbazones listed in Table 11. Various buffer components such as acetate, citrate, formate, citrate-phosphate did not have any significant effect on current-voltage curves. This indicates the absence of any catalytic current. The absence of catalytic currents, during the 492
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
reduction of semicarbazones, was further confirmed by plotting i us. concentration of various semicarbazones. A linear relationship was obtained in all cases for concentrations between 10-6-10-4M. This further demonstrates the absence of catalytic currents, since catalytic currents in the majority of cases are not a linear function of concentration (211. All the above investigations indicate that the adsorption, kinetic, or catalytic currents do not significantly contribute toward total peak currents during the reduction of various semicarbazones. Reversibility of the Electrode Process. These investigations were restricted to semicarbazones of formaldehyde, benzaldehyde, and 2,4-dimethoxybenzaldehyde because they were found to be representative of all compounds, listed in Table 11, and gave low, intermediate, and highest response, respectively. Solutions containing 2.5 X 10-jM of each semicarbazone were carried through the same procedure as described previously in this paper and the reduction waves were recorded using a PAR 174 Polarographic Analyzer. The rate of potential scan for all measurements was maintained a t 2 mvlsecond with the same dropping mercury electrode characteristics as described above. The wave for formaldehyde semicarbazone was characterized by a slight distortion-ie., there was an indication of a slight splitting of the wave a t one-third the height level of the wave. The potential difference - E314) for formaldehyde, benzaldehyde, and 2,4-dimethoxybenzaldehyde semicarbazones were 65, 55, and 30, respectively. Plots of log i / ( i d - i) us. E, a t 30 "C, gave straight lines with slopes corresponding to the reversible 1-electron for benzaldehyde. However, in the case of 2,4-dimethoxybenzaldehyde, a value for a 2-electron transfer was obtained. E 112 for these semicarbazones did not alter with changing the concentrations of the depolarizers, variation of the drop rate, or when the temperature was increased from 20-60 "C. Temperature coefficients of 1.8, 1.5, and 1.6% were obtained for formaldehyde, benzaldehyde, and 2,4-dimethoxybenzaldehyde, respectively. Further confirmation of electrode reversibility was obtained by scan reversal pulse polarography (29). Differential pulse polarography was also used to confirm the reversibility of the electrode reaction (30). A comparison of half widths (W1/2)for these semicarbazones, using a small amplitude with thallium and cadmium, indicated the reversible nature of the electrode process. The values of 1- and 2-electron transfer were confirmed for benzaldehyde and 2,4-dimethoxybenzaldehyde semicarbazones. Formaldehyde semicarbazone gave a slight shoulder in addition to the major peak. Typical results are shown in Figure 6. These results indicate that various semicarbazones probably reduce a t the dropping mercury electrode involving different numbers of electrons depending upon the nature of these semicarbazones. However, further work should be undertaken to confirm the above statement and elucidate the reduction mechanism of the overall process by isolation of parent compounds and their reduction products, and also by carrying out additional work using cyclic voltammetry, coulometry, and mass spectrometry. Quantitative Estimation of Carbonyl Compounds. I t should be possible to use various media, as listed in Table 11, to determine and possibly to differentiate between various classes of carbonyl compounds in natural waters or industrial effluent. As shown in Table 11, it is possible to detect only aromatic aldehydes and ketones using citrate buffers. In alkaline mediums, other aliphatic aldehydes in addition to aromatic aldehydes and ketones can be measured. The alkaline medium can also be used to distinguish
3
A
Table 111. Comparison of Various Polarographic Techniques Technique
Conventional dc polarography Tast polarography Pulse polarography Differential pulse polar ugr aphy Twin cell potential sweep Voltammetry
E
-
2.5 x
Precision, /o
Quantitative range, M
5.0
X
10-'-1.0
X
4-10
X
2-5 2-5 lom4 1-10
1.0 x 2.5 X 10-'-1.0 5.0 x 1 .O X 10m5-1.O 2.5 X l o m 7 5.0 X O.l'-
X
1 .O x lo-* 5 .O x 10-*-1.O
X
X
1-5
--
\--J
Dectection l i m i t , hi
----------i
POTENTIAL
Table IV. Analysis of Carbonyl Compounds Amount
Figure 6. Comparison of half peak widths using pulse amplitude of
10 mV, scan = 2 mV/sec, sens. = 2pA/FS
Original Nature of sample
~ g l l .
10 25 50 10 25 10 25 250 500 50 100
( a )Thallium, 10-4M in 0 . l M LICI; ( b )formaldehyde -k semicarbazide. 2.5 X 10-5M formaldehyde; ( c ) benzaldehyde -!- semicarbazide, 215 X 10-5M benzaldehyde; ( d )cadmium, 5 X 10-5M in 0.1M LICI; ( e ) 2,4-dimethoxybenzaldehyde semicarbazide, 2.5 X 10-5M 2,4dimethoxybenzaldehyde
Synthetic lake water
...
Lake water
...
between saturated and unsaturated aliphatic aldehydes such as formaldehyde and acrolein. The medium containing semicarbazide in citrate buffer can prove useful if the total carbonyl content of the sample is required because it is found to be most reactive with the majority of carbonyl compounds and gives maximum response for all carbonyl compounds which are commonly considered as pollutants. The fact that various carbohydrates do not give any polarographic waves, under the experimental conditions, is of advantage in this particular case because these compounds have a different significance in water quality as compared to other carbonyl compounds. Carbohydrates, normally classified as sugars, serve as principal energy food for man and aquatic life. On the other hand, other carbonyl compounds usually impart an odor or taste and, in some instances, can prove fatal to aquatic life. Straight line calibration curves were obtained for all of the carbonyl compounds, listed in Table 11, using all three media. Since different calibration curves are sometimes obtained in distilled water and natural waters, other calibration curves were obtained using synthetic lake water and actual lake waters spiked with known amounts of these compounds. These curves were identical to those obtained in distilled water. The coefficient of variation varied between 2.5-10.0%, depending upon the type of medium and the nature of the carbonyl compound used. Sensitivities for the determination of various carbonyl compounds vary depending upon the type of medium used and the type of carbonyl compound to be determined. Some carbonyl compounds, such as formaldehyde, vanillin, 2,4-dimethoxybenzaldehyde,can be detected as low as 0.25 pg/liter without any separation of preconcentration of a sample. I t is also possible to analyze these carbonyl compounds using other polarographic techniques. However, the sensitivity and the speed of analysis will vary depending upon the nature of the sample, various extraneous ions that might be present in the sample, and the type of polarographic technique used. For example, it is possible to obtain comparable sensitivities for differential pulse polarography ( D P P ) and twin cell potential sweep voltammetry (TCPSV) using standard solutions in deionized water.
River water
10
+
Industrial effluent (X) River water receiving (X)
added,
content, u d l .
240
40
Recobery, rn 0
106 98 99 94 102 100
93 99 100 106 97
Average of five determinations. Indicated the presence of acetophenone type compounds in addition to formaldehyde.
However, when actual samples are analyzed by DPP, they produce high currents, due to reduction of unknown constituents in the samples, in that range where the majority of semicarbazones are reduced. Attempts to eliminate these high currents by the use of twin cell setup in conjunction with D P P was not successful because it was very difficult to match the relatively fast drop rates of two capillaries which are normally employed in DPP. Therefore, to minimize the effect of such background currents, it was essential to operate the instrument a t lower sensitivity and the apparent detection limit using D P P appears to be higher than TCPSV. The figures in Table I11 give a fairly realistic picture of the detection limits and the range in which good quantitative results can be obtained using various polarographic techniques. Analysis of Actual Samples. In our laboratories, twin cell potential sweep voltammetry was used to determine various carbonyl compounds since it was found to be the most sensitive and fastest technique for routine analysis. This technique was operated in both the subtractive and the eliminative mode (12) to obtain the desired selectivity and sensitivity. In most cases, particularly when analyzing industrial effluents, the eliminative mode was used because it is possible to remove the majority of background currents, arising from the reduction of unknown constituents in the samples, using this mode of operation. Table IV shows the results obtained for lake water and for some industrial waste waters. In most of the waters, relatively low concentrations of these compounds were found. However, in some industrial effluents, very high levels of formaldehyde and related compounds were found. ANALYTICAL C H E M I S T R Y , VOL. 47, NO. 3 , M A R C H 1975
493
LITERATURE CITED
(16)W. Horwitz, Ed., "Official Methods of Analysis," Eleventh ed., Association of Official Analytical Chemists, Washington, D.C., 1970,p 89. E. Barrett and A. Lapworth, J. Cbem. Soc., 93, 85 (1908). , A. Olander, Z.Physik. Cbem., 129, 1 (1927). J. G. Conant and P. D. Barllett. J. Amer. Cbem. SOC..54. 2881 11932). 6 . M. Anderson and W. P. Jencks, J. Amer. Chem. Soc., 8'2, 1773 (1960).
(1)L. Klein, "Aspects of Water Pollution,'' Academic Press, New York, N.Y., 1957. (2)6. K. Afghan, A. V. Kulkarni, et a/., fnviron. Led., 7, (l),53 (1974). (3)J. D. Roberts and M. C. Caserio. "Basic Principles of Organic Chemistry," W. J. Benjamin, New York, N.Y., 1964 p 451. (4)J. Heyrovsky and P. Zuman, "Practical Polarography," Academic Press, New York, N.Y., 1968,p 138. ( 5 ) J. Sourd, Mem. Poudres, 40, 453 (1958);Chem. Abstr., 55, 1267e (1961). (6) P. Zuman and L. Meites, "Progress in Polarography," Vol. 3,Wiley, New York, N.Y., p. 103 (1972). (7) B. Fleet and P. Zuman, Collect. Czech. Chem. Commun., 32, 2066 (1967). (8)H. Lund, Acta Chem. Scand. 13, 249 (1959). (9)6. Fleet, Anal. Cbem. Acta, 36, 304 (1966). (10)M. Pribyl and J. Nedbalkova, Fresenius' Z. Anal. Chem., 224, 244 (1969). (11) E. K. Afghan and P. D. Goulden, Environ. Sci. Techno/., 5,601 (1971). (12)6. K. Afghan, "Proceedings of International Symposium on Identification
P. Zumna, "Organic Polarographic Analysis, "The Macmillan Company, New York, N.Y., 1964,pp 19,121. R. Pasternack, Helv. Chim. Acta, 31, 753 (1948). M. Tokyoka. Collect. Czech. Chem. Commun., 7, 392 (1935). P. Zuman and 0. Exner, Collect. Czech. Chem. Commun., 30, 1832
(1965). I. M. Kolthoff and J. J. Lingane, ''Polarography,'' Vol. 11, lnterscience Publishers, New York, N.Y., 1952,pp 509,682-83,6-52-98, S. G. Mairanovskii, "Catalitic and Kinetic Waves in Polarography." Plenum Press, New York, N.Y.. 1968,Chapter 111. J. Heyrovsky and J. Kuta, "Principles of Polarography," Academic Press, New York, N.Y., 1966,p 314. P. Zuman, D. Barnes etal., Discussions Farraday Soc., 45,202 (1968). K. E. Oldham and E. P. Parry, Anal. Chem., 42, 229 (1970). E. P. Parry and K. B. Oldham, Anal. Cbem., 40, 1031 (1968).
and Measurement of Environmental Pollutants," Campbell Printing, Ottawa, Ontario, Canada, 1971,p 391. (13)6. K. Afghan, P. D. Goulden, and J. F. Ryan, Anal. Chem., 44, 354
(1972). (14)6. K. Afghan and I. Sekerka, Chem. Can., 26, 21 (1974). (15)B. K. Afghan and P. D. Goulden, Can. Res. Develop., 4 (4),21 (1972).
RECEIVEDfor review July 29, 1974. Accepted November 15. 1974.
Nonaqueous Potent iometric Titration of Pheno1s with PaIladiumHydrogen Electrodes Samuel Kaufman Naval Research Laboratory, Washington,D.C. 20375
Earlier electrochemical research has shown that palladium metal impregnated with hydrogen gas can be used as a reliable hydrogen-ion indicating electrode for aqueous solutions. However, this electrode has not enjoyed general use in potentiometric titrations. It has been found in this study that the palladium-hydrogen electrode is especially suited to potentiometric titration in a nonaqueous solvent because of its rapid response, sensitivity to hydrogen ions, low electrical resistance, stability, and compatibility with the nonaqueous system. Phenols are titrated in methyl ethyl ketone with an alcoholic potassium hydroxide titrant. Unusual and unstable reagents are avoided, and there is no requirement for the presence of water, as in many procedures which depend upon the glass electrode. The potential change observed in the end-point region of the titration is 250 to 500 millivolts, and in the system described, methyl ethyl ketone is a differentiating solvent.
The acid-base titration of phenols poses special problems because of the extremely weak acidic properties of phenols. In water, the titration is unsatisfactory and, in most anhydrous inert organic solvents, a glass electrode is slow to respond and therefore insensitive. Introduction of water or other protogenic solvent makes it difficult or impossible to distinguish the phenol electrometrically from the protogen in the solvent and, in the organic solvent alone, the glass electrode is very noisy because of its extremely high resistance. Use of a calomel reference electrode in nonaqueous titrations is not recommended because it can introduce water and potassium chloride into the solution to be titrated, and because the potential a t the liquid junction between the aqueous and nonaqueous phases may become irregular or slow to stabilize (1-3). 494
The problems of nonaqueous titrations in general have been discussed in the literature (1, 4-6), and the nonaqueous titrations of weak acids, including phenols in particular, have been studied in considerable detail (2, 3, 7-10). The potentiometric titration of phenols has been investigated in numerous solvents with various electrode combinations and with different titrating reagents. Solvents have included tertiary butanol (7), ethylenediamine (2, 3), dimethylformamide (3, 8 ) , pyridine (8, g), benzene-isopropano1 ( B ) , acetonitrile ( 8 ) ,dimethyl ether (9), chloroform (9), acetone (9),methyl ethyl ketone (9),piperidine (9),methyl, ethyl, and propyl alcohols (9), and butylamine (10). Electrode combinations studied have included glass-calomel (2, 3, 7-9), glass-modified calomel ( 8 ) ,glass-platinum (2, 9), platinum (anodically and cathodically polarized)-platinum (2, 9), and glass-antimony (10).Among the titrants investigated have been tetrabutylammonium hydroxide in water (B), in benzene-methanol (8), in isopropanol ( 9 ) ,in isopropanol-water (3, 8 ) , and in benzene-isopropanol-water (7); potassium hydroxide in isopropanol (3, 9); and sodium methylate in benzene-methanol (10). The potentiometric systems described in the literature all suffer in varying degree from one or more inconveniences and difficulties associated with the acquisition, preparation and stability of the solvents and titrants, the sensitivity and stability of the electrode pair, and the compatibility of the electrodes with the solvent and titrant. Under circumstances of repetitious analysis of phenols, perhaps the reagents, solvents, electrodes, and equipment can be maintained routinely. However, when the analysis is performed infrequently and a t unpredictable intervals, the effort can be excessive. In a study requiring the preparation of several metallic salts of phenols ( I I ) , it was required that the preparations be stoichiometrically neutral, because excesses of either the
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
