1126
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
Polarographic Characterization of Nitrohumic Acids Prepared by Nitric Acid Oxidation of Coal John B. Green and Stanley E. Manahan" Department of Chemistry, 723 Chemistry Building, University of Missouri, Columbia, Missouri 652 1 7
Humic acids prepared by the partial oxidation of bituminous coal with nitric acid have been analyzed polarographically. A major shift in the half-wave potential for the polarographic reduction of the nitro group at pH 5 indicates that the most probable neighboring group is phenolic -OH. Nitrobenzoic acids have pK,'s that are too low to account for the observed effect. It is possible that the preparation of humic acid by the nitric acid oxidation of coal also introduces some nitroso groups, the presence of which cannot be determined with the methods employed in this study. Humate salts prepared from coal are of interest for pollutant removal because of their chelation, flocculation, acid neutralization, and acid gas sorption properties.
Humic acids (HA) constitute a class of naturally occurring organic compounds with complex, variable structures that result from the decomposition of vegetation. They occur in nature where the residues of decayed vegetation have accumulated, including soil, marsh sediments, peat, lignite, and coal. HA is soluble in base, and base extraction is used to isolate HA from soil, peat, and sediments. Addition of mineral acid precipitates HA from solution. Because of their acidbase. complexing, and sorptive properties, humic acids have a strong influence upon aquatic environmental chemistry ( I ) . T h e structure of a hypothetical HA molecule is shown in Figure 1. This figure shows that naturally occurring HA has a partially aromatic structure, as well as carboxyl. phenolic hydroxyl, alcoholic hydroxyl, and carbonyl functionality. In recent years, interest has increased in humic acids for wastewater treatment and treatment of acid stack gases (2). Among the HA properties conducive to such uses are strong chelation of heavy metal ions. flocculating properties. and (in t h e basic humate salt form) ability to neutralize acid and to sorb acid gases, such as sulfur dioxide. Most HA used in these applications has been prepared from the nitric acid oxidation of coal; this preparation and its uses have been summarized ( 2 ) . Nitric acid oxidation produces nitrohumic acids, forms of HA with an elevated nitrogen content ( 3 ) . This increased percentage of N is due mostly to nitro, -NO2, groups introduced ontc, aromatic rings during the oxidation of coal (4,s). However, a t least one other type of nitrogen, inferred to be nitroso or isonitroso groups. has been shown to be present in nitrohumic acid structures (n). Coal HA, particularly that prepared via oxidation with nitric acid, shows polarographic reduction waves in aqueous and nonaqueous media (4),whereas HA extracted from soils seldom s h o w such activity (6, 7). One investigation ( 4 ) reported three distinct waves in polarograms of HA prepared from coal oxidized with nitric acid. T h e first, most distinct wave was attributed t o reduction of nitro groups. Soil HA oxidized with nitric acid showed a similar wave, also attributed to nitro groups (7). T h e original soil extract showed only one extremely weak wave in DMSO, and none in water. More recently. HA extracted from river water and heated with sodium nitrite was found to exhibit polarographic activity (8. 0003-2700/79/0351-i 126$O1 OO/O
9). The observed wave, presumably due to reduction of nitroso groups, had a half-wave potential similar to those of coal HA prepared by nitric acid oxidation. This observaLion, coupled with other work cited above, infers that not all of the polarographic activity of coal humic acid can be attributed to the reduction of nitro groups. Aside from increased polarographic activity and a tendency to decompose violently when heated ( I O ) , very little is known about the effects of nitro or nitroso groups on t h e overall properties of nitrohumic acids as compared to other HA. Obviously, the effects of these groups on properties such as metal ion complexation and total acidity could be postulated more accurately if more were known about their orientation on aromatic rings with respect to other functional groups present. Thus, this investigation was conducted to (1)compare the polarographic behavior of different humic acids. (2) determine the effect of p H and other solution parameters, and (3) evaluate the potential of polarography as a means for quantitative analysis of HA.
EXPERIMENTAL Preparation and Purification of HA. Nitrohumic acid employed in this study was prepared from Illinois No. 6 hituminous coal via preoxidation of moist coal with concentrated nitric acid and subsequent dissolution in NaOH. (c.Ac:TioN. This procedure results in the evolution of large quantities of toxic NO, gas from a potentially explosive mixture and should not be attempted without following a detailed, careful procedure employing adequate hoods and shielding.) Details of the synthesis are given elsewhere ( 1 1 , 1 2 ) . In addition, separate batches of the same cod were oxidized with KMn04, H202,and air, followed by isolation and purification procedures identical to those used for the nitric acid oxidized coal. Finally, dried leaves and a crude shale oil extract were oxidized with nitric acid using modifications of the procedure employed for the coal. The HA product was washed with 1 S HCI and deionized water to remove inorganic impurities and water-Poluhle organics. Moreover, the infrared spectra and elemental analyses were performed on HA that had been extensively treated with ionexchange resins to remove inorganic impurities (13)and vacuum dried at room temperature over P205. Infrared Spectra. Spectra were recorded in a Nujol mull and in a KBr pellet (0.5 mg H A / l 5 0 mg KBr). These spectra were taken to identify the presence of nitro and/or nitroso groups. Polarographic Studies. Initial studies were made to determine polarographic activity at the dropping mercury electrode (DME) of the H A prepared from coal hy various oxidizing agents, as well as the HA prepared from leaves and shale oil. In addition, a polarogram was taken of HA extracted from peat moss. Only HA prepared by HNO:3oxidation gave distinct reduction waves. Regardless of the source of organic material, all these waves had similar shapes and half wave potentials ( E detailed study was confined to a single preparation of HA from nitric acid-oxidized coal. All polarograms were taken in water Fvith a standard dc polarograph, Sargent Model 15. Polarographic parameters employed. such as drop time, were the same a5 those described in a previous study ( 3 ) . Solutions were prepared by pipetting 50 m L of supporting electrolyte into the sample side of a standard "H" cell equipped with a standard calomel electrode (SCE) reference, followed by the addition of 0.5 mI, of relatively concentrated stock HA. The pH of the solution was then adjusted Sy :.mall additions c 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
1127
Table I. Main Infrared Absorption Bands for Humic Acid frequency, cm-' I
CO,H
OH
H
I CQ,H
Figure 1.
2940 2900-2500
Hypothetical molecule of humic acid
9'3
t?
I
very broad, strong, partially due t o traces of H,O weak indistinct
hydrogen-bonded 0 - H stretch
3400
H-c-H
comment
assignment
1715 1625 1540 1450 1350 1270
aliphatic C-H stretch intramolecular H-bonded 0 - H stretch from COOH C=O stretch from ketone, ester, COOH aromatic C=C?, H-bonded C=O?, H - 0 from traces of H,O? NO, stretch aliphatic C-H bend NO, stretch C - 0 stretch from ester, C-OH, etc.
strong strong medium, sharp weak, broad weak, broad broad. medium
Portion of humic acid infrared spectrum showing NOz stretching bands (KBr pellet) Figure 2.
of H N 0 3 or KOH, oxygen was removed with a nitrogen purge. and the polarogram was run, scanning to more negative potentials from 0.0 V vs. SCE.
RESULTS AND DISCUSSION Elemental analysis of t h e HA employed (Galbraith laboratories, Knoxville, Tenn.) gave 55.5% C, 3.87% H, 5.3% N, 1.07% S, and (by difference) 34.3% 0. These values are similar to those from other HA samples prepared by HNO,< oxidation of coal ( 2 , s ) .Since there was 1.7% N in the original coal. the nitrated HA may contain u p to 3.670 N in nitro groups. T h e average equivalent weight of this HA is 308 f 4 g/equiv. (23). Its molecular weight is not known; published values of HA molecular weights range from 1000 to more than 50000 ( 1 4 ) . T h e IR spectrum of HA showing bands likely to be due to nitro groups (1.5) is shown in Figure 2. However, part of the absorption at 1540 cm-' could be due to the presence of nitroso groups which absorb strongly in t h e 1500-~1600cm-' region (15). Table I summarizes the frequencies of other major bands in the spectrum. Except for N - - 0stretching bands, the spectra are similar to others reported for HA ( 1 5 ) . Representative polarograms run in a variety of supporting electrolytes and a t different p H values are shown in Figure 3. T h e polarographic reduction waves observed are largely due to aromatic nitro groups many of which are on the same rings as phenolic -OH groups. T h u s the electrode reaction is similar to t h a t of paranitrophenol.
a four-electron reduction. T h e product is not further reduced a t p H values above 4. In addition to Reaction 1, some of the polarographic activity could be due to the reduction of nitroso groups. T h e spreading of t h e polarographic waves over a rather wide range of potential is due to irreversibility of the electrode reactions a n d , more importantly, the diverse chemical environments of the nitro groups . on the complicated HA molecule. T h e p H dependence of the half-wave potentials is due to
.i . _9
> .' - -
.I
i
-
1
.
-
..
-
Traced polarograms of humic acid (vertical line segments mark €,,A polarsupporting [Cu(II)], ogram E l , * ,V PH electrolyte ppm
Figure 3.
a
-0.36
4.30
0.1 F KN0,1 0 - 3 EDTA ~ 0 , 2 F KCI
0
4.98 0 -0.74 7.90 0.1 F KNO, 0 c 0.60 0 . 2 F KC1 0 d -0.67 9.84 7.51 0.2 F KCI 27 e -0.53 10.55 0 . 2 F KCI 27 f --0.69 ionization of nearby acidic substituents. At p H values appreciably below the ph', of these substituents, El$',values near -4.35 V are obtained; a t pH's near 5, two waves due to reduction of groups near both ionized and non-ionized neighboring groups are observed; and at pH's exceeding approximately 5 . 5 , only t h e reduction wave for groups near ionized neighboring groups ( E l $ around 4 . 6 5 V) are observed. The more negative potential required for reduction a t higher pH's can be explained by the increased negat>ivecharge on N O p (or NO) groups resulting from ionization of acidic groups on HA. Addition of agents that induce precipitation of HA decreases, or completely eliminates, the HA wave (diffusion current). Removal of HA was demonstrated hy acidification b
-0.30,
-
1128
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
Table 11. Selected Polarographic Dataa
result no.
1
2 3 4 5 6 7
0.2 F KCl 0.2 F KC1
[HA], mg/L
71 71 71 1 41 211 71 71
15
0.2 F KC1 0.2 F KC1 0.2 F KC1 0.2 F KC1 0.2 F KC1 0.2 F KC1 0.2 F KC1 0.1 F KNO, 0.1 F KNO, 0.1F KNO; 0.1 F KNO, 0.1 F KNO, 0.1 F KNO,
16
0.1 F KNO,
71
17
0.1 F KNO, 71 0.1 F KNO, 71 0.1 FKNO, + 35 10 - 3 F EDTA 0.1 F K N O , t 71 1 0 - 3 F EDTA 0.1 F KNO, + 123 10-3 F EDTA
8
9 10 11
12 13 14
18
19 20 21 a
supporting electrolyte
71
71 71 71 71 71 71 71
E , , , values are 20.02 V,
id
PH E,,,, v .-2.5-4.1 - 0.28 4.98 -0.74 -0.62 5.77 -0.62 5.66 - 0.66 5.64 -0.62 6.99 -0.66 9.35 -0.67 9.96 10.26 -0.71 - 0.24 3.48 3.83 -0.37 4.21 -0.37 -0.36 4.67 - 0.36 4.84 -0.30 5.10 -0.70 5.41 -0.30 -0.66 - 0.59 7.90 9.84 -0.65 4.31 - 0.33
i d >PA
0
0.20 0.15 0.53 0.90 0.99 0.68 0.64 0.60 0.66 0.58 0.85 0.89 0.67 0.50 0.22 0.15 0.06 0.18 0.53 0.53 0.27
4.18
-0.36
0.56
4.11
-0.34
0.57
uncertainty is at least t 5 % .
and by addition of Cu2+ between pH's 4 and 8, to form a n insoluble copper humate (compare polarograms d and e on Figure 3). However, at pH's higher than approximately 9 in t h e presence of copper, the polarographic wave reappeared as shown in Figure 3f. This is due t o the resolubilization of copper humate as an anion due to the presence of -OH- groups on t h e coordinated copper ions (12, 16). Addition of excess Cu(I1) to HA at a p H below 7 resulted in polarograms with Cu(I1) reduction waves and shifted the El/*values of the HA reduction waves, as shown in Figure 3e. T h e shapes of t h e polarograms suggest irreversible reduction. T h e relatively wide voltage range between the onset of reduction and attainment of diffusion-limited current is due to t h e presence of nitro (and perhaps nitroso) groups in different chemical environments in the HA. T h e dependence of diffusion current (id) on HA concentration is a function of the supporting electrolyte used. Use of 0.2 F KC1 supporting electrolyte resulted in lower id's compared t o 0.1 F K N 0 3 electrolyte. Humic acid molecules aggregate t o a greater extent a t higher ionic strengths ( 171, causing a reduced diffusion rate for HA. Table I1 compares polarographic data for t h e three supporting electrolytes employed. Nitroso groups on t h e humic acid employed in this study could arise from several sources. Nitrous acid produced during coal oxidation could bring about nitroso substitution in a manner analogous to the synthesis of p-nitroso phenol from phenol, sodium nitrite, and sulfuric acid (18). Another possibility is autoreduction of nitro groups; HA is known t o be a reducing agent capable of reducing metal ions t o lower oxidation states (19). Unfortunately, this work and work reported in the literature is consistent with either nitro or nitroso HA derivatives. For example, t h e infrared band a t 1360 cm-' could be due to several functional groups other than nitro groups. Also, the of the polarographic waves are in the range of both nitro a n d nitroso groups. As noted previously, t h e polarographic data indicate that nitro groups reduced a t the DME are next to ionizable acidic
7
.
.
4.:
5.0
5.:
E.:
i.;
-.
Figure 4. Diffusion current (id) dependence of pH. (A) E l / * = -0.35 f 0.02 V, (B) €112 = -0.65 f 0.02 V
groups. T h e phenol group is most likely to be ortho to a nitro group because of t h e known ability of the -OH group to activate aromatic rings toward nitration in the ortho and para positions (21). Carboxylic groups, on t h e other hand, deactivate benzene rings toward nitration; hence any nitration occurs meta to a -C02H substitutent. However, it is possible t h a t nitration could occur ortho or para t o aliphatic substituents, which weakly activates rings toward nitration ( 2 1 ) , followed by oxidation of t h e aliphatic groups t o -C02H. T h e orientation of the nitro groups relative to -OH or -C02H groups may be inferred from analysis of the polarographic data. Figure 4 shows a plot of relative id vs. p H for polarograms taken on the same HA solution a t different p H values. Zuman (22) has shown t h a t the point of intersection of t h e plots from waves having El,* around -0.35 V and Ell*around -0.65 V vs. SCE occurs a t a p H equal to the ph', of the neighboring acidic substituent groups. In this case, the average pK, of the neighboring groups must be approximately 5.0. Since the pK,'s of o-nitrobenzoic acid, onitrophenol, 2,4-dinitrophenol, and 3,6-dinitrophenol are, respectively, 2.16, 7.17, 3.96, and 5.15 ( 2 3 ) ,i t appears most likely that nitration occurs ortho and para to phenol groups. Introduction of a nitro group a t any position on benzoic acid lowers the pK, below 3.5 (23);hence it seems quite unlikely that polarographically active -NO2 groups are near carboxylate functions. T h e resonance structures,
explain the large jump in the HA reduction potential to more negative values near p H 5 . This occurs because the greatly increased negativity of the nitro group allowed by ionization of the phenolic -OH group makes the nitro group harder to reduce. The consumption of hydrogen ion in the reduction of nitro or nitroso groups (22) explains the p H dependence of Eli* (other than the major shift at p H 5). A similar p H dependence for Eli2and a lesser one for id had been reported ( 4 ) in a study performed at p H values exceeding 7 , such that the major shift in Ell*a t p H 5 was not observed. Similar results were observed for nitric acid oxidation products of soil HA, nitro phenols, and nitrobenzoic acids (7). Existing data for nitrophenols (24)indicate that they form only moderately stable metal complexes; for example, log Kf values for Fe3+ complexation range from 2 to 8. Hence, the importance of nitro or nitroso groups in determining metal binding properties of HA is probably minimal. ACKNOWLEDGMENT T h e authors thank E. E. Pickett for his assistance in obtaining infrared spectra. LITERATURE CITED (1) Manahan, Stanley E. "Environmental Chemistry", 3rd ed: Willard Grant Press: Boston, Mass., 1979; pp 74-76.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979 Manahan. S.E.; Poulson. R. E.; Green, J. B.; Farrier, D. S. "Coal Humic Substances and their Application to Pollution Control in the Synthetic Fuels Industry". LETC/RI-76/5, USDOE Technical Information Center: Oak Ridae, Tenn., 1978 Thornson, G. H. J . Appl. Chem. 1952, 2 , 603. Cody, A. F.; Miiliken, S. R.; Kinney, C. R . Anal. Chem. 1955, 27, 362. Charmbury, H. B.; Eckerd, J. W.; LaTorre. J. S.; Kinney, C. R. J . Am. Chem. SOC. 1945. 67. 625. Lucena-Conde, F.; Gonzaiez-Crespo, A. Trans, Int. Congr. Soil Sci., 7th, Madison, Wis., 1960 1961, 2 , 59. Lindbeck, M. R.; Young, J. L. Soil Sci. 1966, 101, 366. Stoeber, H.; Eberie, S. H. Kernforschungszentrum Karlsruhe (Ber.). KFK 1969 UF 1974, 58-81; Chem. Abstr. 1975, 82, 4 7 5 6 5 ~ . Ref. 8 , pp 44-57; Chem. Abstr. 1975, 82, 4 7 5 6 4 ~ . Mazumdar, B. K.; Chatlerjee, A. K.; Lahiri, A,, Fuel 1967. 4 6 , 379. Green, J. 8.; Manahan, S. E. J . Inorg. Nucl. Chem. 1977, 39, 1023. Green, J. 8. M A Thesis, University of Missouri, Columbia, Mo., 1975. Green, J. 8. Ph.D. Thesis, University of Missouri, Columbia, Mo., 1977. Schnitzer, M.;Khan, S. U., "Humic Substances in h e Environment"; Marcel Dekker: New York, 1972: pp 71-72. Williams, D. H.; Fleming, I. "Specb-oscopic Methods in Organic Chemistry". McGraw-Hill: New York. 1966: ChaDter 3. Stevenson, F J. So!/ Sci 1977, 123, 10
1129
Fiaig. W. A. J.; Beutelspacher, H. Roc. Symp. Use Isotop. and Rad. in Sod Org. Mat. Studies, Vienna, 1968, Int. Atomic Energy Agency: Vienna, 1968; pp 23-30. Bate, S. C. "Synthesis of Benzene Derivatives", E. Benn Ltd.: London, 1926; Chapter 5. Sziiagyi, M. Fuel 1974, 53, 26. Zuman, P. "Organic Pohrographic Anabsis"; Pergamon: New York. 1964; p 105. Boyd, R. N. "Organic Chemistry". 2nd ed.;Allyn and Bacon: Mwrison, R. T.; Boston, Mass., 1966. Zuman, P. in "Progress in Physical Organic Chemistry"; Streitwieser, A,; Taft, R. W., Eds.; J. Wiiey and Sons: New York, 1967, Vol. 5, pp 81-206. "Handbook of Chemistry and Physics", 50th ed.; Chemical Rubber Go.: Cleveland, Ohio, 1969, pp D-118-119. Martell, A. E. (compiler), "Stability Constants of Metal Ion Complexes", Spec. Pub/. 17 (part 2), Chemistry Society: London, 1964.
RECEIVED for review February 7, 1979. Accepted April 17, 1979. This research was supported by the US.Department of t h e Interior Office of Water Research and Technology Matching Grant B-115-MO.
Application of Recursive Estimation to the Real Time Analysis of Trace Metal Analytes by Linear Sweep, Pulse, and Differential Pulse Anodic Stripping Voltammetry Paul F. Seelig' and Henry N. Blount" Brown Chemical Laboratory, The University of Delaware, Newark, Delaware
The application of a Kalman filter to the analysis of data derived from the determination of trace quantities of lead in municipal and sea water analytes using differential pulse, pulse, and linear sweep anodic stripping voltammetry at the hanging mercury drop electrode and linear sweep anodic stripping voltammetry at the thin film mercury electrode is demonstrated. A critical comparison of this recursive estimator to other analysis techniques is presented. For these real analytes where substantial faradaic and nonfaradaic modeling errors occur, the concentration estimates computed by the real time Kalman filter and nonreal time multiple regression were generally more precise than those estimates computed by other digital methods for analyses where S / N ,< 5. At higher S/N, concentrations computed using more standard analysis algorithms were comparable to those determined by the real time Kalman filter or nonreal time multiple regression.
1977 1
tained in noise corrupted transient signals has recently been demonstrated by the use of an optimal estimation technique, the Kalman filter ( 1 ) . Application of this algorithm to trace metal determinations by linear sweep anodic stripping voltammetry (LSASV) a t hanging mercury drop (HMDE) and thin film mercury (TFME) electrodes has been found to substantially increase t h e precision and accuracy of these analyses (2). This present work demonstrates application of a Kalman filter to the analysis of data derived from t h e determination of trace quantities of lead in municipal and sea water analytes using pulse anodic stripping voltammetry (PASV) and differential pulse anodic stripping voltammetry (DPASV) at H M D E and using LSASV at both H M D E and TFME. A critical comparison of several nonreal time (NRT), pseudo-real time (PRT), and real time (RT) numerical analysis techniques applied to voltammetric transients resulting from these electrometric methods is presented.
EXPERIMENTAL In all areas of chemical analysis, there is a pronounced need to improve the quantity and quality of information extracted from each experimental measurement. Various computational algorithms executable by on-line computer systems commonly incorporated in modern instrumentation provide the means whereby high fidelity information can be derived. Full utilization of the control capabilities inherent in such on-line systems can only be realized in situations where the analytical result is generated in real time (Le., as t h e experiment is in progress) and without loss of either accuracy or precision. The real time determination of t h e analytical information con-
'Present address: Department of Chemistry, Reiss Science Center, Georgetown University, Washington, D.C. 20057. 0003-2700/79/0351-1129$01.00/0
Apparatus. The electrochemical apparatus and the computer system employed for data acquisition and analysis have been previously described ( 2 ) . All potentials are reported relative to the saturated calomel electrode (SCE). All measurements were carried out at 25.0 (f0.2) "C. Atomic absorption determinations were carried out using a Jarrell-Ash model 810 atomic absorption spectrophotometer equipped with a model FLA-10 flameless atomizer attachment. Citrate buffers, pH 5.0, were prepared by dissolving 352.9 g sodium citrate and 147.1 g citrate acid in distilled water with subsequent dilution to 1 L. This buffer was electrolytically purified as previously reported ( 2 ) . Reagent grade methyl isobutyl ketone (MIBK. Fisher) was used as received. Ammonium pyrrolidine dithiocarbamate (APDC) solutions were prepared by dissolving 0.50 g reagent grade APDC (Fisher) in 25 mL distilled water, filtering to remove an un-
0 1979 American
Chemical Society
