Removal of Iron, Copper, Cadmium, Cobalt, and Nickel from Sodium Hydroxide by Precipitation and Extraction with Phenyl-2-Pyridyl Ketoxime Daniel Reiner and Donald P. Poe" Department of Chemistry, University of Minnesota, Duluth, Minnesota 558 12
Reagent grade sodium hydroxide usually contains several parts per million of iron and nickel, along with other transition metals. Determinations of traces of these metals in which sodium hydroxide is used as a reagent, therefore, require large blank corrections. While many extraction procedures have been developed for removal of transition metals from aqueous solutions ( I , 2 ) ,most methods fail in highly alkaline solutions, or leave the solution contaminated with other chemicals. The procedure described here was developed in response to a need for "iron-free'' sodium hydroxide, but we have found that copper, nickel, cadmium, and cobalt are also effectively removed. Phenyl-2-pyridyl ketoxime forms slightly-soluble, colored compounds with a number of transition metals a t high p H (3-5). The metal complexes can be removed by filtration or by extraction into an immiscible solvent. The method described here employs phenyl-2-pyridyl ketoxime as a precipitant, and leaves the sodium hydroxide free of all reagents used in the procedure.
EXPERIMENTAL Reagents. Phenyl-2-pyridylketoxime, iron- and copper-free isopentanol, and redistilled hydrochloric acid were obtained from G. F. Smith Chemical Co., Columbus, Ohio. Phenyl-2-pyridyl ketoxime also was prepared according to Trusell and Diehl (5). Ethanol (95%) from the University of Minnesota Chemical Storehouse was found to be essentially free of iron. The 10% palladium on carbon powder was from Baker Catalysts, Newark, N.J. The 10 M sodium hydroxide was prepared from reagent grade pellets. Hydrogen and high-purity nitrogen gas were obtained locally. Apparatus. Polyethylene, polypropylene, or polytetrafluoroethylene containers were used to handle the sodium hydroxide solutions. A Beckman Acta I11 UV-visible spectrophotometer was used with 1-ern silica or 10-cm Vycor cells for solution absorbance measurements. Procedure for Removal of Metals. To 3 L of 10 M sodium hydroxide in a 4-L Pyrex filter flask, add 0.2 g of 10% Pd on carbon (wetted with water) and 2.0 g of phenyl-2-pyridylketoxime dissolved in a minimum amount of hot ethanol. Heat to 100 "C and bubble with hydrogen gas with stirring for 2 h. Allow to cool overnight. Filter the mixture through a glass fiber filter which has been rinsed with 1:l hydrochloric acid and deionized water. Extract each liter of the filtrate with two 50-mL portions of a mixture containing 25% ethanol and 75% isopentanol. Transfer the aqueous phase to a 1-L Teflon bottle fitted with a length of Teflon tubing which extends from above the cap to near the bottom of the bottle. Add 0.6 g of phenyl-2-pyridylketoxime dissolved in a few milliliters of hot ethanol. Place the bottle in an oven set at 100 "C so that the tubing protrudes through a vent hole in the top of the oven, and bubble with nitrogen for 12 h. When cool, repeat the extraction and bubbling steps. Atomic Absorption Analyses. The purified samples of sodium hydroxide were adjusted to 10 M by addition of deionized water. Samples of 20 mL each were neutralized with redistilled hydrochloric acid in a polyethylene beaker, adjusted to pH 2.5, and the metals extracted into 20 mL of methyl isobutyl ketone with ammonium pyrrollidine carbodithioate (6). One sample was neutralized with Baker Ultrex ultrapure analyzed hydrochloric acid to provide a determination of the blank due to the acid. Standard metal ion solutions were prepared by dilution of stock solutions containing 0.100% each of Fe, Cu, Ni, Cd, Co, Mn, and Zn, prepared from the corresponding sulfate salts. The ionic strength of the standards was adjusted with sodium chloride to 2.0 to match the ionic strength of the neutralized samples. The
Table I. Effect of Concentration of Phenyl-2-pyridyl Ketoxime on Removal of Irona Mol oxime/ mol Fe
Final [Fe], mol/L (mg/LIb 3 14 X (0.08) 10 16 x 10-7 (0.09) 50 11 x 10-7 (0.06) 100 8 X lo-' (0.05) a Initial concentration of iron = 7 X lo-' M (3.9 mg/L). Palladium not removed. standards were adjusted to pH 2.5 and extracted as described above. The prepared samples were analyzed by the Lake Superior Basin Studies Laboratory using an Instrument Laboratories 353 atomic absorption spectrophotometer.
RESULTS AND DISCUSSION Effect of Conditions on Removal of Iron. Oxidation State of Iron and Concentration of Sodium Hydroxide. Iron(I1) and iron(II1) form slightly soluble red compounds with phenyl-Zpyridyl ketoxime in aqueous solutions. Little or no formation of the iron(1II) compound occurs above pH 10, The iron(I1) compound is formed completely even in concentrated sodium hydroxide, and its solubility decreases with increasing concentration. Sodium hydroxide (10 M) provides low solubility of the iron(I1) compound, is concentrated enough to use as a stock solution for preparation of more dilute solutions, and its viscosity is not so great as to make filtration and extraction difficult. Reducing Agent. Hydrogen gas was chosen as a reductant because the excess is easily removed by boiling, and the oxidation product, water, does not constitute a contaminant. Hydrogen gas alone is not effective, but addition of a palladium catalyst (10% Pd on carbon powder) is required. The catalyst is easily filtered off along with the insoluble iron compound. Some palladium dissolves in the sodium hydroxide. Additional steps required for its removal are discussed below. Time and Temperature. Reduction of iron(II1) with hydrogen proceeds slowly at room temperature, but the rate is considerably increased a t 100 "C. After the purification procedure (without removal of dissolved palladium) was carried out on solutions initially containing 7 X M iron(III), the iron which remained in solution was estimated spectrophotometrically using 4,7-dihydroxy-l,lO-phenanthroline (7) with 10-em cells. Figure 1 shows absorbance as a function of time and temperature. Disregarding the presence of palladium in these samples, and assuming that Beer's law holds, the lowest reading obtained corresponds to 0.05 mg Fe/E. Concentration of Phenyl-2-pyridyl Ketoxime. Table I shows that the removal of iron becomes more complete as the molar ratio of phenyl-2-pyridyl ketoxime to iron is increased. In each case, the initial concentration of iron was adjusted to 7 X M, and the final concentration of iron was estimated spectrophotometrically as described above. Because the amount of iron in reagent grade sodium hydroxide is much less than that used in this experiment, we recommend using 0.7 g of phenyl-2-pyridyl ketoxime per liter of sodium hydroxide, which corresponds to a molar ratio greater than 501. ANALYTICAL CHEMISTRY, VOL. 49,
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Figure 1. Effect of time and temperature on reduction of iron(II1). See text. (A)Room temperature, (0)100 O C
Removal of Excess Reagents. Most of the excess phenyl-2-pyridyl ketoxime separates from the solution as fine, mol/L (UV analysis) after white needles, leaving 3 X filtration. After one extraction with the alcohol mixture, the concentration of phenyl-2-pyridyl ketoxime is below the detectable limit with ultraviolet spectrophotometry (A, = 278 nm, E = 2.0 X lo4L mol-* cm-', detection limit = 6 X lo-' mol/L) using 1-cm cells. Except for a slightly higher absorbance around 223 nm due to the presence of carbonate, the ultraviolet spectrum of the finished material is identical to that of the untreated sodium hydroxide. After the initial filtration the solution is a faint pink, indicating the presence of some iron and possibly other metals. The iron compound is slowly oxidized by air to form the iron(II1) derivative of phenyl-2-pyridyl ketoxime, which is unstable in sodium hydroxide. Therefore, it is necessary to carry out the extraction immediately after the filtration. The presence of a slightly soluble yellow compound was observed in some early preparations of the treated sodium hydroxide after they had been filtered but not extracted with the alcohol mixture. Formation of the compound was very slow, being first noticeable after several days, and increasing over a period of several weeks. A solution of the yellow compound in chloroform matched the spectral characteristics of the Pd(I1) derivative of phenyl-Bpyridyl ketoxime, reported by Sen ( 4 ) . Because of slow formation of the compound, palladium is not completely removed in the initial extraction. Therefore, following the initial filtration and extraction, additional phenyl-2-pyridyl ketoxime is required to chelate with the residual palladium. Heating the mixture promotes rapid formation of the palladium-oxime complex. It is also necessary to remove the alcohol at this point by bubbling with nitrogen; otherwise the alcohol forms a separate liquid phase, extracting the oxime from the aqueous phase before all of the palladium has been complexed, the result being incomplete removal of the palladium. It is also recommended that the bubbling operation be carried out immediately after the extraction to prevent oxidation of the alcohols by air. Removal of Other Transition Metals. Analyses for seven metals in six different preparations of 10 M sodium hydroxide are shown in Table 11. The metals in samples 2 and 3 were added as the sulfate salts of Fe(III), Cu(II), Cd(II), Co(II), Ni(II), Mn(II), and Zn(I1). Removal of iron, copper, cadmium, nickel, and cobalt was very effective, while manganese and zinc were not removed at all. 890
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The anomalous behavior of manganese and nickel is interesting. In the case of manganese, filtration of the insoluble hydrated oxides removes most manganese (sample 2), but the purification treatment leaves over 90% of the manganese in solution (sample 3). We suspect that in the former case, insoluble manganese dioxide is formed via oxidation by dissolved oxygen and is filtered off. In the latter case, the reduction step keeps manganese in the +2 oxidation state, which apparently remains in solution as an hydroxo complex. The precipitation of Ni(I1) in sample 2 probably results from coprecipitation with other metal ions.
in carrying out the metal analyses.
LITERATURE CITED (1) G. H. Monison and H. Freiser, ‘‘Solvent Extraction in Anawcai Chemlstty”, Wiley, New York, 1957. (2) J. Stary, “The Solvent Extraction of Metal Chelates”, Macmillan, New York, 1964. (3) B. Sen, Chem. Ind. (London), 1058, 562. (4) B. Sen, Anal. Chem., 31, 881 (1959). (5) F. Trusell and H. Diehi, Anal. Chem., 31, 1978 (1959). (6) D. Long, “Current Water Anatysis Methods”, Lake Superior Basin Studies Center, University of Minnesota, Duluth, Minn., October 1973. (7) A. A. Schik, G. Frederick Smith, and Alvin Heimbuch, Anal. Chem., 28, 809 (1956).
ACKNOWLEDGMENT
RECEIVED for review December 15,1976. Accepted February
We thank Duane Long of the Lake Superior Basin Studies Center, University of Minnesota, Duluth, for his assistance
11, 1977. Financial support was provided by the Graduate School of the University of Minnesota.
Computer-Assisted Furnace Atomic Absorption Spectrometric Analysis J. E. Poldoski United States Environmental Protection Agency, Environmental Research Laboratoty- Duluth, 620 1 Congdon Boulevard, Duluth, Minnesota 55804
The popularity of furnace atomic absorption spectrometry (FAAS) has resulted in the performance of an ever-increasing number of determinations by this method in environmental studies (I). In addition, this has prompted studies of performance evaluations (2,3)for commercial instrumentation. Although high sensitivity is perhaps one of its most outstanding features, some well-known serious disadvantages are the lengthy times for analysis, including data analysis, and the presence of matrix interferences that necessitate the use of standard additions to samples. Presently, there exists no readily available description of a data acquisition and handling method that is both flexible and designed to conveniently satisfy the specific needs of production analysis. The objective of this work is to report a method for using a turnkey data system with FAAS instrumentation. Use of the system’s high-level language programming capabilities together with the peak integration software results in a combination providing both on-line and high-volume post-run calculation capabilities. As applied to natural water analysis, the following discussion will describe the performance of a particular instrumental configuration and a method for the acquisition and reduction of large numbers of atomization signals near instrumental detection limits.
EXPERIMENTAL Apparatus. Atomic absorption instrumentation (PerkinElmer) consisted of a Model 305B atomic absorption spectrophotometer equipped with a deuterium arc background corrector, an HGA-2100 graphite furnace atomizer, and a Model 56 recorder. Hollow cathode lamps were employed for copper and lead analyses, and an electrodeless discharge lamp was used for cadmium. The argon gas was interrupted during atomization for lead, whereas the noninterrupted mode was used for cadmium and copper. Other operating conditions and trace analysis considerationsare given elsewhere (4). In order to start the analog to digital (AID) converter, an ultrasensitive SPDT relay (Dl-962 Calectro) was installed at pole 6 of the recorder switch on the HGA-2100 power supply (used in auto mode), as indicated in Figure 1. This allowed for AID conversion to start automatically during the furnace program charring cycle by simply providing a contact closure about 15 s prior to the start of atomization. It was convenient that the charring time be set to any value greater
than 15 s. An identical relay was installed in parallel with the aforementioned relay to provide contact closure for the terminals at the spectrophotometer auto zero control, except for during atomization and 15 s prior to it. This provided automatic instrument zeroing, and thus the 0 10 mV baseline required for the computer was assured. Since it was advantageous to be able to manipulate the spectrophotometerbaseline voltage from a value other than zero, a simple voltage offset (summing)circuit (5) was constructed from a low-cost operational amplifier (741HC, Fairchild) and was placed between the spectrophotometer output and the AID input (Figure 2). As indicated, this circuit also provided for approximately 100-fold amplification to 5-fold deamplification of the signal from the spectrophotometer. The computer hardware-software was a Model 3352B gas chromatography data system (Hewlett-Packard), which included 32K of core memory, a punch and reader-equipped HP2752A teleprinter, a 2748B photo-tape reader, a 9866A line printer, an 18562A remote A/D module, and the Lab BASIC language interpreter. A 1-V full-scale analog signal from the spectrophotometer was input to the AID converter, which operated at a sampling rate of 32 Hz. This sampling rate was the highest available but it was sufficient to provide for accurate integration of relatively rapid atomization signals. Programs and Procedure. All relevent analytical information is recorded on paper tape. If experiment-identification information is desired, it is entered before all other data. The analysis sequence is begun by simply initiating a program cycle on the furnace. At the conclusion of each cycle, the system prints the current sample number, estimated concentration, and requests input for the sample code. The sample code indicates whether the injection is a standard, a sample, or a sample with a spike. A misentered sample number or code can be easily corrected. The design of the post-run calculation requires that a set of any random number of sample or spiked sample injections be grouped during the analytical run since only consecutive signals for a sample are averaged. Each sample name is unique and possesses eight or less characters, either alpha or numeric, or both. The name BLK is reserved to indicate a general analysis blank that can be analyzed and subtracted from all samples in the analytical run. Corresponding spiked and unspiked samples require the same sample name and have to differ by the sample code which designates the concentration of the spike. The data-manipulation scheme requires that any number of spiked samples immediately follow a particular sample in the analysis sequence to allow for calculation of the average recovery. Any
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