Once again it is evident that, with exception of copper, there was a high percentage of loss in the spiked samples. This study shows that the method of analysis is effective for dissolved metals in a distilled deionized water system. Lead, cadmium, zinc, iron, and copper were recovered simultaneously with acceptable accuracy. The study also shows that six 500-ml aliquots from a river sample gave dissolved metal levels of desirable precision. However, extreme caution must be used in interpreting extractable metal data. A large number of samples must be run to obtain an average concentration value because of the extremely poor reproducibility of individual results. The problem appears to be a result of the variability of metal content within the sediment, not of poor precision in sampling of the water and sediment. When river water was prepared for both extractable and dissolved metal analysis and then spiked, a quantitative recovery of that spike could not be achieved. Adjusting the pH of samples stored seemed to have little constructive effect on spike recovery. Based on these experiments, the method of solvent extraction was considered unacceptable for use for multiple metal analysis of a natural water system. Copper is the only trace metal which can be quantitatively determined by this method.
ACKNOWLEDGMENT The authors thank Robert Koob for his professional assistance. LITERATURE CITED (1) K. H. Mancy, "Analytical Problems in Water Pollution Control", National Bureau of Standards Special Publication 351, U.S. Government Printing Office, Washington, D.C., 1972. (2) J. A. Plane, "Trace lnorganics in Water", Adv. Chem. Ser., 73, 247 (1968). (3) "Methods for the Analysis of Water and Wastewater", Environmental Protection Agency (EPA), Cincinnati, Ohio, 197 1. (4) J. L. Robinson, R . G. Barnekow, and P. F. Lott, At. Absorp. News/., 6, 60 (1969). (5) B. Montford, Can. Spectrosc.. 4, 2 (1968). (6) "Operator's Guide for the Model 303," Perkin-Elmer Corp., Norwalk, Conn., 1969. (7) V. S.Sastri, K. I. Aspila, and C. L. Chakrabarti, Can. J. Chem., 47, 2320 (1969). (8) J. Stary and K. Kratzer, Anal. Chim. Acta, 40, 93 (1968). (9) J. T. Pyle and W. D. Jacobs, Anal. Chem., 36, 1796 (1964). (10) P. G. Stecher. Ed., "Merck Index", 8th ed., Merck and Co., Inc.. Rahway, N.J., 1968, p 585. (11) A. W. Struempler. Anal. Chem., 45, 2251 (1973). (12) R. B. Dean and W. J. Dixon, Anal. Chem., 23, 636 (1951).
RECEIVEDfor review August 15, 1975. Accepted September 19, 1975. Acknowledgment is made to the Office of Water Resources Research for support of this work.
Preconcentration of Certain Anions Using Reagents Immobilized via Silylation Donald E. Leyden,' G. Howard Luttrell,' William K. Nonidez, and Dennis B. Werho Department of Chemistry, University of Georgia, Athens, Ga.30602
improved methods of recovery of ions from solution with simultaneous preconcentration of either a broad range or selected few ions is a significant goal in trace elemental determinations. Ion exchange resin beads have limitations for this purpose. In this study, functional groups are attached to the surface of controlled pore glass beads via silylation reactions. Many reagents are commercially available and chemical modifications may be done after immobilization. This report demonstrates that oxyanions such as arsenate, dichromate, selenate, molybdate, tungstate, and vanadate present at the ng/mi Concentration level may be recovered using controlled pore glass treated with N-@-aminoethyl-yaminopropyltrimethoxysilane (Dow-Corning 2-6020). The recoveries were investigated as a function of time and pH. By placing 100 mg of the treated glass beads in a small column and pumping test solutions through the column at 50 ml/mln, an average recovery of 103% was observed for 1.5 ng/ml solutions of selenate.
There are two approaches to trace elemental determinations. One is to attempt to devise instruments which are capable of determinations directly on the sample when concentrations are very low. Instrumental developments have produced an increasing number of methods suitable for Present address, Center Laboratories, Division of Alcon Laboratories, Port Washington, N.Y.
trace determinations. A second approach is to use chemical means to increase the concentration of the elements or functional groups to be determined. Techniques such as solvent extraction and various chromatographic procedures have been used for this purpose. Recently, several novel methods have been applied to immobilize reagents on solid substrates so that selective or general preconcentration may be obtained. Several papers have been published which demonstrate that reagents immobilized in polyurethane foam may be used for preconcentration ( I , 2). Filter papers impregnated with ion exchange resin have also been used for this purpose (3, 4 ) as well as chelating ion exchange resin beads ( 5 ) . Weetall et al. reported the immobilization of an 8-hydroxyquinoline on controlled pore glass beads (6).This was done using y-aminopropyltriethoxysilane which was bonded to the glass via a silylation reaction. The amino functional group was then converted to the 8-hydroxyquinoline by a series of chemical reactions. A similar procedure has been used in our laboratory to immobilize a variety of functional groups on both silica gel and glass beads (7). This paper reports further studies of these materials and demonstrates their potential application to the preconcentration of certain important anions.
EXPERIMENTAL Reagents. Two types of stock solutions were used. One of these had a concentration of 10-3M in the anion of interest while the other contained 100 mgh. of the metal as the oxyanion. Working ANALYTICAL CHEMISTRY, VOL. 48,
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solutions were made by diluting these stock solutions to an appropriate volume with deionized water. In all cases, reagent grade chemicals were used. The alkoxysilanes immobilized on the silica gel (E.M. Reagents, Type G) and the controlled pore glass beads (Electro-Nucleonics CPG-10, 200/400 mesh) were obtained from. Dow-Corning or Union Carbide. These were N-fl-aminoethyl-y-aminopropyltrimethoxysilane, 2-6020; N-methyl- y-aminopropyltrimethoxysilane, XZ-2024; y-aminopropyltriethoxysilane, A-1100; and N,N,N-trimethyl-y-aminopropyltriethoxysilane,Y-5817. The silylation procedure has been discussed in detail (7). The dithiocarbamate of Z6020 was prepared as reported earlier (7). Apparatus. All x-ray fluorescence data were obtained using a Philips model PW-1410 x-ray fluorescence spectrometer. In all cases, a LiF-PO0 analyzing crystal was used along with gas flow and scintillation detectors in tandem. P-10 gas (Selox, Inc.) was used in the flow detector. Either a Mo or W, or Cr x-ray tube was used as a source of x radiation and was operated a t 50 kV and 50 mA. The counting times varied, but were less than 40 seconds in all cases. Spectrometer parameters were optimized using the usual procedures of step scanning to locate the peak and background 2 9 angles and a PHA scope to determine the pulse height selector window. Pellet Preparation. The silica gel was pelletized for use in the x-ray spectrometer. Because of the mechanical instability of pressed silica gel or glass bead pellets, a binder of powdered cellulose was mixed with the silica gel or glass beads before pressing to form pellets. Equal weights of cellulose and silica gel or glass beads were used. Rate of Anion Extraction. To ascertain the time needed for optimal extraction, experiments were undertaken to determine extraction rate a t constant pH and ionic strength. In each case, 100 mg of the immobilized complexing agent was placed in a KHP buffer along with 2 pmol of the anion salt of interest. The P H in each case was 3.5 f 0.2, and the final volume was 27 ml. These solutions were allowed to stir for various periods of time. The solid extractant was then removed by filtration, rinsed with water, mixed with 100 mg of powdered cellulose, pelletized, and the amount of metal uptake determined by x-ray fluorescence. Percent Extraction as a Function of pH. For this study, 100 mg of the immobilized diamine was placed into a solution containing 2 fimol of the metal oxyanion of interest. The pH of the solution was adjusted by the addition of 0.1M HC104 or 0.1M NaOH. In cases in which the pH was below 2,2M HC104 was used. A constant ionic strength was maintained by the addition of suitable amounts of 0.1M NaC104. The solutions were allowed to stir for 10 min, the solid extractant removed and prepared for analysis by x-ray fluorescence. Capacity Study. The capacity of the protonated immobilized diamine was determined with resPect to C r ~ 0 7 ~and - M004~-.The immobilized diamine (200 mg) was stirred with an excess of 0.1M Na2Cr207 for 10 min. The material was removed by filtration and placed in 25 ml of a dilute HC104 solution. To this solution was added 0.5 g of K&08 and 5 drops of 10-3M AgN03. The solution was boiled to oxidize all the chromium to the +6 state and to destroy all organic matter. After filtering to remove the silica gel, 10 ml of this solution was diluted to 100 ml with 0.1N KHP. The amount of chromium extracted was then determined by determining the absorbance of this solution at 340 nm compared to a set of standard dichromate solutions prepared in a similar manner. For the capacity of Mood2-, the immobilized diamine was equilibrated with 0.1M NazMo04 and stirred for 15 min a t a pH of 2.9. The substrate was washed with water, filtered, and then placed in beakers containing NaOH a t a pH of 11 for 20 min. The resulting filtrate was diluted to volume and the amount of Mood2- present determined by the method of Sandell (8). Standard Curves and Recoveries. Standard solutions were prepared that contained respectively 1, 2, 5, and 10 ug/ml As and Se in the form of Add3- and Se0d2-. A synthetic test solution was also prepared that contained 3 pg/ml As and 9 fig/ml Se as the oxyanions. To a 25-ml sample of each of these solutions was added 100 mg of the immobilized diamine. The mixture was buffered a t pH 5.0 f 0.2 with a KHP buffer, stirred for 10 min, and the solid extractant prepared for analysis by x-ray fluorescence. The standard curves were plotted for each anion, and the percent recovery of “unknown” solution was determined from these curves. Column Recovery Studies. For preconcentration work in the ng/ml range, batch extractions are not particularly convenient; therefore recovery studies using small columns packed with 100 mg of the diamine immobilized on controlled pore glass were in68
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
vestigated. The columns themselves were of glass with a length of 6 cm and an i.d. of 0.5 cm. The height of the extractant in the column was about 1 cm. The sample solutions and the column were connected with Tygon tubing (i.d. 3h in.). ,The tubing passed through a variable speed peristaltic pump (Harvard Co.) which maintained a constant flow rate. For this study, Se0d2- was used. Standard solutions (2 1.) containing 1, 2, 5, and 10 ng/ml selenium as Se0d2- were prepared by serial dilution of a 100 fig/ml stock solution. These solutions were analyzed within minutes after preparation. Two synthetic test solutions (2 1.) containing 1.50 ng/ml of selenium were also prepared. The pH of each of these solutions were adjusted to 1.9 with HC104, and each solution was pumped through a column at 50 ml/min. After x-ray analysis, the standard curves were plotted and the percent recovery of the test solutions was determined from these curves.
I
RESULTS AND DISCUSSION Rate of Anion Extraction. Studies of the rate of metal oxyanion extraction were undertaken using the 2-6020 diamine and Seck2-, MnOd-, and Cr2072-. These studies were performed using a batch extraction process. The results are illustrated in Figure 1. The selenate and permanganate were 90% extracted in about 2 min. However, the Mn04- immediately began to come off the substrate and after 15 min was only about 60% extracted. A possible explanation of this loss is given later in this paper. The dichromate is about 90% extracted after about 4 min and, like the selenate, remains on the substrate. The h o d 3 - after about 8 min was optimally extracted but, like the permanganate, quickly began to come off the substrate. An additional experiment was performed using ( 2 1 3 0 7 ~ as an illustrative oxyanion to compare the relative extraction rates among the various immobilized functional groups. The results of this study are given in Figure 2. The fastest extractive rate was observed for the diamine and N-alkyldithiocarbamate, which extracted 90% of the C1307~- in about 2 min. The primary amine A-1100 and the CPG-10 immobilized diamine follow with about 5 min needed for 90% extraction. The slowest extraction rates were observed for the XZ-2024 secondary amine and the Y-5817 quaternary amine, these providing 90% extracted in about 30 min. After an hour, only the dithiocarbamates and the primary amine showed significant loss from the substrate. I t should be remembered that all of the studies in both the above experiments were performed at the same pH to obtain a relative view of the rate with regard to both the anions and the substrates. The p H was not optimal in any one study. However, the studies were executed with multielement analysis in mind. In general, the results indicate that probably the best substrate with regard to rate is the immobilized 2-6020 diamine. Percent Extraction as a Function of pH. To obtain optimal conditions for extraction, studies were undertaken to determine what effect pH would have on the procedure. The silica gel immobilized 2-6020 diamine was used as the substrate for extraction of arsenate, permanganate, dichromate, selenate, tungstate, molybdate, and metavanadate. The results of this study are given in Figure 3. The Se042is completely extracted a t a pH of 2. The extractability of this anion decreases with increasing pH to a,value of about 10% at a pH of 7 . The molybdate is optimally extracted at a pH of about 2.9. Both the dichromate and tungstate are best extracted between pH 4 and 5 , the arsenate between pH 5 and 6, while the metavandate and permanganate are best extracted between pH 7 and 9. Dichromate, permanganate, and selenate are strong oxidizing agents and probably oxidize the diamine and then either complex with the oxidized form of the diamine or with other diamine groups. In the case of the dichromate and permanganate, evidence obtained concerning the oxi-
100 90
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.-A.
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Flgure 2. Percent extraction as a function of time with
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Flgure 1. Percent extraction as a function of time for the immobilized diamine
dation state of the metal on the substrate from ESCA spectra indicated the presence of either Cr(1V) or Cr(V) and Mn(I1). Cr(VI), Cr(III), and Mn(VI1) were not detected on the substrate using ESCA. Since the Mn04- is actually on the substrate as the Mn(I1) form, the desorption phenomenon of the MnOd-ldiamine rate study can be explained. The Mn(I1)-diamine complex is not stable at the low pH value a t which the rate study was performed (7). I t is doubtful that arsenate would have the oxidizing power to oxidize the diamine a t the p H a t which the extractions are conducted. The rising part of the extraction curve for arsenate vs. pH coincides closely with the formation curve of the HAs0d2- species, while the falling portion of the curve very closely follows the disappearance of the diprotonated diamine species as a function of increasing pH. This would indicate a stable complex forms between the HAs04*- and H2En2+ species. This leads to the possibility that the complex is the result of hydrogen bonding between the protonated nitrogen atoms of the diamine and the oxygen of the HA SO^^-. Studies of extraction of Mo04'-, a poor oxidizing agent, by the diamine also indicated a nonoxidative extraction process. In this case, the Mo was shown to be in the +6 state from studies of its ESCA spectra. The diamine was investigated in a qualitative way to determine if it would extract other common anions. It would not extract I-, Br-, IO3-, IO4-, or BrOs-. I t seems that the central atom in extractable anions must be metallic in nature resulting in more basic anions. Additionally, untreated silica gel would not extract any of the studied anions. Capacity Study. The capacity of the immobilized diamine was determined with respect to the dichromate and molybdate anions. The capacity study resulted in values of 0.54 and 0.67 mmol per gram of substrate for dichromate and molybdate, respectively. This value is in accordance with previously determined capacities of this material for various cations (7). Standard Curve and Recovery Studies. This study sought to duplicate the conditions under which an actual multielement trace analysis might be performed. Standard solutions containing 1,2, 5, and 10 pg/ml of both Se and As as Se04'- and A s O ~ ~were - used along with a synthetic test
0
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Flgure 3. Percent extraction as a function of pH for the immobilized 2-6020diamine
solution containing 9 pglml Se and 3 /*g/ml As. The pH of these solutions was approximately 5 . The pH value was not optimal for either species but did allow both anions to be extracted. The average recovery for five samples in the range 0.1 to 10 pg/ml was 101% for selenate and 98% for arsenate. Column Recovery Studies. This study was undertaken to determine the applicability of the general method using immobilized complexing agents for very dilute solutions (1 to 10 ng/ml range). The diamine immobilized on CPG-10 glass beads was used to extract 2-1. test solutions containing 1.50 ng/ml Se as NazSe04. The pump maintained the flow rate a t a 50 ml/min. The amount of selenium extracted from the test solution was determined from the standard curve. These recovery studies are summarized in Table I. Table I. Results of Column Recovery Study for Selenate Sample
1 2 AV
Present, nglml
Found, nglml
Recovery,
1.50
1.57
105
1.50 1.50
1.53 1.55
103
%
102
_-._____---------.-----------__----I-..--------.
ANALYTICAL CHEMISTRY, VOL. 48,
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As the results indicate, the recoveries are very good indeed, considering the very low concentration levels and the very rapid flow rate. These materials would appear to show considerable promise as easily prepared agents for the preconcentration of selected ions.
ACKNOWLEDGMENT We thank J. C. Carver for acquiring the ESCA data. LITERATURE CITED (1) T. Braun and A. B. Farag, Talanta, 19, 129 (1972). (2) T. Braun and A. E. Farag, Anal. Chim. Acta, 61, 265 (1972).
(3) W. J. Campbell, E. F. Spano, and T. E. Green, Anal. Chem., 38, 987 (1966). (4) W. J. Campbell, T. E. Green, and S.L. Law, Amer. Lab., 2, 26 (1970). (5) C. W. Blount. D. E. Leyden, T. L. Thomas, and S. Guill, Anal. Chem., 45, 1045 (1973). (6) K. F. Sugawara, H. H. Weetall. and D. G. Shucker. Anal. Chem.. 46, 469 (1974). (7) D. E. Leyden and G. H. Luttrell. Anal. Chem., 47, 1612 (1975). (8)E. B. Sandell, "Colorimetric Determlnatlons of Traces of Metals", 2 ed.. Interscience Publishers, New York, 1950, p 455.
RECEIVEDfor review July 16, 1975. Accepted September 29, 1975. This work was supported in part by Research Grant GP-38396X from the National Science Foundation.
Investigation of the Wet Oxidation Efficiencies of Perchloric Acid Mixtures for Various Organic Substances and the Identities of Residual Matter Gary D. Martinie and Alfred A. Schilt' Department of Chemistry, Northern Illinois University, DeKalb, Ill. 60 1 15
The effectiveness of nitric and perchloric acid mixtures for wet oxidizing 85 different organic substances was evaluated. After prolonged wet oxidation, each sample was screened for organic residue by proton magnetic resonance spectrometry, ultraviolet spectrophotometry, and carbon microanalysis. Most samples exhibited some ultraviolet absorption, and approximately one half retained measurable carbonaceous matter in solution. Residues were isolated and identified whenever possible. Compounds with N-methyi, Smethyl, Cmethyi, and pyridyl moieties proved the most resistant toward wet oxidation. Of the less easily oxidized substances studied, all but pyridine and 2,4,6-trimethyipyridine were totally destroyed by wet oxidation with sulfuric, nitric, and perchloric acids in a reasonable period of time. Vanadlum(V), cerium(Ill), and copper( II) with seienlted Hengar granules were found to exert catalytic influences on certain wet oxidations.
As a result of pioneering researches by G. F. Smith in this country and by E. Kahane in France, perchloric acid has found numerous applications and widespread use of the destruction of organic matter prior to elemental analysis. These and other studies have been extensively reviewed by Smith (1-3), Kahane ( 4 ) , Diehl and Smith (5, 6), and Gorsuch (7,8 ). The use of perchloric acid, alone and in combination with other acids, for the wet oxidation of organic matter affords a number of advantages over dry ashing. These include simplicity of apparatus, speed, less risk of loss of volatile elements, and considerable less tendency for retention by the vessel of sought-for elements. Disadvantages include the need for greater safety precautions and attention by the operator, difficulty of handling large samples, cost, and the possibility of increased blanks due to impurities in the acids used. An extensive investigation by Gorsuch (7) of eight different ashing methods led to the following conclusions regarding wet oxidations of organic matter with perchloric acid mixtures: 1) a mixture of nitric and perchloric acids is trouble-free and particularly suitable for recovery 70
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
of trace elements (except mercury, which forms a volatile compound in the presence of organic matter), and 2) a mixture of nitric, perchloric, and sulfuric acids is particularly effective for oxidation of obdurate materials without loss of trace metals (except for loss of lead in the presence of calcium by coprecipitation of PbS04 on CaS04). Although many studies have been devoted to evaluating recoveries of trace metals and certain nonmetals from various compositions, relatively little has been done to establish how completely various organic substances are destroyed by wet oxidation with perchloric acid and its mixtures with other acids. Studies to date have been essentially qualitative or indirect in this respect. If clear and colorless solutions result, or if total recovery of some trace element is achieved, it has been tacitly assumed that oxidation of the organic matter has been complete for practical purposes. However, such assumptions are not necessarily reliable in all cases. More conclusive and direct information as to the presence and identities of any residual matter retained by the acid solution is certainly desirable, especially if such matter might interfere in any subsequent measurement. The present study was undertaken to provide information of this kind. Various model compounds and common substances of a representative nature were subjected to wet oxidation by a mixture of nitric and perchloric acid (one of the safest and most commonly used wet ashing combination), and the final solutions were examined for residual organic content. Samples that proved particularly resistant toward oxidation were also subjected to more stringent wet oxidation conditions provided by the combination of sulfuric, nitric, and perchloric acids, with and without catalysts. The results provide information useful for analytical purposes as well as for possible synthetic approaches to certain compounds. A previous study (9) somewhat parallels the present one, to the extent that it attempted to determine inorganic compositions after evaporation to dryness of nitric and perchloric acid wet-oxidized residues. The results of that study are of interest in conjunction with preparing organic samples for spectrographic analysis.
