Anal. Chsm. 1983, 55, 1205-1210
1205
Determination of Arsenic and Phosphorus Compounds in Groundwater with Reduced Molybdenum Blue Robert E. Stauffer Water Chemistry Laboratow, University of Wisconsln, Madison, Wisconsin 53706
Chemlcal studies were made to optimize reduced molybdenum blue for arsenic and phosphorus determlnations In chemically diverse groundwaters. Phosphate strongly catalyzes the development of the arsenlc(V) molybdate complex; increasing H+ or H+:Mo retards complexation, but the effect Is greater for the potentially Interfering siilcomolybdate complex. The oxidation of As( 111) to As(V) by iodate partially lnhlbits the subsequent color development of sllica. Iodide has no effect. Fluoride ion partlally suppresses silica color development, probably because of competitive SI-F complexlng at low pH. The molybdate responses of As(V), P, and SO2 mixtures are not color addltlve. Optimum reaction condltlons are H+ = 0.30 N; H+:Mo = 88 for arsenic and phosphorus determinations In the presence of slllca. Silica Is an important anaiytlcai Interference for the lower H+ and H+:Mo conditions recommended by several popular procedures.
Because of the notable ease, stability, sensitivity, and precision of the technique, phosphate determinations in natural waters are now normally made by using “molybdenum blue” with ascorbic acid as the reducing agent. This technique dates from 1962 (1)but has undergone innumerable seemingly minor modifications accompanying its popularization. The original procedure was designed for ocean waters with low arsenate:phosphate ratios because it had been known for several decades that arsenate, As(V), also forms a blue complex with molybdate analogous to phosphomolybdate. For the reaction conditions originally specified by Murphy and Riley ([H+] = 0.4 N; [H+]/[Mo] = 74; room temperature), the low silica concentrations in seawater did not present a significant interference. One of the most popular variations of the original procedure (2) features lower acidity (0.227 N) and the same [H+]/ [Mol ratio during the color-developing stage. Furthermore, the popular “Strickland and Parsons” (2) scheme for fractionating the total phosphorus pool into its various organically bound and ”soluble-reactive”components uses the same “ascorbic acid” procedure for the final colorimetric step. Going and Eisenreich ( 3 )systematically studied the spectrophotometric responses of the “ascorbic acid” procedure in solutions free of silica and As(V). In particular, they showed that response was insensitive to change in both Sb and ascorbic acid (at the levels specified by 1and 2). More importantly, they showed how optimal “plateau responses” depended on H+ and H+:Mo. From this important study it became clear that satisfactory spectrophotometric results can be obtained for a range of H+ and H+:Mo conditions in the absence of potential analytical interferences. The reaction conditions employed by both (1)and (2) proved nearly optimal because the phosphate complexation reaction proceeded at a satisfactory rate at room temperature, and final plateau spectrophotometric response was insensitive to minor shift in H+ concentration. This insensitivity to final H+ is important because natural waters have varying buffering capacities and acid is often used to preserve samples for later analysis. Portmann and Riley (4) determined As at low levels in the ocean with molybdenum blue by first concentrating and
separating As (from P) by using cocrystallization with thionalide. Phosphorus determinations in the presence of As(V) require selectively reducing As(V) to As(II1) with thiosulfate (5). This procedure was subsequently improved (6) and then automated (7). Johnson and Pilson (8)determined all three species, As(V),As(III), and P, in the ocean, by oxidizing As(II1) with iodate, reducing As(V) with thiosulfate, and differencing the results. Arsenic species can now be separately determined at low levels (027‘ >072‘ 214 -2.8 424 -1.2 829 -1.7 131 (94.0%)b -2.2 181 (95.2%)b -2.2 471 (98.8%)b -0.6 519 (98.6%)b -1.0 a Responses are absorbances x103 at 865 nm, 5 cm lightpath; corrected for blank (003 units). The percentage is 100 times the ratio of As response (mixed As:P standards) over fully developed As response (As standard). The P response in the mixed standard is assumed equal to the P response for the separate P standards. Inequalities indicate plateau response was not attained during duration of the experiment (17 h). Replicated positions in design indicate very high precision. would be expected to be faster than in the present work. The late color development may have resulted from an unsuspected silicon interference (see below). Varying H+ from 0.30 to 0.35 had no effect on plateau As(V) response. However, a slight (1.4%) decrease occurred accompanying H+increases from 0.25 to 0.40 N (Table I). In a separate experiment (see below) it was found that reducing H+ from 0.25 to 0.20, while holding H+:Mo constant a t 73.5, caused a 0.8% increase in As(V) response. Table I shows that increasing H+ and H+:Mo has a more important, but still modest, effect on P response. The As(V) plateau response is notably insensitive to H+ and H+:Mo throughout the range 0.20-0.40 N and 73.5-118, respectively. Mixed As:P standards have lower molybdate absorbances than the sum of absorbances for the two elements in separate solution. This effect is significantly less at [H+] = 0.40 N than at [H+] = 0.25 N (Table I). Johnson and Pilson (8) also observed this phenomenon but did not investigate the effect of changing H+. Experimental evidence suggests a discrete As:P mixed complex with Mo with a reduced molar absorptivity at 865 nm. Plateau arsenomolybdate absorbance deviates very slightly from Beer’s law; the departures from linearity are negative for all values of H+tested (Table I). The slight curvature is detectable because of the very small “pure” analytical error at a fixed point in the As(V)-P domain (cf. Table I). Thus, the pure analytical C.V. is only 0.3% a t the 500 mg m-3 As(V) concentration level. The slight response curuature (model lack of fit for Beer’s law) is important in evaluating spike recovery tests (11). Experiment 2. By comparing the spectrophotometric responses of autoclaved As(II1) vs. As(V) standards, it was shown that persulfate completely oxidized the As(II1) during autoclave treatment. Autoclave treatments with or without 100 mg L-l C1- resulted in slight but consistent (1.5%) increases in final colorimetric response for both As(V) and P standards. The slight increases (despite capping with foil) may have been due to vapor losses during autoclaving and cooling (conservation of H 2 0 assumed in calculations). Fluoride at the 20 mg L-l raised As responses by an additional 2.5-4%. Thus, no evidence was found of As losses by volatilization in the presence of either C1- or F-. Experiment 3. The less rigorous iodate oxidation conditions resulted in 084‘ 215 425 835
Table 11. Arsenic-Silica-Fluoride-Molybdate Interactions: Effects of Time and Two Ht Levels
(As(V), SiO,, F, H+)‘
nj
responses b t, = t, = 150 min 855 rnin 423.5 425.5 429.0 427.5 427 428 528 858 662 1230 430 433 505.0 764.0 1077.5 603.5 962.0 562.5 006.0 004.5 005.0 004.5 005.0 004.0 513.5 124.5 900.0 260.0 750.0 200.5 615.5 154.0 259.5 865.0 1033.5 345.0 589.0 153.5 402.8 91.0
(300, 0, 0, 0.25) 2 (300, 0, 0, 0.20) 2 1 (300, 0, 10, 0.25) 1 (300, 100, 0, 0.25) 1 (300, 200, 0, 0.25) 1 (300, 0, 20, 0.25) 4 (300, 100, 10, 0.25) 2 (300, 200, 10, 0.25) (300, 200, 20, 0.25) 2 3 (0, 0, 0, 0.25) 2 (0, 0, 10, 0.25) 2 (0, 0, 20, 0.25) 2 (0, 100, 0, 0.25) 2 (0, 200, 0, 0.25) 2 (0, 200, 10, 0.25) 2 (0, 200, 20, 0.25) 2 (0, 200, 20,0.20) 2 (0, 200,10, 0.20) 2 (0, 100,10, 0.20) 4 (0, 100, 10, 0.25) a Units are: mg m-3;mg L-I; mg L-l; N. H+:Mo= Means of n j replicates; pooled estimates of 73.5. variance are the following: (1)points with SiO, = O:uz = 0.5; (2) points with 100 SiO, at t , : G 2= 7 ; (3) points with 100 SiO, a t t , : G 2 = 21. increased monotonically with increasing natural water dissolved solids content, or equivalently, with the fraction of hot spring water in the original sample (20). The stronger iodate oxidizing conditions resulted in 100% recoveries of As(II1) as As(V) in distilled-deionized water standards. The less rigorous conditions were also adequate for seawater (8). Detailed studies of the catalysis were not made. Experiment 4. The statistical resolution of “main” factor effects and binary ”interactions” is discussed elsewhere (18, 21). In this experiment B, F-, and I- had negligible main effects. Nor were the B*Si02 or I-*SiO, interactions statistically significant. The important effects were the SiO, main effect, the H+*SiOzinteraction, the As(V)*SiO2 interaction, and the Si02*F- interaction (Table 11). Whereas As(V) at the 300 mg m-3 level developed >99% of its plateau response within 150 min at [H+] = 0.25 N, SiOz continued to develop color throughout the 911 min of the
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
-
Table 111. Color Nonadditivity of Silica and Arsenic Reduced Complexes with Molybdate SO,-As(V) nonadditivity responsesa effectsb
(As(V), SiO,, F,H+)
-~
(0, 100, 10, 0.25) (0, 100, 0, 0.25) (0, 200, 20, 0.25) (0, 200, 10, 0.25) (0, 200, 0, 0.25)
91 125 154 201 260
A,/
A,
A,
s,
403 514 616 750 900
-12 -25 -20 -24 -25
-68
0.13 0.20 0.13 0.12 0.10
-87 -101 -102
absorbancea (acidity ( N ) and molar ratio H+:Mo)
ratios
s,
-88
Table IV. Silicomolybdate Color Development as a Function of H', H+:Mo, and F Concentrations
a,/ 8, 0.17 0.17 0.14 0.13 0.11
a S, and S, are 865 nm responses at t , = 150 min and t , = 855 min (without As(V)). Reductions in response for mixed As:SiO, standards (SiO,, F,and H*levels as shown in col 1, 300 mg m-3 As(V), vs. separate Standards).
experiment. However, the rate of color development slowed substantially during the second time interval. At tl the 200 mg L-l SiOz level had slightly more than twice the response of the 100 mg L-l level; at t 2the response ratio was 1.75. Reducing H+ from 0.25 to 0.20 N while holding H+:Mo = 73.5 caused a dramatic and consistent increase in silica color development. Time tl responses at the lower acidity were 169%, 172%,and 169'%of the comparable 0.25 N responses. At t,, these percentages had declined to 141%, 139%, and 147%, respectively (Table 11). The As*Si02 interaction is negative, with a magnitude typically 10-17% of the SiO, main effect (Table 111). The ratio of the interaction to the main effect increases only slightly between tl andl tz,and, in the absence of F, decreases with increasing Si02 concentration. This latter trend accompanies an increasing Si:As(V) ratio and suggests that Si and As(V) are either slowly forming a joint complex with a reduced 865 nm absorptivity or that silica is decreasing the activity of the Mo available for As(V) complexation, hence indirectly shifting the arsenomolybdate response. The trinary interaction among As(V), €',and Mo proceeds rapidly (Figures 1 and 2) and is almost certainly the result of a joint complex. The interaction between SiO, and F is large and negative (Table 11). Both the present and an earlier experiment showed evidence of a small ( 1 4 % ) positive As(V)*F interaction as registered in colorimetiric response. The interaction of F with SiO, probably results from competitive complexing of Si by F at acid pH values (19). The SiO,*F interaction increases with increasing t , and ]increasingconcentrations of both SiOz and F. At [H"] = 0.20, increasing F concentration (SiO, held fixed) had a larger effect on response than at [H+] = 0.25 N. Experiment 5. Experiment 4 showed that decreasing [W+] from 0.25 to 0.20 N while holding H+:Mo at 73.5 (hence symmetrically bracketing the H+ level recommended by ref 2 at the same H+:Mo ratio) caused a dramatic increase in the silicomolybdate response. Experiment 5 was then designed to test the practicality of operating at higher acidities and/or higher H+:Mo ratios to suppress silica as a nuisance interference. Increasing acidity (whole holding H+:Mo constant) decreases silica response. Columns 2-4 of Table IV show that the mean response ratios for 0.30 vs. 0.35 N and 0.25 vs. 0.30 N are 1.57 and 1.69, respectively, continuing a trend previously described for the range, 0.20 I[H+] 5 0.25 N. Increasing H+, while holding Mo constant, resulted in a much greater diminution of response (Table IV). Thus, changing from [H+] = 0.25 N, H+:Mo = 73.5 to [H+] = 0.30 N, H+:Mo = 88 resulted in approximately a 5-fold reduction in silica response at 420 min. The same change in H+ and H+:Mo resulted in ~ 7 5 % increase in the time required for As(V) complexing by molybdate. Thus the experimental strategy developed by (11)for total As determinations tends
(0.25; (0.30; (0.35; 73.5) 73.5) 73.5) 340 295 262 186 68 1 628 551 433
207 178 154 119 392 352 309 251
131 111 100 81 240 214 194 158
(0.30; (0.35; 88.2) 103) 66 59 59 55 12gC 115 93d 71
39 44 46 35 38 35 32 26
a Responses are l o 3 absorbances (815 nm) after 420 Replicates = 66, 65. min (corrected for blank). Replicates = 92, 93. Replicates = 131, 127.
to separate As response from its silica interference. A further increase in H+:Mo to 103 was less efficacious in further reducing the silica response (Table IV). The rows of Table IV show that increasing F- level results in a pronounced reduction in Si02 response, providing the H+:Mo ratio is held at 73.5. The last two columns indicate that this relationship breaks down as the H+:Mo ratio is increased to 103, a departure which is evidenced much more strongly at the 100 mg of SiOz level. After 420 rnin of color development, and in the absence of F-,silica response is nearly proportional to silica concentration. The preaddition of iodate, as in the procedure to oxidize As(II1) before the color development step, markedly reduces silica response, the effect increasing with iodate level. In the presence of 0.40 mM iodate, 100 and 200 mg L-l Si02solutions featured reductions of 46 and 53%. At the 0.8 mM iodate level, silica responses were reduced 64 and 65%. The use of iodate is an expedient technique for oxidizing As(II1) and making it reactive toward molybdate. The mechanism of the inhibitory effect of iodate on silica complexing remains enigmatic. Because each molecule of ascorbic acid transfers two electrons in its reduction of I(V) to I-, 19% of the added ascorbic acid is consumed by the 0.40 mM iodate. The reaction proceeds through an Iointermediate in 6 1 s. However, as shown by experiment 4,the I- created has no effect on the silica complexing. Because the Mo:Sb and Mo:ascorbic acid ratios were kept invariant throughout the entire series of experiments, response shifts attributed to changes in H+:Mo may have resulted in part from parallel changes in H+:Sb and H+:ascorbic acid. Based on (3),plateau responses for As(V) and mixtures of As(V) with P are likely to be independent of both Sb and ascorbic acid at the levels used here, and adopted earlier ( 1 , 2 ) . However, the concentrations of Sb and ascorbic acid may influence the kinetics of the two slower developing (As(V) and SiOz) molybdate complexes. Modeling the Silica Interference for As(V) Analyses. The 50 nm difference in the spectral maxima for Si-molybdate and As(V)-molybdate can be exploited for estimating As concentration in the presence of silica. Equations 1 and 2
Yl = X I +
x 2
Yz = 0.865X1 + 1.416X2
(1)
(2) model spectrophotometric responses by ignoring the As*SiOz interaction, where Yl and Y , are the observed responses at 865 and 815 nm corrected for the blank, and Xl and X 2 are the 865-nm responses due to As(V) and Si02 (unknown). The coefficients in eq 2 were derived from the mean 865 vs. 815 nm response ratios for As-free silica standards (R = 0.708) and silica-free As(V) standards (R = 1.156). Solve eq 1 and 2 for
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
X1 and X,;_call these initial estimates 2, and X,. An improved estimate, X1 2 1=
Y1 - 0.8522
(3)
accounts for the_negative As*SiOz interaction. From Table I11 the bias in XI is unlikely to exceed o.05xz.
CONCLUSIONS Applications: Arsenic. The unbiased application of reduced molybdenum blue to As and P determinations in natural waters requires knowledge of As(V) and P interactions with molybdate and an assessment of the potential of SiOz to interfere analytically. The silica interference is minimized by using iodate to oxidize As(II1) and/or suppress SiOz complexing (11,this work), operating at [H+] = 0.30-0.35 N and H+:Mo = 88-103, and applying kinetic information to minimize color development time, while still ensuring that As and/or P have reached plateau absorbance at all concentrations being analyzed (cf. Figures 1 and 2). In the absence of P, and for an As(V) concentration (during color development) of 30 to 60 mg m-3, [H+] and H+:Mo should be kept at 0.30 N and 88, respectively, in order to ensure complete As(V) color development within 6 h at 23 "C. Table IV shows that silica present at 100 mg L-l will develop the same 865 nm absorbance as 30 mg-3 As(V) during this same development period. Spiking with iodate to achieve a final concentration (50.0 mL) of 0.8 mM should reduce this silica interference by 65%. Thus, in the presence of only 10 mg L-I SiOz, a level representative of many lake and stream waters, the silica color component can be kept below -3% for As = 30 mg m-3. For natural waters with As(V) < 30 mg m-3 and SiOz > 20 mg L-l, I do not recommend the molybdate procedure. Fortunately, many geothermal waters where As is of special interest have much more satisfactory As:Si ratios (11, 12). In natural waters containing even low P concentrations, the P greatly accelerates the development of the As(V) molybdate complex and so reduces the potential for a silica interference. An 8-fold rate enhancement was found for a 60 mg m-3 As(V) solution in the presence of 15 mg m-3 P, both at [H+] = 0.25 N, H+:Mo = 73.5 and a t [H+] = 0.40 N, H+:Mo = 118. Johnson and Pilson's (8) investigation of As(V) and P joint interactions (with molybdate) should be extended to [H+] = 0 . 3 0 . 3 5 N and H+:Mo = 88-103, a domain where the silica interference is much more effectively suppressed. Table I indicates that color nonadditivity decreases as H+and H+:Mo increase. Because of geochemical interest in playa lakes and geothermal brines, the kinetic effects of higher ionic strength would also have to be investigated for certain applications. This precaution applies to many of the lakes and groundwaters in the Rift Valley region of East Africa (15, 16). Applications: Phosphorus. Because phosphate develops its molybdate complex so rapidly, avoiding a silica interference is easier than in the case of As(V). Because of the higher called-for acidity, the original ascorbic acid procedure (1) circumvents the interference (1,this work), unless the Si:P ratio of the sample is extraordinarily high (e.g., certain geothermal waters, cf. ref 11-13), and the analyst is careless in allowing excessively long color development times. Because of the dramatic acceleration of the interference as acidity drops, a silica bias is possible using a popular procedural variant (2). The bias can both be avoided, and analytical efficiency simultaneously increased, by applying the Strickland and Parsons (2) procedure to samples that were acidified in the field. Field acidification to pH 1.0 is frequently a conventient technique for preventing Fe and Mn precipitation (accompanying oxidation or coagulation) and biological uptake of P during sample transit and storage. If 25 mL of this acidified sample is reacted with 5 mL of saturated persulfate in an autoclave (without additional acid), subsequently diluted
1209
with 15 mL of ddw, and finally reacted with 5 mL of the Strickland and Parsons (2)"mixed reagent", the final volume is 50.0 mL and the optimal reaction conditions, [H+] = 0.30, H+:Mo = 88, have been obtained. To increase analytical sensitivity, 40 mL of sample can be used instead and the acid spike corresponding reduced to ensure the sought-for ratio. Sulfuric acid (10 N) is particularly desirable for field acidification (unless Ca determinations are also made on the sample) because HC1 is volatile at the higher temperatures in the autoclave, and HN03 can cause later color instabilities using molybdenum blue (Stauffer, unpublished). Naturally, if samples are acidified in the field, only the total-P, and total-soluble-P fractions can be determined on filtered and unfiltered sample splits. For most uncontaminated natural waters the P:As ratio is so high that As(V) interferences can be ignored in P determinations. The situation is reversed in many geothermal waters, contaminated groundwaters, and lake waters that have been treated with sodium arsenite for weed control in the past (Stauffer, unpublished; Wisconsin Department of Natural Resources). Automated Methods. Because the development of the silica interference is kinetically controlled, it is obvious that increasing the reaction temperature may differentially affect the various molybdate complexes. Thus, the use of "automated" methods for environmental phosphorus determinations invites very serious silica-related biases for the unwary analyst. Chan and Riley (22)reported a 1.8 mg m-3 P04-Pbias due to SiOz at the 8.6 mg L-l SiO, level. Their automated procedure featured H+:Mo = 73.6 (the usual value) and a 70 "C heat bath. Because many streams and groundwaters contain higher SiOz concentrations (particularly in volcanic rocks) and equal or lower phosphate concentrations than specified above, the automated procedure introduces an unacceptable bias. The automated procedure of Campbell and Thomas (23)introduces a silica-related bias of 7.3 mg m-3 P04-Pfor each mg L-' SiOz in the sample, despite an H+:Mo ratio of 70.6 and a final (color development) acidity of 0.47 N. The high temperature of the heating bath (95 "C) or long residence time may have been responsible. Downes' (24) automated phosphorus procedure eliminates the SiOz interference by operating at moderate temperature (70 "C)in the presence of a very high H+:Mo ratio (206). However, the high H+:Mo ratio is near the edge of the plateau response surface for phosphomolybdate as a function of H+and H+:Mo ( 3 ) . The procedure is thus poorly poised. The present paper shows both why Downes' procedure acts to suppress the SiO, interference, and why such a procedure could not be used for arsenic determinations. Biased Environmental Data. Unfortunately, it appears that many phosphorus determinations of natural waters throughout the world have been seriously, and sometimes needlessly, compromised because of the generally unappreciated analytical subtleties of the reduced molybdenum blue procedure. This is particularly serious because it has been strongly suspected for decades (25)that phosphorus is the key element regulating the trophic development of most natural waters. Hence, accurate determinations of this element are essential. Furthermore, the reduced-molybdate procedure (earlier using Sn2+instead of ascorbic acid) dates back more than half a century (26,27). Thus, total and soluble P concentrations reported earlier (26,27)for culturally unmodified lakes in the Northern Highlands of Wisconsin are two to five times higher than recently determined values (Stauffer, unpublished). Other indicators of lake tropic state (hence P) in that region (secchi transparencies, areal hypolimnetic oxygen deficits) are unchanged from the early years (Stauffer, unpublished). Most of the drainage lakes in that region have moderately high SiOz concentrations (often >10 mg L-') and
Anal. Chem. 1983, 55, 1210-1215
1210
Si:P ratios >1000. Thus, special care must be exercised to avoid a silica bias. More recently, P determinations by using popular "Hach Kits" have yielded soluble phosphate estimates an order of magnitude higher than accurate total-P estimates in the same uncontaminated cold spring water (Stauffer, unpublished). In the total-P determination a silica bias failed to develop because of retained acidity from the persulfate digestion step. This carried over into the colorimetric step and raised the H+:Mo ratio; the lower ratio in the soluble-P determination allowed the development of a silica bias in that silica-rich groundwater. Elsewhere, P determinations for hot spring drainage waters in Yellowstone National Park published prior to 1979 were biased by up to 3 orders of magnitude (11,12). Phosphate-P concentrations in silica-rich lake water and groundwaters of East Africa have been reported (15,16,28,29)ranging up to 50 mg L-l P. However, some of these estimates have not been substantiated by radio-tracer methods (30). Until further checked for a possible silica and/or As bias, these reported P concentrations in volcanic regions should be treated with caution.
(4) Portmann, J. E.; Rlley, J. P. Anal. Chim. Acta 1964, 31, 509-519. (5) Van Schouwenburg, J. S.; Walinga, I. Anal. Chim. Acta 1967, 37, 271-274. (6) Johnson, D. L. Environ. Sci. Technol. 1971, 5,411-414. (7) Goulden, P. D.; Brooksbank, P. Llmnol. Oceanogr. 1974, 19, 705-707. (8) Johnson, D. L.; Pllson, M. E. Q. Anal. Chim. Acta 1972, 58,289-299. (9) Andreae, M. 0. Anal. Chem. 1977, 49,820-823. (10) Grabinski, A. A. Anal. Chem. 1981, 53,966-968. (11) Stauffer, R. E. Environ. Sci. Technol. 1980, 14,1475-1481. (12) Stauffer, R. E.: Jenne, E. A.: Ball, J. W. Geol. Surv. Prof. Pap. ( U S . ) 1960 NO. 1044-F,20 p. (13) Fournler, R. 0.; Rowe, J. J. Am. J . Sci. 1966, 264,685-697. (14) Holland, H. D. "The Chemistry of the Atmosphere and Oceans"; Wlley: New York, 1978. (15) Kllham, P.; Hecky, R. E. Limnol. Oceanogr. 1973, 18,932-946. (16) Jones, B. F.; Eugster, H. P.; Rettlg, S. L. Geochim. Cosmochim. Acta 1977, 41,53-72. (17) Menzel, D. W.; Corwln, N. Limnol. Oceanogr. 1965, 10, 280-282. (18) Box, G. E. P.; Wllson, K. B. J . Roy. Stat. SOC. Ser. E 1951, 13, 1-45. (19) Slllen, L. G.; Martel, A. "Stability Constants of Metal-Ion Complexes", 2nd ed.; Chemical Society: London, 1964; Spec. Publ. 17. (20) Fournler, R. 0.: White, D. E.; Truesdell, A. H. Proc. 2nd U.N. Geothermal Symposium, San Franclsco, CA, 1975, pp 731-739. (21) Searle, S.R. "Linear Models"; Wiley: New York, 1971. (22) Chan, K. M.; Riley, J. P. Deep Sea Res. 1966, 13,467-471. (23) Campbell, F. R.; Thomas, L. T. Environ. Sci. Technol. 1970, 4 , 602-604. (24) Downes, M. T. Water Res. 1978, 12,743-745. (25) Hutchinson, G. E. "A treatise on Llmnology, Vol. 1"; Wiley: New York,
ACKNOWLEDGMENT Both James Ball and David Johnson made helpful criticisms of the draft manuscript. This paper is dedicated to the statistician G. E. P. Box. Registry No. Arsenic, 7440-38-2;molybdenum blue, 6677143-5; silica, 7631-86-9; water, 7732-18-5.
(26) Juday, C.; Blrge, E. A,; Kemmerer, 0. I. Trans. Wis. Acad. Scl. 1927, 23, 233-248. (27) Juday, C.; Blrge, E. A. Trans. Wis. Acad. Sci. 1931, 26,353-382. (28) Talling, J. F.; Talling, I. B. Int. Rev. Gesamten Hydrobioi. 1965, 50, 421-463. (29) Melack, J. M.; Kilham, P. Limnol. Oceanogr. 1974, 19,743-755. (30) Peters, R. H.; MacIntyre, S. Oecologia 1976, 25,313-319.
LITERATURE CITED (1) Murphy, J.; Rlley, J. 1'. Anal. Chim. Acta 1962, 27,31-36. (2) Strickland, J. D. H.; F'arsons, T. R. A Practical Handbook of Seawater Analysis; Bull., Fish Res. Board. Can. 1968, Spec. Publ. No. 167. (3) Going, J. E.; Eisenreich, S. J. Anal. Chim. Acta 1974, 70,95-106.
.--. .
lQS7
RECEIVED for review January 20, 1983. Accepted April 15, 1983. Partial financial support was provided both by the U.S. Geological Survey and by the U.S. EPA Research Grant R805281010.
Cation-Exchange Concentration of Basic Organic Compounds from Aqueous Solution J. R. Kacrvlnsky, Jr., Koichl Saitoh, and James S. Fritz" Ames Laboratory and D@partmentof Chemlstry, Iowa State lJnlversl& Ames, Iowa 5001 1
A macroreticular poly(styrene-dlvlnyibenzene) catlon-exchange resin Is prepared. An aqueous sample is passed through a column of thls resin In the H+ form and organlc bases are taken up as catlons. Washlngs wlth methanol and ethyl ether remove sorbed neutral and acldlc compounds. Ammonla gas Is introduced into the column prlor to elutlon of the bask organics with either methanol or ethyl ether containing ammonla. The eluate is concentrated by evaporation, and the concentrated sample Is analyzed by gas chromatography. Over 50 organlc bases are recovered from water at the 1 ppm and 50 ppb levels. Recoverles of over 85% are achieved for most of the compounds studled wlth at least one of the eluents. The resin procedure shows Improved recoveries and reproduclbllity over a simple ether extraction procedure. Real samples of river water, shale process water, and supernatant from an agricultural chemlcal disposal plt are analyzed for basic organlc materlal by using the resin procedure.
The analysis of complex aqueous mixtures of trace organic compounds usually involves preliminary isolation and concentration steps (1-6). Any preliminary separation should be selective, while minimizing analyte loss. This separation may consist, entirely or in part, of a division of the compounds of interest into one of several general classes (3, 4). This classification is usually based on a particular compound characteristic, such as hydrophobicity or acidity. A preconcentration step is often necessary to permit direct analysis by conventional methods without appreciable loss of sensitivity (5, 6). The transfer of the analytes from aqueous solution to an organic solvent also facilitates the use of several common analytical techniques, such as gas chromatography. An ideal preparative method for organics in aqueous solution would combine these processes (isolation, concentration, transfer) into a single step. Many techniques have been developed to simultaneously fractionate and concentrate organics from aqueous solution.
0003-2700/83/0355-1210$01.50/00 1983 American Chemlcal Soclety
