Anal. Chem. 1982, 5 4 , 59-62
59
Salt Effects on the Surfactant-Sensitized Spectrophotometric Determination of Beryllium with Chrome Azurol S John H. Callahan anld Kelsey D. Cook" School of Chemical Scienroes, University of lllinols, 44 Roger Adems Laboratory, Box 49, 1209 West California, Urbana, Illinois 61801
The effects of added ellectrolytes on the spectrophotometrlc determlnatlon of berylllim by absorbance of Its complex wlth Chrome Arurol S (CA!;) In the presence of aqueous hexadecyltrlmethylammonluim bromide (CTAB) were explored. The molar absorptlvlty Increased wlth the addition of salt, In the order no salt C Na,,804 C NaCl < NaBr 5 NaNO,. Variation of the cation (LI', Na', K', NH4") of added chloride was red rhltted 12-14 nm for sans produced no effect. all salts. The exact magnitude of thls shilft depended on beryllium concentration. Increaslng the concentratlon of the added electrolyte resulted In lncreaslng absorbances whlch leveled off at salt concentrations greater than 0.2 M. Thus, addltion of salts at hlgh concentrations swamped out thelr Interferences and loweired the detectlori llmlt for the determlnatlon of beryllium by the surfactant-sensltlzed technique to 2.0 X lo-' M. The r~esuItssuggest that lonsxchange processes at the micellar rrurface may be lnlvolved In the mechanism of the salt effect,,
The sensitivity of spectrophotometric determinations of various metal ions by absorption of thieir complexes with metallochromic dyes can sometimes be markedly improved by the addition of surfactants. (For general discussions of surfactant and micellai*chemistry, see rlef 1and 2.) One- to tenfold increases in absorptivity ( E ) and 50-100 nm red shifts in the wavelength of maximum absorbance (Amm) have frequently been observed; as much as a 61-fold increase in E (3) and 150 nm red shift ( 4 ) have been reported. Hinze ( 5 ) recently reviewed applications of surfactants in analysis, citing over 40 different sensiti:zed spectrophotometric systems in 130 publications. Despite this extensive utilization, the mechanism of these sensitizations has not belen determined. In fact, careful study of the impact of variation of solution parameters known to affect the solution chemistry of surfactanits (e.g., pH and ionic strength) has not been reported. The beryllium-Chroime Azurol S (Be-CAS) system (Table I) is typical of these applications and their level of characterization. For the Be-CAS complex at pH 6.7 in the absence of surfactant, A- = 540 1 mand E = 1.5 X 104 (6). Under these conditions, there is considerable overlap with free dye absorbance (Amm = 430 nm). The overlap can be relieved and the determination sensitized by a number of surfactants, although sensitization may be accompanied by increased matrix effects, as reported by Strelow and Weinert (7). In fact, these authors were among the very few reporters of sensitization to include aome considerat ion of possible "salt effects". They observed an unexplained dependence of, , X of the Be-CAS complex sensitized by benzyldimethylhexndecylammonium chloridle on total berylllium concentration. They also observed that deletion of ammonium chloride from this system resulted in a 15% decrease iin the maximum absorbance. Although they attributed thiw desensitization to a salt effect, closer examination of their raystem components reveals that NH4+ was, acting as a proton source in their
hexamethylenetetramine (HMT) buffer system; thus, it is unclear whether a salt (ionic strength), pH, or other effect was actually operative. In subsequent (unpublished) studies (8), Strelow has observed that sodium chloride had a sensitization effect similar to that of ammonium chloride in these systems, suggesting that action of the ammonium ion as a proton source makes a t most a minor contribution to the sensitization. A similar observation was made for this system in preliminary studies in our llaboratory (9). Although similarly complex behavior has only occasionally been reported for other systems, this is almost certainly attributable more to the depth of investigation than to the nature of the chemistry involved. As a result, the systems are widely used for quantitative determinations in a context in which important matrix effects may be hidden or disregarded. Ultimately, the analytical utility of the methods depends on the ability to control or compensate for these effects, which in turn requires better understanding of the sensitization mechanism(s). Such understanding is a major goal of research in this laboratory. This report summarizes results from the first stages of this program, an examination of the effects of solution parameters on surfactant-sensitized absorptiometric determinations. In particular, this report deals with the effects of added electrolytes on the determination of Be2+by CAS in aqueous hexadecyltrimethylammonium bromide (CTAB). To at least partially separate pH and ionic strength effects, a relatively low ionic strength buffer (HMT/HCl) has been employed. It must be emphasized that this system has been examined as a representative of the family of surfactantsensitized methods rather than as a result of any special interest in this particular example. Other studies presently under way involve variation of several solution parameters in this and other systems. While it is not yet possible to generalize the quantitative or even qualitative aspects of these matrix effects, there is no doubt that the effects themselves are quite general. This report is intended to alert the user of these methods of the existence and potential magnitude of these effects by illustrating one example in detail. In this instance, the information obtained has also provided some insight into the nature of the interference and suggested a means of controlling the effect. Subsequent studies will attempt to provide more details of the sensitization and interference mechanisms.
EXPERIMENTAL SECTION Apparatus. A GCA McPherson Model 707 spectrophotometer was used for all absorbance measurements. One-centimeter matched quartz cells were used. Readings were made against a distilled deionized water blank (none of the salts, surfactants, or buffers exhibited any absorbance in the 400-700 nm region). pH measurements were made with an Orion Model 701 pH meter with a Model 90-01-00 pH (glass) electrode. All glassware was cleaned with 10% "OB and triply rinsed with distilled deionized water. Reagents. Distilled deionized water was used for making all solutions. The Be2+used in this study was obtained from a 1.104 X 10-1 M BeClz atomic absorption standard (Aldrich). A 1.104 X M stock solution was prepared by appropriate dilution of the AA standard.
0003-2700/82/0354-0059$01.25/00 1981 American Cheimicai Society
60
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
Table I. Absorbance of Unsensitized and Sensitized Be-CAS Complexes PH 4.8 6.7
a
surfactant none none none poly(viny1 alcohol)
Amax,
nm
ref
€
568 2.1 x 104 6 54 0 1.5 x 104 6 10.0 490 2.5 X l o 4 6 6.7 615 6 5.2 x 104 benzyldimethylhexadecylammonium chloride 6.65 6 10-61 5' 9 . o x 104 7 cetylpyridinium bromide 5.0 9.6 x 104 605 10 4.5 polyoxyethylenedodecylamine 605 9.0 x 104 11 benzyldimethyltetradecylammonium chloride 5.1 610 9.9 x 104 12 cetyltrimethylammonium chloride 5.46 619 9.1 x 104 13 benzyldimethyltetradecylammonium chloride 5.1 1.1 x 105 610 14 hm, is reportedly sensitive to total Be concentration in the presence of BDHA, although dye is present in excess.
Table 11. Effect of Sodium Salts on Spectroscopy of the Be-CAS-CTAB Systema added electrolyte
€608
'608
5.50 x 104 1700 7 . 0 2 ~104 300 7.32 X l o 4 500 7.12 x 104 500 6.64 x 104 600 a Be, 2.2 x 10-6-2.2 x l o - * M; CAS, 1 x M; CTAB, 1 x Measured for a Be concentration of 2.2 X 10.' M. none 0.1 M NaCl 0.1 M NaBr 0.1 MNaNO, 0.1 M Na,S04
CAS was obtained from Eastman (practical grade) and purified by using a modification of the procedure described by Langmyhr and Klausen (15). Tetrabasic dye was precipitated from aqueous solution with 9 M HzS04, followed by suction filtration and washing with 6 M HCl. The dye was redissolved in distilled deionized water and then precipitated and washed a second time. Finally, the precipitate was vacuum dried over PzOsfor several days. The purified CAS was then used to make a 1 X lo* M stock solution. Dissolution required addition of NaOH (Mallenkrodt) M in NaOH. to make the final concentration 3 X Hexadecyltrimethylammonium bromide (CTAB)was obtained from Sigma Chemical Co. Prior to use it was recrystallized three times from ethanol and diethyl ether. Injection of an ethanolic solution of CTAB and sodium methoxide (Fisher) into the injection port (250 "C) of a Varian 1400A gas chromatograph (OV-101 column) was used to check for homologue surfactant impurities (16). Coinjection of 1-alkene standards (Sigma) confirmed the presence of the expected major Hoffman degradation product, 1-hexadecene. Neither 1-tetradecene nor 1-octadecene was present in either unpurified or purified surfactant (as would be expected if homologue impurities were present). An unknown impurity of longer retention time than 1-octadecene was present in equal proportion prior to and following recrystallization. Elemental analysis of the unpurified and purified surfactant yielded the following results: sought, C, 62.7%; H, 11.5%; Br, 22.0%; N, 3.8%; obtained for Sigma CTAB, C, 62.7%; H, 11.4%; Br, 22.2%; N, 3.9%; obtained for purified CTAB, C, 62.7%; H, 11.4%; Br, 22.0%; N, 3.9%. A 1 X lo-* M stock solution was prepared from purified CTAB. The buffer system used in this study was prepared from Aldrich hexamethylenetetramine (HMT) and Baker HC1. The system was prepared by dissolving 1mol of HMT in 800 mL of H20 and adjusting the solution to pH 6.7 with 17 mL of 1 M HCI. The solution was then diluted to 1L to give a 1M HMT/0.017 M HC1 buffer system. All other salts used in this study were reagent grade (Mallenkrodt) and were used as received. Salts were chosen whose anions and cations do not interact strongly with Be2+or CAS, respectively, in solution (i.e., no interfering complexes were formed). Procedure. Because of the sensitivity of color development in surfactant-sensitized systems to order of mixing (kinetic) effects (17), all solutions were prepared for spectrophotometric analysis by pipetting the stock solutions into a 50-mL volumetric flask in the following order: Be2+,CAS, buffer, CTAB, and salt (if present). The solutions were then diluted to volume with distilled deionized HzO and allowed to "develop" for 1h prior to analysis. Final solution concentrations were as follows: Be2+,2.2 X lo-'
104
104 104 104 104
2600 1900 1600 1700
608 622 622 622 622
1800
M;pH 6.7 (1x 10'' R.I HMT/1.7 x
O156o1
M HCl).
'' '' 600
nm
Amam
'622
€622
4.70 x 7.94 x 8.37 x 8.41 x 7.65 x
1
640
WAVELENGTH, nrn
1
'
Figure 1. Spectra of Be-CAS complex In CTAB with (A) 0.1 M NaNO,, (B) 0.1 M NaBr, (C) 0.1 M NaCI, (D) 0.1 M Na2S0,, and (E) no salt; Be, 2.2 x 10-5 M; CAS, 1 x 104 M; CTAB, 1 x 10-3 M; p~ 6.7 (1 x 10-1 M HCI). M HMT/1.7 X
to 2.2 X M; CAS, 1 X lo4 M; buffer, 0.1 M HMT/0.0017 M HC1; CTAB, 1 X M; salts, 0-0.4 M.
RESULTS Control Experiment: Effect of Salts on Unsensitized System. Preliminary studies of the Be-CAS system at pH 6.7 indicated that added salts (up to 0.4 M NaCl, NaBr, NaN03, and Na2S04)do not affect the spectroscopy of the unsensitized system, except for slight decreases in all absorbances. These decreases can be attributed to decreases in activities which occur in solutions of relatively high ionic strength (>0.01 M). Effects of Sodium Salts on the Sensitized System. Figure 1 shows spectra of the Be-CAS-CTAB system for a series of solutions to which sodium salts were added in 0.1 M concentrations. Addition of salts resulted in a shift of,,A from 608 to 622 nm. The absorbances of the solutions also varied with the anion of the sodium salt added. Results of
ANALYTICAL CIHEMISTRY, VOL. 54, NO. 1, JANUARY 1982 81
Table 111. Effect of Clhloride Salts on S,pectroscopyof the Be-CAS-CTABSystema added electrolyte 0.1 M NaCl 0.1 M LiCl 0.1 M KC1 0.1 M NH,Cl Be, 2.2 X 10-6-2.2x 1X M; pH 6.7 (1x a
'612
7.94 x l o 4 7.93 x 104 7.89 x 104 7.95 x 104
lo-'
6 '
12
1900 2100 1900
1700
M; CAS, 3. x M HMT/1.7 x
M ; CTAB,
M HCl).
working curves for the determination of Be2+by CAS in CTAJ3 with no added salt and with 0.1 M NaC1,O.l M NaBr, 0.1 hl NaN03, and 0.1 M Na2S04are summarized in Table 11. A11 other reagents (except Be2+)were maiintained at constant concentration. Two working curves were prepared for each set of solutions. The slopes of the working curves and their standard deviations were calculated by linear regression assuming a nonzero intercept. e values were calculated at both 608 and 622 nm. The t values reported are the average of two slopes, and the standard deviation was the pooled value for the two curves.
L
L
OO
J
0.I 02 0.3 ELECTROLYTE CONCENTRATION, M
0.4
L
Figure 2. Absorbance of Be-CAS complex in CTAB vs. electrolyte concentration: (A) equimolar concentration of NaCl and NaBr (concentration of each salt given on abscissa),(B) NaBr, (C) NaCi; Be, i. I
x 10-5 M; CAS, 1 x 10-4 M; CTAB, 1 x 10-3 M; PH 6.7 (1 x 10-1 M HMT/1.7 X I W 3 M HCI); A = 608 nm.
s2 = --(nl - 1)S12+ (n2- 1)SZ2 nl+ n2 - 2
The results of Table I1 and Figure 1 rihow that the molar extinction coefficient of the Be-CAS-CTAB system increases and A- shifts to longer wavelengths upon the addition of each of the sodium salts. The relative degree of sensitization is dependent on the anion of the salt added. The degree of sensitization (as measured by the value of e over the 13e concentration range of 2.2 X lo* to 2.2 X M) increases in the following order: no salt 0.1 M Na2S04< 0.1 M NaCl < 0.1 M NaBr 6 0.1 IM NaN03. Statiratical analysis using standard t tests indicates that the differences between t values for no salt, Na2S04,NaC1, and NaBr are all significant at the 95% confidence level. The difference between E values for NaBr and NaN03 was not significant a t this level. Effect of Chloride Salts on the Sensitized System. To test for cation-specific effects, various chloride salts were added to the Be-CAS-CTAB system. The salts tested were LiC1, NaC1, KCl, and NH4CI. Two working curves were prepared for each salt tested, and the results in Table I11 are the averages of the t values as calculated from the slope of each curve. The standard deviation was calculated as above. ,A, was 622 nm for all of the chloride salts (at [Be2+]= 2.2 X lom5
M).
The results in Table I11 indicate that the sensitization process is not dependent on the type of cation present. Standard t tests confirm that the differences between E values for any two sets of salts in Table I11 are not significant at the 95% level. Dependence of Sensitization on Concentration of Added Electrolyte. In order to test the dependence of the sensitization on the concentration of salt present, the concentration of an added electrolyte was varied while holding all other concentrations constant. Three series of solutioins were prepared in which the concentrations of NaCl and NaBr or an equimolar mixture of NaCl and NaBr was varied. The results are shown in Figure 2. The data in Figure 2 show that, as the concentration of added electrolyte increases, the absorbance of a fixed amount of Be-CAS in CTAB also increases. However, the slopes of these curves are greatest at low concentrations of salt and approach zero as the concentration of sallt approaches 0.4 Ed. Figure 2 also shows that NaBr always produced greater absorbances than NaCl far a given concentration of added salt. Additionally, when equal concentrations of NaBr and NaCl
Be CO~CENTRATIONx IO',
M
Flgure 3. Absorbance of Be-CAS complex in CTAB vs. Be concentratlon: (A) with 0.1 M NaBr, A = 622 nm, (B) no salt, A = 608 nm; CAS, 1 X IO-' M; CTAB, 1 X M; pH 6.7 (1 X IO-' M HMT/1.7
x
io-3
M HCI).
were added simultaneously, the absorbance produced was only slightly higher than for NaBr alone. In fact, as the concentrations of NaCl and NaBr increased, the small increment due to NaCl virtually disappeared. Detection Limits in the Absence and Presence of Added Electrolyte. Working curves for the determination of Be by CAS in CTAB were prepared in the absence and presence of added salt (NaBr), over a Be concentration range M. Other concentrations were held of 2.2 X lo-' to 2.2 X constant. The results for these determinations are plotted in Figure 3. The slopes of these curves increase with increasing Be concentration and the plots are not strictly linear in any concentration region. An estimate of the improvement in detection limit achieved upon the addition of salt can be obtained from the results presented here. The detection limit is taken to be the Be2+ concentration at which the absorbance exceeds the base line by twice the standard deviation of the noise. The standard deviation of the noise in these experiments was found to be 0.005 absorbance units, and the background was 0.01 absorbance units (the absorbance of free dye in buffered CTAB at 608 nm). Thus, the detection limit was reached at 0.02 absorbance units, which, by graphical extrapolation of the calibration curve (Figure 3) corresponds roughly to an analyte concentration of 1.0 X lo* M. Similar treatment of the BeCAS-CTAB system in which salt was added (where the background absorption due to free dye at 622 nm was 0.04 absorbance units) gives a detection limit of 0.05 absorbance units, corresponding to a concentration of 2.0 X lo-' M. The addition of 0.1 M NaBr to the surfactant-sensitized system results in a 5-fold improvement in the detection limit
62
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
of the technique, to a level comparable to that for determination of Be by flame atomic absorption (1.1X M) (18). It should be noted that the detection limit reported for the surfactant-sensitized system with no salt (1.0 X lo4 M) is somewhat higher than the detection limit reported in the M at A = literature for the unsensitized system (6.7 X 0.01) (6),in spite of the increase in c accompanying sensitization (e = 5.5 X lo4 in this work, VS. 1.5 X lo4 for the unsensitized complex, as reported in ref 6). This discrepancy probably arises from differences in the methods used to calculate detection limits. The limit reported in ref 6 evidently arises simply from dividing an estimated “measurability limit” (0.01 absorbance unit) by an absorptivity estimated from the overall calibration curve (1.5 X lo4 cm-’ M-l). A similar treatment of the data from this study results in an estimated limit of 1.8 X lo-’ M, which is (as expected) lower than the value reported for the unsensitized system. However, such an estimate ignores both the dependence of E on [Be2+]and the background absorbance due to free dye. Both of these factors raise the detection limit estimates, and their proper consideration for the data of ref 6 would probably have a similar effect (corrections due to background from dye absorption may be especially large in the unsensitized system).
strongly binding counterions is involved in the salt effects mechanism. The concentration dependence and nonadditivity of the salt effects support this conclusion, as does the absence of cation-specific effects (cations do not interact strongly with the CTA micellar surface). The above considerations deal mainly with effects on absorptivity. However, added salts also resulted in a red shift in A,=. This red shift is in addition to the general red shift with increasing Be concentration which has been obin A,, served in the presence of surfactants (7). Both of these shifts indicate the possibility of changes in the nature (stoichiometry) of the complex and/or the environment around it (i.e., a solvent effect). On the basis of the data presented in this study, it is not possible to distinguish between these (and other) possibilities. Further studies aimed at elucidating the detailed mechanism of interactions involved are under way and should advance our understanding of these phenomena.
DISCUSSION The results of this study carry important implications for analytical applications of surfactant-sensitized photometric determinations. Clearly, “spectator” salts constitute important matrix interferents in these methods. However, the data of Figure 2 suggest that these matrix effects can be swamped out by increasing the total electrolyte concentration to 0.2 M or greater. In this region, the change in absorbance per change in salt concentration is relatively small, and errors due to differences in salt concentration in sample and standards will also be small. Additionally, the results show that for large interferent concentrations, the salt effects of a combination of salts are not additive but are instead dependent on the identity of the most strongly interfering anion present. Therefore, when swamping out matrix effects, an anion such as NO3- or Br- should be used to minimize anion-specific effects. Furthermore, the addition of relatively high concentrations of salts offers the additional advantage of maximizing the sensitivity of the surfactant-sensitized technique. These observations suggest that the results obtained in this study may also lend some insight into the mechanism of salt effects in surfactant-sensitized systems. The order of increasing anion-specific sensitization (Sod2-< C1- < Br- 5 NO3-) roughly parallels the order in which counterions bind to the surface of CTA micelles (19). This suggests that displacement of hydroxide from the micellar surface by more
Tanford, C. “The Hydrophobic Effect: Formation of Micelles and Biological Membranes”, 2nd ed.; Wiley: New York, 1980. Fendler, J. H.; Fendler, E. J. “Catalysis in Micellar and Macromolecular Systems”; Academic Press: New York, 1975. Evtimova, B.; Nonova, D. Anal. Chim. Acta 1973, 6 7 , 107. Vekhande, C.; Munshi, K. N. Mlchrochem. J . 1978, 23, 28. Hinze, W. L. I n “Solution Chemlstry of Surfactants”; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 79 and references therein. Sommer, L.; Kuban, V. Anal. Chlm. Acta 1989, 4 4 , 333. Strelow, F. W. C.; Weinert, C. H. S. Anal. Chem. 1975, 4 7 , 2292. Strelow, F. W. C., private comrnunlcation, 1981. Baxter-Hammond, J.; Cook, K. D. I n “Solution Chemistry of Surfactants-Theoretical and Applled Aspects”; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, In press. Mulwanl, H. R.; Sathe, R. M. Analyst (London) 1977, 102, 137. Nlshida, H.; Nlshida, T.; Ohtomo, H. Bull. Chem. SOC.Jpn. 1976, 49, 571. Horiuchi, Y.; Nlshida, H. BunseklKagaku 1969, 18, 180. Shijo, Y.; Takeuchi, T. BunsekiKagaku 1971, 20, 137. Nlshida, H. BunseklKagaku 1971, 20, 1080. Langmyhr, F. J.; Klausen, K. S. Anal. Chlm. Acta 1963, 29, 149. Chalmovich, H.; Boniiha, J. B. S.; Pollti, M. J.; Quina, F. H. J . Phys. Chem. 1979, 83, 1851. Kudryavtseva, L. M.; Chernova, R. K. Zavod. Lab. 1978, 4 4 , 522. Christian, G. D.; Feldman, F. J. Appl. Spectrosc. 1971, 25, 660. Bunton, C. A.; Romsted, L. S.; Sepulveda, L. J . Phys. Chem. 1980, 8 4 , 2611.
ACKNOWLEDGMENT We gratefully acknowledge the assistance of J. BaxterHammond in the early stages of this research. LITERATURE CITED
RECEIVED for review August 11, 1981. Accepted October 7 , 1981. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Support from the Research Corporation and from the University of Illinois Campus Research Board is also gratefully acknowledged.
