940
Anal. Chem. 1904, 56,948-950
(17) Reijn, J. M.; Van der Linden, W. E.; Poppe, H. Anal. Chim. Acta 1980, 714, 105. (18) Abramowitz, M.; Stegun, I . A. “Handbook of Mathematlcal Functions”; Dover: New York, 1963. (19) Huber, F. J. K.; Jonker, K. M.; Poppe, H. Anal. Chem. 1080, 52, 1. (20) Reijn, J. M. Ph.D. Thesis, University of Amsterdam, 1982. (21) Synder, L. R. Anal. Chlm. Acta 1080, 114, 1.
(22) Reijn, J. M.; Van der Linden, W. E.; Poppe, H.Anal. Chim. Acta 1983, 245, 59.
RECEIVED review March 4, 1983. Resubmitted January 26, 1984. Accepted February 1, 1984.
Solvent Extraction-Spectrophotometric Determination of Boron with 3,5-Di-ferf-butylcatechol and Ethyl Violet Mitsuko Oshima,* Shoji Motomizu, a n d Kyoji Tiiei Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka Okayama-shi, 700 Japan
Boric acid and 3,5-di-terl-butyicathechol formed a complex anion at pH 6.3-8.7 which was extracted into toluene as an ion pair with ethyl violet (EV); the excess EV coextracted into toluene was removed by adding 3 % hydrogen peroxide. Boron was determined spectrophotometrically by measuring the absorbance of EV of the ion pair in toluene at 610 nm. The calibration curve was linear within a range of 0-0.45 pg of boron, and the molar absorptivity was 10.5 X I O 4 L mol-’ cm-‘. This method, using the new compiexlng agent, was applied to the determination of boron In seawater and found to be appreciably satisfactory compared with other methods.
Table I. Comparison of Sensitivities of Catechol Derivatives substituent of catechols 3,5-di-tert-butyl 4-tert-butyl 3-methyl 4-meth yl 4-chloro 4,5-dichloro 3-nitro 3,5-dinitro
lo4€,L mol-’ cm-I 10.5 5.4
reagent blank abs 0.05
7.7
0.06 0.06
3.5 3.5
0.13 0.29
a a a
a The absorbances were not measured because of the high reagent blanks.
Determination of boron is of importance particularly in the fields of the nuclear reactor materials, industrial metallurgy, pharmacy, and agriculture. Many spectrophotometric methods for boron have been reported so far. Of these, the method based on extraction of tetrafluoroborate with methylene blue was the most popular. This method, however, contained some practical difficulties such as in the formation of the boron complex and the relatively large absorbance of the reagent blank. In an attempt to improve such disadvantages, methods by extracting a boron-organic reagent complex anion as an ion pair with a dye cation have been studied. Thus, we reported the extraction methods with 2,4-dinitro-l,%naphthalenedioland brilliant green (1,2)and with 2,6-dihydroxybenzoic acid and malachite green (3). The former was highly sensitive (e = 10 x lo4 L molw1cm-l) and the reagent blank was small, but required an evaporation procedure, whereas the latter provided a quick procedure due to the rapid formation of the boron-2,6-dihydroxybenzoicacid complex in an weakly acidic aqueous solution, but required a washing procedure. Sat0 et al. determined boron by using 2,3-naphthalenediol and crystal violet ( 4 ) and mandelic acid and malachite green (5). The former method was highly sensitive but troublesome to remove the reagent blank, whereas the latter provided a simple procedure but the sensitivity was not satisfactory. Hakoila et al. (6),in the mean time, reported that 4-nitrocatechol reacted with aqueous boric acid to form a boron complex, the absorption spectrum of which, however, closely resembled that of 4-nitrocatechol, and showed low sensitivity. In this paper, we report a very useful complexing agent, 3,5-di-tert-butylcatechol (DBC), which was found after examining various catechol derivatives. The reagent blank in the organic phase can be diminished by adding aqueous hy0003-2700/84/0356-0948$0 1.5010
drogen peroxide. Though the formation of boron complex anion in the proposed method is somewhat time comsuming, this extraction method coupled with ethyl violet (EV) is apparently simpler, more selective, and more sensitive than any other method reported previously.
EXPERIMENTAL SECTION Apparatus. The absorptions were measured with a Hitachi Perkin-Elmer Model 139 spectrophotometer and a Shimadzu UV-300 recording spectrophotometer in a 10-mm quartz cell. A Hitachi-Horiba M-5 pH meter, an Iwaki Model KM shaker, and a Shimadzu CPN-005 centrifuge were used. Reagents. Standard boron solution (LOO0 X 10” M) was made by dissolving 0.1546 g of boric acid in 250 mL of deionized water. Working solutions were prepared by accurate dilution. 3,5-Ditert-butylcatechol (DBC) was synthesized from pyrocatechol and tert-butyl alcohol according to literature (7) without making use of glassware, and the purity of the reagent was comfirmed by the elemental analysis, melting point, and the ‘HNMR spectrometry. A 1.75 X M solution of DBC in toluene was prepared; this solution was stable for 3 days when stored in a brown bottle. Ethyl violet (EV) (Tokyo Kasei Kogyo CO., Ltd.) was dissolved in water M solution. Phosphate (pH 5.5-9.5) and to make a 5 x carbonate (pH 10) buffer solutions were used. Deionized water was used throughout and all solutions except toluene were stored in polyethylene bottles. All other reagents employed were of analytical reagent grade. Selection of Catechols. Eight derivatives of catechol listed in Table I were examined by the following procedure. To 2 mL of 1.258 X M boric acid solution in a soda lime glass test tube with a stopper were successively added 0.5 mL of pH 5.5 (or 8.0) phosphate buffer, 0.5 mL of 5 X M EV solution, and 5 mL of the 2 X M catechol solution in toluene. After the contents were shaken for 50 min, 1 mL of 3% hydrogen peroxide solution was added and the mixture was then shaken for 30 s by hand. 0 1984 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
A
94s
Table 11. Tolerable Limit of Coexisting Ions without EDTA concn? M ion
with EDTA concn? M ion
2.5 x
2.5 x
Na+, NO;, BY, HCO,-, SO,'-
1.25 X Cl-, CO,*2.5 X Ca2+,Mg" 2.5 X l o T 4 Baz+,ClO; 1.25 x 10-5 I2.5 X lauryl sulfate, lauryl benzenesulfonate
Co2+,MnZt, Zn2+
1.25 x 10-3 ~ 1 3 + 5.0x NiZ+ 2.0 x Fe3t 5.0 x Cu2+ 2.5 x PbZ+
a Differences of absorbances between coexisting ions added and none were within 0.01.
Wavelength, nrn
Figure 1. Absorption spectra: (a) boron 0.435 pg; (b) reagent blank.
Table 111. Determination of Boron in Seawater samplea
abs
Ushimado A
0.389 0.380 0.379 0.382 0.389 0.381 0.387 0.388 0.371 0.375 0.372 0.369
Nishiwaki 0.21
Ushimado B o /
b
Shibukawa
found, PPm
recovery,b
4.23
97.2
4.30
100.0
4.13
99.1
4.13
100.9
%
a Samples were obtained in Okayama prefecture and diluted 50-fold. These values were obtained by simultaneous experiments conducted by measuring a mixture of 1 mL of seawater diluted 50-fold and 1 mL of boric acid solution containing 0.109 p g of boron; the recovery percentage is based on the amount of boron added.
addition of an excess volume of EV, 0.5 mL of 5 X lo-' M EV solution was added throughout this experiment. Shaking Time. The effects of shaking time on the boron complex formation and on extraction of the ion pair were examined. The absorbance remained constant on shaking for more than 40 min. For safety, 50 min of shaking time was employed. Effect of Hydrogen Peroxide. The effect of the concentration of hydrogen peroxide on diminishing the reagent blank was examined by adding 1 mL of aqueous hydrogen peroxide of various concentrations (1-5%), and the absorbances were found to remain constant in this range. The ion pair of the boron-DBC complex and EV tended to decompose gradually at a higher concentration of hydrogen peroxide; 1 mL of 3% hydrogen peroxide was added throughout. Standing Time. Standing time after centrifugation was examined. As the absorbance of the toluene layer gradually became lower after 40 min, the measurement was performed within 40 min. Calibration Curve. The calibration curve obtained by this standard procedure became linear in a range of 0-0.45 yg of boron. The molar absorptivity calculated from the slope was 10.5 X lo4 L mol-1 cm-l and the absorbance of the reagent blank was 0.05. Mean absorbance for ten measurements at 0.272 pg of boron was 0.577, the relative standard deviation being 0.67 % . Effect of Coexisting Ions. The effect of coexisting ions was examined and the results are summarized in Table 11. The metal ions that tend to precipitate as hydroxides at pH 8 were studied after having been treated with 0.5 mL of 0.1 M EDTA solution; EDTA did not interfere with the determination a t
950
Anal. Chem. lg84, 56,950-957
this concentration. Bulky ions, such as I- and Clod-, showed a positive error because their ion pair with EV was extracted into toluene.
Application to the Determination of Boron in Seawater. As described above, the coexisting ions usually present in seawater did not interfere with the determination when the seawater was diluted 50-fold. The results obtained by the standard procedure are shown in Table 111. The recovery tests conducted by adding a known amount (0.109 pg) of boron to the sample solution confirmed that the recoveries were satisfactory (97.2 to 100.9%). Reaction and Extraction Mechanism. The reaction mechanism of the formation of the boron complex with DBC (HzR) and the extraction mechanism of the ion pair with EV are considered to be as follows:
HzR + (H&)org
+ dye+
+ H+
+
( ~ Y ~ + ) ( B R ~ - ) 3, ,H~z 0 (dye+)(HR-),,,
+ HzO
slow
(Dye-OH),,, (carbinol)
(2)
+ H+ (3)
+ (HZR)org (4)
HA
LITERATURE CITED
(1)
dye+ + (H2R)org+ (dye+)(HR-),,,
HSB03 + 2(HzR)o,g
The spectroscopic study indicates that reactions 3 and 4 are rather time-consuming states. The presence of hydrogen peroxide markedly facilitates reaction 5, yielding the colorless carbinol within 1 min which shows its characteristic absorbance at 350 nm. On the other hand, the absorption spectra of DBC around 290 nm in toluene remained unchanged after adding hydrogen peroxide. These results suggest that hydrogen peroxide presumably catalyzes the transformation of the colored EV to the colorless carbinol. Registry No. Boron, 7440-42-8; 3,5-di-tert-butylcatechol, 1020-31-1;ethyl violet, 2390-59-2; water, 7732-18-5.
(dye+)(HR-)org + HzO fast- (dye-OH),,,
+ (H2R)org
(5)
(1) Kuwada, K.; Motomlzu, S.; Toei, K. Anal. Chem. 1978, 50, 1788-1792. (2) Toei, K.; Motomizu, S.; Oshima, M.; Watari, H. Analyst (London) 1981, 106, 778-78 1. (3) Oshima, M.; Motomlzu, S.; Toei, K. Bunseki Kagaku 1983, 32, 268-272. (4) Sato. S.; Uchikawa, S. Anal. Chim. Acta 1082, 143, 283-287. (5) Sato, S. Anal. Chim. Acta 1983, 757, 465-472. (6) Hakoila, E. J.; Kankare, J. J.; Skarp, T. Anal. Chem. 7972, 4 4 , 1857-1860. (7) Schulze, H.; Flalg, W. Justus Liebigs Ann. Chem. 1052, 575, 231-241.
RECEIVED for review November 3, 1983. Accepted January 16, 1984.
Determination of Pore Size Distributions of Liquid Chromatographic Column Packings by Gel Permeation Chromatography F. Vincent Warren, Jr., and Brian A. Bidlingmeyer* Waters Associates, 34 Maple Street, Milford, Massachusetts 01757
A method for the determlnatlon of “effective” pore slre dlstrlbutlons (PSDs) and median pore dlameters (MPDs) by gel permeatlon chromatography Is applled to 37 commerclally avallable columns. A practlcal dlscusslon of the technlque addresses preclslon, alternatlves for data presentatlon, and the use of n-hydrocarbons to supplement the polystyrene standards ordlnarlly used. The resultlng MPD values are In general agreement with the nominal pore sires speclfled by column manufacturers. I n a few cases, the MPDs are as much as 50% below the antlclpated value.
There is presently a need for more information concerning the median pore diameter (MPD) and pore size distribution (PSD) of commercially available chromatographic column packings. The need for this information is quite significant as many chromatographers are now suggesting that differences in chromatographic performance are due to pore size differences among the various packings (1-7). Unfortunately, the chromatographic behaviors attributed to pore size are frequently baaed on information which is limited. Often, specific details concerning MPDs and PSDs are not available to chromatographers. In some situations these are proprietary
to the manufacturer. In others, a specification comes from the silica supplier, and in still other cases, some information is not even known. Another problem is that the average pore size information may result from one of several different methodologies; therefore, comparisons based upon these data become meaningless. When a nominal pore size (NPS) is available, some ambiguity may still remain as this value may not reflect batch to batch variations. If the measurement technique used by a manufacturer to determine the NPS is not stated and if the width of the pore size distribution is not provided, it is difficult to make accurate claims regarding pore size effects. There is no guarantee that a nominal pore size specified by the manufacturer is closely related to the mean of the pore size distribution. For instance, if the nominal pore diameter is determined by the Wheeler equation (8),then the stated values give only a rough indication of the mean of the pore size distribution (9). Clearly, for chromatographic columns, there is a need to standardize the method used to determine pore size distribution and mean pore diameter. There are two common techniques for the direct measurement of pore size distributions: mercury intrusion and gas condensation/evaporation (9). Using either af these techniques has not been practical for most chromatographic
0003-2700/84/0356-0950$0 1.50/0 0 1984 American Chemical Society
