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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Spectrophotometric Determination of Boric Acid in Boron Powder with Curcumin E. W. Grotheer' Bendix Kansas City Division, Materials Evaluation,
P.O.Box
1159, Kansas City, Missouri 64 14 I
A surface film of boric acid on boron particles may adversely affect the suitability of boron powder in some end uses. Boron quickly oxidizes and becomes coated with boric acid after manufacture. Boric acid contents in a range from 0.02 to 0.3% are common. A rapid and accurate method was needed to determine trace amounts of boric acid for quality control and specification testing of elemental boron. The method previously used involves the titration of boric acid in a water slurry after formation of a mannitol-boric acid complex. This procedure requires as much as 20 g of boron per sample when low levels of boric acid are present. This requirement made the method prohibitively expensive and necessitated development of a more sensitive method. Colorimetric methods based on the reaction of boric acid with curcumin were found to be rapid, accurate, and sensitive enough to require less than 2 g of boron per determination. Spicer and Strickland ( I ) demonstrated the utility of curcumin for making boron determinations in the 1950's, and many attempts have been made to improve the use of this reagent since that time. The most significant improvement occurred in 1962 when Hayes and Metcalfe ( 2 ) developed a method involving the reaction of boric acid with curcumin in the presence of sulfuric and acetic acids. Modifications of this method for particular applications have been reported by Kowalenko and Lavkulich ( 3 ) ,Uppstrom ( 4 ) ,and Ostling ( 5 ) . T h e reaction between boric acid and curcumin occurs at a measurable rate only when the curcumin molecule is protonated. Protonation takes place a t the carbonyl groups in the presence of a strong acid and occurs completely and rapidly when sulfuric acid is added to a solution of curcumin in acetic acid. In its protonated form, curcumin absorbs strongly at the same wavelength (555 nm) as the curcumin/boric acid complex. Curcumin changes from yellow to an intense purple-red when complexed with boric acid. The acidity of the solution therefore must be sufficiently reduced to allow the curcumin to revert t o the nonprotonated form and thus remove the interference. This can be accomplished by diluting the mixture with methanol, or by adding a suitable buffer. Many compounds, including methanol, water, and glycols or diols, have been used by other experimenters to extract boric acid from different sample media. The extraction of boric acid by the use of a diol is particularly effective with boron because many of the complexes formed are high-boiling and are soluble in organic solvents. The organic solvents can be easily evaporated to concentrate the sample with less probability of oxidizing the boron surface. In 1961, Agazzi (6) reported the use of 2-ethyl-l,3-hexanediol to extract boric acid from aqueous solutions into chloroform for analysis by flame photometry. Extractions from aqueous solutions by the use of 2-ethyl-1,3-hexanediol also have been reported by Peterson and Zoronski (7), and Mair and Day (8). Mair and Day analyzed the extracted boric acid using curcumin. For the boric acid extraction, a 10% (v/v) solution of 2-ethyl-1,3-hexanediol in chloroform was used. Because the complex formed between boric acid and 2-ethyl-1,3-hexanediol boils a t 244 "C, the chloroform can be removed by evaporation without volatilizing the boric acid/diol complex. Water also can be used for the extraction of boric acid. However, since boric acid will volatilize upon heating in an Present Address: IBM, P.O. Box 12195, Research 'Triangle Park, N.C. 27709. 0003-2700/79/0351-2402$01 0010
aqueous solution unless the solution is basic ( 9 ) ,sodium hydroxide or calcium hydroxide must be added to the solution before the water is evaporated. EXPERIMENTAL Apparatus. Spectrophotometric measurements were made with a Bausch and Lomb Spectronic 20 spectrophotometer. Polypropylene beakers were used for the evaporation of the samples and the formation of the curcumin/boric acid complex. Reagents. All standard boric acid solutions were prepared by dissolving reagent-grade boric acid in the diol solution ( l o % , v / v , solution of 2-ethyl-1,3-hexanediolin reagent-grade chloroform). The curcumin reagent was a 0.125%, w/v, solution of curcumin (Eastman Organic Chemicals) in glacial acetic acid. The pH of the solutions, after formation of the boric acid/curcumin complex, was raised by using 100 mL of reagent-grade methanol or 20 mL of an acetate buffer (consisting of 90 mL of ethanol, 180 g of ammonium acetate, and 135 mL of glacial acetic acid, diluted with water to 1 L,). Procedure. A sample of boron powder of appropriate size (less than 2 g) was accurately weighed into a 250-mL flask, 100 mL of the diol solution was added, and the flask was stoppered and swirled. After 2 h, a portion of the solution was filtered through Whatman No. 40 or 42 filter paper. If the acetate buffer was to be used t o raise the pH of the final solution, a sample aliquot containing 1 to 5 pg of boric acid was transferred to a 100-mL polypropylene beaker; if methanol was to be used to raise the pH of the final solution, the aliquot contained 5 to 20 pg of boric acid. A blank was carried through the process with each set of samples. The chloroform was evaporated from the samples through the use of an 80 "C hot-water bath. After the chloroform had been completely removed, 3 mL of curcumin reagent and 3 mL of a 1:1 mixture of sulfuric and acetic acids were added, and the samples were thoroughly mixed. The samples then were allowed t o stand 15 min to ensure the complete formation of the curcumin/boric acid complex, after which their pH was raised by adding 100 mL of methanol or 20 mL of the acetate buffer. The chromophore was allowed to stabilize for 5 min. The percent transmittance for each sample then was recorded at a wavelength of 555 nm with the blank set to 100% transmittance, and the readings obtained were compared to a standard curve which had been prepared by analyzing standard boric acid/diol solutions in the same manner. When water was used as the extraction solvent, the solutions were filtered through 0.45-~mGA-6 Metricel membrane filters before sampling, and 1 mL of 1070sodium hydroxide was added to each sample before it was evaporated to dryness. RESULTS AND DISCUSSION The extraction of boric acid from boron powder was found to be complete within 2 h when either water or the diol solution was used. Whatman No. 40 or 42 filter paper was used to obtain diol samples free of boron particles. However, fine boron particles proved more difficult to remove completely before analysis when water was used for the extraction. Filter paper removed the larger boron particles, but the very small particles left the filtrate a muddy color. Because of the dilution of the sample during analysis, the particles present in the filtrate should not have caused significant optical interference. However, analysis for boric acid after extraction with water and filtration through filter paper yielded results which were both erratic and higher than the results obtained when extracting with diol solution or with an acid-base titration method (Table I). A clear filtrate and reproducible data were obtained when 0.15-pm Metricel filters were used. These results may indicate that elemental boron, if it has not been completely removed, is oxidized during evaporation of the 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table I. Analysis Results of Boron Powder Using Different Filtering Techniques extraction method diol diol
diol water water water water
filter
boric acid, 5%
Whatman 40a Whatman 40 Whatman 4U Whatman 40 Whatman 40 Whatman 40
0.051 0.050
0.45-pm Metricelb 0.4 5-1m Metricel
water water
0.45-pm
Metricel
0.068 0.084 0.055 0.051
0.057
0.052
acid-base tit rat ion a
0.050 0.099
W. R. Balstron Ltd.
Gelman Instrument Company.
powder. The water then was evaporated a t room temperature and the samples were extracted with diol solution, after which the original boron powder and the samples spiked with boric acid were analyzed. The data that were obtained are given in Table 11. These data have been corrected for the original boric acid content of the boron powder. The actual boric acid content of the spiked samples was in close agreement with the theoretical content. The extraction efficiency also was evaluated by determining the boric acid content of boron which had been recovered from a previous extraction and boric acid determination. No boric acid was found in this boron, indicating the boric acid had been quantitatively extracted during the first determination. The determination of boric acid using curcumin is unaffected by the presence of other compounds, except for fluoride and nitrate ions. A procedure for removing the nitrate interference is reported by Hayes and Metcalfe ( 2 ) . No satisfactory method has been found to prevent the interference caused by fluoride.
Table 11. Extraction Efficiency of 2- Eth yl-l,3-Hexanediol
LITERATURE CITED
boric acid, w g
sample 1
added 250
2 3
250 2 50
recovered recovery, 256 102.1 246 9s.4 258
average
2403
%
103.2 101.3
water. Careful filtration of the samples, therefore, is necessary before analysis. The extraction efficiency of 2-ethyl-1,3-hexanediol was evaluated by adding 1mL of 500 ppm aqueous boric acid and 1 drop of 10% NaOH to accurately weighed samples of boron
Spicer, G. S.; Strickland, J. D. H. J . Chem. SOC. 1952, 4644. Hayes, M. R.; Metcalfe, J. Analyst (London) 1962, 87, 956. Kowalenko, C. G.; Lavkulich. L. M. Can. J . Soil Sci 1976, 56, 537. Uppstrom, L. R. Anal. Chlm. Acta 1968, 43, 475. Ostling, G. Anal. Chim. Acta 1975, 78, 507. Agazzi, E. J. Anal. Chem. 1987, 39, 233. Peterson, H. P.; Zoronski, D. W. Anal. Chem. 1972, 4 4 , 1291. Mair, J. W.; Day, H. G. Anal. Chem. 1972, 4 4 , 2015. Feldman, C. Anal. Chem. 1961, 33, 1916.
RECEIVED for review May 25, 1979. Accepted August 20,1979. This work is a result of work performed by the Bendix Corporation a t the Kansas City Division which is operated for the U S . Department of Energy under Contract Number DEAC04-76-DP00613.
Evaluation of Solute Vaporization Interference Effects in a Direct Current Plasma G. W. Johnson, H. E. Taylor,' and R.
K. Skogerboe"
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Quantitative measurements by atomic emission spectrometry depend on the production of free and excited atom populations. Processes which cause shifts in these populations may be broadly labeled as physiochemical interference effects. Solute vaporization effects are included among these. These originate from the conversion of the analyte population(s) to compounds which exhibit some degree of stability in the excitation medium. Classical examples include the suppression of calcium atom populations when phosphorus and/or aluminum are present (2-8). Several previous investigations (4-12) dealing with dc plasma excitation sources have indicated that both phosphorus and aluminum cause supression of calcium excitation. These reports have also shown that the extent of the interferences observed is dependent on operational conditions used. A previous publication (13) from this laboratory has demonstrated that a three-electrode dc plasma can be used with a single set of operating conditions for the simultaneous determination of 18 elements in natural and effluent waters a t requisite concentration levels. Although it was shown in that report that reliable analyses were obtained for reference water 'Present address, U S . Geological Survey, 5293 Ward Road, Arvada, Colo. 80002. 0003-2700/79/0351-2403$01.00/0
samples having reasonably diverse compositions, data that specifically addressed the possibility of solute vaporization interference effects were not presented. The present report summarizes the results of experiments used to define: the significance of the interferences of phosphorus on barium, calcium, and strontium emission; the effects of excess barium, calcium, and strontium on phosphorus emission; the interference effect of calcium on aluminum emission; and the extent of the aluminum effect on calcium emission. EXPERIMENTAL Apparatus. The Spectraspan I11 (Spectrametrics, Inc.) dc plasma spectrometer was used. The unit consists of a dc power supply, a three-electrode plasma torch ( I ' $ ) , a gas regulation and sample nebulization system, a direct-readingechelle spectrometer, and a microprocessor-based control and data acquisition system (13). The operating conditions used in the present study have been described (13). Reagents. All test solutions were prepared from 99.999% pure (or better) metals, oxides, or carbonates. Master solutions were prepared by dissolving the solids in doubly redistilled nitric acid and diluting to volume with distilled-deionizedwater to maintain a final HN03 concentration of 0.1 M. Appropriate dilutions of the master solutions were made with the same water maintaining the same " 0 3 concentration. IC 1979 American Chemical Society
