Anal. Chem. 1999, 71, 2551-2553
Determination of Boron by Flow Injection Analysis Using a Conductivity Detector Sangita D. Kumar, B. Maiti, and P. K. Mathur
Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
A flow injection method for the determination of boron using a conductivity detector has been described. Boric acid injected into the flow system reacts with mannitol (0.3 M) in the mobile phase and an equivalent amount of H+ is liberated in the stream. The increase in the conductance of the mobile phase due to the liberated H+ has been equated to the boron concentration in the sample. A linear calibration for light- and heavy-water samples containing 0-20 µg/mL boron was obtained. Boron concentrations in the samples of light and heavy water and lithium pentaborate solution have been measured. The interferences due to various ions such as Na+, Li+, Cu2+, Ni2+, Co2+, Fe3+, Al3+, SO42-, NO3-, F-, and Cl- could be eliminated by adopting a two-step sample pretreatment procedure. In the first step, all the anions were converted to Cl- by treating the sample solution with a strong anionexchange resin. In the second step, the solution obtained from the first step was passed through a silver-guard cartridge to remove interfering cations and Cl-. The relative standard deviation was (0.25% for the determination of 1 µg of boron in light water, and the limit of detection was 0.01 µg present in an injection volume of 100 µL. The corresponding values for heavy water were (0.38% and 0.1 µg, respectively. The determination of boron is very important due to the application of many of its compounds in food preservation, agriculture, metallurgy, electronics, and in many industries such as glass and ceramics, health care products, and nuclear power production.1-4 Boron plays a very important role in the nuclear power industry because of its high neutron absorption cross section. Boron in the form of carbide is extensively used in control rods for nuclear reactors for controlling the neutron flux. In Canadian deuterium-uranium pressurized heavy water (CANDUPHW) type reactors, boron compounds also serve as readily removable neutron poisons added to the moderator (heavy-water) system for reactivity control. Since a small amount of boron often shows a remarkable influence on the physical and chemical properties of the host matrix, the determination of the element at low concentration levels is extremely important. (1) Mark, H. F.; McKetta, J. J., Jr.; Othmer D. F. Encyclopedia of Chemical Technology, 2nd ed.; John Wiley & Sons Inc.: New York, 1968; Vol. 3, pp 602-737, and Vol. 4, pp 86-87. (2) Spicor, G. S.; Slickland, J. D. H. Anal. Chim. Acta 1958, 18, 231. (3) Kunin, R.; Preuss, A. F. Ind. Eng. Chem. Prod. Res. Dev. 1964, 3, 304. (4) Official Methods of Analysis of the Association of Official Analytical Chemists, 2nd ed.; Association of Official Analytical Chemists: Washington, DC, 1975. 10.1021/ac981309s CCC: $18.00 Published on Web 05/22/1999
© 1999 American Chemical Society
There are several methods,5,6 e.g., atomic absorption spectrophotometry, X-ray fluorescence, etc., available for the determination of boron at low concentration. However, these methods are often not sensitive or precise enough and are generally prone to severe matrix interferences. The ICPAES7,8 is perhaps the most sensitive, accurate, and precise method for the determination of boron, but the high cost of the instrument and its operation does not permit its use in most laboratories. The chemical method for the quantitative analysis of boron is simple, reliable, accurate, and cost-effective. In this method, the element is chemically converted to boric acid which in turn is determined by titration or by some instrumental method. Boric acid is such a weak acid (pK ) 9.3) that as such it cannot be successfully titrated directly with alkali. However, it reacts with polyhydric alcohols such as mannitol, sorbitol, or glycerol and liberates 1 equiv of proton which can be titrated with alkali using a suitable acid-base indicator. Obviously, the titrimetric method is suitable for the determination of boron only at high concentration levels. The determination of traces of boron by a chemical method involves the separation of the element from the matrix by distillation as methyl borate, followed by the spectrophotometric determination9,10 using chromogenic reagents such as curcumine, carminic acid, and methylene blue. This is a lengthy procedure, and the method is prone to error due to incomplete recovery and possible loss of highly volatile methyl borate during distillation. Moreover, the color development needs stringent reaction conditions. An ion chromatographic determination of boron by converting the element to BF4- has been described in the literature,11 but the quantitative conversion of boron to BF4 at a low concentration level is doubtful. A HPLC method based on the reversed-phase separation and conductometric detection has also been proposed,12 but the study has been limited to the determination of pure boric acid. A flow injection method, based on the decrease in the absorbance of chromotropic acid solution due to the reaction of boric acid with the reagent, has been reported13 but the slow reaction kinetics is (5) Kilroy, W. P.; Moniham, C. T. Anal. Chim. Acta 1976, 83, 381. (6) Andrew, B. E. Ceram. Bull. 1961, 55, 145. (7) Barba M. F.; Valle F. J. Inst. Ceram. 1984, 38, 389. (8) Walsh, J. N.; Howe, R. A. Appl. Geochem. 1986, 1, 161. (9) Nemodruk A. A.; Karalova Z. K. Analytical Chemistry of Boron; Ann ArborHumphrey Science Publishers: Ann Arbor, London, 1969. (10) Ruggeri, R. Anal. Chim. Acta, 1961, 29, 145. (11) Wilshire J. P., Brown W. A. Anal. Chem. 1982, 54, 1647. (12) Zenki, M., Ohlani, J., Ikeda, T., Tooei, K. Chem. Lett. 1990, 885. (13) Lussion T., Gilbert R., Hubert J. Anal. Chem. 1992, 64, 2201.
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the major drawback of the method. Determination of boron by flow injection analysis using spectrophotometric detection has recently been reported,14 where the decrease in the absorbance of bromocresol green due to proton liberation during the reaction of boric acid and mannitol in the flow line has been equated to the boron concentration. However, the bleaching is nonlinear and the absorption values are very sensitive to the initial pH of the mobile phase. This affects the precision and accuracy of the method. In the present paper, a conductometric flow injection analysis (FIA) of boron, based on the increase in the conductance of the mobile phase, due to the liberation of protons resulting from the reaction of boric acid with excess of mannitol present in the mobile phase, has been described. The interfering cations and anions have been removed by treating the sample solution with anionexchange resin and passing the resultant solution through a silverguard column prior to the injection into the flow system. EXPERIMENTAL SECTION Reagents. All chemicals were of analytical reagent grade. Nanopure water (specific resistance 18.3 MΩ) was obtained from Barnstead Nanopure System. All the solutions were prepared using Nanopure water. Solutions of mannitol (Sisco Research Laboratories, India) were prepared by dissolving 54.65 g of the solid in 1000 mL of Nanopure water. Stock standard solutions of boric acid (1000 µg/mL); (Sarabhai Chemicals, India) were prepared by dissolving 0.5715 g of boric acid in 100 mL of Nanopure water. Working standard solutions were prepared by appropriate dilution of the stock standard solution. Heavy water with isotopic purity of >98% and chemical purity of >99% was obtained from the Heavy Water Division, Bhabha Atomic Research Centre, Mumbai. Stock solutions of metal salts containing ∼1000 µg/mL metal ion were prepared by dissolving the corresponding salts in Nanopure water. The solutions were suitably diluted to obtain a solution of the required concentration. A strong anion-exchange resin (Indion) in chloride form was procured from Ion Exchange India Ltd., India. The silver-guard column was procured from Dionex Corp., USA. It had the capacity of 2-2.5 mequiv/cartridge on a waterswollen basis. Approximately 9 mL of 1% NaCl may be treated with one on-guard Ag cartridge. Apparatus. Flow System. A Dionex model 16 (Dionex Corp., USA) ion chromatograph was converted into a single-channel flow injection system by removing the separator column and suppressor unit and joining the flow line to the conductivity detector via the injector valve. The detector cell body was made up of machined Kel-F and the electrodes were of passivated 316 SS. The detector cell volume was 6 µL when the cell constant was set at 1. A thermistor, installed slightly downstream of the electrodes, provided on-line temperature compensation through electronic circuitry on the conductivity boards. The system was equipped with a sample injection loop of 100-µL capacity. The mobile phase from the reservoir was pumped into the flow system, (14) Sanchoz- Ramos, S, Medina - Hernandez, M. J., Sagrado, S. Talanta 1998, 45, 835.
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which carried the sample from the injector loop and entered directly into the conductivity detector cell. The length of the flow line between the injector and the detector was ∼130 cm and the inner diameter of the liquid transfer line was 0.5 mm. The selected values of different experimental parameters, which led to a good compromise between sensitivity (peak height), reproducibility and peak shape were, flow rate 1 mL min-1, injection volume 100 µL, and the mannitol concentration in the mobile phase 0.3M. Procedure. For the analysis of pure boric acid, the aqueous solution was directly injected into the flow injection system and the conductance/peak height was measured. These were used to calculate the boric acid concentration from the calibration curve. However, for the samples containing interfering ions, the following two-step pretreatment was essential. Step 1. Adequate aliquots of stock sample solution were taken and equilibriated with a known weight of the anion-exchange resin for 1 h. The solution was filtered through a No. 41 Whatman filter paper and the resin was given four to five washings with deionized water. The final volume was made up in a 25-mL volumetric flask. Step 2. The on-guard Ag column was first washed with 8-10 mL of deionized water at a flow rate of 2 mL min-1. The solution after treatment as described in step 1 was taken in a 10-mL syringe and passed through the Ag column at a flow rate of 2 mL min-1. The first 4 mL was discarded and the rest collected in a clean and dry beaker. This was then injected into the flow injection system for boron analysis. After the pretreatment, analysis time was merely 30-40 s and the detection limit was 0.01 µg of boron when the signal was ∼3 times higher than the noise. RESULTS AND DISCUSSION In the presence of a polyol like mannitol (LH2), boric acid forms a stable anionic complex L2B- and liberates an equivalent quantity of proton according to the reaction
H3BO3 + 2LH2 f H+ + L2B- + 3H2O
The flow injection analysis of boron described here involves direct conductometric determination of the concentration of H+ ion liberated in the flow stream. The background conductance was very low, baseline was practically free of noise, and there was no significant vacancy peak due to blank injection. Because of these favorable conditions, a single-channel flow system was satisfactory for the analysis. The reaction of boric acid with the mannitol in the flow line was very rapid and quantitative.The conductance values were reproducible and linear in the range of 0-20 µg of boron. The straight line had a zero intercept and a slope of 0.5 µS/ppm of boron, and the relative standard deviation was (0.25%. Any ionic species, if present in the solution, would contribute to the conductance value and must be absent. In the absence of any ionic interference in the sample, the determination of boron present as boric acid in the solution was simple, rapid, and straightforward. It takes ∼30 s only for each analysis, and the concentration of boron could be directly obtained from the calibration plot. If the boric acid is present along with other ions, a two-step sample pretreatment is absolutely essential. In the first step, the sample was equilibrated with an excess of the anionexchange resin in chloride form. For quantitative exchange, the equilibration should be carried out with neutral or slightly acidic
Table 1. Interference of Different Ions in the Determination of 0.1 µg of Boron no.
cation
in the form
concn of cation (µg)
1 2 3 4 5 6 7 8
Li+ Na+ Cu+2 Ni+2 Co+2 Al+3 Fe3+ Cu+2, Ni+2, Co+2 Al+3, Fe+3
LiCl NaCl CuSO4 NiSO4 CoCl2 AlCl3 Fe2(SO4)3 Cl- & SO4-2
1000 1000 100 100 100 100 100 20 (each)
Table 2. Determination of Boron in Various Samples boron found (µg) 0.101 0.101 0.102 0.097 0.102 0.104 0.098 0.099
solution (pH 3-7). All common anions, namely, SO42-, NO3-, Br-, I-, and F- are converted to Cl- while boric acid remains unaffected. However, weak acid anions such as acetate and formate could not be quantitatively removed. These ions, therefore, interfere in the recovery of boron from a standard solution. In the second step, an aliquot of the sample solution after treating with the anion-exchange resin in the chloride form was passed through the Ag-guard cartridge column to remove all cations and anions. The interfering cations exchange with Ag+ and remain in the column while the chloride is precipitated within the cartridge as AgCl.
Res-Ag+ + M+Cl- f Res-M+ + AgClV
The cartridge could be successfully used to remove the interfering ions quantitatively, provided that the concentrations of the ions were well below the total capacity of the packing material of the cartridge. The solution emerging out of the cartridge was used for the determination of boron by injecting a measured volume (100 µL) into the flow system. The recovery of boric acid after the two-stage operation was tested both in the presence and absence of interfering cations and anions. Table 1 shows the quantitative recovery of 0.1 µg of boron from synthetic water samples containing different interferring ions. Analysis of Heavy Water. The determination of boron in heavy water is of particular interest due to the use of boric acid in the moderator systems of nuclear reactors. The concentration of the boric acid, added to the heavy water used as moderator, is closely monitored to ensure optimum reactor operation. Thus, the need for rapid determination of boron in synthetic samples was felt and the concentrations of boron in several synthetic samples were determined. As the conductance values as well as the slope
sample
boron sought
boron found
heavy water 1 heavy water 2 lithium pentaborate decahydrate
10.0 µg/mL 15.0 µg/mL 19.38%
10.0 µg/mL 16.2 µg/mL 19.3%
of the calibration curve for boron in heavy water are expected to be much lower, the calibration curve obtained for light-water standards was not valid for the analysis of boron in heavy water. A separate calibration graph using the standard solutions of boron in heavy water was, therefore, obtained. The slope of the straight line plot of conductance vs boron concentration in heavy water was found to be 0.2 µS/ppm boron. The results of the analysis using this calibration plot are given in Table 2. Analysis of Lithium Pentaborate. Lithium pentaborate is often used in place of boric acid as a neutron poison in nuclear reactors. It was, therefore, of interest to determine the concentration of boron in the solution of lithium pentaborate. The aqueous solution of lithium pentaborate was first neutralized with dilute HCl and the Li+ and Cl- ions were removed by passing the solution through the Ag cartridge. The concentration (%) of boron obtained is also included in Table 2. This value was found to be in excellent agreement with that obtained by a standard titrimetric method using an acid-base indicator. CONCLUSION The flow injection method for the determination of boron is simple and rapid and has several advantages over the other existing methods particularly at low concentrations. In addition to the simplicity and rapidity, it can be fully automated and can be used for on-line analysis of boric acid in the absence of a complex matrix. The use of Ag-guard cartridges has simplified the sample pretreatment process to a great extent. The low detection limit (0.01 µg of boron) coupled with the low operational cost make it a very useful method for determination of boron even at low concentration levels. However, the method cannot be used “on line” for samples where a two-step cleanup is essential. If the concentration of electrolytes is high, a large amount of anionexchange resin is needed and a single Ag-guard cartridge is not enough to remove all the cations and chloride. Second, boron cannot be quantitatively determined if the sample contains weak acids and weak acid anions. Received for review November 30, 1998. AC981309S
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