Extraction-Flame Photometric Determination of Boron E. J, Agazzi Shell Development Co., ii?meryviIle, Calif.
FLAME PHOTOMETRY offers a rapid and sensitive means for the determination of many elements. In the case of boron, adequate sensitivity is achieved only in organic solvents or aqueous-organic solvent mixtures ( I ) . Maximum sensitivity has been obtained with organic solvents. Organic boron compounds offer little difficulty since they may be dissolved directly in an organic liquid and measured with the flame photometer (2). When boron is in aqueous solution it may be extracted into an organic phase as described by Maeck et al. ( 3 ) . These authors extracted boron into methyl isobutyl ketone as the tetrabutylammonium-boron tetrafluoride ion association complex. The organic phase was aspirated into a special oxygen-sheathed oxyhydrogen flame and the emission at 548 mp measured. A limit of detection of about 10 ppm of boron was reported. It has come to our attention that boron reacts with 2-ethyl1,3-hexanediol to form a weak chelate which is extractable into chloroform. Our investigation of the extraction showed it to be simple, highly selectivl: and not to require careful control of conditions for quantitative extraction. The boron content of the chloroform was readily measured by flame photometry. EXPERIMENTAL
Apparatus and Reagents. The work was done with a Beckman Model D U flame photometer equipped with a photomultiplier, electronic power supply, and means for scanning and recording flame spectra. The boron band maximum is broad and measuring i?s intensity by scanning across the maximum has no advantage so measurement at a preset wavelength was used. A medium-bore Beckman oxyhydrogen burner was the excitation source. If the laboratory is lighted with fluorescent lamps, care should be taken to prevent light from the lamps from entering the monochromator. Fluorescent lamps emit a mercury line which occurs at the wavelength chosen for measurement of boron (546 mp). The resulting positive error was especially severe when operating the flame photometer at high sensitivity. To eliminate this unwanted radiation, the door of the burner housing assembly must be kept closed when taking readings and the monochromator entrance slit should be shielded. A satisfactory shield can be msde by inserting a 3.5 inch length of Thermoflex 2-inch diameter tubing between the burner housing exit and the monochromator entrance. The extraction solvent is a 5 % solution of 2-ethyl-1,3hexanediol in chloroform. The practical grade diol manufactured by Matheson, Coleman & Bell, East Rutherford, N. J., is satisfactory. Procedure. Prepare an acid solution of the sample and transfer an aliquot which contains less than 1 mg of boron to a 50-ml plastic bottle. Dilute to 10 ml and add 5.00 ml of a 5 % solution of 2-ethyl-1.3-hexanediol in chloroform. Shake 5 minutes, then allow the phases to separate. Aspirate the chloroform phase into an oxyhydrogen flame and record the emission at 546 mF. The boron concentration is obtained from a calibration curve established with known aqueous (1) J. A. Dean, “Flame Photometry,” McGraw-Hill, New York,
1960. (2) B. E. Buell, ANAL.CHEM., 30, 1514 (1958). (3) W. J. Maeck, M. E. Kussey, B. E. G. Ginther, G. V. Wheeler, and J. E. Rein, ANAL.CIIEM. 35, 62 (1963).
solutions of boric acid. The calibration curve should be linear and pass through the origin. RESULTS AND DISCUSSION
Extraction of Boron. Boron reacts with 2-ethyl-1,3-hexanediol to form a weak chelate which is very soluble in chloroform. The effects of various parameters on the extraction are shown in Figure 1. In these tests 0.1 mg of boron was used and the parameters not being varied were maintained at the optimum levels, The emission intensities measured were adjusted to yield an emission of 100 scale divisions for the highest observed emission. The concentration of diol is not critical; complete extraction was obtained with solutions containing from 2-10z diol. As the concentration of diol is increased beyond 10% a lowering of emission intensity is noted. This is caused by the increase in viscosity of the mixture which leads to a decrease in the rate of sample aspiration. Extraction is complete over a wide range of acidity but boron cannot be extracted from alkaline solutions. We chose sulfuric acid to study the effect of acidity, since in sample preparation the sample is commonly obtained in a sulfuric acid solution. However, other acids may be used (Table I). The chelate forms and is extracted readily. Extraction is complete in 5 minutes; prolonged extraction has no effect. Phase separation is clean and rapid. The volume ratio of aqueous to organic phase is of importance. A high ratio is desirable as it is a means of concentrating the boron thus lowering the detection limit. Our data show essentially complete recovery of boron up to a ratio of two. The above tests allow the fixing of optimum extraction conditions. However, the completeness of extraction has not been demonstrated. We found that solid boric acid could be dissolved in 2-ethyl-1,3-hexanediol and by diluting this solution with chloroform a known solution of boron in a solution of the same composition as the extraction solvent was prepared. A series of such solutions containing various amounts of boron was compared with solutions obtained by extracting aqueous boric acid solutions. The data indicated up to 0.4 mg of boron are extracted completely and 96% of the boron is recovered from solutions having 0.4 to 1.0 mg of boron. Increasing the extraction time at the 1.0-mg level did not improve recovery. In practice no error is introduced by the incomplete extraction since calculations are based upon a calibration curve prepared by extraction of aqueous boron solutions. However, when organic boron compounds are analyzed by dissolving them directly in the extraction solvent, there is no extraction step. For highest accuracy the calibration curve used for organic samples should be prepared from solutions obtained by dissolving boric acid directly in the extraction solvent as described above. Flame Excitation. The flame excitation of boron gives rise to a series of band spectra with the most intense maxima at 518 and 546 mk. The boron spectrum and that of the extraction solvent are shown in Figure 2. It can be seen that the solvent has interfering emission in the region of 518 mp. The interference could be decreased by measuring emission from a differentregion of the flame; however, the intensity of VOL. 39, NO, 2, FEBRUARY 1967
233
100
a-
\
50
Effect of Concentration of 2-Ethyl-l,3Hexanediol
A 10 20 3
4 8 12 Molality of Sulfuric Acid
0
'70( v / v )2-Ethyl-l,3-Hexanediol in Chloroform
100
-
50
Effect of Ratio of Aqueous to Organic Phase
G
3
Effect of Extraction Time
v)
$ + E
Y
0
2 4 Ratio a q / o r g
6
0
4 8 12 16 20 24 Extraction Time, minutes
Figure 1. Extraction variables In aII cases 0.1 mg o,f boron
boron emission was also decreased, Measurements were therefore made at 546 mp, Background emission from the solvent was appreciable at this wavelength. To correct for the background the flame photometer was adjusted to give a reading of zero while extraction solvent was being aspirated. The optimum pressures for oxygen and hydrogen in the Beckman instrument were established experimentally. For each oxygen pressure there was an optimum hydrogen pressure. The oxygen serves also to aspirate the sample SO the rate of sample consumption and therefore the emission intensity increased with increasing oxygen pressure; the background also increased. As a compromise between sample consumption and background we chose an oxygen pressure of 20 psi and the co-rresponding optimum hydrogen pressure (5 psi). The optimum hydrogen pressure will be unique for each burner and should be determined experimentally. It is well known in flame photometry that there are variations in emission intensity within the flame, so we investigated the effect of burner position. The best position is the same as that of the fixed position of an unmodified Beckman instrument. 234
ANALYTICAL CHEMISTRY
A slit width of 0.03 mm was used for 0.1 to 1 .O mg of boron, With less than 0.1 mg of boron the emission intensity of the boron band head does not provide adequate energy and it is necessary to increase the slit opening to 0.1 mm. With a 0.1 mm slit we obtained a limit of detection of about 0.1 ppm of boron in chloroform. Effect of Various Ions. The effects of a number of extraneous ions are shown in Table I. Data were obtained from single determinations. Also shown in most cases is the amount of cation found in the chloroform phase. The extraction is very selective. Large amounts (100 mg) of cations were practically completely separated from 0.1 mg of boron. From tests with aqueous boron solutions free of interferences, a relative standard deviation of 3 % is calculated for 0.1 mg of boron. Strontium, lead, and iron led to recoveries of boron ranging from 95 to 109% and may cause interference when high accuracy is desired. Manganese, titanium, molybdenum, and tungsten extracted and showed serious interference; the interference is spectral. Titanium and manganese have a series of bands around 546 mp. Molybdenum shows a continuous emission as is probably the
70
I1-
546 mp
5 1 8 w
2t
Table I. Effect of Diverse Elements on Extraction-Flame Photometric Determination of Boron Amount of element in Amount Recovery chloroform added, of 0.1 mg Element boron, % phase me Sn(IV) 100 99 3 Pg No evidence in Sr 100 95 flame Cr(II1) 100 99 5 1.18 99 100 125 1.18 Mg Zn ... 100 98 Mn 100 121 ... ... 101 10 Fe(II1) 100 109 3 Pi3 Co(l1) 100 103 58 I.rg A1 97 100 28 1.1g ... 97 Bi 100 99 CU(I1) 100 41 PCCB Ni 100 101 32 1.1g 100 Pt 100 150 1.1g K 101 100 P8 Pb 104 100 V 97 100 < 3 g; P 100 100 210 Pg Ca 100 101 g Ti 10 >150 ... 1 101 ... 10 Mo ... >150 1 ... >150 W 10 >150 ... 1
c1(added as HCL) N (addedasHNOd
10
F-
S
O
510
520
135 101
3N
100
30
100
... *.. ...
5N
98
...
(added as NaF) (added as H,SOd) U
530
540
U
550
.560
P
Wavelength, mcc Figure 2. Flame spectra (oxyhydrogen ffame) 0.02 mg ]Boron per rnl of solvent A . Boron in 2ethyl.1,3-hexaraediol chloroform S d u t h B. 5 2-Ethyl-l,34hexmdbI in ddorofonu
case with tungsten, Manganese showed no interference when decreased to 10 mj; and titanium no longer interfered at 1 mg. At the 1-mg level tungsten interference was less intense but was still intolerable and molybdenum interfered strongly. Of the anions only fluoride gave trouble. It reacts with boron to form BF4- which is not extractable into the chloroform solution. However, the interference is overcome by complexing the fluoride with zirconium prior to extraction. The sample solution is placed in a plastic bottle and acidified with 2 ml of 1 :1 sulfuric acid, 12 ml of zirconium nitrate solution (5 mg Zrjml) are: added and the mixture is heated in a water bath at 100°C for 10 min. After cooling, extraction solvent is added and the extraction time increased to 20 min. In this manner no interference was observed with samples containing 30 mg of fluoride. Application of Method. Two standard glass samples and two organic boron compounds were used to test the method. The data are shown in 'Table 11. Results agreed well with the certified or calculated values. The glasses were decomposed by fusion with sodium carbonate and the melt was dissolved in a small amoiint of acid. Boron was measured as
...
2N
Table II. Boron Content of Various Materials by ExtractionFlame Photometric Method Boron, % w Resent Found Sample Borosilicate glass 3.95 3.92 NBS Std. Sample No. 93 (certifiedvalue) Soda-lime glass 0.47 0.44 NBS Std. Sample No. 128 (certified value) Menthyl borate 2.27(Calcd) 2.31 n-Butyl borate 4.3. 4.3 Determined by fusion-distillation method. 5
described. The organic compounds were dissolved directly in the extraction solvent and compared to a calibration curve obtained by dissolving boric acid in the extraction solvent In this way the small error introduced by incomplete extraction was avoided. When applied to aqueous solutions of boric acid, the method gave a relative standard deviation of 2 % of the mean at the 1-mg level and 3 % of the mean at the 0,l-mg level. From duplicate analyses of the organic borates, the borosilicate glass listed in Table 11, and many research materials containing 2 4 boron, a relative stamlud deviation of 2z of the mean was obtained. For materials low in boron and when only a small amount of sample may be taken a greater error is observed. RECEIVED for review August 8, 1966. Accepted October 17, 1966. VOL. 39, NO. 2, FEBRUARY 1967
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