Extrac t io n-
FIa me
Photometric Dete rmina ti o n of Boron
WILLIAM J. MAECK, MAXINE E. KUSSY, BURTON E. GINTHER, G. VERNON WHEELER, and JAMES E. REIN Atomic Energy Division, Phillips Petroleum Co., ldaho Falls, ldaho
b Submilligram quantities of boron are completely extracted from aqueous solution into methyl isobutyl ketone as the tetrabutylammonium borontetrafluoride ion association complex. The boron content of the separated organic phase is determined b y direct flame spectrophotometric analysis, using an oxygen-sheathed oxyhydrogen flame as the excitation source. Of 30 metal ions studied, only five interfered at a 1 to 1 mole ratio to boron. Interfering anions are phosphate, perchlorate, and high levels of nitrate. Nitrate interference is eliminated by sulfuric acid fuming prior to extraction.
D
the last decade, Dean (1) has pioneered the solvent extraction-direct flame spectrophotometric determination of metals. Rapid, selective methods with very low detection limits have been developed for many elements. Detection limits by flame spectrophotometry for many elements are much lower in organic media than in aqueous solutions, because of such factors as formation of volatile metal species, lower surface tension of the aspirated medium, and a hotter flame. Desirable characteristics for an extraction are rapidity, quantitative recovery, and freedom from matrix and diverse ion effects. Previously reported extraction methods for boron are based on the formation of the BF4- ion and extraction of neutral ion association complexes formed upon the addition of dyes (6, 7 ) or the tetraphenylarsonium ion (6). The former requires a 2-hour formation time, the latter 15 hours. In a previous study (4), the extraction, into methyl isobutyl ketone (4methyl-2-pentanone), of metal ions as the lower molecular weight tetraalkylammonium complexes from various acid media and from sodium hydroxide, has been reported. Those metal ions that formed monovalent anionic complexes extracted, presumably as neutral species, formed by ion asssociation with singly charged tetraalkylammonium cations. In a subsequent study, boron mas extracted completely in 1 minute from hydrofluoric acid as a tetrabutylammonium complex. Because few other metal ions extract from this medium
62
URIWG
ANALYTICAL CHEMISTRY
(4), a flame analysis of the separated organic phase offered promise as a selective and widely applicable method for boron. EXPERIMENTAL
Apparatus. Extractions were made in 50-ml. polypropylene centrifuge tubes with polyethylene stoppers on a 33-r.p.m. extraction wheel (Spinnerette model, Xew Brunswick Scientific Co., S e w Brunswick, N. J,). A 0.5-meter grating spectrophotometer with flame attachment (Jarrell-Ash Co., Newtonville, Mass.) was used for the flame analysis. The atomizerburner was modified, as described in the text and as shown in Figure 3, to provide an oxygen sheath (3). Reagents. The tetrabutylammonium hydroxide was liM titration grade (Southwestern Analytical Chemicals, Austin, Tex.). All other reagents were reagent grade, used without purification. Boron standards mere prepared by dissolving reagent grade boric acid in distilled water. The metal ion solutions used for the diverse ion studies were prepared from the high purity metals or reagent grade salts. The final media were either sulfuric or hydrochloric acid. Procedure. SEPARATION OF BORON. Pipet the sample aliquot (of 5 ml. or less) containing 100 to 1000 pg. of boron into a 50-ml. polypropylene centrifuge tube. Add 1 ml. of l5M H2SO4, 1 ml. of 27kf HF,3 ml. of lizI tetrabutylammonium hydroxide, and sufficient distilled water to give a total volume of 10 ml. Pipet 10 ml. of methyl isobutyl ketone into the tube. Stopper with a polyethylene stopper and mix for 3 minutes on the extraction wheel. Centrifuge and remove most of the organic phase to capped polyethylene bottles for the flame analysis. For sample aliquots containing more than 10 mmoles of nitrate, pipet the aliquot plus 1 ml. of 15M H2S04 into a 50-ml. borosilicate glass test tube. Place another equal aliquot of the sample in a second test tube. Fume both a t 140’ C. or less in a glycerol bath, stopping when the sample in the second tube reaches a volume of 0.5 ml. or less. Quantitatively transfer the contents of the first tube with 5 ml. of distilled water into a 50-ml. polypropylene tube. Proceed as above, omitting the addition of the sulfuric acid. FLAME ANALYSIS. The spectrophotometer is operated a t 548 mp with entrance and exit slits of 0.05 mm. The atomizer-burner is operated with
15-p.s.i. oxygen pressure and 4-p.s.i. hydrogen pressure with the needle valve closed to give a very lean mixture, and an oxygen sheath flow of 0.68 cu. foot per minute. Light from the upper portion of the flame is reflected to the entrance slit by tilting the mirror. An RCA 931A multiplier phototube operated a t 770 volts is used as the detector. The signal from the multiplier phototube is amplified by a Leeds and Northrup stabilized DC micromicroampere amplifier, then recorded with a 0- to 10-mv. linear response Bristol strip chart recorder a t a chart speed of 0.5 inch per minute, Immediately before analysis, transfer the sample from the polyethylene bottle into a polyethylene 12X Caplug (Protective Closures Go., Inc., Buffalo 23, N. Y.). Samples or standards exposed to the atmosphere give erroneously high readings due to evaporation of the hexone. This effect is negligible up to 5 minutes. Aspirate the sample into the burner in the usual manner and record the flame emission for about I minute. Aspirate a distilled water rinse through the burner between samples. Process standards and samples sequentially through the entire procedure and calculate results by direct comparison. DISCUSSION
Extraction Study. On the basis of a previous study of the extraction of metal ions as quaternary ammonium complexes ( 4 ) ~the extracted boron species is postulated as [But4N+] [BFd-1, in vhich [But4N+] is the tetrabutylammonium ion. The over-all reaction for the formation of BF4- can be depicted as H a 0 3 4F3H+*BF43H20,neglecting polymeric forms of hydrofluoric acid. Based on this, the formation of BF4and the extraction of the [ButcN+] [BF4-] complex can be expected to increase with both increased hydrogen ion concentration and increased fluoride concentration. This duo effect is shown in Figure 1 for the range of 0.1 t o 5M hydrofluoric acid and 0 to 2.5H sulfuric acid. Sulfuric acid \vas chosen to increase hydrogen ion conoentration in preference to other common acids, because few extractable metal complexes form with it (4), thus providing specificity to the extraction. Perchloric acid is not suitable, as perchlorate forms an ion association complex with the tetrabutylammonium ion. The levels of the other variables for this series of
+
+
+
0 0
I 05
I 1.0 SULFURIC
I 15 ACID
I
I
20
2 5
MOLARITY
Figure 1 . Effeci of hydrofluoric acid and sulfuric acid concentrations on extraction of boron 1 0-ml. aqueous volume
0.3M tetrabutylammonium ion, 500 pg. of boron 1 0-ml. methyl isobutyl ketone volume
experiments were the same as in the recommended procedure-namely, equal 10-ml. volumes of the aqueous and the methyl isobutyl ketone phases and 0.3M tetrabutylammoniurn ion added as the hydroxide. The boron level was 500 pg. or 0.004631, giving a tetrabutylammonium ion-boron ratio of 65. The levels of hydrofluoric and sulfuric acid selected for the recommended
procedure were 3111 and 1.5M, respectively. This level of sulfuric acid gives tolerance for as much as 27 meq. of hydroxyl ion in the sample aliquot; 3 meq. are neutralized by the added tetrabutylammonium hydroxide. The extraction of boron as a function of the mole ratio of tetrabutylammonium ion to boron is shown in Figure 2. The experimental conditions were as described in the recommended procedure. The boron level was constant a t 500 pg. Extraction is complete at a 10 to 1 ratio. For the recommended procedure, a ratio of 65 to 1 was selected at this boron level. At a boron level of 1000 pg., the recommended upper level for the procedure, this threefold excess of tetrabutylammonium ion gives tolerance for those diverse ions that form quaternary ammonium complexes. The extractable complex forms rapidly. Under the conditions of the procedure, extraction of boron is complete in 1 minute a t room temperature, with no formation time allowed after the addition of the reagents. The recommended 3-minute extraction time should permit the use of less efficient mixing than with the extraction wheel. In the extraction, the volume of the organic phase increases from 10 ml. to about 11.5 ml. This is attributed to the distribution of tetrabutylammoniurn salts and acid into the organic phase. As the aqueous volume is maintained constant (at 10 ml.) by the addition of water (see Procedure), this volume change is constant and is of no significance. Flame Excitation Study. The flame excitation of boron gives rise to a series of bands in the green portion of the spectrum, with the most intense band heads a t 492, 518, and 546 mp. A preliminary investigation of boron emission from methyl isobutyl
f
130
80
J
"/. n
0 0
n ni
I
ni MOLE R A T I O
I
I
in ._
.in-
I inn
EUTqN*/B
Figure 2. Extraction of boron as a function of mole ratio of tetrabutylammoniurn ion to boron 1 0-ml. aqueous volume
3M hydrofluoric acid 500 pg. of boron 1 0-ml. methyl isobutyl ketone volume
L
Figure 3. assembly
Modified atomizer-burner 1. 2.
3. 4. 5.
6.
Oxygen sheath Oxygen nozzle Fuel jacket Burner base Capillary Mounting block
ketone burned in an oxygen-hydrogen flame showed poor sensitivity, with interfering background in the region of the principal band heads. Spectra studies with a 3.4-meter Ebert spectrograph indicated that most of the background was due to Cz Swan bands. This background was least for the 546-mp band and the signal-background ratio was maximum a t 548 mp, Gilbert (3) has reported that the use of an oxygen-sheathed flame generally increases sensitivity. It was felt that oxygen supplied to the outer part of the flame (oxygen sheath) would give more complete combustion with a corresponding decrease of Swan band background as well as an increased boron emission. An oxygen sheath was added to the atomizer-burner (N.odel4020, Beckman Instruments, Inc., Fullerton, Calif.) as shown in Figure 3. The burner base of another atomizer-burner assembly was drilled to 0.375-inch inside diameter and the portion above the lower edge of the oxygen inlet port was increased to 0.413-inch inside diameter. This was positioned on the top of the burner and held in place by miring the oxygen supply tube to the oxygen and hydrogen supply tubes of the burner on both sides of the burner mounting block. It is planned to add a diffuser plate to distribute the ohygen sheath more uniformly, giving a more symmetrical flame. This oxygen-sheathed flame increased both the emision of boron and background n-ith an optimum signalbackground ratio at a sheath oxygen flow rate of 0.68 cu. foot per minute. This flow rate n-as found to be critical and was controlled t o -10.01 cu. foot per minute. Various portions of the flame were examined for boron sensitivity and for background level with extracted standards and with extracted blanks. The boron emission was relatively VOL. 35, NO. 1, JANUARY 1963
63
constant over the sampled region of the flame, while the background decreased toward the upper part of the flame. I n the upper region, the signal-back-
ground ratio was 4 to 1 a t the 500-pg. boron level at a wavelength of 548 mp. The flame emission was linear with boron concentration through 1000 pg. of
boron. Subtraction of the blank emission gave a linear curve that intercepted the origin. EFFECTS OF DIVERSE IONS
Table 1. Effect of Diverse Ions Millimoles added Diverse ion Added as 0.046 None, boron standard only Number 32 Standard deviation k1.25 95% confidence limits 53.4 t o 58.6
Flame reading 56.0
Anions 30
25
c1-
30 10
po4-3
5 2 0.5 30 15
NOa-
10
6
80"
Metal ions
K+ La +3
5.0 2.5 5.0 5.0 0.25 5.0 2.5 5.0 5.0 5.0 5.0 5.0 5.0 5. 0c 5.0 2.5 5.0 4.0
Mg +z
5.0
~ 1 + 3
Be +2 Bi + 3 Ca +2 Cd f z Cr f 3 Cr f6
c u +2 Fe + * Fe+3
KO3-
c1c1so,c1-
Ii&rZOj
Hgf2
2.0
M o +6
1.1
Na Nb+j
c1-
h-i t 2
SO3 -
+
(KsSbsOis. 15HzO)
Pb + 2 Pd +z Pt +4
NOJ-
Ru +s
c1-
c1-
0.01 5.0 1.0
0.1 0.01 4.0 0.4 5.0 2.5 0.5 0.08 0.008 0.0008
4.0 0.4 0.04 4.0
si t 4
0.8
1.0
Sn 1.2
0.2
5.0
Th+'
1.0
C T +8
5.0 1.0
\T
t5
w +B
WOa in "'OH
Zn + 2
so4
ZrO +2 a
64
0.6 0.06 0.006 1.0
0.1
0.001
-2
NO3 -
5.0 5.0
Nitrate removed by sulfuric acid fuming as described in procedure. Average of three determinations. 15 mmoles of "*OH added.
ANALYTICAL CHEMISTRY
54.0 53.5
>IO0 78.0
65.0 59.5 56.5 34.0
48.0
53.0 56.5 54. O b 61.0 57.0 56.5 80.0
59.0 59.0 57.0 56.5 54.5 >I00 57.0 56.5 >loo
56.5 52.5 52.5 56.5 47.5 56.0 57.0 >lo0
>loo
56.5 >I00 >lo0 60.0 52.0 55.0 50.0 53.0 55.5 >I00 59.0 55.5
> 100
75.5 57.0 63.0 57.0 60.0
S.3. .j 31 .0 56.0
84.5 58.0
>loo
>lo0 61.0 >lo0 99.0 60.0 55.0 56.5
An advantage of the extraction is that diverse anions and cations are separated and the organic phase provides a reproducible matrix such that background flame emission tends to be reproducible. Thus a flame intensity measurement at the emission peak alone should give a reliable analytical result. For this reason, the study of diverse ion effects is based on the flame emission value a t 548 mp with no background correction. In actual practice, spectroscopists probably will wish to employ background correction for this method t o improve the reliability. Background correction also would improve tolerance for those diverse ions that coextract and whose spectra include continuum, but no lines or bands, in the wavelength region of interest. Four anions and 30 cations were studied, as summarized in Table I. The quantity of diverse ion shown in the table and 0.046 mmole (500 pg.) of boron were pipetted into a tube, then analyzed by the outlined procedure. Standards of 0.046 mmole of boron were processed a t the rate of one standard per three diverse ion samples. A large scale extraction of the same boron standard level was made. The instrument response was set to give a value of 56 a t 548 mp with this master standard a t the beginning of each run. Aliquots of the master standard were aspirated periodically during runs and all flame intensity readings both for the diverse ion and extraction standard samples were normalized to this 56 value. The average and standard deviation, based on 32 estracted standards, was 56.0 f 1.25. One can consider a diverse ion effect to be significant if the flame reading falls outside the range of 53.4 to 58.6, the 95% confidence limits for a single determination. Anions. The effects of the common anions are shown in Table I. The level of sulfate is in excess of that added as the sulfuric acid reagent in the procedure. Phosphate enhances the flame intensity by increasing the background. Perchlorate forms a complex with the tetrabutylammonium ion, thereby consuming this reagent and preventing complete extraction of boron. The perchlorate complex also extracts, giving a potential explosion hazard. The flame intensity is depressed by large amounts of nitrate. This is attributed t o its extraction, with a consequent lowered flame temperature and an increased surface tension. A special pretreatment, based on sulfate fuming, was developed for samples high in nitrate and is described in the
procedure. Boron \pas quantitatively recovered by this procedure from 5-ml. sample aliquots of concentrated nitric acid. This confirms the study of Feldman (2), in which he evaporated nitric acid samples without loss of boron. Losses will occur if the heating is allowed to continue after the nitric acid is evaporated. Metal Ions. As shown in Table I, of 30 metal ions studied, only five extracted and caused high emission rc.adings a t a mole ratio to boron equal to or less than 1. Most metal ions had no effect a t a 10 to 1 mole ratio and nianv had no effect a t a 100 to 1 ratio. Iron(II1) as a chloride complex and dichromate also extract and cause high emission readings. Hydroxylamine will reduce both these ions to nonevtractable forms and the hydroxylamine does not interfere with
the flame emission of the extracted boron. Several metal ions were added as nitrate salts (as indicated in Table I). In some cases the total nitrate exceeded the noninterference level of about 10 mmoles, causing the observed low flame readings. OTHER APPLICATIONS
The high flame emission readings for some metal ions, particularly Mo(VI), Nb(V), Pt(IV), Ru(III), V(V), and R(VI), are indicative that this system might be useful for their flame photometric determination. The study of a chloride media extraction system also seems promising. The high flame readings observed with platinum and ruthenium are attributed to their eutraction as chloro complexes.
LITERATURE CITED
(1) Dean, J. A., “Flame Photometry,”
McGraw-Hill, New York, 1960.
(2) Feldman, Cyrus, AXAL. CHEM.33,
1916 (1961).
(3) Gilbert, P. T., Beckman Instruments,
Inc., Fullerton, Calif., private communication, November 1960. (4) Maeck, W.J., Booman. G. L., Kussy, M. C., Rein, J. E., ASAL. CHEM.33, 1775 (1961). (5) Morrison, G . H., Freiser, H., “Solvent Extraction in Analytical Chemistry,” Wiley, New York, 1957. (6) Pasztor, L. C., Bode, J. D., ASAL. CHEM.32, 1530 (1960). ( 7 ) Pasztor, L. C., Bode, J. D., Fernando, Quintue, Zbid., 32, 277 (1960). RECEIVED for reviewed August 8, 1962. Accepted October 22, 1962. First Annual Pacific Meeting of Applied Spectroscopy and Analytical Chemistry, Los Angeles, Calif., October 1962. Work done under Contract .4t( 10-1)-205 t o Idaho Operations Office, IT. S. Atomic Energy Commission,
Determination of Benzyl Chloroformate by Gas Chromatography of Its Amide Extension of the Principle to Other Acid Chlorides CHARLES HISHTA and JOSEPH
BOMSTEIN
Brisfol Laborafories, Division o f Brisfol-Myers Co., Syracuse, N.
b A combined chemical and gas chromatographic method has been developed for determining benzyl chloroformate and carboxylic acid chlorides in the presence of carboxylic acids. The method depends on converting the acid chlorides to amides by the use of diethylamine in chloroform, followed by gas chromatographic separation of the amides on glass beads coated with O.25y0 Carbowax 1500. Applied to benzyl chloroformate and to a-phenoxypropionyl chlo1.9 and ride, precision is shown to be =t2.4%, respectively (relative standard deviations).
*
I
x SYIiTHESIziNG carboxylic acid chlorides by conventional techniques, such as reaction of a carboxylic acid with thionyl chloride or phosphorus pentachloride, a mixture of the acid chloride, carboxylic acid, and hydrochloric acid generally results. Because these acid chlorides are often expensive and important intermediates, considerable attention has been given to determining them separately and in their mixtures. Lohr ( 2 ) has recently reviewed the work which has been done. Methods which have been applied are titrimetric in nature, either differential
Y.
or direct, and depend on neutralizing the acids present. Lohr’s work (2) involved a direct potentiometric titration of the acid chloride with cyclohexylamine; earlier methods required determination of the acid chlorides by difference. The present study was originally undertaken to provide a method for determining the purity of benzyl chloroformate (abbreviated hereafter as CBZ to conform with the name carbobenzoxy chloride in common usage), which appears to be too unstable to determine accurately by existing methods. The technique developed was found to apply as well to carboxylic acid chlorides, alone and in mixtures. Advantages are freedom from interference by other components, ability to measure the acid chloride directly (as the amide), and minimization of decomposition during analysis. PRINCIPLE
OF
THE METHOD
The acid chloride mixture is dissolved in a suitable solvent and allowed t o react with diethylamine. The resulting amide solution is extracted with acid to remove excess amine, and the amide is determined quantitatively by gas chromatography.
Separation of amides by gas chromatography has apparently received scant attention. Saunders and Murray (3) have reported the identification of amides from the reaction of dichlorocarbene with amines, but no detailed studies appear to have been published. Direct determination of the acid chlorides by gas chromatography was attempted in our laboratory, but decomposition of the samples during passage through the column prevented our adopting this approach. PROCEDURES
Procedures described here are for the determination of CBZ, but have been shown to be generally useful, as exemplified by applying them also to other acid chlorides. Preparation of Amide. Approximately 0.015 mole of CBZ is weighed accurately into a 10-ml. volumetric flask and diluted t o volume with Spectro-grade chloroform. A 2-ml. aliquot is pipetted dropwise into a 20-ml. test tube immersed in a n ice bath and containing 2 ml. of chloroform and 2 ml. of diethylamine. The mixture is agitated continually during the addition, and, finally, is stirred for 15 minutes a t room temperature. VOL. 35,
NO. 1 , JANUARY 1963
65
