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Phenylboric acid is an important intermediate in the processing of spent nuclear fuel waste. A colorimetric method was developed for the determination of phenylboric acid in an aqueous process stream which involves liquid-liquid extraction and conversion to a boronate ester. The colorimetric reagent is an anthroquinone dye with vicinal hydroxy groups which react with alkylboric acids, shifting the visible absorption maximum. The structure of the ester is reported, along with a method of using this reaction for quantifying alkylboric acids.
A technology for disposal of Sr-90, Cs-137 and other fission by-products of the nuclear fuel cycle is to disperse them into molten glass which is then cast into stainless steel canisters for burial in a geologic repository. This engineering solution requires several chemical processing steps prior to the introduction of the waste into the glass melting furnace. Sodium tetraphenylborate is used as a precipitating agent for cesium, which introduces organic moieties into the process. Prior to vitrification of the cesium containing waste materials, these organic constituents must be removed, as any high boiling organics which remain in the waste can adversely affect the operation of the glass melter.
The last intermediate in the chemical decomposition of tetraphenylborate ion to benzene and boric acid is phenylboric acid (PBA). Measurement of the residual phenylboric acid concentration in aqueous solution is desired for control of the process, as it not only shows the completeness of the chemical decomposition, but it is also indicative of the level of other organics in the process solution. The current method of analysis is to by HPLC equipped with a photodiode array detector.1 The sample preparation and analysis of one sample takes a half hour. A method was desired which would more quickly determine the concentration of PBA in samples containing significant amounts of Cs-137. This report relates a colorimetric method for measuring the concentration of phenylboric acid, which is also useful for other monoalkyl organoboric acids.
The search for a PBA specific reagent started by considering reagents which are used for the determination of elemental boron. The standard methods for the determination of boron in samples call for the complete oxidation of the sample, which converts all the boron compounds to B2O3 2-4. Boric acid, H3BO3, is then formed by heating the sample with water and this can be quantified by adding a colorimetric reagent and measuring the complex either by direct absorbance or fluorescence. The obvious drawback of this method is that it requires conversion of the organoboron compounds to inorganic borate and would not distingush PBA from B2O3 .
Quinalizarin (1,2,5,8 tetrahydroxy anthroquinone) is a dye which has been used as a colorimetric reagent for the determination of Be, Al, Mg, and Cu in addition to B5,6. Although quinalizarin can be used to detect as little as 60ppb concentrations of borate, it also suffers from requiring a solvent system of 94% H2SO4 for optimum sensitivity. The highly acidic environment is a problem for the stability of organoboron compounds. In this study, quinalizarin and other anthroquinone dyes were investigated for their reactivity with phenylboric acid in organic solvents to give a response which could be adapted to an analytical, colorimetric method.
The mechanism by which the colorimetric reagents work is the formation of a boronate ester with a hydroxy group on the dye molecule, which interacts with the chromophore and shifts the absorption maximum. Vicinal diols are known to undergo esterification reactions with boric acids to give a boronate ester, with either 1:1 or 2:1 stoichiometries. The 2:1 stoichiometry is distinguished from the 1:1 stoichiometry by being an anion with a quadridentate boron. Of the common anthroquinone dyes, quinalizarin, alizarin (1,2 dihydroxy anthroquinone), and alizarin red S (1,2 dihydroxy, 3-sulfonato anthroquinone) have a vicinal diol and were tested as possible colorimetric indicators for organoboric acids.
Alizarin (Acros), alizarin red S (Aldrich), and quinalizarin (Pfaltz&Bauer)
and were used as received without further purification. Solvents used were
reagent grade without further purification. Methylboric, butylboric and
phenylboric acids (Aldrich) were used without further purification.
The anthroquinone (10.0 mmole) was dissolved in 75 ml of hot dimethylformamide
(DMF). An excess of alkylboric acid (11.0 mmole) was added and the mixture was
stirred for 15 minutes. The solvent was then removed by flowing nitrogen gas
into the reaction vessel until it was dry (about 3 hours). The crude product is
difficult to purify by recrystallization because it has low solubility in
common organic solvents and does not show an appreciable increase in solubility
with increasing temperature. A solvent system of isopropanol and xylene gave
the best crystals of the product. Crystals of the boronate ester of
quinalizarin with methyl-, butyl-, and phenylboric acid were isolated using
this procedure.7
FTIR spectra of the solid product was taken on a Nicolet model 210 FTIR
spectrometer, with a nominal resolution of 2 cm-1 . NMR spectra
were taken using a Varian Gemeni 300 instrument with dimethyl sulfoxide as the
solvent.8 HPLC measurements were taken on a Hewlett Packard model
1090 instrument using acetonitrile-water eluent and a Chemco 5-ODS column.
Spectra of the anthroquinone-alkylboric acid reaction were taken on a diode
array spectrophotometer using fiber optics to couple the source to the sample
cell and back to the detector. Due to the high radioactivity of the PBA
containing samples, a remote sample cell for the spectroscopy is desired and
fiber optics provide a method for implementing what would otherwise be routine
visible spectra. The spectrometers had 1024 element detectors, covering a
spectral range of 380 to 800 nm, with a typical resolution of 2 nm/pixel.
Results obtained on three different spectrometers (Hewlett Packard, Zeiss, and
Ocean Optics) were similar.
The FTIR and NMR spectra of the reaction products are identical and independent of whether methyl-, butyl-, or phenylboric acid was the starting material. This indicates that only one product is formed when quinalizarin reacts with an alkylboric acid. The proton NMR spectra of quinalizarin and the product are quite similar, with the only substantial difference being a new peak far downfield at 11ppm. This would be consistent with a hydroxyl group on boron.
The process solution to be measured for PBA content (known as PHA for "process hydrolysis aqueous") contains a variety of organic decomposition by-products and is approximately 0.4 M in formic acid and 900 ppm in Cu+. A preliminary extraction of the PBA is required before analyzing for it using this boronate ester reaction. Liquid-liquid extraction into a nonpolar solvent was tried with methylene chloride. The partitioning of PBA between methylene chloride and either (1) water or (2) process solution (PHA), was determined by HPLC. The following table shows the ratio of the PBA concentration in the methylene chloride to PBA remaining in the aqueous phase.
Trial (ml extractant and ml sample) |
Partition (MeCl2/water) |
Variance (%) |
4ml MeCl2 /4ml water |
0.515 |
2.4 |
3ml MeCl2/3ml PHA |
0.912 |
3.56 |
3 ml MeCl2/5ml PHA |
0.784 |
2.56 |
4 ml MeCl2/5ml PHA |
0.745 |
2.93 |
Table 1. Partitioning of phenylboric acid (PBA) between aqueous and nonpolar
phases.
After extraction into methylene chloride, the extractant solution is dried with a drying agent9 to remove remaining water (as water was found to be an interferent) and added to a known concentration of quinalizarin. The amount of phenylboric acid present is measured by observing the change in the absorbance at 526nm. Recalling that quinalizarin forms colored complexes with a variety of transition metal ions, including copper, this extraction step has the added benefit of removing metal ions which could be possible interferents.
Figure 1 shows the absorption spectra of quinalizarin and phenylboric acid using pure dimethylformamide as the solvent. The maxiumum absorbance of quinalizarin is at 580 nm, and the reaction with phenylboric causes a shift to a maxiumum at 526 nm with an isosbestic point at 576 nm. Comparison of methyl-, butyl-, and phenylboric acids , shows no perceptible difference in the shape of the resulting visible spectrum. In pure DMF, the reaction requires about 15 minutes to go to completion at room temperature, which allows the reaction to be monitored by an on-line spectrometer.
Fig. 1 Visible spectrum of the quinalizarin-phenylboric acid reaction. The isosbestic point for the reaction is at 576nm and the absorption maximum of the boronate ester is at 526nm.
Alizarin also forms a boronate ester with alkylboric acids and the progress of this reaction is shown in Fig. 2. It should be noted that the change in both absorption intensity and wavelength is smaller with alizarin than quinalizarin, making the latter the preferred colorimetric reagent. Esterification was also attempted with alizarin red S, but no reaction was observed. In this case, the presence of the sulfonate anion on the anthroquinone molecule inhibits deprotonation of the hydroxy group on the anthroquinone which participates in formation of the boronate ester.
Fig. 2 Visible spectra of the alizarin-methylboric acid reaction in pure DMF. The isosbestic point for the reaction is at 552 nm and the absorption maximum of the boronate ester is at 452nm. Spectra were taken at 15 second intervals to show the progress of the reaction.
Quinalizarin exhibits acid-base indicator behavior10, being orange in acid form (neutral), blue in mild base (deprotonation of one hydroxy group) and purple in strong base (deprotonation of two hydroxy groups) . In a nonpolar solvent like toluene where deprotonation is unlikely, quinalizarin is orange. Esterification was not observed in any of the solvent systems which were orange in color (isopropanol, toluene, DMF with acetic acid added). In the slightly basic solvent environment of the DMF/MeCl2 system used in this method, quinalizarin is blue and the reaction is complete in 15 minutes. The rate of formation of the boronate ester is highly dependant on the basicity of the solvent, with DMF giving the fastest reaction of any pure solvent. Solvent systems containing at least 50% DMF were basic enough to allow the reaction to go to completion,although the reaction rate varied. In the solvent system of dimethylacetamide/methylene chloride/isopropanol (67%, 24%, 9%, respectively) having an added base concentration of 1.23 x 10-3 M diaminopropane, the esterification reaction is immediate.
The formation of the boronate ester is a condensation reaction producing water, and water present in the solvent system will push the equilibrium of the reaction back to leave unreacted quinalizarin and alkylboric acid. Trials with 4% and 8% water in DMF resulted in a reduction of the absorbance at 526nm of 27% and 50%, respectively.
The possibilty of an alcohol as the leaving group in the reaction was tested by adding methanol to the reaction of methylboric acid and quinalizarin, with no change in the immediacy of the reaction. The inference to be drawn is that methanol is not involved in the reaction kinetics as a leaving group.
A balanced chemical reaction consistent with the observations of the experiment is:
Figure 3 shows the ionization state of the reactants as the solvent pKa changes. In acidic or very nonpolar solvents, both anthroquinone and alkylboric acid are protonated and the reaction is too slow to be observable. In sufficiently strong base, both reactants are anions and their electrostatic repulsion quenches the reaction. In the narrow region where the anthroquinone has lost one proton and the alkylboric acid has not, one finds that the reaction can proceed.
Fig. 3 Ionization State plotted vs. pKa. The pKa of alkylboric acids coincide with the second dissociation of quinalizarin, explaining the small range of basicity under which the reaction can proceed.
A characterization of the method of using quinalizarin to measure alkylboric acid concentrations was not completed, however some parameters of the method were obtained. From the visible spectra, one can extrapolate that to get a change of 0.001 absorbance unit at 526nm, a concentration change of 5x10-8M (6ppb) of alkylboric acid is required. The upper limit for the method corresponds to saturation of the detector at high levels of the dye, or about 5x10-4M. Preliminary results indicate that the method is linear over this range, but full characterization of the calibration curve was not performed.
Quinalizarin has been shown to be a quantitative colorimetric reagent for the measurement of monoalkyl boric acids. Water acts as an interferent and requires that the monoalkyl boric acid be first extracted into a nonpolar solvent and dried. The kinetics of the reaction are highly solvent dependant, requiring deprotonation of one of the anthroquinone hydroxy groups, but no deprotonation of the monoalkyl boric acid. Dimethylformamide provides close to optimal pKa conditions for the reaction to proceed. Since the alkyl substituent is lost in the reaction with the anthroquinone indicator, this cannot be used as a method to differentiate between differently substituted alkylboric acids.