`Vol. 30A, January 1991. pp, 66-69
`
`Effect of complexing reagents on the ioni-
`zation constantofboric acid andits rela-
`tion to isotopic exchangeseparation factor
`
`B K Sharma* & R Subramanian
`
`Chemical Technology Section,
`Indira Gandhi Centre for Atomic Research,
`Kalpakkam 603 102
`and
`PK Mathur
`
`Water and Steam Chemistry Laboratory,
`Applied Chemistry Division, BARC, IGCAR Campus,
`Kalpakkam 603 102
`
`Received 4 April 1990; revised 24 July 1990;
`accepted 22 August 1990
`
`The effect of change in concentration of complexing
`reagents having two or more hydroxyl groups, viz.,
`ethylene glycol, propylene glycol, dextrose and manni-
`tol on the ionization constant of boric acid has been
`studied by pH-metric titration method. The effect of
`increase in ionization constant of boric acid on iso-
`topic exchange separation factor for the separation of
`isotopes of boron by ion exchange chromatography
`has beenstudied by the batch method.
`
`Because of the higher cross-section of '"B for the
`reaction !B (n, a)’Li, boron compounds enriched
`in '’B isotope are generally used for control rods
`of fast breeder reactors (FBRs), neutron counters,
`neutron capture therapy of malignant tissues and
`treatment of melanotic cancers and brain tu-
`mours'*, Natural boron has 18.8 at. % of '°B and
`81.2 at. % of ''B. For efficient control of fast
`reactors, it is required to enrich boron in '’B iso-
`tope. Studies have been carried out to enrich '°B
`isotope by ion exchange chromatography in which
`a strong-base anion exchange resin in hydroxyl
`form is equilibrated with boric acid solution con-
`taining a complexing reagent?*. The increase in
`the isotopic exchange separation factor has been
`attributed to the increased ionization of boric
`acid’,
`The present study was underts'.en to investig-
`ate the effect of a few commercially available di-
`ols and polyols (complexing reagents for boric ac-
`id) on the ionization of boric acid andits relation
`to isotopic exchange separation factor.
`
`Experimental
`AR grade chemicals were used and demineral-
`
`66
`
`ized water, collected from a deionization CA-20
`unit, was used throughout this study. NaOH solution
`was prepared in CO,-free demineralized water
`and was standardized against standard potassium
`hydrogen phthalate solution pH-metrically. A 0.1
`M soloution of boric acid containing the required
`amount of complexing reagents was prepared by
`appropriate dilution of stock solutions of boric
`acid and the complexing reagent. At
`least 3 ali-
`quots from this sample solution were taken and
`titrated pH-metrically against standard NaOH so-
`lution. Another solution with varying concentra-
`tion of complexing reagent was also prepared in
`0.1 M boric acid and the entire procedure wasre-
`peated.
`A Metrohm titroprocessor was used for pH-
`metric titrations. The electrode was calibrated
`with potassium hydrogen phthalate (pH 4.02),
`phosphate (pH 6.18) and borax (pH 9.12) buffers.
`A macroporous strong base anion exchange in-
`digenous resin (Tulsion A-27 MP) having quater-
`nary amine type functional groups wasused in the
`study. The characteristic properties of the resin
`have been described elsewhere’.
`Boric acid was analysed bytitrating it against
`standard NaOH solution after
`the addition of
`mannitol. In the presence of HCI, the analysis was
`carried out alkalimetrically with standard NaOH
`by first
`titrating the sample to methyl
`red end
`point and then to phenolphthalein end point after
`the addition of mannitol.
`
`To determine the isotopic exchange separation
`factor by the batch method, a known quantity of
`the resin in hydroxyl form was taken in a stop-
`pered bottle. To this, an aliquot of the boric acid
`solution containing the required amount of the
`complexing reagent was added. The solution was
`allowed to equilibrate for 10 min with intermittent
`stirring of the contents. The supernate was then
`discarded and a fresh aliquot of the solution was
`added. This procedure was repeated 20-25 times
`to ensure the completion of isotopic exchange
`reaction. The resin was thus converted to borate
`form. This resin was then transferred to a small
`pyrex glass column and was eluted with HCl. The
`effluent was isotopically analysed for '°B/''B ra-
`tios as described below.
`
`To determine isotopic ratios, a sampie of boric
`acid was converted to sodium metaborate by the
`addition of Na,CO,. The isotopic analyses of bor-
`
`000001
`
`Exhibit 1100
`Exhibit 1100
`ARGENTUM
`ARGENTUM
`IPR2017-01053
`IPR2017-01053
`
`000001
`
`
`
`NOTES
`
`ic acid were carried out by using a VG Micro-
`[HA]: YHa
`
`PK,=p BTAya
`mass 30BK mass spectrometer, having a thermal
`ionization chamber and a Daly detector.
`'°B/''B
`ratios were determined by measuring the peak
`unionized acid a the corresponding anion” Te-
`where [HA] and [A™] are the concentrations of
`heights at mass numbers 88 and 89 for sodium
`metaborate ions containing '’B and ''B atomsre-
`spectively, produced by thermal
`ionization of
`Na,BO,.
`
`= pH + lo
`
`spectively and y,,;, and y,- are the corresponding
`activity coefficients. For the dilute solutions the
`activity coefficients of the ionic species may be
`taken as unity. Thus,
`
`When the solution is half-neutralized, [HA]=
`[A~]. Under such conditions pK,= pH. Thus, pH
`of the half-neutralized solution represents the pK,
`ofthe acid.
`From the data obtained during the titrations,
`the volume of NaOH required to neutralize the
`acid was determined by plotting ApH/AV versus
`the volume of the NaOH, and the value of pH at
`half-neutralization and, hence, pK, was noted.
`The relevant data pertaining to dextrose and man-
`nitol are presented in Table 1. As in the presence
`of ethylene glycol or propylene glycol the change
`in pK, of 0.1 M boric acid was found to be quite
`insignificant (0.09 and 0.13 pK, units for ethylene
`glycol and propylene glycol respectively), the data
`obtained in these cases are not included in Table
`
`Table | — Ionization constant of 0.1 M boric acid in presence of
`dextrose and mannitol
`Conc. of polyol
`lonization constant
`(MW)
`pK,
`=
`9.12*
`
`Polyol
`
`Results and discussion
`Out of various commercially available polyhy-
`LAN
`_
`droxy compounds, four complexing reagents, viz..
`pK,=pH + log TAR
`ethylene glycol, propylene glycol, dextrose and
`mannitol were selected for the present study with
`a view to selecting a suitable reagent
`that could
`be employed for an economical separation of iso-
`topes of boron by using ion exchange chromatog-
`raphy. It was observed that there was no signifi-
`cant change in the pH-metric titration profiles of
`0.1 M boric acid in the presence of 0.2 M-1.0 M
`ethylene glycol and propylene glycol. A significant
`change was, however, observed with dextrose and
`mannitol under similar conditions, which indicat-
`ed that boric acid becomesa strongeracid in pres-
`ence of these complexing reagents. This effect was
`much more pronounced in the case of mannitol
`than with dextrose. In fact, sufficiently large effect
`could be observed with mannitol even at relat-
`ively low concentrations (varied in the range 0.1-
`0.5 M). The sharp change in pH observed near
`the end point during the titrations was more in
`the presence of dextrose as comparedto that with
`ethylene glycol and propylene glycol. This change
`was still sharper in the presence of mannitol indi-
`cating thereby that mannitol was moreeffective in
`increasing the ionization of boric acid.
`The ionization ofboric acid or the polyol-boric
`acid complex (HA) maybe representedas:
`
`=
`
`Dextrose
`
`0.2
`
`8.98
`
`HA = H* + A™
`
`where A’ is the corresponding anion. Theioni-
`zation constantfor the above reactionis given by,
`
`x, = HA)
`(HA)
`
`Solving for pK,, we get
`
`Mannitol
`
`0.4
`
`0.6
`
`0.8
`
`1.0
`0.05
`0.10
`
`O.1S
`
`0.20
`
`0.25
`
`8.71
`
`8.56
`
`8.43
`
`8.29
`8.55
`7.88
`
`7.31
`
`6.93
`
`6.62
`
`(HA)
`_
`pK,=pH + log (A)
`* pK, of 0.1 Mboric acid alone
`
`
`0.30
`
`0.50
`
`6.42
`
`5.89
`
`or
`
`000002
`
`67
`
`000002
`
`
`
`INDIAN J CHEM, SEC. A, JANUARY 1991
`
`that pK, of boric
`1. It can be seen from Table 1
`acid gets reduced from 9.12 to 7.93 when the
`concentration of dextrose is varied from 0 to 1.0
`M.In the case of mannitol, however, the change in
`PK,is from 9.12 to 5.89, i.e., by more than 3 pK,
`units when its concentration changes from 0 to
`0.5M. This confirms that among the reagents
`studied, mannitol
`is the best complexing reagent
`for increasing the ionization constant of boric acid
`and, hence, for the separation of isotopes of bor-
`on by ion exchange chromatography. From Fig. 1,
`it can be observed that
`there is an abrupt de-
`crease in values of pK, when concentration of
`mannitol
`increases from 0 to 0.2 M for 0.1 M
`boric acid. Beyond this concentration, the change
`in pK,
`is relatively small
`indicating thereby that
`
`10-0
`
`9-0
`
`8-0
`
`7-0
`
`pKa
`
`6-0
`PE
`
`0
`
`0-2
`
`0-40
`
`0-60
`
`0-80
`
`1-00
`
`Concentration, M
`
`Fig.
`
`1 — Variation of pK, of 0.1 M boric acid with concentra-
`tions of dextrose and mannitol.
`
`2-0
`
`addition of 0.2 M mannitol to 0.1 M boric acid
`must be sufficient for its use as feed solution in
`the separation of the isotopes of boron by ion ex-
`change chromatography.
`The order of four complexing reagents in in-
`creasing the ionization of boric acid (ethylene gly-
`col=propylene glycol<dextrose< mannitol)
`is
`analogous to the order of formation constants of
`the complexes of these polyols with hydrated
`borate ion®~ °.
`The effect of concentration of mannitol on the
`isotopic exchange separation factor was studied
`by batch method’. From the obtained values ofis-
`otopic ratios, isotopic exchange separation factor
`was calculated*. From these values of isotopic ex-
`change separation factor (K),
`the values of pE
`were computed where E=K-— 1. The data so ob-
`tained are presented in Fig. 2. From thisfigure,it
`is found. that the value of isotopic exchange separ-
`ation factor
`increases as the concentration of
`mannitol
`is increased from 0 to 0.2 M. Beyond
`this, there is no significant change in isotopic ex-
`change separation factor. As a similar behaviour
`was observed for pK, of boric acid, an attempt
`was made to derive a relation between pE and,
`hence,
`the isotopic- exchange separation factor
`and pK, of boric acid. It was found that there ex-
`ists a linear relation between pK, and pEas given
`below:
`
`pE = 0.8554 + 0.12635 pK,
`
`The close agreement between experimentally
`observed and computed values is depicted in
`Fig. 3.
`
`
`
`15
`
`0
`
`0-10
`
`0:20
`
`0-30
`
`0-40
`
`0:50
`
`Concentration, M
`
`Fig. 2 — Variation of pE with concentration of mannitol in 0.1 M
`boric acid.
`
`Fig. 3 — Variation of pE with pK,of boric acid.
`
`68
`
`000003
`
`000003
`
`
`
`NOTES
`
`Conclusions
`Isotopic exchange separation factor for isotopes
`of boron increases as a result of addition of com-
`plexing reagents to boric acid. This increase is
`due to the increased ionization of boric acid.
`Among the four complexing reagents under study,
`mannitol is the most suitable. Addition of 0.2 M
`of mannitol is sufficient for 0.1 M boric acid to
`be used as the feed material for the separation of
`isotopes of boron by ion exchange chromatogra-
`phy.
`
`Acknowledgement
`The authors are grateful to Shri S.R. Paranjpe,
`Director,
`IGCAR for his keen interest
`in the
`present work and to Dr. M.K. Ahmed, Shri R.
`Balasubramanian and Shri D. Darwin Albertraj of
`the Radiochemistry Programme, IGCARfor help
`in someof the analytical work.
`
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