Indian Journal of Chemistry
`Vol. 30A. January [991. pp. 66-69
`
`Effect of complexing reagents on the ioni-
`zation constant of boric acid and its rela-
`
`tion to isotopic exchange separation factor
`
`B K Shanna" & R Subramanian
`
`Chemical Technology Section.
`lndira Gandhi Centre for Atomic Research.
`K‘alpakkam 603 IUZ
`and
`P K Malhur
`
`Water and Steam Chemistry Laboratory.
`Applied Chemistry Division. BARC. IGCAR Campus.
`Kalpakltam 603 102
`
`Received 4 April I990; revised 24 July 1990;
`accepted 22 August 1990
`
`The effect of change in concentration of complexing
`reagents having two or more hydroxyl groups, via,
`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 been studied by the batch method.
`
`Because of the higher cross-section of "'B for the
`reaction "’8 (n, (1)7Li, 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.
`”/u of ”B. For efficient control of fast
`
`reactors, it is required to enrich boron in "’8 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 undertaken 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 and. its relation
`to isotopic exchange separation factor.
`
`Experimental
`AR grade chemicals were used and demineral-
`
`66
`
`000001
`
`ized water, collected from a deionization CA-Zt)
`unit, was used throughout this study. NaOH solution
`was prepared in Col-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-mctrically 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 was re-
`peated.
`A Metrohm titroprocessor was used for pH-
`metric titrations. The electrode was calibrated
`
`with potassium hydrogen phthalate (pi-1 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 was used in the
`study. The characteristic properties of the resin
`have been described elsewherei.
`
`Boric acid was analysed by titrating 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 giass column and was eluted with HCI. The
`effluent was isotopically analysed for 1"B/"B ra-
`tios as described below.
`
`To detennine isotopic ratios. a sanipic of boric
`acid was converted to sodium metaborate by the
`addition of Na2C03. The isotopic analyses of bor-
`
`Exhibit 1 100
`
`ARGENTUM
`
`Exhibit 1100
`ARGENTUM
`IPR2017-01053
`
`IPR2017-01053
`
`000001
`
`

`

`NOTES
`
`ic acid were carried out by using a VG Micro-
`mass 30BK mass spectrometer, having a thermal
`ionization chamber and a Daly detector.
`l0B/“B
`ratios were determined by measuring the peak
`heights at mass numbers 88 and 89 for sodium
`metaborate ions containing “JB and "B atoms re-
`spectively. produced by thermal
`ionization of
`NazBOZ.
`
`Results and discussion
`
`Out of various commercially available polyhy-
`droxy compounds, four complexing reagents. viz..
`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-metrie titration profiles of
`0.] M boric acid in the presence of 0.2 M-l.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 becomes a stronger acid 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 compared to that with
`ethylene glycol and prOpylene glycol. This change
`was still sharper in the presence of mannitol indi-
`cating thereby that mannitol was more effective in
`increasing the ionization of boric acid.
`The ionization ofboric acid or the polyol-boric
`acid complex (HA) may be represented as:
`
`HA7—‘H* +A'
`
`is the corresponding anion. The ioni-
`where A'
`zation constant for the above reaction is given by.
`
`K“ = (WM)
`(HA)
`
`Solving for (ma. we get
`
`_
`
`916)
`pH + log (A‘)
`
`pKi,
`
`01'
`
`M. = pH + log
`
`
`lHAl'YHgt
`[A l‘yr\
`
`where [HA] and [A‘] are the concentrations. of
`unionized acid and the corresponding anioni re-
`spectively and yHA and yAr are the corresponding
`activity coefficients. For the dilute solutions the
`activity coefficients of the ionic species may be
`taken as unity. Thus.
`
`M
`z
`pK.I — pH + 10g[A_]
`
`lHAl=
`When the solution is half-neutralized.
`[A‘]. Under such conditions pKu=pH. Thus. pH
`of the half—neutralized solution represents the pKa
`of the 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. pKa 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 pKa of 0.1 M boric acid was found to be quite
`insignificant (0.09 and 0.13 pKa units for ethylene
`glycol and propylene glycol respectively). the data
`obtained in these cases are not included in Table
`
`Table l — Ionization constant of 0.1 M boric acid in presence of
`dextrose and mannitol
`
`Poiyol
`
`_
`
`Dextrose
`
`Mannitol
`
`Cone. of polyol
`{Ml
`._
`
`Ionization constant
`pK,
`9.13'
`
`0.2
`
`0.4
`
`0.6
`
`0.8
`
`1.0
`
`0.05
`0.10
`
`0. [5
`
`0.20
`
`0.25
`
`0.30
`
`0.50
`
`8.98
`
`8.7l
`
`8.56
`
`8.43
`
`8.29
`
`8.55
`7.88
`
`7.3]
`
`6.93
`
`6.63
`
`6.42
`
`5.39
`
`‘pK, of 0.! Mboric acid alone
`
`
`000002
`
`67
`
`000002
`
`

`

`INDIAN J CHEM. SEC. A, JANUARY 1991
`
`that pK, of boric
`1. it can be seen from Table I
`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 pKa when concentration of
`mannitol
`increases from 0 to 0.2 M for 0.1 M
`boric acid. Beyond this concentration, the change
`in pKa is relatively small
`indicating thereby that
`
`10-0
`
`9-0
`
`PM
`
`3-0
`
`7-0
`
`6-0
`
`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" ' 3.
`The effect of concentration of mannitol on the
`
`isotopic exchange separation factor was studied
`by batch method". From the obtained values of is-
`otopic ratios, isotopic exchange separation factor
`was calculated“. From these values of isotopic ex-
`change separation factor (K),
`the values of p13
`were computed where E = K - l. The data so ob-
`tained are presented in Fig. 2. From this figure, 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 pKa 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 pE as given
`below:
`
`pE = 0.8554 + 0.12635 pK,
`
`The close agreement between experimentally
`observed and computed values is depicted in
`Fig. 3.
`
`
`
`5-0
`
`6-0
`
`7-0
`
`8-0
`
`9-0
`
`10-0
`
`0
`
`O-E
`
`IMO 0-60
`
`am LN
`
`(men tuition, M
`
`Fig.
`
`l — Variation of pK. of 0.1 M boric acid with concentra-
`tions of dextrose and mannitol.
`
`2-0
`
`1-9
`
`1.5}
`
`1-7
`
`J
`
`1-6 4
`
`Ia
`
`,
`
`1-5 Ly—y—r—v—v—v—v—r—r—‘t
`
`0
`
`0-10
`
`D-N
`
`0-30
`
`0410
`
`0'50
`
`Cortentmtionm
`
`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
`
`References
`
`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, IGCAR for help
`in some of the analytical work.
`
`1 Blue T E. Robert T C. Barth R F. Talnagi J W 8: Alam F.
`Nucl Tech. 77 (1987) 220.
`2 Kakihana H. Kotaka M. Satoh S. Nomura M & Oka—
`mato M. Bullchem Sochn. 50(1977) 158.
`3 Yoneda Y. Uchijima T & Makishima S. J phys Chem. 63
`(195902057.
`
`4 Sharma B K. Separation of boron isotopes by ion exchange
`chromtography. M. Phil. Dissen. Punjabi University. Pa-
`tiala. 1986.
`
`S Bassett J. Denney R C. Jeffery G H & Mendham J. Vogel‘s
`text book of quantitative inorganic analysis. 4th Edn {The
`English Language Book Society. London ), 1979. 19.
`
`6 PaalT,Ac!a chem hung, 103(1980) 181.
`7 Roy G L. Laferriere A L & Edward J 0. J inorg nucl
`Chem. 4(1957) 106.
`8 Mikan A & Bartusek M. Call Czach chem Commun. 45
`(1980)2645.
`
`9 Shanna B K & Subramanian R. Symposium on mdiochem—
`isrry & radiation chemistry.
`lGCAR. Kalpakkam. 1989.
`paper ST~03.
`
`000004
`
`69
`
`000004
`
`