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Inorganic Chemistry Communications 10 (2007) 20–22
www.elsevier.com/locate/inochepyPreparation of highly concentrated aqueous solution of sodium borate
Isao Tsuyumoto a,*, Tomokazu Oshio a, Kenji Katayama b

Department of Environmental Chemistry, College of Environmental Engineering and Architecture, Kanazawa Institute of Technology,
7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japanb
Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japanal

Received 10 July 2006; accepted 25 August 2006
Available online 8 September 2006

coaAbstractrson

A highly concentrated aqueous solution of sodium borate with 5.24 mol/kg of boron was prepared at 20 °C by adjusting the Na/B
ratio at 0.22 (pH 6.9). This is more than twice more concentrated than the most soluble known borate solution (Na2O Æ B2O3 Æ 8H2O,
2.49 mol/kg at 20 °C). Analysis using the Raman spectroscopy and the APCI/MS (atmospheric pressure chemical ionization/mass spec2
22trometry) suggested the formation of polyborate ions such as B9 O10 ðOHÞ2
9 , B10 O12 ðOHÞ8 , B11 O14 ðOHÞ7 , B12 O16 ðOHÞ6 .
Ó 2006 Elsevier B.V. All rights reserved.

peKeywords: Boron; Borate; Fire retardant; Raman; Sodium; Boric acid; Polyborate

Au

thor'sBorax (Na2[B4O5(OH)4] Æ 8H2O) shows ﬁreprooﬁng
eﬀect for cellulose-based materials such as wood, paper,cotton, and Gay-Lussac ﬁrst investigated scientiﬁcally theﬁreprooﬁng properties of borax and boric acid (H3BO3)
in 1823 [1]. The solubilities of boric acid and borax in water
are not suﬃciently high at room temperature (4.90 forboric acid, and 5.14 for borax at 20 °C), and thus there
are technical diﬃculties in impregnating suﬃcient amountof the active constituent into wood. Since their solubilities
in hot water are rather high (23.6 for boric acid, 79.60 for
borax at 80 °C), some researchers have proposed animpregnation treatment at high temperature (80 °C). However, this causes the precipitation of crystals on surfaces ofwood and results in the deterioration of wood. In order toestablish eﬀective ﬁreprooﬁng of wood, a highly concentrated aqueous solution of borate at room temperature isdesirable. It is known that the sodium borate of the composition Na2O Æ B2O3 Æ 8 H2O [2] showed the highest solubility, 41.9 in water at 20 °C, corresponding to 2.49 mol/kgof boron. In the present study, we have successfully pre-*Corresponding author. Tel.: +81 76 294 6712; fax: +81 76 248 9417.
E-mail address: tsuyu@neptune.kanazawa-it.ac.jp (I. Tsuyumoto).

1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.08.019

pared a highly concentrated aqueous solution of sodium
borate with 5.24 mol/kg of boron at 20 °C.
When we prepare a saturated solution at high temperature by dissolving boric acid or borax into hot water andthen cool it to a room temperature, the solution recrystallizes and becomes a saturated solution at room temperature. However, when the mixture of boric acid and borax
was dissolved at a certain ratio in hot water, we found thatthe solution did not show recrystallization after cooling toroom temperature. This was not oversaturation because
the solutions remained transparent without any precipitation. The Na/B ratio at which the solutions more concentrated than the known borate were formed was in therange of 0.19–0.40. The most concentrated sodium borate
solutions were formed in the range 0.22–0.27 (pH 6.9–6.5). For example, the solution prepared by dissolving
20 g of boric acid and 25 g of borax into 100 g of hot waterover 80 °C did not recrystallize at 20 °C. This corresponds
to the Na/B ratio 0.22 and 5.24 mol/kg of boron. Similarly,
the solution prepared by dissolving 25 g of boric acid and
35 g of borax into 100 g of hot water did not recrystallize
at 20 °C. This corresponds to the Na/B ratio 0.27 and6.62 mol/kg of boron. The properties of the former solution (Na/B 0.22, 5.24 mol/kg) at 20 °C are surface tension,

I. Tsuyumoto et al. / Inorganic Chemistry Communications 10 (2007) 20–22onalcopy

in the middle of those of borax and boric acid solutions.
This suggests that mixing of the boric acid solution andthe borax solution at room temperature does not accompany the reaction of borate anions. On the other hand,the present borate solution prepared by dissolving 20 g ofboric acid and 25 g of borax in 100 g of water at 80 °C
showed the peak at a much smaller wavenumber than theother three. It has been reported that the peaks of Raman
spectrum for vibration of the metaborate ring ðB3 O3
6 Þ arein the range 382–778 cm1 [3]. The peaks for B–O–H bending appears at 1077 cm1 and 1163 cm1 [4], and those for
B–O stretching at 878 cm1 and 1365 cm1 [5]. The reduction of the wavenumber observed for the present borate
solution indicates that B–O–H was transformed into themetaborate ring (B3O6). Polyborate ions are considered
to be formed by condensation of the conventional borate
2ions such as BðOHÞ
derived
4 , B3 O3 ðOHÞ4 , B4 O5 ðOHÞ4from boric acid and borax.
The APCI/MS (atmospheric pressure chemical ionization/mass spectrometry) [6] spectrum of the present borate
solution is shown in Fig. 3. Polyborate ions such as2222
B7 O8 ðOHÞ7 , B8 O10 ðOHÞ6 , B8 O8 ðOHÞ10 , B9 O10 ðOHÞ9 ,22
2
B10 O12 ðOHÞ8 , B11 O14 ðOHÞ7 , B12 O16 ðOHÞ6 , B13 O18 2
2
ðOHÞ2
were observed
5 , B14 O20 ðOHÞ4 ; B15 O22 ðOHÞ3along with the conventional ions such as B4 O5 ðOHÞ3,4
B3 O3 ðOHÞ4 . This regularity leads to B18 O2
andBO20 32 ,28
though they have not been clearly observed. It is interesting4
to note that B18 O2
28 and B20 O32 are considered to form
spherical shapes, because they do not have terminal oxygen. B20 O4
32 may form the spherical structure similar tothe pentakis dodecahedron having 60 isosceles triangles
and 32 vertices. One possible structure for B10 O12 ðOHÞ2
8pers

67.5 mN/m; viscosity, 4.5 mPa s; density, 1.13 g/cm3; andpH, 6.86. These solutions were transparent and showed
the Tyndall phenomenon when irradiated by laser light.
This indicates that the solute forms colloidal dispersion
consisting of polyborate ions.
Evaporation of the solution (5.24 mol/kg) by heatingyielded the residue which appeared to be solid foam withthe density 0.1–0.3 g/cm3. The XRD pattern of the residue
showed that it is an amorphous phase of sodium borate(Fig. 1). When the solution was dried extremely slowly inthe air at room temperature, Na2B18O28 precipitated in asmall quantity. The resultant amorphous sodium borate
was easily soluble in water at room temperature, and its
concentration reached that of the original solution before
evaporation. It should be noted that the mixture of boric
acid and borax does not dissolve into water at room temperature to the level of the highly concentrated solution.
This is considered to be due to the problem of kinetics.
The Raman spectrum for the present highly concentrated borate solution is shown in Fig. 2 along with thoseof the saturated borax solution (Na2[B4O5(OH)4] Æ 8H2O,4.9 wt%), and the saturated boric acid solution (H3BO3,4.7 wt%). The present borate solution showed a broad peak
centered around 910 cm1, whereas the borax and boric
acid solutions showed broad peaks around 1350 cm1and 1450 cm1, respectively. The mixed solution of borax
and boric acid showed a broad peak around 1400 cm1,21100thoFig. 1. Powder XRD pattern of the residue obtained by heating thepolyborate solution (5.24 kg/mol of boron).

2-80
B10 O 12 (OH) 8I/ %r's

B9 O 10 (OH) 92-60
40
B11 O 14 (OH) 72-

B12 O 16 (OH) 6202-Au

I/ a. u.

Polyborate

01000

050

Boric acid

100

150

200
250m /e

300

350

400

Fig. 3. APCI/MS spectrum of the highly concentrated polyboratesolution (5.24 kg/mol of boron).

Borax
Mixture

2000

3000

k /cm-1
Fig. 2. Raman spectra of the highly concentrated polyborate solution(5.24 kg/mol of boron), the saturated borax solution (Na2[B4O5(OH)4] Æ8H2O, 4.7 wt%), the saturated boric acid solution (H3BO3, 4.7 wt%), andthe mixture of the saturated boric acid and borax solutions.

Fig. 4. One possible structure for B10 O12 ðOHÞ2
8 .

22

I. Tsuyumoto et al. / Inorganic Chemistry Communications 10 (2007) 20–22

is shown in Fig. 4. The Raman and APCI/MS spectra indicate that the drastic increase in the borate concentration isdue to formation of the polyborate ions containing moreboron atoms.
These concentrated sodium borate solutions are usefulas ﬁreprooﬁng agents because of a remarkable ﬁreprooﬁng
eﬀect for cellulose-based materials such as wood, paper,cotton, rayon. It is also expected that it will be widely used
for new ﬁre retardant for polymers because it contains nohazardous heavy metals.

References

Au

thor'spe

rson

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[5] R.E. Mesmer, C.F. Baes Jr., F.H. Sweeton, Inorg. Chem. 11 (1972)
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