Green and Sustainable Chemistry, 2012, 2, 8-13
http://dx.doi.org/10.4236/gsc.2012.21002 Published Online February 2012 (http://www.SciRP.org/journal/gsc)
Synthesis, Characterization and Application of ZS/HMS
Catalyst in the Esterification of Gossypol
Shihong Dong, Mingyuan Zhu*, Bin Dai*
Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan,
School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, China
Received November 3, 2011; revised December 8, 2011; accepted December 17, 2011
ABSTRACT
A solid acid catalyst of zirconium sulfate (ZS) on a pure hexagonal mesoporous silica (HMS) sieve was prepared and
characterized by small angle X-ray diffraction, NH3-temperature programmed desorption, and thermogravimetric analy-
sis. The obtained ZS/HMS catalyst displayed a typical mesoporous structure, ZS was well dispersed on the HMS sup-
port, and the acidity increased with the amount of ZS loading. Gossypol was extracted from cottonseed cake with ace-
tone as solvent, and then the gossypol solution was esterified with ZS/HMS as catalyst to yield products of acetic acid
gossypol. Under the optimal conditions, the conversion efficiency of gossypol was as high as 96.7%.
Keywords: Gossypol; Zirconium Sulphate; Hexagonal Mesoporous Silica; Esterification
1. Introduction
Gossypol is a polyphenolic compound derived from the
yellow pigment glands of cotton plants that plays an im-
portant role in pest resistance [1,2]. Acetylated gossypol
is the esterification product of gossypol and acetic acid.
Acetylated gossypol has been studied in a wide range of
biological and medicinal fields due to its purported anti-
tumor and antifertility activities [3-5]. Gossypol is usual-
ly extracted from cottonseed kernels using aniline as in-
termediates [5,6]. The toxic residue in the cottonseed cake
is harmful to both humans and animals. Therefore, deve-
lopment of a non-toxic extraction method for acetylated
gossypol from cottonseed cake is of great importance.
Solid acid catalysts are widely used in esterification
reactions because of their low acute toxicity. Furthermore,
there is no evidence of carcinogenicity to humans. Sev-
eral recent studies reported that zirconium sulfate (ZS)
showed high activity and selectivity as a solid acid cata-
lyst for the esterification of fatty acids [7]; however, ZS
is easily soluble in water and difficult to remove from the
reaction mixture. To circumvent this problem, many re-
cent studies have investigated the synthesis of immobi-
lized ZS supported on various supports such as activated
carbon [8], γ-alumina [9], silica [10], carbon nanotubes
[11], or MCM-41 [12] as a solid catalysts.
Acetylated gossypol is the esterification product of
gossypol and solid acid catalyst may be able to enhance
the conversion ratio of this reaction. In this paper, we
describe the preparation of immobilized ZS on hexagonal
mesoporous silica (HMS) sieve. The ZS/HMS was char-
acterized and applied as a solid acid catalyst for the es-
terification of gossypol. The loading of ZS on HMS, the
amount of catalyst, the reaction time, and reaction tem-
perature were investigated to optimize the reaction con-
ditions for gossypol esterification.
2. Experimental
2.1. Catalyst Preparation
Pure siliceous HMS material was synthesized according
to a procedure described previously [13]. Briefly, do-
decylamine (DDA, 4.9 mmol) was dissolved in 5 mL of
ethanol, and 45 mL of distilled H2O was then added to
afford a 90:10 (V/V) H2O/EtOH solution of the surface-
tant. At room temperature, tetraethyl orthosilicate (TEOS
19.6 mmol) was added to the solution to yield a reaction
mixture. Into this reaction mixture, we added 7 mL etha-
nol during the mixing process to compensate for the vo-
latilization. The reaction flask was sealed with cling film
and shaken at 220 rpm in a heated water bath at room
temperature for 20 h. The reaction product was then fil-
tered, washed, and dried at room temperature for 24 h. The
obtained samples were calcined in a muffle furnace at
about 600˚C for 4 h.
The ZS/HMS materials were synthesized following
procedures similar to those of Joon and his co-works [14].
The ZS-impregnated HMS catalyst was prepared by first
dispersing the HMS in a water solution and then adding
*Corresponding author.
C
opyright © 2012 SciRes. GSC

S. H. DONG ET AL. 9
ZS. The mixture was stirred overnight and then dried at
120˚C for 10 h. The samples obtained after drying were
denoted X% ZS/HMS (X% refer to loading of the ZS).
Before use in the reaction, the ZS/HMS catalyst was
stored in desiccators.
2.2. Characterization
The powder X-ray diffraction (XRD) data was collected
on a Brunker D8 Advance X-ray diffractometer using
Cu-Kα irradiation (λ = 1.5406 Å) as source at 40 kV and
40 mA. The samples were recorded from 1 to 20˚ (2θ)
with a step scan of 0.01˚/s.
Brunauer-Emmett-Teller (BET) surface area analysis
was performed from the nitrogen adsorption isotherms at
77 K using a Micromeritics Model ASAP 2020 instru-
ment. All samples were degassed at 110˚C under vacuum
for 6 h. Average pore diameter (d) and pore volume were
calculated based on the Barret-Joyner-Halenda (BJH) me-
thod.
The NH3-temperature programmed desorption (NH3-
TPD) data were collected using a ChemBet 3000 analy-
zer. About 200 mg samples were activated under flowing
heat (500˚C) for 2 h, and cooled to 120˚C under conti-
nuous evacuation. The sample was equilibrated with ga-
seous NH3 at 0.04 kPa and the temperature increased by
10˚C/min under flowing heat 20 cm3/min.
Thermogravimetric analysis (TGA) was performed on
a Netzsch STA-449F3 (Jupiter OR, Germany) analyzer
under an oxygen atmosphere. Heating rates were typi-
cally 10˚C/min under an oxygen atmosphere. The gas
flow through the system was 20 mL/min.
2.3. Extraction of Gossypol
A sample of cottonseed cake raw material was weighed.
After grinding, cottonseed cake was put into a grind port
flask. An appropriate amount of acetone was added to the
flask. The cottonseed cake extracting solution was remo-
ved from the residue by vacuum filtration. The remaining
residue was washed three times with 50 mL of distilled
water to remove residual solvents. The liquid product
was filtered through a 0.45 μm organic membrane and 20
μl of the filtrate was analyzed by high performance liquid
chromatography. The chromatographic conditions, an C18
reversed-phase column was used. The particle size was 5
μm, and the column dimensions were 4.6 mm × 150 mm,
the mobile phase were methanol and 2% phosphoric acid
solution (90:10), the column temperature was 40˚C, the
flow rate was 1 mL/min and the samples were detected
by Waters a 2487 UV detector at 235 nm. The extraction
yield of gossypol from cottonseed cake was calculated by
the following formula:

12
R %WW100% (1)
The formula: R—extraction yield of gossypol;
W1—extracting amount of gossypol;
W2—the total content of gossypol in cottonseed cake.
2.4. Esterification of Gossypol with Acetic Acid
Esterification was performed at atmospheric pressure in a
grinding glass bottle. A typical esterification reaction con-
sists of acetic acid (1 mL), 0.6 mg/mL of gossypol (5
mL), distilled water (3.2 mL), and fresh ZS/HMS solid
acid catalyst (0.1 g). After acetic acid and gossypol were
mixed and stirred for 15 minutes, distilled water was ad-
ded drop by drop. The reaction mixture was kept on stir
for esterification. At the beginning of the reaction, the
catalyst was handed by ultrasonic dispersion. Speed of
agitation had only a small effect on the reaction rate. All
subsequent experiments were conducted at a stirrer speed
of 200 rpm to ensure that there was no external mass
transfer resistance. After standing for 10 minutes, the pro-
duced acetic acid gossypol was precipitated due to its low
solubility in water, and then the upper supernatant was
taken using HPLC to detect the remaining amount of gos-
sypol and computed the conversion of gossypol. The yel-
low precipitation was dissolved in acetone with a certain
amount. After stirring for 10 minutes, the sample was fil-
tered in order to obtain a pure gossypol acetic acid.
3. Results and Discussion
Figure 1 shows the typical XRD pattern for the ZS/HMS
reaction product with different ZS loading. The patterns
all contain an intense diffraction peak 2θ = 2.3˚ corre-
sponding to the diffraction of (100) plane, indicating that
these samples had the typical characteristic peaks of a me-
soporous molecular sieve in agreement with previous re-
ports [15].
246810
a
b
c
d
e
Intensity(a.u.)
2θ(Degree)
Figure 1. XRD patterns of ZS/HMS material for different
ZS/HMS ratios: a: HMS; b: 30% ZS/HMS; c: 40% ZS/
HMS; d: 50% ZS/HMS; and e: 60% ZS/HMS.

S. H. DONG ET AL.
10
The textural properties of samples were examined by
surface area and pore size distribution (Table 1). The ave-
rage surface area of the HMS was 637 m2/g. Surface area
and pore volume decreased with increasing ZS loads,
while the average pore size of ZS/HMS expanded from
3.2 to 4.1 nm with increasing ZS. At higher ZS loads, ZS
particles will enter into the surface the microporous struc-
ture and block superficial pores, reducing surface area and
surface pore volume. The pore volume reduced from 0.60
cm3/g to 0.20 cm3/g. Pore size distributions are shown in
Figure 2 and measured by N2 adsorption technique. The
pore sizes of catalyst decrease with increment of ZS load-
ing. As these surface microporous are blocked, the re-
maining pores of the support must be larger, so the aver-
age pore size underestimates the average pore diameters
within the ZS/HMS solid acid catalyst.
The nitrogen adsorption isotherms for HMS and ZS/
HMS samples with different ZS loading are shown in Fi-
gure 3. All samples exhibited the shape of II isotherms
according to the IUPAC classification for nitrogen ad-
sorption-desorption isotherms. The adsorption and desor-
ption branches were not parallel and hysteresis loops
emerge between 0.9 and 1. The hysteresis loops of ZS/
HMS were similar to those of parent of HMS, indicating
that ZS had good loading dispersion, consistent with the
XRD and results.
Table 1. The structure of various ZS/HMS samples.
Sample SBET (m2/g) d (nm) Pore volume (cm3/g)
HMS 637 3.2 0.60
20% ZS/HMS 571 3.4 0.48
30% ZS/HMS 315 3.7 0.29
40% ZS/HMS 253 4.0 0.26
50% ZS/HMS 213 4.1 0.22
60% ZS/HMS 193 4.1 0.20
30 40 50 60 70 80 90100
dV/dlogD
Pore size
(
nm
)
a
b
c
d
e
f
Figure 2. Pore size distributions for a: HMS; b: 20% ZS/
HMS; c: 30% ZS/HMS; d: 40% ZS/HMS; e: 50% ZS/HMS;
and f: 60% HMS.
Volume Adsorption [cm3/gSTP]
f
e
d
c
b
a
R e lative P re s su re (P /P o )
0.0 0.2 0.4 0.6 0.8 1.0
Figure 3. Nitrogen adsorption isotherms for a: HMS; b:
20% ZS/HMS; c: 30% ZS/HMS; d: 40% ZS/HMS; e: 50%
ZS/HMS, and f: 60% HMS.
There are many methods for acidity measurements [16,
17]. Ammonia TPD is widely used to determine the acid-
ity of solid acid. The NH3-TPD profiles of HMS and ZS/-
HMS are shown in Figure 4. Gannapati et al. [18] re-
ported that desorption temperature could be divided into
three categories: 1) intermediate or medium (100˚C -
200˚C); 2) strong (200˚C - 400˚C); 3) very strong acid
(>400˚C). The HMS exhibited one peak at 125˚C, reflec-
ting the physical adsorption of NH3 on the HMS surface.
When zirconium was loaded onto the HMS, peaks of dif-
ferent intensities (heights) at 242˚C will appear that indi-
cate strong acid strength. As the zirconium content in-
creased, the peak intensity also increased. The peak hei-
ght was equivalent at 50% zirconium and 60% zirconium
loads, indicating that these two load levels deposited equal
amounts of ZS on the HMS.
Thermogravimetric analysis was conducted under at-
mospheric conditions to test the thermal stability of bulk
ZS and HMS-supported ZS. The thermograms of HMS
and ZS/HMS materials with various ZS loadings are shown
in Figure 5. The first change in TGA curve between room
temperature and 100˚C is ascribed to removal of physic-
cally adsorbed and structural water. At higher tempera-
tures, TGA curves yield the decomposition pattern of
ZS/HMS. The profiles indicated that the unmodified HMS
showed little loss; however, TGA curves of ZS/HMS re-
vealed degradation between 650˚C and 750˚C, reflecting
breakdown of ZS groups on the silica surface, in agree-
ment with earlier reports [19].
The orthogonal test results for optimal extraction of
gossypol from cottonseed cake are listed in Tables 2 and

S. H. DONG ET AL. 11
0100 200 300 400 500 600 700
TCD Signal (a.u.)
Temper a ture (
o
C )
b
a
c
d
e
f
Figure 4. NH3-TPD curves for a: HMS; b: 20% ZS/HMS; c:
30% ZS/HMS; d: 40% ZS/HMS; e: 50% ZS/HMS; and f:
60% HMS.
0200 400 600 8001000
Weight loss (%)
a
b
c
d
Temperature (oC)
Figure 5. TGA curves for a: 40 wt% ZS/HMS; b: 50 wt%
ZS/HMS; c: 60 wt% ZS/HMS; and d: HMS.
Table 2. Variables and levels for orthogonal test.
Level A (time, h) B (Phosphoric
acid, mol/L) C (Acetone, %) D (solvent, mL)
1 8 1.0 60 25
2 16 1.4 70 30
3 24 1.8 80 35
3. Table 2 is variables and levels for orthogonal test;
Table 3 is three factors three levels (L9(34)) orthogonal
experiment and through 9 sets of experiments to deter-
mine the optimal conditions for gossypol extraction from
cottonseed cake using acetone. The influence of each va-
riable on the extraction yield of gossypol was determined
for extraction time (A), phosphoric acid concentration
(B), acetone concentration (C), and the amount of solvent
(D). From Table 3, it can be seen that the rank order of
influence was A > C > D > B. Extracting time had the
strongest influence on gossypol extraction, while the pho-
sphoric acid concentration was the least influential vari-
able. The results of range analysis revealed that the op-
timal combination of these four factors was A3B1C3D1.
The optimum conditions for gossypol extraction from cot-
tonseed cake using acetone were an extraction time of 24
h, phosphate concentration of 1.0 mol/L, 80% acetone, and
solvent dosage of 25 mL. At these optimal values, the ave-
rage removal rate of gossypol reached 63.75%.
The effects of the ZS/HMS catalyst on gossypol es-
terification are presented in Figure 6. It is obvious that
the conversion of gossypol to acetylated gossypol increa-
sed with increasing ZS loading on the catalyst and was as
high as 97% at 50 wt% ZS loading on HMS. No further
increase was observed at high ZS loading on HMS. Again,
this high efficiency indicated that the loaded ZS was well
dispersed on the HMS surface, thus maximizing catalytic
surface area. At ZS loading below 50%, the weaker ca-
talytic activity was due to lower dispersion of ZS on
HMS.
The amount of ZS/HMS catalyst used in the esterifica-
tion process also affected the acetylated gossypol con-
version efficiency. The effect of the catalyst dose on the
conversion efficiency is shown in Figure 7. The catalyst
amount was varied from 10 - 70 mg in 10 mg increments.
The conversion efficiency of acetylated gossypol increa-
Table 3. The L9(34) orthogonal experiment for determining
the optimal conditions for gossypol extraction from cotton-
seed cake using acetone.
Experiment no.A B C D
Extraction of
gossypol (mg/g)
1 8 1.0 60% 25 0.123
2 8 1.4 70% 30 0.184
3 8 1.8 80% 35 0.388
4 16 1.0 70% 35 0.159
5 16 1.4 80% 25 0.646
6 16 1.8 60% 30 0.146
7 24 1.0 80% 30 1.173
8 24 1.4 60% 35 0.327
9 24 1.8 70% 25 0.831
K1 0.2320.4860.199 0.533
K2 0.3170.3860.391 0.501
K3 0.7770.4550.736 0.291
R 0.5450.0990.537 0.242
K1: Average extraction of gossypol for the four factors in level 1; K2: Av-
erage extraction of gossypol for the four factors in level 2; K3: Average
extraction of gossypol for the four factors in level 3; R: mean range.

S. H. DONG ET AL.
12
20 30 40 50 60
90
91
92
93
94
95
96
97
98
Conver sion rate (%)
ZS loading on HMS (wt %)
Figure 6. Catalytic activities of solid acid catalysts with dif-
ferent ZS loading on HMS for esterification of gossypol.
10 20 30 40 50 60 70
80
85
90
95
100
Conversion rate (%)
Catalyst amount (mg)
Figure 7. Effect of catalyst amount on the esterification of
gossypol.
20406080100 120 140 160
92.5
93.0
93.5
94.0
94.5
95.0
95.5
96.0
96.5
97.0
Conversion rate (%)
Time (min )
Figure 8. Effect of reaction time on the esterification of gos-
sypol.
sed to reach the maximum conversion efficiency at 50
mg ZS/HMS, while addition of additional catalyst did not
significantly increase gossypol esterification efficiency.
These results confirmed that the average size of catalyst
particle (60 - 80 mesh) was small enough to limit the in-
ternal mass transfer inside the catalyst pores.
The reaction was carried out at room temperature with
50 mg ZS/HMS (50 wt%) and 2.5 mL gossypol (0.6 mg/
mL). The mixture was stirred at 200 rpm for 15 min.
Then 1.6 mL distilled H2O was added to the solution and
it was left standing. Samples were removed every 30 min
to determine gossypol ester production (Figure 8). The
conversion rate increased with reaction time and gradu-
ally peaked at 120 min. At 120 min, the conversion rate
of acetylated gossypol reached 96.5%.
4. Conclusion
The synthesized ZS/HMS shows good dispersion by TG,
XRD and BET characterizations. The acid activity of the
catalysts increases with the increment of the ZS loading.
When ZS loading on HMS was 50%, the optimal amount
of catalyst was 50 mg catalyst for an esterification reac-
tion of 120 min, the yield of acetylated gossypol reached
up to 96.7%.
5. Acknowledgements
The authors would like to acknowledge the support of
Doctor’s Special Fund of Xinjiang Production and Con-
struction Corps (No. 2010JC13), and Initial Fund for high-
level talents of Shihezi University (RCZX20036).
REFERENCES
[1] R. D. Stipanovic, D. W. Altman, D. L. Begin, G. A. Green-
blatt and J. H. Benedict, “Terpenoid Aldehydes in Upland
Cottons: Analysis by Aniline and HPLC Methods,” Jour-
nal of Agricultural and Food Chemistry, Vol. 36, No. 3,
1988, pp. 509-515. doi:10.1021/jf00081a026
[2] A. A. Nomeir, M. B. Abou-Donia and J. Am, “Photode-
composition of Gossypol by Ultraviolet Irradiation,” Jour-
nal of the American Oil Chemists Society, Vol. 62, No. 1,
1985, pp. 87-89. doi:10.1007/BF02541497
[3] L. L. Hua, J. J. Zhou and H. Y. Han, “Direct Electro-
chemiluminescence of CdTe Quantum Dots Based on
Room Temperature Ionic Liquid Film and High Sensitiv-
ity Sensing of Gossypol,” Electrochimica Acta Electron,
Vol. 55, No. 3, 2010, pp. 1265-1271.
doi:10.1016/j.electacta.2009.10.038
[4] W. A. Pons, J. Pominski, W. H. King, J. A. Harris and T. H.
Hopper, “Recovery of Gossypol from Cottonseed Gums,”
Journal of the American Oil Chemists Society, Vol. 36,
No. 8, 1959, pp. 328-332. doi:10.1007/BF02640046
[5] R. D. Stipanovic, J. C. Donovan, A. A. Bell and F. W.
Martin, “Factors Interfering in Gossypol Analysis of Okra
and Glandless Cottonseed Using Direct Aniline Extrac-

S. H. DONG ET AL.
13
tion,” Journal of Agricultural and Food Chemistry, Vol.
32, No. 4, 1984, pp. 809-810. doi:10.1021/jf00124a027
[6] W. A. Pons, R. A. Pittman and C. L. Hoffpauir, “3-Amino-
1-propanol as a Complexing agent in the Determination
of Total Gossypol,” Journal of the American Oil Chem-
ists Society, Vol. 35, No. 2, 1958, pp. 93-97.
doi:10.1007/BF02672661
[7] S. Furuta, H. Matsuhashi and K. Arata, “Catalytic Action
of Sulfated Tin Oxide for Etherification and Esterification
in Comparison with Sulfated Zirconia,” Applied Catalysis
A: General, Vol. 269, No. 2, 2004, pp. 187-191.
doi:10.1016/j.apcata.2004.04.017
[8] J. C. Juan, J. C. Zhang, Y. J. Jiang, W. L. Cao and M. A.
Yarmo, “Zirconium Sulfate Supported on Activated Car-
bon as Catalyst for Esterification of Oleic Acid by n-Bu-
tanol under Solvent-Free Conditions,” Catalysis Letters,
Vol. 117, No. 3-4, 2007, pp. 153-158.
doi:10.1007/s10562-007-9128-3
[9] J. R. Sohn and D. H. Seo, “Preparation of New Solid
Superacid Catalyst, Zirconium Sulfate Supported on γ-Alu-
mina and Activity for Acid Catalysis,” Catalysis Today,
Vol. 87, No. 1-4, 2003, pp. 219-226.
doi:10.1016/j.cattod.2003.09.010
[10] C. L. Chen, T. Li, S. Cheng, H. P. Lin, C. J. Bhongale and
C. Y. Mou, “Direct Impregnation Method for Preparing
Sulfated Zirconia Supported on Mesoporous Silica,” Mi-
croporous and Mesoporous Materials, Vol. 50, No. 2-3,
2001, pp. 201-208. doi:10.1016/S1387-1811(01)00453-X
[11] J. C. Juan, Y. Jiang, X. J. Meng, W. L. Cao, M. A. Yarmo
and H. C. Zhang, “Supported Zirconium Sulfate on Car-
bon Nanotubes as Water-Tolerant Solid Acid Catalyst,”
Materials Research Bulletin, Vol. 42, No. 7, 2007, pp.
1278-1285. doi:10.1016/j.materresbull.2006.10.017
[12] C. L. Chen, S. Cheng, H. P. Lin, S. T. Wong and C. Y.
Mou, “Sulfated Zirconia Catalyst Supported on MCM-41
Mesoporous Molecular Sieve,” Applied Catalysis A: Gen-
eral, Vol. 215, No. 1-2, 2001, pp. 21-30.
doi:10.1016/S0926-860X(01)00504-X
[13] T. R. Pauly and T. J. Pinnavaia, “Pore Size Modification
of Mesoporous HMS Molecular Sieve Silicas with Worm-
hole Framework Structures,” Chemistry of Materials, Vol.
13, No. 3, 2001, pp. 987-993. doi:10.1021/cm000762t
[14] J. C. Juan, J. C. Zhang and M. A. Yarmo, “Study of Cata-
lysts Comprising Zirconium Sulfate Supported on a Me-
soporous Molecular Sieve HMS Foresterification of Fatty
Acids under Solvent-Free Condition,” Applied Catalysis
A: General, Vol. 347, No. 2, 2008, pp. 133-141.
doi:10.1016/j.apcata.2008.06.004
[15] P. T. Tanev and T. J. Pinnavaia, “A Neutral Templating
Route to Mesoporous Molecular Sieves,” Science, Vol.
267, No. 5199, 1995, pp. 865-867.
doi:10.1126/science.267.5199.865
[16] S. M. Rlseman, F. E. Massoth, G. M. Dhar and E. M.
Eyring, “Fourier Transform Infrared Photoacoustic Spec-
troscopy of Pyridine Adsorbed on Silica-Alumina and γ-
Alumina,” The Journal of Chemical Physics, Vol. 86, No.
10, 1982, pp.1760-1763.
[17] L. Forni, “Comparison of the Methods for the Determina-
tion of Surface Acidity of Solid Catalysts,” Catalysis Re-
views—Science and Engineering, Vol. 8, No. 1, 1974, pp.
65-115. doi:10.1080/01614947408071857
[18] G. D. Yadav and A. D. Murkute, “Preparation of the
Novel Mesoporous Solid Acid Catalyst UDCaT-4 via
Synergism of Persulfated Alumina and Zirconia into Hex-
agonal Mesoporous Silica for Alkylation Reactions,” Ad-
vanced Synthesis and Catalysis, Vol. 346, No. 4, 2004, pp.
389-394. doi:10.1002/adsc.200303212
[19] D. P. Quintanilla, A. Sánchez, I. Hierro, M. Fajardo and I.
Sierra, “Functionalized HMS Mesoporous Silica as Solid
Phase Extractant for Pb(II) Prior to Its Determination by
Flame Atomic Absorption Spectrometry,” Journal of Se-
paration Science, Vol. 30, No. 10, 2007, pp. 1556-1567.
doi:10.1002/jssc.200600540