Spectrophotometric determination of gold after separation and preconcentration with a solid-phase extraction method

Article abstract of DOI:10.1246/bcsj.81.1103

A simple, sensitive, and inexpensive method for separation and preconcentration of gold ions from aqueous solutions prior to its spectrophotometric determination is reported. The proposed method includes two procedures. In the first one, sodium 8-aminoquinoline-5-azobenzene-4'-sulfonate (SPAQ) was immobilized directly on an alumina column which adsorbed gold ions from aqueous solutions. Then, Au^III was desorbed by HCl solution, and the absorbance of the alkaline solution in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTMAB), was measured at 611 nm. In the second procedure, Au-SPAQ complex in the presence of CTMAB in alkaline media was passed through the column containing alumina (washed with nitric acid and neutralized by washing with distilled water). The complex was then desorbed with ethanol, and the absorbance was measured immediately at 611 nm. The proposed methods were applied successfully to the determination of gold in Azerbaijan ores. The relative standard deviations for five replicate determinations of 0.2μgmL^-1 Au^III for the first and second procedures were 3.01% and 2.66%, and the corresponding limits of detection were 1.37 and 3.70 μg L^-1, respectively.

Full text of DOI:10.1246/bcsj.81.1103

ꢀ 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 9, 1103–1107 (2008) 1103
Spectrophotometric Determination of Gold after Separation
and Preconcentration with a Solid-Phase Extraction Method
ꢀ
Mohammad-Hossein Sorouraddin and Masoud Saadati
Analytical Chemistry Department, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
Received February 4, 2008; E-mail: soruraddin@tabrizu.ac.ir
A simple, sensitive, and inexpensive method for separation and preconcentration of gold ions from aqueous solu-
tions prior to its spectrophotometric determination is reported. The proposed method includes two procedures. In the first
one, sodium 8-aminoquinoline-5-azobenzene-40-sulfonate (SPAQ) was immobilized directly on an alumina column
which adsorbed gold ions from aqueous solutions. Then, AuIII was desorbed by HCl solution, and the absorbance of
the alkaline solution in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTMAB), was mea-
sured at 611 nm. In the second procedure, Au–SPAQ complex in the presence of CTMAB in alkaline media was passed
through the column containing alumina (washed with nitric acid and neutralized by washing with distilled water). The
complex was then desorbed with ethanol, and the absorbance was measured immediately at 611 nm. The proposed meth-
ods were applied successfully to the determination of gold in Azerbaijan ores. The relative standard deviations for five
replicate determinations of 0.2 mg mLꢁ1 AuIII for the first and second procedures were 3.01% and 2.66%, and the cor-
responding limits of detection were 1.37 and 3.70 mg Lꢁ1, respectively.
Due to wide industrial applications as well as its role in eco-
nomic activities, gold is one of the most important noble met-
als. Regardless of the type of detection method, separation and
preconcentration of gold ions prior to its determination is an
essential step. Even with the high sensitivity and selectivity
of modern instrumental methods, such as atomic absorption
spectrometry (AAS), X-ray fluorescence (XRF), inductively
coupled plasma-mass spectrometry (ICP-MS), and neutron ac-
tivation analysis (NAA), preconcentration and separation steps
are often necessary because of the complex compositions of the
real samples and trace or ultra trace concentrations of gold.1–4
The separation and preconcentration processes of gold ions
from different matrices are mainly based on the utilization
and application of available techniques including membrane
disks,5 liquid–liquid extraction,6,7 ion-exchange and sorp-
tion,1,8 fire assay and co-precipitation,9 activated carbon,10
HPLC11 and CE,12 and solid-phase extraction (SPE).
on aluminum oxide or alumina as an inexpensive and accessi-
ble material is a suitable process for preconcentration of metal
ions. In our previous works, dithizone (1,5-diphenylthiocarba-
zone) and 1-(2-pyridylazo)-2-naphthol were immobilized on
surfactant-coated alumina for successful preconcentration of
Cu2þ 14 Hg2þ 15 and Co2þ 16 We have also reported the utiliza-
, , .
tion of surfactant-coated alumina for preconcentration of CrVI–
diphenylcarbazide complex17 and chromium speciation.18 Oth-
er alumina-based SPE methods, have been applied for separa-
tion and preconcentration of silver,19 chromium(VI),20 lead,21
uranium,22 mercury,23 and some other metal ions.24 In these re-
ports, a suitable ligand was immobilized on the surfactant-
coated alumina prior to the adsorption of metal ions. Such sys-
tems could also be applied for determination of organic mate-
rials.25,26 Alumina may also directly be used for separation and
preconcentration of metal ions without using any surfac-
tant.27,28
Since methods commonly used for determination of gold are
generally complex and expensive, solid-phase extraction (SPE)
technique has increasingly become popular because of several
major advantages such as simple to operate, high preconcen-
tration factor, rapid phase separation, and the ability to com-
bine with different detection techniques. Combining solid-
phase extraction (SPE) with spectrophotometry as a simple
and inexpensive detection method makes it possible to intro-
duce a simple and sensitive method for preconcentration and
determination of gold in geological samples.
Among the various adsorbents used in SPE methods, alu-
mina has been extensively studied for the adsorption of metal
ions. Alumina has many desirable ion-exchange properties. It
undergoes little swelling or shrinking in electrolyte solution,
has good resistance towards strong oxidizing and reducing
agents, and can act both as an anion or a cation exchanger de-
pending upon solution pH.13 Immobilizing adsorbing reagents
In spectrophotometric determination of gold with most of
the chromogenic agent, as the sensitivity is very poor, evapo-
ration or extraction with an organic solvent for concentration is
necessary. Sodium 8-aminoquinoline-5-azobenzene-40-sulfo-
nate (SPAQ), a derivative of 8-aminoqunoline, was synthe-
sized for spectrophotometric determination of gold in ores
and slimes, and found to be more sensitive and selective com-
pared with other chromogenic agents.29 Later, SPAQ was used
for spectrophotometric determination of gold in a flow injec-
tion (FI) system.30 This ligand recently has been used for cop-
per determination in water and mineral samples.31
The aim of this work was to study the utilization of SPAQ
immobilization on alumina for separation and preconcentration
of gold from ore sample solutions. Au–SPAQ complex was
also studied. The spectrophotometry coupled with the advan-
tages of SPE methods implied a simple and sensitive method
to determine gold in part per billion (ppb) levels.
Published on the web September 11, 2008; doi:10.1246/bcsj.81.1103

1104 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Determination of Gold in Ore Samples
cible) at 700 ꢄC. The fused sample was washed with distilled wa-
ter into a 250-mL beaker and 50 mL of concentrated hydrochloric
acid was added. The mixture was heated to decrease the volume
and then compensated with distilled water. Evaporation followed
by distilled water addition was repeated several times. Remaining
solution was transferred into a 100-mL flask and filled with distill-
ed water. In gold determination using the first procedure, a needed
volume of sample was passed through the column, containing
0.8 g of SPAQ-coated alumina, and eluted with 2 mL of 4 M
HCl into a beaker containing 1.2 mL of 0.2% CTMAB and 2 mg
of solid EDTA (disodium ethylenedinitrilo-N,N,N0,N0-tetraacetate:
Na2edta), as masking agent for Cu2þ ion. After alkalination of the
solution with NaOH, the absorbance was measured at 611 nm. In
similar methods reported in the literature, it is necessary to add a
masking agent such as NaF to eliminate Fe3þ ion interference29
but with our procedure, Fe3þ ions up to 150 mg mLꢁ1 did not in-
terfere with the determination of 0.3 mg mLꢁ1 of AuIII. In higher
concentrations of Fe3þ, addition of NaF seems to be required. In
gold determination in ores using the second procedure, an appro-
priate volume of sample was transferred to a beaker containing
3 mL of 0.2% CTMAB, 2 mL of 2 ꢂ 10ꢁ4 M SPAQ, 5 mL of
5% NaF, and 6 mL of 2% EDTA solution and alkalinated with
2 mL of 2 M NaOH solution. After standing for 30 min, the com-
plex formed was passed through a column containing 1 g of acti-
vated alumina. The adsorbed complex was then eluted from the
column with 3 mL of ethanol followed by immediate absorption
measurement at 611 nm.
Experimental
Apparatus. A Shimadzu UV-265-FW type UV–vis spectro-
photometer was used for acquiring the spectra, a UNICO model
2150 spectrophotometer was used for the absorption measure-
ments; a Phoenix 986-AA type flame atomic absorption spectro-
photometer was used for comparing the results and a Metrohm
pH meter was used for measuring pH.
Reagents. All chemicals were of analytical reagent grade or
the highest purity available, except SPAQ which was synthesized
according to a method reported in the literature.29 The procedure
included diazotization of 4-aminobenzenesulfonic acid in ice-cold
distilled water with sodium nitrite and coupling it to 8-amino-
quinoline. The precipitate obtained was recrystallized and salted
out with sodium chloride several times from water. The data ob-
tained from NMR, FTIR, and UV–visible spectroscopy confirmed
the SPAQ formation. About 2 ꢂ 10ꢁ4 M (M = mol dmꢁ3) SPAQ
solution was prepared by dissolving 0.037 g of purified synthe-
sized SPAQ in 500 mL of distilled water. 1000 mg mLꢁ1 AuIII so-
lution was prepared by dissolving 0.145 g of H[AuCl4] xH2O
ꢃ
(49% Au) in a few mL of water and making up to 10 mL in a grad-
uated flask. A 0.2% solution of cetyltrimethylammonium bromide
(CTMAB) was prepared by dissolving 0.5 g of solid CTMAB
in 250 mL of distilled water. Doubly distilled water was used
throughout. Alumina (aluminum oxide anhydrous, ꢀ-alumina)
was activated with 4 M HNO3 by shaking for a few seconds and
rinsing with sufficient amount of distilled water for its neutraliza-
tion was used for adsorption. All the reagents used were purchased
from Merck (Darmstadt, Germany).
Results and Discussion
Immobilization of SPAQ on Alumina for AuIII Adsorp-
General Procedures.
Gold Adsorption on Immobilized
SPAQ in a Column: SPAQ solution (2 ꢂ 10ꢁ4 M, 3 mL) was
added into a beaker containing 0.8 g of activated alumina particles
and shaken for a few seconds. The adsorption capacity of SPAQ
on alumina was determined to be 0.147 g per gram of alumina.
10–100 mL of gold stock solution containing 0.05–0.5 mg mLꢁ1
AuIII with pH 3–8 was passed through a column packed with
SPAQ-coated alumina, with a flow rate of 2.5–5 mL minꢁ1. The
column was then washed with 2 mL of distilled water and the ad-
sorbed AuIII was eluted with 2 mL of 4 M HCl followed by 4 mL
of distilled water to a 10-mL beaker containing 1.2 mL of
CTMAB 0.2% and alkalinated by adding 2.3 mL of 4 M NaOH so-
lution. After standing for 25 min the absorbance of final solution
was measured at 611 nm.
tion.
Mixing 2 mL of 2 ꢂ 10ꢁ4 M SPAQ, 1 mL of 0.2%
CTMAB, and 1 g of alumina powder in alkaline media showed
strong adsorption of SPAQ on alumina. Since AuIII solution is
acidic, the procedure was repeated in acidic media (by adding
1 mL of 2 M HCl). Even after stirring for five minutes, no con-
siderable adsorption was observed. An SPAQ solution contain-
ing an anionic surfactant, sodium dodecylsulfate (SDS), mixed
with alumina powder showed enhanced adsorption of SPAQ.
However, light absorption at 611 nm by Au–SPAQ in the pres-
ence of SDS has been reported to be lower in the absence of
SDS.29 Thus, SPAQ was immobilized directly on alumina
without using any surfactant. Mixtures of 1 g of alumina pow-
der, and 3 mL of 2 ꢂ 10ꢁ4 M SPAQ solution with different
concentrations of NaOH, were shaken in a shaker for a few
minutes, however, no adsorption of SPAQ on alumina was ob-
served. The same procedure was repeated with acidic solutions
with pH (1–5). Maximum adsorption was observed for a mix-
ture of 5 mL of 2 ꢂ 10ꢁ4 M SPAQ and 2 mL of 0.2 M HCl so-
lution, pH 2.6, and 1 g of alumina. The pH of the solution was
changed to 4.6 after SPAQ adsorption on alumina. The color of
SPAQ solution is orange in pH above 3.6 where most of SPAQ
is in basic form (Aꢁ), and red at pH below 3.6 where most of
SPAQ is in acidic form (HA). Decoloration of the solution ac-
companied by conversion of the alumina color from white to
orange indicates almost complete adsorption of the ligand
(Aꢁ form). In other words, the alumina surface which becomes
positively charged by adsorbing proton at lower pH has strong
affinity for SPAQ in Aꢁ form but not for HA form.
Au–SPAQ Complex Adsorption on Alumina Column: To
10–100 mL of 0.05–0.7 mg mLꢁ1 AuIII solution in a 100-mL beak-
er; 3 mL of 0.2% CTMAB, and 2 mL of 2 ꢂ 10ꢁ4 M SPAQ solu-
tion were added in order, and alkalinated by adding 2 mL of 2 M
NaOH. After 30 min standing for the complex formation, the solu-
tion was passed through a column (containing 1 g of activated alu-
mina) at 2.5–5 mL minꢁ1 flow rates. Preliminary experiments re-
vealed that, if the column is treated with 0.01 M NaOH (2 mL) fol-
lowed by rinsing with sufficient amount of distilled water, prior to
the work, Au–SPAQ adsorption on the column was slightly in-
creased. After washing the column with distilled water, the com-
plex was eluted from the column with 3 mL of ethanol and the
light absorption was immediately measured at 611 nm.
Determination of AuIII in Azerbaijan Ores. 5 g of crushed
ore sample was transferred into a 250-mL beaker containing
50 mL of concentrated hydrochloric acid. The beaker was covered
and heated gently on an electric heater to dissolve the sample. The
mixture was filtered, the residue was mixed with 5 g of sodium
peroxide, 2 g of sodium hydroxide, and fused (in a porcelain cru-
Figure 1 shows SPAQ spectra in three pH values (2.5, 6,
and 9.5). As shown in the figure spectra are similar in pH 6

M. H. Sorouraddin et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008) 1105
Figure 3. Effect of volume of NaOH solution (2 M) on for-
mation and adsorption of Au–SPAQ complex.
Figure 1. Absorption spectra of SPAQ solution using
distilled water as a blank: A, pH 6; B, pH 9.5; and C,
pH 2.5. Concentration of SPAQ solution is 5 ꢂ 10ꢁ5 M
in A and B and 1 ꢂ 10ꢁ4 M in C.
existence of the chloride form of AuIII in the sample hydro-
chloric acid was used as desorbing agent. It was observed that
the recovery increased with increase in the concentration of
HCl and remained constant in concentrations greater than
4 M HCl.
Adsorption of Au–SPAQ Complex on Alumina. Au–
SPAQ complex formed in alkaline media showed a strong af-
finity for adsorption on alumina. Solutions containing 50 mL
of 0.2 mg mLꢁ1 AuIII, 3 mL of 0.2% CTMAB, and 2 mL of
2 ꢂ 10ꢁ4 M SPAQ, alkalinated by adding 0.5–4 mL of 2 M
NaOH solution, passed through a column packed with acti-
vated alumina. The result obtained indicated that maximum
adsorption was achieved with 2 mL of 2 M NaOH solution
(Figure 3). It was also shown that the complex adsorption on
alumina decreases by adding more than 2 mL of 2 M NaOH.
The adsorption efficiency of Au–SPAQ on the column at the
optimum conditions was found to be 96.5–97.8%.
Elution of the Adsorbed Au–SPAQ Complex from the
Alumina Column. A series of elute solutions were used
in order to find a selective elute for desorption of Au–SPAQ
complex. Some of the eluting agents used were based on for-
mer experiences in our laboratory.14–18 50 mL of 0.3 mg mLꢁ1
AuIII solution was transferred into a beaker containing 3 mL of
0.2% CTMAB and 2 mL of 2 ꢂ 10ꢁ4 M SPAQ solution and
alkalinated with 2 mL of NaOH solution. After standing for
30 min, the formed complex was passed through a column,
containing 1 g of activated alumina. 2 mL of portions of listed
eluants (Table 1) were used for desorbing Au–SPAQ complex.
The absorbance of eluted solutions was measured at 611 nm
and the percent recoveries of gold with respect to its adsorp-
tion efficiency (96.5–97.8%) were calculated by comparing
with the absorbance of the same solution without passing
through the column. The effect of ethanol as the best eluant
(Table 1) was also examined. The results indicated that
97.1–98.4% recoveries can be obtained with 3 mL or more
of ethanol if it was added in three 1 mL of portions followed
by distilled water (1 mL) after each portion.
Figure 2. Effect of solution pH on adsorption of AuIII on
SPAQ-coated alumina. 50 mL of 0.1 mg mLꢁ1 AuIII solu-
tion at different pH values, 0.8 g of SPAQ-coated alumina.
Elution and determining conditions are as described in
Experimental section.
and 9.5 but in pH 2.5 the maximum absorbance shifted to a
longer wavelength. This is expectable because the pKa3 of
SPAQ is 3:65 ꢅ 0:03 based on the literature.29
Adsorption of AuIII on SPAQ-Coated Alumina. The pH
of AuIII solution is an important factor affecting adsorption of
AuIII on SPAQ-coated alumina. Standard solutions with differ-
ent pH but the same concentration of AuIII passed through a
column packed with alumina (Experimental section) for AuIII
adsorption. The results (Figure 2) indicated that for solutions
of pH 3–8, AuIII is retained constantly on the column. It was
also shown that below pH 2, SPAQ can not be adsorbed on
alumina (above section) while above pH 8, since the alumina
surface is negatively charged, and is not capable of SPAQ ad-
sorption.
Elution of Adsorbed AuIII from SPAQ-Coated Alumina.
Desorption of AuIII from SPAQ-coated alumina was studied by
using different eluants. Common solvents such as acetone and
alcohol did not prove to be suitable eluants. Sodium hydroxide
solution and ammonia water were tested as available bases and
their effectiveness was found to be very low. Because of the
Optimization of Other Parameters. There were some
other parameters to be optimized such as sample flow rate,
amount of alumina loaded in the column, quantities of
CTMAB and SPAQ, and standing time for reading the com-
plex absorbance in the first procedure. For parameters like

1106 Bull. Chem. Soc. Jpn. Vol. 81, No. 9 (2008)
Determination of Gold in Ore Samples
Table 1. Recovery of Au–SPAQ Complex Extracted from
the Column Containing Alumina with Different Elutes
Table 2. Tolerable Concentration Ratios for the Spectro-
photometric Determination of 0.3 mg mLꢁ1 of AuIII with
Proposed Procedure (Relative Error ꢅ5%)
Solvent
Recovery of Au–SPAQ/%
0
Tolerable
Ion added
Acetone
Acetone/HCl (1 M)
(1:3)
concentration ratio
10.3
ꢁ
NO3
Pb2þ, Ba2þ, Agþ,a) Fe3þ
Clꢁ, Ni2þ, Hg2þ, Naþ, Fe3þ
Al3þ
2000
1000
500
300
200
100
50
20
10
5
Hexane
Ethanol
Acetonitrile
Ethanol/Acetonitrile
(4:1)
0
88
71
Mg2þ, Zn2þ, Cd2þ
Ca2þ, Co2þ, Cr3þ
EDTA
79
0
H2O/Ethanol
(1:1)
Fꢁ
HCl (1 M)
Methanol
Propanol
36
46.5
72.5
CrVI, Pd2þ, PtIV
Cu2þ
a) Masked with 10 mg of NaF added to beaker containing des-
orbed AuIII.
Ethanol/Methanol
(1:1)
Acetone/Ethanol
(1:3)
HCl (1 M)/Ethanol
(1:1)
HCl (0.1 M)/Ethanol
(1:20)
41.2
46.5
41.5
83
through a column containing SPAQ-coated alumina. A maxi-
mum error of 5% in the absorbance reading was considered
tolerable. The results of interference studies are presented in
Table 2. The results indicated that Cu2þ was the major inter-
ferant, while CrVI, PtIV, and PdII interfere to a lesser extent.
Interference of Cu2þ was eliminated by adding chelating
agent, EDTA (disodium ethylenedinitrilo-N,N,N0,N0-tetraace-
tate: Na2edta), into eluted sample solution. The real sample
(ore) used for analysis did not contain CrVI, PtIV, and PdII
above the tolerable levels listed in Table 2.
Applications. The applicability of the proposed procedures
for determination of gold in ores of Azerbaijan was examined.
For Au, the most common AAS method is the extraction of
AuIII by methyl isobutyl ketone (MIBK) from aqueous solu-
tions of HCl and subsequently determination of the analyte
by FAAS and GFAAS.32,33 For the FAAS method three repli-
cates were used, obtaining an average of 0.735 mg mLꢁ1 with a
standard deviation of 0.055. For the first and second proce-
dures three replicates were carried out obtaining averages of
0.695 and 0.702 mg mLꢁ1 with standard deviations of 0.081
and 0.063, respectively. The results obtained by the proposed
procedures were compared with FAAS standard method by
two-sided t-test. Statistical comparison at the 95% confidence
level showed no significant difference between the results ob-
tained with the proposed procedures and those of the FAAS
standard method.
pH of the complex solution, amount of CTMAB, amount of
alumina loaded in the column, and volume of ethanol in the
second procedure, they were optimized employing procedures
described earlier. It was found that the 0.8 and 1 g of alumina
is the optimum amount for the first and second procedures re-
spectively. The flow rates of 2.5–5 mL minꢁ1 were optimum
for both procedures. Volume of 3 and 2 mL of 2 ꢂ 10ꢁ4 M
SPAQ and 1.2 and 3 mL of 0.2% CTMAB was found as opti-
mum volumes of ligand and surfactant for the first and the sec-
ond procedures respectively. It was also found that, desorption
of the complex from the column was maximum if 3 mL of
ethanol is used and the maximum absorbance at 611 nm is
achieved in 25 min standing after NaOH addition in the first
procedure.
Analytical Parameters. Under the optimum conditions,
calibration graphs were obtained for AuIII determination by us-
ing a series of six standard solutions in both procedures. In the
first procedure, the calibration graph was found to be linear in
the range 0.05–0.50 mg mLꢁ1 of AuIII. The relative standard
deviation (RSD) calculated for five replicate determinations
of 50 mL of 0.2 mg mLꢁ1 of AuIII was 3.01%. The limit of de-
tection (LOD) based on IUPAC definition (CLOD ¼ 3Sb=m,
where Sb is the standard deviation of blank, and m denotes
Conclusion
A method using alumina as a substrate for a solid-phase ex-
traction has been developed for determination of AuIII. The
two proposed procedures provide a simple, sensitive, and inex-
pensive method for separation and preconcentration of mg Lꢁ1
amount of gold from ore samples. Utilizing the first procedure,
AuIII can be determined in the presence of Fe3þ without any
need for masking agent. In higher concentrations of Fe3þ, ad-
dition of NaF seems to be necessary.
the slope of the calibration line) was found to be 1.30 mg Lꢁ1
.
For the second procedure, linearity of the calibration graph
was in the range of 0.05–0.70 mg mLꢁ1 of AuIII. The RSD
for 50 mL of 0.2 mg mLꢁ1 AuIII was 2.66% based on five
replicated measurements and the estimated LOD value was
3.70 mg Lꢁ1
Interferences.
.
Preconcentration and separation of ele-
ments can be affected by the matrix constituents of the sample,
so the influence of some ions on the preconcentration and de-
termination of gold was examined. The ions tested were added
individually to a 0.3 mg mLꢁ1 solution of AuIII and passed
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