molecules
Article
Determination of Ultra-trace Rhodium in Water
Samples by Graphite Furnace Atomic Absorption
Spectrometry after Cloud Point Extraction Using
2-(5-Iodo-2-Pyridylazo)-5-Dimethylaminoaniline as
a Chelating Agent
Quan Han 1,2, *, Yanyan Huo 1 , Jiangyan Wu 2 , Yaping He 1 , Xiaohui Yang 1 and Longhu Yang 2
1
2
*
School of Chemical Engineering, Xi’an University, Xi’an 710065, China; [email protected] (Y.H.);
[email protected] (Y.H.); [email protected] (X.Y.)
School of Chemistry and Chemical Engineering, Yan’an Universty, Yan’an 716000, China;
[email protected] (J.W.); [email protected] (L.Y.)
Correspondence: [email protected]; Tel.: +86-029-8825-8506
Academic Editor: Derek J. McPhee
Received: 12 February 2017; Accepted: 15 March 2017; Published: 24 March 2017
Abstract: A highly sensitive method based on cloud point extraction (CPE) separation/
preconcentration and graphite furnace atomic absorption spectrometry (GFAAS) detection has been
developed for the determination of ultra-trace amounts of rhodium in water samples. A new reagent,
2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline (5-I-PADMA), was used as the chelating agent and
the nonionic surfactant TritonX-114 was chosen as extractant. In a HAc-NaAc buffer solution at
pH 5.5, Rh(III) reacts with 5-I-PADMA to form a stable chelate by heating in a boiling water bath
for 10 min. Subsequently, the chelate is extracted into the surfactant phase and separated from bulk
water. The factors affecting CPE were investigated. Under the optimized conditions, the calibration
graph was linear in the range of 0.1–6.0 ng/mL, the detection limit was 0.023 ng/mL for rhodium
and relative standard deviation was 3.67% (c = 1.0 ng/mL, n = 11). The method has been applied to
the determination of trace rhodium in water samples with satisfactory results
Keywords: 2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline; rhodium; TritonX-114; cloud point
extraction; graphite furnace atomic absorption spectrometry
1. Introduction
As a precious element of the platinum group elements (PGEs), rhodium is widely used in catalysts
in the chemical and automotive industries due to its excellent catalytic activity. More than 80% of the
world production of rhodium in the last decade was used in the production of automobile catalysts [1].
As a component in the typically used three-way catalysts, rhodium is indeed effective in reducing
NOx emissions, but as a result more and more rhodium is being released into the environment during
the operation of converters [2]. Hence, a reliable and efficient analytical technique for studying and
evaluating its future risk to human health and the ecosystem is required. As rhodium often occurs in
environmental samples in complex matrices at low concentrations, its analysis often requires a highly
sensitive analytical method with a pre-concentration step.
Several sensitive techniques such as graphite furnace atomic absorption spectrometry
(GFAAS) [3,4], cathodic stripping voltammetry (CSV) [5–7], neutron activation analysis (NAA) [8,9],
inductively coupled plasma-atomic emission spectrometry (ICP-AES) [10,11], and inductively coupled
plasma-mass spectrometry (ICP-MS) [12–14] are available for the determination of rhodium in
Molecules 2017, 22, 487; doi:10.3390/molecules22040487
www.mdpi.com/journal/molecules
Molecules 2017, 22, 487
Molecules 2017, 22, 487
2 of 9
2 of 10
environmental
samples.
though
it is highly
sensitive,
it suffersbecause
from organic
matter
interference. NAA
couldCSV,
hardly
be considered
a routine
approach
it is both
timeinterference.
consuming
NAA
could
hardly
be
considered
a
routine
approach
because
it
is
both
time
consuming
and
laborious.
and laborious. ICP-MS is a powerful technique for the determination of ultratrace amounts
of
ICP-MS
is but
a powerful
technique
for the
determination
ultratrace amounts
of GFAAS
rhodium,isbut
is very
rhodium,
it is very
expensive
to operate
and a of
sophisticated
method.
an itefficient
expensive
operate and aofsophisticated
GFAAS issensitivity
an efficientwith
method
of determination
of
method of to
determination
rhodium for method.
its appropinquity
ICP-MS
and lower cost.
rhodium
for
its
appropinquity
sensitivity
with
ICP-MS
and
lower
cost.
Furthermore,
the
concentration
Furthermore, the concentration of rhodium in environmental samples is very low, so separation and
of
rhodium in environmental
samples
is very
so separation
and
pre-concentration
steps are often
pre-concentration
steps are often
needed
priorlow,
to analysis.
Widely
used
techniques for separation
and
needed
prior
to
analysis.
Widely
used
techniques
for
separation
and
pre-concentration
of
trace
amounts
pre-concentration of trace amounts of rhodium include liquid-liquid extraction [15,16], solid–liquid
of
rhodium[12,17,18],
include liquid-liquid
extraction
extraction
[12,17,18],
and techniques
extraction
and techniques
based [15,16],
on ion solid–liquid
exchange [3,6,19].
Cloud
point extraction
(CPE),
based
exchange [3,6,19].
Cloud point
extraction
(CPE),has
which
an alternative
to conventional
which on
is ion
an alternative
to conventional
solvent
extraction,
beenis extensively
used
in analytical
solvent
extraction,
has
been extensively
used
analytical
chemistry
in the last decade [20,21].
CPE,
as a
chemistry
in the last
decade
[20,21]. CPE,
as ainnew
separation
and pre-concentration
technique,
offers
new
separation
and pre-concentration
technique,convenience,
offers severallow
advantages,
such
as higher
simple extraction
operation,
several
advantages,
such as simple operation,
cost, safety,
and
convenience,
low cost, safety,
and[22].
higher
extraction
pre-concentration
factors
[22]. CPE
also in
and pre-concentration
factors
CPE
is also and
in agreement
with the
principles
ofis“Green
agreement
with
the
principles
of
“Green
Chemistry”,
because
it
does
not
use
toxic
organic
solvents.
Chemistry”, because it does not use toxic organic solvents.
The aim of this study was to combine
combine CPE, as
as aa highly
highly efficient
efficient separation
separation and pre-concentration
pre-concentration
approach, with
withGFAAS,
GFAAS,asas
a highly
sensitive
detection
technique,
and develop
a new method
simple
a highly
sensitive
detection
technique,
and develop
a new simple
method
for the determination
of trace amounts
of rhodium
water samples.
In the developed
for the determination
of trace amounts
of rhodium
in water in
samples.
In the developed
system, a
system,
a newly synthesized
2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline
(5-I-PADMA),
newly synthesized
reagent, reagent,
2-(5-iodo-2-pyridylazo)-5-dimethylaminoaniline
(5-I-PADMA),
which
which
forms
a
hydrophobic
complex
compound
with
Rh(III)
and
is
proved
to
be
a
highly
sensitive
forms a hydrophobic complex compound with Rh(III) and is proved to be a highly
613 = 1.86 × 5105 L·mol
−1−1·cm−1 ) [23], was used as the chelating
chromogenic reagent
reagent for
for rhodium
rhodium(ε
(ε613
= 1.86 × 10 L·mol−1·cm
) [23], was used as the chelating agent.
agent.
The structures
of the reagent
and its rhodium
areinshown
in1.Scheme
1. Triton
The structures
of the reagent
and its rhodium
complexcomplex
are shown
Scheme
Triton X-114
wasX-114
used
was
as a nonionic
surfactant.
The
main affecting
factors affecting
thewere
CPE evaluated
were evaluated
and optimized.
as a used
nonionic
surfactant.
The main
factors
the CPE
and optimized.
The
The
method
has been
successfully
applied
to determination
the determination
of trace
rhodium
in water
samples.
method
has been
successfully
applied
to the
of trace
rhodium
in water
samples.
Scheme
and its
its rhodium
rhodium complex.
complex.
Scheme 1.
1. The
The structure
structure of
of 5-I-PADMA
5-I-PADMA and
2. Result and Discussion
2. Result and Discussion
2.1. Effect
2.1.
Effect of
of pH
pH
As shown
shown in
in Scheme
Scheme 1,
1, the
the chelating
chelating agent
agent 5-I-PADMA
contains three
three nitrogen
nitrogen atoms
atoms (the
(the ring
ring
As
5-I-PADMA contains
nitrogen
atom
and
the
two
amino
group
nitrogen
atoms)
all
of
which
can
be
protonated.
It
can
nitrogen atom and the two amino group nitrogen atoms) all of which can be protonated. It can therefore
+
2+
+
+, H R
therefore
in the
in four
states:
R,2+HR
,H
3R . The concentration distribution
exist
in theexist
solution
insolution
four states:
R, HR
, and
H23RR+,.and
The H
concentration
distribution of the four
2
of
the
four
species
in
solution
is
determined
by
its
acidity.
Therefore,
pH ofaffects
the solution
affects
species in solution is determined by its acidity. Therefore, the pH of the the
solution
the formation
the
formation
of
the
chelate
as
well
as
its
hydrophobicity,
and
is
a
critical
parameter.
According
to
of the chelate as well as its hydrophobicity, and is a critical parameter. According to the procedure, the
the procedure,
influence
of evaluated
pH on thebyCPE
was evaluated
changingbuffer
the pH
of HAc-NaAc
influence
of pH the
on the
CPE was
changing
the pH of by
HAc-NaAc
solution
added in
buffer
solution
added
in
the
range
of
3.5–8.0.
Figure
1
shows
the
effect
of
pH
on
extraction
of rhodium
the range of 3.5–8.0. Figure 1 shows the effect of pH on extraction of rhodium chelate. It can
be seen
chelate.
It can be
seen that value
maximum
absorbance
value
was
at pH range
of5.5
5.0–6.0.
Hence,
that
maximum
absorbance
was achieved
at pH
range
ofachieved
5.0–6.0. Hence,
a pH of
was selected
a
pH
of
5.5
was
selected
for
subsequent
experiments.
for subsequent experiments.
Molecules 2017, 22, 487
Molecules 2017, 22, 487
3 3ofof109
3 of 10
0.4
0.4
AA
0.3
0.3
0.2
0.2
0.1
0.1
0.0
3.0
0.0
3.0
4.5
4.5
pH
pH
6.0
6.0
7.5
7.5
Figure 1. Effect of pH on absorbance. 8.0 × 10−6 mol/L 5-I-PADMA; 0.080% (m/v) Triton X-114;
Figure 1.
of
10
mol/L
5-I-PADMA; 0.080%
0.080% (m/v)
(m/v) Triton
Triton X-114;
X-114;
1. Effect
of pH
pH on
onabsorbance.
absorbance.
8.0
10−−66 mol/L
temperature:
60 °C;
heating
time:
15 min;8.0
2.5××ng/mL
Rh. 5-I-PADMA;
◦ C; heating time: 15 min; 2.5 ng/mL Rh.
temperature: 60 °C; heating time: 15 min; 2.5 ng/mL Rh.
2.2. Effect of the Amount of Chelating Agent
2.2. Effect of the Amount of Chelating Agent
5-I-PADMA is employed to form a hydrophobic complex with rhodium. According to the
5-I-PADMA
form
hydrophobic
complexon
with
rhodium.
According
to the
−3 mol/L 5-I-PADMA
is employed
to form
hydrophobic
complex
with
rhodium.
According
procedure, the effect
of the amount
of 1 ×aa10
absorbance
was
investigated
by
−33mol/L 5-I-PADMA on absorbance was investigated by
−
procedure,
the
effect
of
the
amount
of
1
×
10
procedure,
the volumes
effect of the
of 1 × added
mol/L
5-I-PADMA
absorbance
wasininvestigated
changing the
of amount
the 5-I-PADMA
from
20 to 250onµL.
As shown
Figure 2, the
changing
the
volumesof
of
5-I-PADMA
added
20 µL.
to 250
µL. Asinshown
inthe
Figure
2, thea
the
volumes
thethe
5-I-PADMA
from from
20 to
250
As5-I-PADMA
shown
Figure
2,
absorbance
absorbance
increased
significantly
with added
the increasing
amounts
of
added
and
reached
absorbance
increased
significantly
with
the
increasing
amounts
of
5-I-PADMA
added
and
reached
a
increased
significantly
with
the
increasing
amounts
of
5-I-PADMA
added
and
reached
a
constant
value
constant value within the range of 60–120 µL of 5-I-PADMA. However, the absorbance decreased
constant
value
within
the
range
of
60–120
µL
of
5-I-PADMA.
However,
the
absorbance
decreased
within
the range
µL of 5-I-PADMA.
the 120
absorbance
gradually
when
the
gradually
when of
the60–120
amount
5-I-PADMA However,
was beyond
µL. As decreased
5-I-PADMA
also had
strong
gradually
the amount
of 5-I-PADMA
beyond
120
As
5-I-PADMA
also
had
strong
amount of when
5-I-PADMA
was more
beyond
120 µL. Aswas
5-I-PADMA
also µL.
had
strong
hydrophobicity,
there
was
hydrophobicity,
there
was
5-I-PADMA
and
less
complex
in the
surfactant-rich
phase
with
the
hydrophobicity,
there
wascomplex
more
and
less 80
complex
in
surfactant-rich
phase
with
more 5-I-PADMA
and
less
in the surfactant-rich
phase
the
further
increasing
of amounts
further
increasing
of amounts
of 5-I-PADMA
5-I-PADMA.
Hence,
µL 1 ×with
10−3the
mol/L
5-I-PADMA
was
used the
for
−3
−
3
further
increasing
of amounts
Hence, 80 µL
1 ×used
10 mol/L
was used for
of 5-I-PADMA.
Hence,
80 µL 1of
× 5-I-PADMA.
10 mol/L 5-I-PADMA
was
for the5-I-PADMA
subsequent experiments.
the
subsequent
experiments.
the subsequent experiments.
AA
0.3
0.3
0.2
0.2
0.1
0.1 0
0
100
100 V
(L)
V (L)
200
200
Figure
2. Effect
Effect of
of the
theamount
amountofof5-I-PADAM
5-I-PADAM
cloud
point
extraction
of Rh.
0.08%
(m/v)
Triton
XFigure 2.
onon
cloud
point
extraction
of Rh.
0.08%
(m/v)
Triton
X-114;
Figure
2. Effect of
the°C;
amount
oftime:
5-I-PADAM
on cloud
point
extraction
of Rh. 0.08% (m/v) Triton X◦
114;
temperature:
60
heating
15
min;
pH
=
5.5;
2.5
ng/mL
Rh.
temperature: 60 C; heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh.
114; temperature: 60 °C; heating time: 15 min; pH = 5.5; 2.5 ng/mL Rh.
2.3.
X-114 Concentration
Concentration
2.3. Effect
Effect of
of Triton
Triton X-114
2.3. Effect of Triton X-114 Concentration
The
of of
itsits
advantages
such
as
The nonionic
nonionic surfactant
surfactantTriton
TritonX-114
X-114was
wasused
usedasasextractant
extractantbecause
because
advantages
such
The
nonionic
surfactant
Triton
X-114
was
used
as
extractant
because
of
its
advantages
such
as
commercial
availability
with
high
purity,
point
as commercial
availability
with
high
purity,low
lowtoxicity
toxicityand
andcost,
cost,and
and relatively
relatively low
low cloud
cloud point
commercial
availability
withconcentrition
high purity,oflow
toxicity
and
cost,
and relatively
lowefficiency
cloud point
temperature
(23–26
°C).
The
Triton
X-114
not
only
affected
extraction
◦
temperature (23–26 C). The concentrition of Triton X-114 not only affected extraction efficiency but
but
temperature
(23–26of°C).
The
concentrition
of Triton
X-114
not
only
affected
extraction
efficiency
but
also
the
sensitivity
the
cloud
point
extraction.
The
effect
of
the
amount
of
1%
(m/v)
Triton
X-114
on
also the sensitivity of the cloud point extraction. The effect of the amount of 1% (m/v) Triton X-114
also
the
sensitivity
of
the
cloud
point
extraction.
The
effect
of
the
amount
of
1%
(m/v)
Triton
X-114
on
the
absorbance
was
investigated.
AsAs
shown
in in
Figure
chelate
on the
absorbance
was
investigated.
shown
Figure3,3,the
theabsorbance
absorbanceof
of the
the rhodium
rhodium chelate
the
absorbance
was investigated.
As shown
in Figure
3,increases
the absorbance
of
the
rhodium
chelate
significantly
increases
when
the
volume
of
Triton
X-114
from
0.1
to
0.6
mL.
The
optimal
significantly increases when the volume of Triton X-114 increases from 0.1 to 0.6 mL. The optimal
significantly
increases
the
volumetoof
TritonHowever,
X-114 increases from
0.1 to 0.6
mL. Theofoptimal
amount
X-114when
ranges
from
amount of
of Triton
Triton X-114
ranges
from 0.6
0.6 to 1.0
1.0 mL.
mL. However, further
further increase
increase in
in the
the volume
volume of Triton
Triton
amount
of
Triton
X-114
ranges
from
0.6
to
1.0
mL.
However,
further
increase
in
the
volume
of Triton
X-114
leads
to
gradual
decease
in
the
absorbance
signal
because
of
the
increment
in
the
X-114 leads to gradual decease in the absorbance signal because of the increment in the volume
volume of
of the
the
X-114
leadsphase.
to gradual
decease
inmL
theof
absorbance
signal X-114
because
ofemployed.
the increment in the volume of the
remaining
Therefore,
0.8
1%
(m/v)
Triton
was
remaining phase. Therefore, 0.8 mL of 1% (m/v) Triton X-114 was employed.
remaining phase. Therefore, 0.8 mL of 1% (m/v) Triton X-114 was employed.
Molecules 2017, 22, 487
4 of 9
Molecules 2017, 22, 487
4 of 10
Molecules 2017, 22, 487
4 of 10
0.3
A A
A
Molecules 2017, 22, 487
4 of 10
0.3
0.2
0.3
0.2
0.1
0.2
0.1
0.0
0.0
0.1
0.4
0.0
0.0
0.4
0.8
1.2
1.6
V (mL)
0.8
1.2
1.6
Figure 3. Effect of the amount of Trition
X-114 on cloud point extraction of Rh. 8.0 × 10−6 mol/L
5-I0.0
Figure 3. Effect of the amount of Trition
cloud1.2
point1.6
extraction of Rh. 8.0 × 10−6 mol/L
0.0 X-114
0.4 on
V0.8
(mL)
PADMA; temperature: 60 °C;
heating
time:
15
min;
pH
=
5.5;
2.5
ng/mL
Rh.
◦
5-I-PADMA; temperature: 60 C; heating time: 15 V
min;
pH = 5.5; 2.5 ng/mL Rh.
(mL)
Figure 3. Effect of the amount of Trition X-114 on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-I2.4. Effect
of 3.
Equilibration
and
Time
Figure
Effect of theTemperature
amount
of Trition
X-114
on pH
cloud
point
PADMA;
temperature:
60 °C; heating
time:
15 min;
= 5.5;
2.5 extraction
ng/mL Rh.of Rh. 8.0 × 10−6 mol/L 5-I-
2.4. Effect PADMA;
of Equilibration
Temperature
and time:
Time min; pH = 5.5; 2.5 ng/mL Rh.
temperature:
60 °C; and
heating
Equilibration
temperature
time are15
two important and highly effective parameters in CPE.
2.4.
Effect
of Equilibration
Temperature
and Time
It has
been
suggested
that
CPE
processes
based
the temperature-driven
phase separation
typical
Equilibration
temperature
and
time
are
twoon
important
and highly effective
parameters
in CPE.
2.4.
Effect
of
Equilibration
Temperature
andbe
Time
of
nonionic
micellar
solutions
should
conducted
at
temperatures
well
above
the
cloud
point
Equilibration
and time are
twoon
important
and highly effective
parameters
in CPE.
It has been
suggestedtemperature
that CPE processes
based
the temperature-driven
phase
separation
typical
of thesolutions
system
[24].should
Thus,
the
effect
of
equilibration
wasabove
investigated
within
temperature
and time
two
andtemperature
highly effective
parameters
in
CPE.point
Ittemperature
hasEquilibration
been
suggested
that CPE
processes
based
onimportant
the temperature-driven
phase
separation
typical
of nonionic
micellar
beare
conducted
at temperatures
well
the cloud
the
range
40–80 °C solutions
according
to
the procedure.
The
results
are shown well
in Figure
4.separation
Itthe
was
found
that
It has
been
suggested
that
CPE
processes
based
the
phase
typical
of
nonionic
micellar
should
conducted
attemperature-driven
temperatures
above
cloud
point
temperature
ofofthe
system
[24].
Thus,
thebeeffect
ofon
equilibration
temperature
was
investigated
within
absorbance
increased
with
increase
inbe
equilibration
from
toinvestigated
50the
°C,cloud
andwithin
reach
of nonionic of
micellar
solutions
should
conducted
attemperature
temperatures
well40
above
point
temperature
the
system
[24].
Thus,
the
effect
of equilibration
temperature
was
◦
the range of 40–80 C according to the procedure. The results are shown in Figure 4. It was found
maximum
the
range
of 50–80
°C.
equilibrium
time
wasin
also
studied
time
interval
temperature
of
the
system
[24]. to
Thus,
theeffect
effectof
ofThe
equilibration
temperature
was
investigated
within
the
range ofin
40–80
°C
according
theThe
procedure.
results are
shown
Figure
4.
It in
was
found
that
that absorbance
increased
with
increase
in equilibration
temperature
from
40over
to 5010◦ C,
and
of
(Figure
5).
It has
been
observed
that
when
equilibration
time
is
min,
thereach
the5–30
rangemin
of increased
40–80
°C according
to
the
procedure.
The
results
are
shown
in
Figure
4.
It
was
found
that
absorbance
with
increase
in
equilibration
temperature
from
40
to
50
°C,
and
reach
◦ C. The effect of equilibrium time was also studied in time interval of
maximum
in
the
range
of
50–80
quantitative
extraction
is50–80
achieved.
Therefore,
60 °C temperature
and 15
min
were
selected
equilibrium
absorbancein increased
with
increase
ineffect
equilibration
from
to 50inas
°C,
and
reach
maximum
the
range of
°C. The
of equilibrium
time
was
also 40
studied
time
interval
5–30 of
min
(Figure
5).
has
been
observed
that
when
equilibration
over
10over
min,
themin,
quantitative
temperature
time,
maximum
the It
range
ofIt50–80
°C. The
effect
ofthat
equilibrium
timetime
was is
also
studied
in time
interval
5–30
mininand
(Figure
5).respectively.
has
been
observed
when equilibration
time
is
10
the
extraction
is min
achieved.
60 ◦Therefore,
Cobserved
and 15 min
were
as equilibrium
of 5–30
(FigureTherefore,
5).is Itachieved.
has been
that
when
equilibration
time
is over
10 min, the and
quantitative
extraction
60 °C
and selected
15 min were
selected
as temperature
equilibrium
0.35
time,temperature
respectively.
quantitative
extraction
is achieved. Therefore, 60 °C and 15 min were selected as equilibrium
and
time, respectively.
temperature and time, respectively.
0.35
A
0.30
0.35
0.30
AA
0.25
0.30
0.25
0.20
0.25
40
50
60
o
70
80
T ( C)
0.20
40
50
60
70
80
Figure 4. Effect of equilibration
temperature
on cloud
point
extraction
of Rh. 8.0 × 10−6 mol/L 5-Io
0.20
40
50
60T ( C)
70
80
PADMA; 0.08% (m/v) Triton X-114;heating time: 15 min; pH
=
5.5;
2.5
ng/mL
Rh.
o
T ( C)
Figure 4. Effect of equilibration temperature on cloud point extraction of Rh. 8.0 × 10−6 mol/L 5-I-
Figure 4. Effect of equilibration0.35
temperature on cloud point extraction of Rh. 8.0−6× 10−6 mol/L
Figure 4. 0.08%
Effect(m/v)
of equilibration
temperature
on15cloud
point
extraction
of Rh. 8.0 × 10 mol/L 5-IPADMA;
Triton X-114;heating
time:
min; pH
= 5.5;
2.5 ng/mL
5-I-PADMA;
TritonX-114;heating
X-114;heating
time:
15 min;
5.5;
2.5 ng/mL
Rh.
PADMA;0.08%
0.08% (m/v)
(m/v) Triton
time:
15 min;
pH =pH
5.5;=2.5
ng/mL
Rh.
0.30
0.25
0.30
A A
A
0.35
0.30
0.35
0.25
0.20
0.25
10
t (min)
20
30
0.20
10 point extraction
20
30 8.0 × 10−6 mol/L 5-I-PADMA;
Figure 5. Effect of equilibration
time on cloud
of Rh,
0.20
t
(min)
10
20
0.08% (m/v) Triton X-114; temperature; 60 °C; pH = 5.5; 2.5 ng/mL Rh.30
t (min)
Figure 5. Effect of equilibration time on cloud point extraction of Rh, 8.0 × 10−6 mol/L 5-I-PADMA;
Figure(m/v)
5. Effect
ofX-114;
equilibration
time on
Rh, 8.0 × 10−6 mol/L 5-I-PADMA;
0.08%
Triton
temperature;
60 cloud
°C; pHpoint
= 5.5;extraction
2.5 ng/mLof
Rh.
Figure 5. Effect of equilibration time on cloud point extraction of Rh, 8.0 × 10−6 mol/L 5-I-PADMA;
0.08% (m/v) Triton X-114; temperature; 60 °C; pH = 5.5; 2.5 ng/mL Rh.
0.08% (m/v) Triton X-114; temperature; 60 ◦ C; pH = 5.5; 2.5 ng/mL Rh.
Molecules 2017, 22, 487
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2.5. Interference Studies
Molecules 2017, 22, 487
5 of 10
Under the optimized conditions identified in Section 2.1 to Section 2.4, the influence of various
2.5. Interference
Studies
interfering
species on
the determination of 1.0 ng/mL of rhodium was studied. A foreign species was
considered
to cause
interference
whenidentified
it caused in
a variation
lesstothan
5%influence
in the absorbance
Under
the no
optimized
conditions
Sections 2.1
2.4,±the
of variousof the
sample.
The tolerance
limits
of various species
were shown
in Table
1. The results
indicated
that the
interfering
species on
the determination
of 1.0 ng/mL
of rhodium
was studied.
A foreign
species was
common
coexisting
species
did not have
a significant
effect onless
thethan
determination
of rhodium,
and the
considered
to cause
no interference
when
it caused a variation
±5% in the absorbance
of the
sample.
The
tolerance
limits
of
various
species
were
shown
in
Table
1.
The
results
indicated
that
the
developed method has very good selectivity.
common coexisting species did not have a significant effect on the determination of rhodium, and the
developed
method
hasofvery
goodspecies
selectivity.
Table
1. Effect
foreign
on the pre-concentration/determination of rhodium.
Table 1. Effect of foreignForeign
speciesSpecies
on the pre-concentration/determination
of rhodium.
Foreign Species to Rh
to Rh
Species
Species
Ratio (w/w)
Ratio (w/w)
Foreign Species to Rh
Foreign Species to Rh
Species
Species
+
+
+
2+
2+
2+
Li , Na , K , Mg , Ca , Sr ,
Ratio
(w/w)
Ratio (w/w)
2500
500
Cu2+ , Ni2+ , W(VI)
2+ , F− , Cl− , Br− , NO −
Zn
+, K+, Mg2+, Ca2+,3Sr2+,
Li+, Na
2++
2+, W(VI)
2500
Cu
,
Ni
500
Ag , Pt(IV), Ir(IV),
2+2+
−, Br2−,−NO3−
Zn
, F,−I,−Cl
200
2000
Ba
, SO
4
Ru(III), Fe3+
Ir(IV),
Ag+, Pt(IV), 2+
2+
3
−
2+
−
2−
1500
4
2000
200100
Cd Ba
, Cr, I ,, SO
As(V)
Co 3+
Ru(III), Fe
2+
1000
80
Pb2+3−
Pd
2+
2+
1500
Co
100
Cd , Cr , As(V)
800
Mn2+ , Al3+ , Mo(IV)
1000
Pd2+
80
Pb2+
2+
3+
800
Mn , Al , Mo(IV)
2.6. Calibration Curve, Detection Limit and Precision
2.6. Calibration Curve, Detection Limit and Precision
Under the optimized conditions, a calibration curve was obtained by pre-concentrating standard
the optimized
conditions,
a calibration
waslinear
obtained
by pre-concentrating
standard
solutions Under
according
to the given
procedure
(Figurecurve
6). The
equation
was A = 0.00998
+ 0.1304c
solutions
according
to
the
given
procedure
(Figure
6).
The
linear
equation
was
A
=
0.00998
+
0.1304c
(c: ng/mL), with a correlation coefficient R = 0.9989. The calibration line was linear within the rhodium
(c: ng/mL), with a correlation coefficient R = 0.9989. The calibration line was linear within the rhodium
concentration range of 0.1–6.0 ng/mL. The limit of detection for rhodium, calculated as three times the
concentration range of 0.1–6.0 ng/mL. The limit of detection for rhodium, calculated as three times
standard deviation of the blank signals (3s), was determined to be 0.023 ng/mL. The relative standard
the standard deviation of the blank signals (3s), was determined to be 0.023 ng/mL. The relative
deviation
(RSD)
was 3.7%
= 1 ng/mL,
= 11). nThe
enrichment
factor,
defined
as the
ratio of
standard
deviation
(RSD)(for
wasc 3.7%
(for c = 1nng/mL,
= 11).
The enrichment
factor,
defined
as the
the aqueous
solution
volume
(10
mL)
to
that
of
the
surfactant
rich
phase
volume
after
dilution
ratio of the aqueous solution volume (10 mL) to that of the surfactant rich phase volume after dilution with
HNOwith
solutionsolution
(0.05 mL),
HNO3-methanol
(0.05was
mL),200.
was 200.
3 -methanol
0.8
A
0.6
0.4
0.2
0.0
0
2
4
6
c (ng/mL)
Figure 6. Calibration graph for Rh.
Figure 6. Calibration graph for Rh.
A comparison of the present method with previously reported methods involving rhodium
determination
thethe
literature
terms of with
detection
limits and
instruments
employed
is given
in
A
comparisoninof
presentinmethod
previously
reported
methods
involving
rhodium
Table 2. Theindetection
limit ofin
theterms
described
method islimits
superior
to those
given in employed
the table, with
the
determination
the literature
of detection
and
instruments
is given
in
exception
of
CPE-ICP/MS.
However,
ICP/MS,
as
a
multi-elemental
analytical
technique,
is
not
Table 2. The detection limit of the described method is superior to those given in the table, with
available in many laboratories due to the high costs, while GFAAS has the advantages of simplicity,
the exception of CPE-ICP/MS. However, ICP/MS, as a multi-elemental analytical technique, is not
high sensitivity and low cost.
available in many laboratories due to the high costs, while GFAAS has the advantages of simplicity,
high sensitivity and low cost.
Molecules 2017, 22, 487
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Table 2. Comparison of the proposed method with reported methods using CPE prior to
rhodium determination.
Reagent
5-(40 -Nitro-20 ,60 -dichlorophenylazo)-6hydroxypyrimidine-2,4-dione
2-propylpiperidine-1-carbodithioate
O,O-Diethydithiophosphate
2-Mercaptobenzothiazole
1-(2-Pyridylazo)-2-naphthol
2-(5-Iodo-2-pyridylazo)-5-dimethylaminoaniline
Detection System
Surfactant
Limit of Detection
(ng/mL)
SP
Triton X-114
0.15
[25]
FAAS
FAAS
ICP/MS
ICP/MS
TLS
GFAAS
Span 80
Triton X-114
Triton X-114
Triton X-100
Triton X-114
Triton X-114
1.2
0.052
0.003
0.001
0.06
0.023
[26]
[27]
[28]
[29]
[30]
This work
Reference
3. Application of Real Water Samples
In order to validate the proposed methodology, the optimized procedure was applied to the
determination of rhodium in tap water, well water, spring water and river water samples. Its accuracy
was checked by spiking known different concentrations of cobalt into the samples and calculating the
recoveries defined as the ratio of measured amount of rhodium to the adding amount of rhodium.
The results are shown in Table 3. As shown, the recoveries for the spiked samples were in the range of
96.6%~104.0%, and relative standard deviation was below 5%.
Table 3. Determination results of rhodium in the water samples.
Sample
Added
(ng/mL)
-
Tap water
1.0
3.0
-
Reservoir water
2.0
4.0
-
Well water
1.0
3.0
-
River water
2.0
4.0
Found (ng/mL)
ND
0.950, 0.960, 0.930
1.02, 0.970, 1.01
3.22, 3.10, 2.94
2.96, 3.16, 3.22
ND
1.94, 2.05, 2.08
1.98, 1.94, 2.08
3.92, 4.06, 3.90
3.92, 3.86, 4.07
ND
0.970, 0.980, 1.00
0.930, 0.960, 1.04
3.02, 2.86, 2.90
2.82, 2.88, 2.92
ND
1.96, 1.95, 2.08
1.92, 2.06, 1.92
3.88, 3.80, 3.88
4.06, 3.92, 3.98
Average
(ng/mL)
Recovery
(%)
RSD (%)
-
-
-
0.973
97.3
3.6
3.10
103.3
3.9
-
-
-
2.01
101
3.3
3.96
98.9
2.2
-
-
-
0.980
98.0
2.9
2.90
96.7
2.4
-
-
-
1.98
99.0
3.6
3.92
98.0
2.3
ND: Not detected. Tap water: c(Mn2+ ) = 10 ng mL−1 , c(Fe3+ ) = 30 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng
mL−1 , c(Cu2+ ) < 50 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . Well water: c(Mn2+ ) = 10 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 ,
c(Cd2+ ) < 50 ng mL−1 , c(Cu2+ ) < 50 ng mL−1 , c(Fe3+ ) < 30 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . Reservoir water: c(Fe3+ )
= 30 ng mL−1 , c(Cu2+ ) = 50 ng mL−1 , c(Mn2+ ) = 230 ng mL−1 , c(Zn2+ ) < 50 ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Pb2+ )
< 200 ng mL−1 . River water: c(Fe3+ ) = 30 ng mL−1 , c(Cu2+ ) = 50 ng mL−1 , c(Mn2+ ) = 230 ng mL−1 , c(Zn2+ ) < 50
ng mL−1 , c(Cd2+ ) < 50 ng mL−1 , c(Pb2+ ) < 200 ng mL−1 . All the water samples and their analytical results were
provided by Xi’an Hydrographic Bureau, Xi’an, Shaanxi Province, China.
4. Experimental Section
4.1. Apparatus
Atomic absorption measurements were performed with a model TAS-990 atomic absorption
spectrophotometer (Beijing Purkinje General Instrument Co. Ltd, Beijing, China) equipped with a
GFH-990 graphite furnace atomizer system (Beijing Purkinje General Instrument Co. Ltd, Beijing,
Molecules 2017, 22, 487
7 of 9
China). A rhodium hollow cathode lamp (Heraeus Noblelight Ltd, Shenyang, Shandong, China)
was used as the radiation source. Argon was used as the purge and protective gas. The work
condition of graphite furnace atomic absorption spectrophotometer was listed in Table 4. The pH
values were measured by a model PB-10 pH meter (Sartorius Scientific Instrument Co. Ltd, Beijing,
China) furnished with a combined electrode. A model HH-2 thermostatic bath (Kewei Yongxing
Instrument Co. Ltd, Beijing, China), maintained at the desired temperature, was used for cloud point
temperature experiments. A model 800-1 centrifuge (Pudong Physical Instruments Factory, Shanghai,
China) was utilized to accelerate the phase separation process.
Table 4. Operating conditions for GFAAS.
Lamp current
Wavelength
Slit
Filter coefficient
Pressure (Ar)
Injected volume
Drying temp.
Ashing temp.
Atomization temp.
Cleaning temp.
6.0 mA
343.5 nm
0.2 nm
0.10
0.60 Mpa
10.0 µL
120 ◦ C (Ramp 5 s, hold 10 s)
1100 ◦ C (Ramp 10 s, hold 15 s)
2300 ◦ C (Ramp 0 s, hold 3 s)
2500 ◦ C (Ramp 1 s, hold 3 s)
4.2. Reagents and Solutions
A standard stock solution of rhodium (1000 µg/mL) was prepared by dissolving spectroscopically
pure (NH4 )2 RhCl5 ·H2 O in 1 mol/L HCl. Working solutions were prepared by appropriate dilution of
the stock solution with water. A 1.0 × 10−3 mol/L 5-I-PADMA (Laboratory-synthesized [23]) solution
was prepared by dissolving appropriate amounts of this reagent in ethanol. The solution of nonionic
surfactant (1% m/v) Triton X-114 (Sigma-Aldrich, Milwaukee, Wisc., USA) solution was prepared by
dissolving 1.0 g of surfactant in 100 mL of water. A buffer solution of pH 5.5 was prepared by 0.2 mol/L
HAc and 0.2 mol/L NaAc, corrected by using a pH meter. The 0.1 mol/L HNO3 -methanol solution was
prepared by mixing of 0.2 mol/L HNO3 and methanol in equal volumes. All the chemicals used were
of analytical reagent grade unless otherwise mentioned. All solutions were prepared with ultrapure
water (18.2 MΩ cm) obtained from a Simplicity 185 (Millipore Company, Billerica, MA, USA) water
purification system.
4.3. Procedure
An aliquot of the sample or working standard solution containing rhodium in the range of
0.1–6.0 ng/mL, 2 mL of pH 5.5 HAc-NaAc buffer solution and 80 µL of 1.0 × 10−3 mol/L 5-I-PADMA
chelating solution were placed in a 10 mL graduated conical centrifuge tube. For the formation of
rhodium complex, the mixture was heated in boiling water bath for 10 min. Afterwords for cloud
point extraction, 0.8 mL of 1% (m/v) Triton X-114 solution was added and the mixture was diluted to
10 mL with ultrawater. The resultant solution was shaken and equilibrated at 60 ◦ C for 15 min in a
thermostated bath. Separation of the two phases was then accomplished by centrifugation for 5 min at
3500 rpm. The bulk aqueous phase was easily discarded by inverting the tube. The surfactant phase
(the remaining phase) in the tube was heated in water bath at 100 ◦ C to remove the remaining water,
and then dissolved with 50 µL 0.1 mol/L HNO3 -methanol solution. Then, 10 µL samples were injected
into the graphite tube for GFAAS determination of rhodium.
5. Conclusions
By combination of CPE as a technique for the separation and pre-concentration, with GFAAS as
a detection method, we proposed a novel and sensitive method for the determination of rhodium.
GFAAS is a highly sensitive analytical technique. CPE is a powerful technique for pre-concentration
Molecules 2017, 22, 487
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and separation of metal ions and it has many advantages including low cost, convenience, simplicity,
safety, low toxicity and high extraction efficiency. Triton X-114 was selected for the formation of
the surfactant-rich phase because of its convenient cloud point temperature and high density of the
surfactant-rich phase which facilitates phase separation by centrifugation. 5-I-PADMA was chosen as
the chelating reagent because it is a new and good chromogenic reagent for rhodium. The described
method has been successfully applied to the selective determination of rhodium at trace levels.
Acknowledgments: The present work was supported by National Natural Science Foundation of China
(No. 21545014, 21445004), and the Xi’an Science and Technology Plan Project (No. 2016CXML09).
Author Contributions: Quan Han conceived and designed the experiments; Yanyan Huo, Jiangyan Wu and
Longhu Yang performed the experiments; Yaping He and Xiaohui Yang analyzed the data; Jiangyan Wu, Yaping He
and Longhu Yang contributed reagents/materials/analysis tools; Quan Han and Xiaohui Yang wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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Sample Availability: Samples of 5-I-PADMA, are not available from the authors, but can be synthesized according
to literature [23].
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