Catalytic spectrophotometric determination of molybdenum

Article abstract of DOI:10.1007/s007060270004

A highly selective, sensitive, and simple catalytic method for the determination of molybdenum in natural and waste waters was developed. It is based on the catalytic effect of Mo(VI) on the oxidation of 2-aminophenol with H₂O₂. The reaction is monitored spectrophotometrically by tracing the oxidation product at 430 nm after 10 min of mixing the reagents. Addition of 800 μg·cm^-3 EDTA conferred high selectivity; however, interfering effects of Au(III), Cr(III), Cr(VI), and Fe(III) had to be eliminated by a reduction and co-precipitation procedure with SnCl₂ and Al(OH)₃. Mo(VI) shows a linear calibration graph up to 11.0 ng·cm^-3; the detection limit, based on the 3S_b-criterion, is 0.10 ng·cm^-3. The unique selectivity and sensitivity of the new method allowed its direct application to the determination of Mo(VI) in natural and waste waters.

Full text of DOI:10.1007/s007060270004

Monatshefte fuÈr Chemie 133, 31±40 (2002)
Catalytic Spectrophotometric Determination
of Molybdenum
Ã
Ashraf A. Mohamed , Saleh A. Ahmed, and Mohamed F. El-Shahat
Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia,
Cairo 11566, Egypt
Summary. A highly selective, sensitive, and simple catalytic method for the determination of
molybdenum in natural and waste waters was developed. It is based on the catalytic effect of Mo(VI)
on the oxidation of 2-aminophenol with H2O2. The reaction is monitored spectrophotometrically by
tracing the oxidation product at 430 nm after 10 min of mixing the reagents. Addition of 800 mg Á cmÀ3
EDTA conferred high selectivity; however, interfering effects of Au(III), Cr(III), Cr(VI), and Fe(III)
had to be eliminated by a reduction and co-precipitation procedure with SnCl2 and Al(OH)3. Mo(VI)
shows a linear calibration graph up to 11.0 ng Á cmÀ3; the detection limit, based on the 3Sb-criterion, is
0.10 ng Á cmÀ3. The unique selectivity and sensitivity of the new method allowed its direct application
to the determination of Mo(VI) in natural and waste waters.
Keywords. Molybdenum; Catalytic determination; 2-Aminophenol; Hydrogen peroxide; Redox
reaction; Natural and waste waters.
Introduction
Determination of molybdenum as a micronutrient is receiving increased interest in
environmental and biochemical studies. The USEPA drinking water health
advisories recommend long-term limits of 10 ng Á cmÀ3 for children and 50 ng Á cmÀ3
for adults [1]. Several methods for the sensitive determination of molybdenum in
natural and waste waters have been described, based on neutron activation analysis
[2, 3], inductively coupled plasma mass spectrometry [4, 5], inductively coupled
plasma atomic emission spectrometry [6±8], atomic absorption spectrometry [9±
11], stripping voltammetry [12, 13], and high performance liquid chromatography
[14, 15]. Typical detection limits are 2.5±10.0, 0.2±0.3, 0.07±8.0, 0.01±1.4, 0.34±
0.5, and 0.4±4.6 ng Á cmÀ3 of Mo(VI). However, the need for preconcentration
[2±15] and relatively high costs [2±8] are common disadvantages of these methods.
Catalytic methods with spectrophotometric monitoring offer low-cost and
simple alternatives for the determination of traces of elements [16±49]. Many
catalytic methods for molybdenum determination are based on its effects on the
oxidation of a substrate with a suitable oxidant such as ClOÀ3 [18], IO4À [19], or
Ã
University, Abha 9033, Kingdom of Saudi Arabia. E-mail: aamohamd@hotmail.com
Corresponding author. Present address: Department of Chemistry, Faculty of Science, King Khalid

32
A. A. Mohamed et al.
H2O2 [20±33, 44, 45]. Other methods rely on its catalytic effect on the reduction of
a dye with a suitable reducing agent such as NH2NH3Cl [34±42] or SnCl2 [43].
However, poor selectivity [18, 20, 26, 27, 31±33, 37±41, 44, 45] and high detection
limits [18±29, 31±45] are common disadvantages. Therefore, there is still a need
for a more sensitive, selective, and precise method.
The catalytic oxidation of 0.05 M 2-aminophenol (1) with H2O2 in the presence
of acetate buffer of pH ˆ 5.0±6.0 has been used to determine Mo(VI) and W(VI) in
highly pure inorganic compounds and synthetic samples [44, 45]. However, this
method is characterized by its poor precision resulting from the very high blank
values and the use of a completely unstable aminophenol reagent. Therefore, a
thorough examination of the Mo(VI) catalyzed 2-aminophenol-H2O2 reaction was
undertaken.
Results and Discussion
Preliminary considerations
The oxidation of 1 with H2O2 is a slow process that can be catalyzed by Au(III)
[46], Cr(III) and Cr(VI) [47], Cu(II) [48], Mo(VI) and W(VI) [44, 45], or Nb(V)
and Ta(V) [49]. The oxidation product exhibits a maximum absorbance at 430 nm
and has been isolated and identi®ed as 2-amino-3-phenoxazinone [50].
Aqueous solutions of 1, prepared by dissolving the reagent in dilute HNO3,
HCl, or H2C2O4 [44±46, 48, 49] are completely unstable and readily darken after
preparation because of the rapid auto-oxidation catalyzed by ultra-trace amounts of
ions. Therefore, sodium metabisul®te and EDTA were tested as stabilizing and
masking agent, respectively. Preliminary experiments showed that the sensitivity
of Mo(VI) determination was not affected by the presence of EDTA (up to
2 Â 10À3 M) and sodium metabisul®te (up to 5 Â 10À3 M) in the reaction cell.
Therefore, several working solutions of 1 were prepared containing EDTA
(2 Â 10À3 M) and sodium metabisul®te (0.1±12 Â 10À2 M). The changes in the
absorbances of these solutions as a function of time were taken as measures of
their stability. A solution was considered stable when its absorbance remained
essentially constant for at least 24 hours. It was found that working solutions of 1
containing less than 1 Â 10À2 M metabisul®te were completely unstable. Moreover,
the solution stability increased with metabisul®te concentration, reaching its
maximum above 3 Â 10À2 M metabisul®te. To provide a stable reagent solution and
to confer enhanced selectivity, the working solution of 1 was thus prepared to
contain 2 Â 10À3 M EDTA and 6 Â 10À2 M metabisul®te, respectively; the working
phosphate buffer solution contained 3 Â 10À3 M EDTA.
The ®xed time method is usually more sensitive compared with the initial rate
method [16, 17]. Therefore, ®xed time measurements after 10 min of mixing of the
reagents was adopted in the recommended procedure to provide a reasonable
sensitivity and a moderate blank reading.
Effects of reaction variables
The absorbances of the uncatalyzed reaction mixture (Au) and of the reaction
catalyzed by 10 ng Á cmÀ3 Mo(VI) (Ac) varied exponentially with pH in the range

Spectrophotometric Determination of Mo
33
Fig. 1. Effect of pH; Au: absorbance of the uncatalyzed reaction, Ac: absorbance of the reaction cata-
lyzed by 10 ngÁ cmÀ3 Mo(VI); except for the abscissa, the conditions were as given in the recommended
procedure
of 6.0±7.0. The sensitivity (Ac±Au) achieved its highest values at pH ˆ 6.70±7.10
(Fig. 1). Therefore, a pH of 6.75 Æ 0.05 was adopted in the recommended
procedure to provide a moderate buffering capacity and a low blank reading and to
minimize the catalytic effect of Cr(VI) ions which is neglegible at pH ꢀ 6.60 [47].
The effect of buffer type at pH 6.75 was studied using HCl/NaOH, CH3COOH/
NaOH, glycylglycine/NaOH, tris-(hydroxymethyl)-aminomethane/HCl,andH3PO4/
NaOH buffers (10 ng Á cmÀ3 Mo(VI), 2.5 Â 10À2 M of the tested buffer). All of
them gave an almost constant Au value; however, Ac values of 0.433, 0.461, 0.298,
0.314, and 0.575 were observed. Therefore, phosphate buffer was used which
offers enhanced sensitivity and selectivity.
The Ac values increased sharply with phosphate concentration up to
2.0 Â 10À2 M; the Au values increased gradually in the studied phosphate concen-
tration range. The sensitivity reached an almost stable maximum in the range of
2.0±3.0 Â 10À2 M phosphate (Fig. 2). The appearance of such a maximum is a charac-
teristic feature for all types of activation reactions where the activator forms a
ternary complex of the type activator-metal-substrate [16, 17]. Therefore, a ®nal
phosphateconcentrationof2.5 Â 10À2 Mwasadoptedintherecommendedprocedure
in order to provide a reasonable activating effect and a buffering capacity.
The Ac and Au values gradually increased with the concentration of 1 resulting
in a maximum sensitivity in the range of 4.0±6.0 Â 10À3 M (Fig. 3). Thus, in order
to provide high sensitivity and a low reagent blank, a concentration of 4.0 Â
10À3 M of 1 was adopted in the procedure.

34
A. A. Mohamed et al.
Fig. 2. Effect of phosphate buffer concentration; conditions and symbols are as given in Fig. 1
Fig. 3. Effect of the concentration of 1; conditions and symbols are as given in Fig. 1
The absorbances Ac and Au and the sensitivity Ac±Au sharply increased with
H2O2 concentration up to 2.0 Â 10À2 M. However, they were almost independent of
the H2O2 concentration in the range of 2.0±10.0 Â 10À2 M. Therefore,
4.0 Â 10À2 M H2O2 were employed.
Ac and Au increased exponentially with temperature; a working temperature
of 40ꢁC was adopted in the recommended procedure because of the moderate
sensitivity and reagent blank and its convenience for operation.

Spectrophotometric Determination of Mo
35
The possible enhancing effects of some water-soluble solvents were tested in
the determination of 5.0 ng Á cmÀ3 Mo(VI) (ethanol, methanol, 2-propanol,
acetonitrile, dimethylsulfoxide, dimethylformamide, 1,4-dioxane; 5±10% v/v;
higher concentrations of these solvents gave low Ac and sensitivity values). The
above solvents gave high Ac and Au values, but lower sensitivity compared to that
obtained in their absence. The effects of added salts was studied using suprapure
(NH4)2SO4, KCl, and NaNO3. The tested salts had almost no effect on the sensi-
tivity up to 0.2 M; however, both Ac and Au very slightly increased with increasing
salt concentration.
The absorbance of the catalyzed reaction was independent of the order of
mixing the reagents. However, H2O2 was added before 1 to avoid the possible auto-
oxidation of 1 catalyzed by Mo(VI). Moreover, Ac values were almost independent
of the time of contact between Mo(VI) and H2O2 before the addition of 1. For
example, in the determination of 10 ng Á cmÀ3 of Mo(VI), the recoveries after 1, 5,
and 10 minutes of contact between Mo(VI) and H2O2 were 100, 99, and 97%.
The effects of 67 potential interfering species which may accompany
molybdenum in natural and/or waste waters were studied using 5.0 ng Á cmÀ3
Mo(VI). The maximum tolerable concentrations of these species are shown in
Table 1, where the tolerance level is de®ned as the concentration of foreign ion
that produced a changÀe in the absorbance of the catalyzed reaction of less than
À
5%. Except for NOÀ2 , I , S CN , Au(III), Cr(III), Cr(VI), and Fe(III), all ions tested
were tolerated at reasonably high concentrations without any speciaÀl precautionÀ.
Sulfamic acid and Hg(II) effectively masked the effects of NOÀ2 , I , and SCN ,
respectively as described in the procedure. However, EDTA and/or oxalate could
not improve the selectivity towards ng Á cmÀ3 levels of Au(III), Cr(III), Cr(VI), and
Table 1. Tolerance levels of foreign species in the determination of 5 ng Á cmÀ3 Mo(VI)a
Tolerance
level/mg Á cmÀ3
> 500
Foreign speciesb
acetate,Àsulfamate, EDTA, CyDTA, NTA, glycylglycine, Tris, NOÀ3 ,
À
‡
‡
‡
Cl , Br , ClOÀ4 , HCO3À, S O , S2O52À, Na , K , NH4 , Li
2À
‡
4
> 100
CNÀ, FÀ, B4O27À, Mg(II), Ca(II), As(III), As(V), Al(III), Zn(II)
50
citrate, oxalate, tartrate, Tl(I)c, Ba(II), Cd(II)c, Cu(II)c, Ni(II)c, Sr(II),
Cr(III)c
d
c
c
2À
5
NH2NH3Cl, NOÀ2 , S O , Ag(I) , Be(II), Co(II) , Hg(II), Pd(II),
3
Pb(II)c, Sn(II), Bi(III)c, Fe(III)c, Ce(III)c, Hf(IV), Sn(IV), Th(IV)c,
Ti(IV)c, Zr(IV)c, Nb(V)c, Ta(V)c, U(VI), W(VI)
1
I
À d, S CNÀ d, Fe(II)c, V(V)c, V(IV)c
Au(III)c, Cr(VI)c
0.10
0.01
NOÀ2 , S CN, S2À, Fe(II), Fe(III), Cr(III), V(V), Nb(V), Ta(V),
À
Cr(VI),
a
Reaction conditions as given in the recommended procedure; b species with buffering action were
adjusted to pH 6.75 before studying their effects; EDTA: ehtylenediamine tetraacetic acid disodium
salt; CyDTA: trans-1,2,-diaminocyclohexane-N,N,N0,N0-tetraacetic acid; NTA: nitrilotriacetic acid;
c
d
after reduction and coprecipitation steps; using sulfamic acid and/or Hg(II) masking solutions

36
A. A. Mohamed et al.
Fig. 4. Effect of pH of precipitation on the recovery of Mo(VI); conditions and symbols are as given
in Fig. 1
Fe(III) ions. On the other hand, up to 5 mg Á cmÀ3 Sn(II) had no effect on the
determination of Mo(VI); however, at a 1:1 ratio it destroyed the catalytic effects
of Au(III) and Cr(VI) ions. Therefore, 0.10 mg Á cmÀ3 Sn(II) were added to the
samples to reduce these ions. Otherwise, traces of Cr(III), Fe(II), and Fe(III) may
be coprecipitated quantitatively from neutral media in presence of a collector such
as Fe(III), La(III), Mn(II), or Al(III) [16]. However, Fe(III), La(III), and Mn(II) are
well known to coprecipitate Mo(VI) ions [51], resulting in very poor recoveries of
Mo(VI). Therefore, Al(III) was employed in the procedure as a collector due to its
ability to give a quantitative and rapid coprecipitation of Cr(III) and Fe(III) ions,
but not creating interferences in the analytical determination of Mo(VI) due to its
high tolerance level (Table 1).
The effects of the different precipitation variables on Mo(VI) recovery were
thoroughly investigated. Quantitative recoveries of Mo(VI) were obtained in the
pH range of 8.20±8.80 in presence of 10±50 mg Á cmÀ3 Al(III) (Fig. 4). At higher
pH values, partial dissolution of the precipitate was observed. Moreover, con-
centrations of 10, 50, 100, 150, 200, and 250 mg Á cmÀ3 Al(III) at pH ˆ 8.40 gave
recoveries of 100, 98.8, 96, 91, 87, and 83%. On the other hand, a salt such as
(NH4)2SO4 added during the coprecipitation step in concentrations up to 0.10 M had
no effect on the recovery of Mo(VI). Therefore, a pH of 8.40Æ0.1 in the presence
of 30 mg Á cmÀ3 Al(III) was adopted in the procedure. Such a treatment successfully
eliminated the interfering effects of 5 mg Á cmÀ3 of Cr(III) and Fe(III) ions and
0.10 mg Á cmÀ3 of Au(III) and Cr(VI) ions.
Despite of its simplicity and high sensitivity, the most interestÀing feature of the
À
implemented method is its selectivity, especially towards NOÀ2 , I , S CN , Cu(II),
Au(III), Fe(III), Nb(V), Ta(V), Cr(III), Cr(VI), and W(VI) which are known to
catalyze the 2-aminophenol-H2O2 reaction [44±49].

Spectrophotometric Determination of Mo
37
Table 2. Determination of Mo(VI) in natural, ground, and waste watersa
Sampleb
Molybdenum conc.c/ng Á cmÀ3
Recovery/%
No.
1
Type
Added
Found
Nile River
±
2.27Æ0.16
6.33Æ0.18
±
4.00
101.5
2
3
4
5
6
7
a
Natural water (Siwa)
Natural water (Mineral)
Well water
±
1.15Æ0.12
±
4.00
5.06Æ0.08
97.8
±
1.64Æ0.13
5.60Æ0.12
±
4.00
99.0
±
2.70Æ0.14
±
4.00
6.75Æ0.10
101.3
Well water
±
1.95Æ0.15
5.88Æ0.15
±
4.00
98.3
Waste water
±
6.42Æ0.10
±
4.00
10.54Æ0.11
103.0
Waste water
±
4.19Æ0.14
±
4.00
8.08Æ0.12
97.3
Reaction conditions as given in the recommended procedure; b No. 1: Nile River, downstream about
1 km from Kasar El-Nile bridge on June 21st, 2000 (pH ˆ 8.17); Nos. 2 and 3: natural waters from
the local market; Nos. 4 and 5: ground waters collected on June 27th, 2000 from Matrouh city; No. 4:
30ꢁ5308.5500 N and 28ꢁ32021.6600 E, Sawani Samalouth, pH ˆ 7.41; No. 5: 31ꢁ04014.1300 N and
28ꢁ28010.2300 E, Saleh Gali, pH ˆ 7.63; Nos. 6 and 7: waste waters collected on August 10th, 2000
from cement and steel industrial plants of Helwan, Cairo; c added to or found in the original sample;
found ˆ meanÆstandard deviation (n ˆ 3)
Calibration graph and detection limit
The calibration graph prepared following the recommended procedure gave a
linear relationship (r ˆ 0.998) between absorbance and Mo(VI) concentration up to
11.0 ng Á cmÀ3 of Mo(VI). The detection limit was determined as 0.10 ng Á cmÀ3
Mo(VI) (three times the standard deviation of the blank; 3Sb-criterion). The preci-
sion of the method was assessed by analyzing 1,5, and 10 ng Á cmÀ3 Mo(VI) in
aqueous solutions; it gave an average recovery of 98.7% with relative standard
deviations below 1.1% (n ˆ 5).
Applications
The new method was applied to the determination of Mo(VI) in Nile River water,
ground water, and industrial waste waters (Table 2). Recovery experiments for
the analyzed samples proceeded quantitatively with standard deviations of
Æ0.15 ng Á cmÀ3 Mo(VI) (n ˆ 3), indicating the high accuracy and precision of the
technique.
Conclusions
The suitability of the 2-aminophenol-H2O2-Mo(VI) system allowed the develop-
ment of a simple and low-cost method for molybdenum determination down to

38
A. A. Mohamed et al.
0.10 ng Á cmÀ3, a level that is eighty times lower than that achieved by the standard
ICP-AESmethod [6]. The simplicity, sensitivity, and freedom from most inter-
ferences are signi®cant advantages of the method compared with the high-cost NAA,
ICP-MS, and ICP-AES methods. Moreover, the procedure surpasses most kinetic
methods [18±29, 31±45] for Mo(VI) determination in sensitivity and selectivity.
Experimental
Apparatus
Absorbance measurements were performed on a precalibrated Spekol-11 spectrophotometer
(Germany) equipped with 50 mm matched cells. The temperature of the cell compartment of the
spectrophotometer was controlled by circulating water from a PolyScience thermostatted water bath
(Niles, IL, USA) with a stability of Æ0.1ꢁC. Eppendorf vary-pipettes (Westbury, NY, USA; 10±100
and 100±1000 mm3) were used to deliver accurate volumes. Measurements of pH were made on a
calibrated EDT (Dover, Kent, UK) pH-mV meter model GP 353 equipped with an EDT combined
glass electrode with an accuracy of Æ0.01. All glassware and storage bottles were soaked in 10%
HNO3 overnight, thoroughly rinsed with fresh distilled, deionized H2O, and dried prior to use.
Reagents
All chemicals were of analytical reagents grade (Merck, Darmstadt, Germany; Fluka, Buchs,
Switzerland). Freshly distilled de-ionized H2O was used throughout.
A stock standard solution of 1000 mg Á cmÀ3 of Mo(VI) was prepared from Na2MoO4. A
50 ng Á cmÀ3 working standard solution was freshly prepared from the stock solution. A 2.0 M H2O2
solution was used. A 0.10 M working solution of 2-aminophenol (1) was prepared by mixing 0.545 g
of the reagent with 0.6 cm3 of suprapure HCl and dissolving in about 20 cm3 of H2O, followed by
addition of 0.037 g of EDTA, 0.57 g of sodium metabisul®te, and H2O and adjusting the pH to
4.0Æ0.2 (partial turbidity appeared at higher pH values). The resulting solution was diluted to 50 cm3
in a calibrated ¯ask which was wrapped with aluminum foil and stored at 4ꢁC when not in use. This
reagent is stable for at least one week.
A phosphate buffer solution was prepared by mixing 6.25 cm3 of 1.0 M H3PO4 with 0.112 g
EDTA and diluting to about 90 cm3 with H2O. The pH of this solution was adjusted to 6.75Æ0.02,
and the solution was diluted to 100 cm3 in a calibrated ¯ask.
Solutions of 2% (w/v) sulfamic acid and 2 mg Á cmÀ3 Hg(II) were used as masking agents for
NOÀ2 , IÀ, and SCNÀ ions. A collector solution for the coprecipitation step was prepared by
dissolving 2.0 g of AlCl3 Á 6H2O in a 100 cm3 calibrated ¯ask. A 5% (w/v) solution of NaOH was
also used.
Treatment of water samples
Collected samples were ®ltered and acidi®ed to pH ˆ 2.0Æ0.2. To 70±80 cm3 of the acidi®ed sample,
1.0 cm3 of the sulfamic acid solution were added. The mixture was shaken well and set aside for
5 min to decompose nitrite ions. After addition of 1 cm3 of 10 mg Á cmÀ3 Sn(II) solution, the sample
was shaken to reduce Au(III) and Cr(VI) ions if present. Addition of 1.0 cm3 of the Al(III) collector
solution and adjustment of the pH to 8.40Æ0.2 using the working NaOH solution led to coagulation.
The formed precipitate was ®ltered off and washed with three 5 cm3 portions of H2O. Filtrate and
washings were collected, the pH was adjusted to 6.75Æ0.2, 1 cm3 of the Hg(II) solution was added to
mask IÀ and SCNÀ ions, and the mixture was diluted with H2O in a 100 cm3 calibrated ¯ask.

Spectrophotometric Determination of Mo
39
Recommended procedure
Keep the working solutions, treated samples solutions, H2O, and 20 cm3 stoppered glass test tubes
(Pyrex, grade A) at 40ꢁC in the thermostatted water bath. To one of the test tubes, transfer ꢂ 2.70 cm3
of the treated sample or the working standard Mo(VI) solution and dilute with H2O to 2.70 cm3. Add
2.00 cm3 of the phosphate buffer solution and 0.10 cm3 of the working H2O2 solution. Start the reac-
tion by adding 0.20 cm3 of the working solution of 1, shake, and immediately transfer a portion of the
reacting mixture to the 50 mm thermostated cell of the spectrophotometer. Record the absorbance
value at 430 nm after 10 min of mixing against water as a reference. The Mo(VI) concentration
of the unknown sample is determined from a calibration graph, similarly prepared with the standard
Mo(VI) solution.
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Received April 11, 2001. Accepted (revised) June 18, 2001