1) A kinetic method is proposed for the determination of uranium(VI) based on its catalytic action on the decomposition of hydrogen peroxide in alkaline media. 2) The method was optimized and the kinetic expression was derived. Uranium(VI) can be determined in the concentration range of 0.8 to 6.4 μg/ml using the tangent method. 3) The method is selective and enables determination of uranium(VI) in the presence of high concentrations of various interfering ions. It was applied to determine uranium in phosphoric acid and phosphate ores with results matching another method.

1. MICROCHEMICAL JOURNAL 37, 263-267 (1988)
A Novel Kinetic Method for the Determination of Uranium(VI)
Based on the Decomposition of Hydrogen Peroxide
G. A. MILOVANOVIC,' M. A. SEKHETA, AND A. VASSILIOS
Department of Chemistry, Faculty of Sciences, University of Belgrade, Studentski trg 16, POB 550,
YV-11001 Belgrade, Yugoslavia
Received July 20, 1987;accepted January 19, 1988
A kinetic method is proposed for the determination of uranium(VI), based on its catalytic
action on the decomposition of hydrogen peroxide in alkaline media. The reaction rate was
followed by the stopped-flow spectrophotometric technique. On the basis of data obtained
by kinetic investigations, the kinetic expression was postulated and the conditional rate
constant was calculated. The method developed can be applied to the determination of
uranium(W) by the tangent method in the concentration range from 0.8 to 6.4 kg/ml. The
method is selective since it enables the determination of uranium(W) in the presence of
W-1000 times higher concentrations of NH,(I), Ca(II), Al(III), Fe(III), Mo(VI), phosphate,
sulfate, and fluoride. The method is applied for the determination of uranium in phosphoric
acid and phosphate ores. 8 1988 Academic PRSS, IIIC.
INTRODUCTION
In view of the increasing application of uranium for the production of atomic
energy and in regard to the control of environmental contamination connected
with it, there is a growing need for sensitive methods for the determination of
uranium.
The determination of traces of uranium(W) has recently been performed by
spectroscopic methods (Z-4), fluorescence spectrophotometry with laser induc-
tion (5, 6), and indirect fluorometry (7) aswell asby flow injection analysis (8, 9).
The determination of uranium(V1) by indirect atomic-absorption spectrophotom-
etry with the application of flow injection analysis (0.05-l kg/ml) afforded a 2000
times higher sensitivity relative to the direct atomic-absorption determination.
However, the former method is suitable only for relatively pure uranium(V1)
solutions or after suitable separation procedures have been applied. In addition,
uranium(V1) has been determined by plasma emission spectroscopy (20) and re-
versed-phasehigh performance liquid chromatography with spectrophotometric
detection (II).
The possibility of the kinetic determination of uranium(VI), based on catalytic
reduction of the pigment Victoria blue V by titanium(III), has been described by
Arkosi (12). However, this reaction is catalyzed also be vanadium(VI), vanadi-
um(V), molybdenum(VI), tungsten(VI), and osmium.
In the present paper a kinetic method, based on the catalytic action of urani-
um(W) on the decomposition of hydrogen peroxide, is proposed.
’ To whom correspondence should be addressed.
263
0026-265X/88$1SO
Copyright 0 1988by Academic Press, Inc.
All rights of reproduction in any form reserved.

2. 264 MILOVANOVIk, SEKHETA, AND VASSILIOS
EXPERIMENTAL
Reagents. Analytical grade reagents and redistilled water were used for the
preparation of all solutions. Concentrations of the stock solutions were hydrogen
peroxide (Merck), 3.3 X IO-* M; UO,(N03, * 6H2O (Merck), 1 X 10m3M; so-
dium hydrogen carbonate and sodium carbonate (Merck), 1M; sodium chloride
(Merck), 1M.
Apparatus. The reaction rate was followed on a Zeiss Specol equipped with a
l-cm path flow cell, connected to the stopped-flow set-up. A simple and inexpen-
sive stopped-flow mixing module consisting of mixing, thermostatting, and pneu-
matic propelling systemswas used (Fig. 1). The inner diameter of the used PVC
tube was 0.5 mm and the length of the mixing coil was 300mm. The flow rate of
the used pump was 25 ml/min, and the approximate time neededto fill the cell up
was 3 s.
The pH measurementswere performed on a Radiometer 4C pH meter, whereas
the temperature was kept constant by use of an ultrathermostat type NBE (VEB
Prtifgerate-Werk, Medingen).
Procedure. One reactant was a mixture of uranium(V1) solution and the buffer,
whereas the other was hydrogen peroxide solution. These solutions were sucked
through the T-shapedvalve Vi, by meansof a pump, and mixed inside the coil M,
in a 1:l volume ratio. The mixture was driven into the thermostatted closed-
system cell C, and the rate of the colored reaction product formation was mea-
sured at 340nm, 40 sfrom the onset of the reaction. During the measurementthe
valve V, was closed, and the pump switched off. The T-shaped valve V2 enables
emptying or refilling of the cell, in order to wash the systemand to prepare it for
the next measurement.
All measurementswere performed at 25 + 0.l”C, at constant ionic strength, Z =
0.2 (NaCl).
Decomposition of the prosphate sample. An aliquot of the phosphate sample
(50 ml of crude phosphoric acid and 20 g of phosphate ore, respectively) was
placed in a 250-mlbaker and 30ml of cone HNO, and 10ml of cone HC104added.
The solution was evaporated to 30 ml and cooled, and 40 ml of HCl and HNO,
(1:3) added and heated to destroy nitric oxides. The cooled solution was trans-
ferred to a 250-ml volumetric flask and diluted to the mark with double-distilled
water.
For kinetic determination of uranium in natural samples, 1ml of 90% Al(N03X
and 0.5 ml of 2% NaF was addedto an aliquot of 2 ml of this solution, diluted with
buffer to 25 ml, and centrifuged (10 min, 4000g). The clear solution was trans-
ferred to the test tube for kinetic determination.
FIG. 1. Diagram of the stopped-flow system. A and B, Reactants; M, mixing coil; T, thermostat; C,
closed-system cell; P, pneumatic propelling system (pump); V, and V2, T-shapped valves.

3. KINETIC DETERMINATION OF URANIUM(W) 265
(b)
I ’ ’ ’ ’ ’
10 20 30 Lo mffcr1t.lx10*
6.0 6.5 9.0 9.5 IO 10.5 p"
FIG. 2. Effect of variables on the reaction rate. Initial concentrations: hydrogen peroxide, 8.20 x
10m5M ((b) and (c)); carbonate buffer (pH 10.5), 1.34 x 10-l M(a) and 1.67 x 10-l M(c); urani-
um(X), 5.88 x 10m5M(a) and 3.34 x 10m5M ((b) and (c)).
RESULTS AND DISCUSSION
The kinetics of the decomposition of hydrogen peroxide in the presence of
uranium(V1) and carbonate buffer have been investigated. Figure 2 shows the
effect of variables on the reaction rate. From the results obtained it can be seen
that the catalytic action of uranium(V1) on the decomposition of hydrogen per-
oxide is maximal at hydrogen peroxide concentration of 8.20 X lo-’ M, and
buffer concentration of 1.67 x 10-i M. The dependenceof the reaction rate on
TABLE 1
Summary of the Kinetic Data for the Decomposition of Hydrogen
Peroxide Catalyzed by Uranium(W)
Variable and concentration range
[H,O,] < 8.2 x 1O-5 M
8.2 x lO-5 M < [H,O,] < 11.0 x lO-5 M
[H,O,] > 11.0 x lo-‘M
[Buffer] < 1.67 x 10-l M
[Buffer] > 1.67 x 10-l M
0.3 x lo-” M < [H,O+] < 16.0 x lo-"M
Partial order
314
-3
- 315
l/2
- l/3
- 114

4. 266 MILOVANOVIC, SEKHETA, AND VASSILIOS
TABLE 2
Kinetic Determination of Uranium(V1)
Taken
(t.d4
Found”
Wml)
Number of
determinations
Relative standard
deviation (%)
0.80 0.80 -t 0.03 5 3.7
3.18 3.30 -I-0.93 5 2.8
6.37 6.08 ” 0.20 5 3.2
@Mean and standard deviation.
uranium(V1) concentration ranging from 0.20 x 10W5to 8.5 x IO-’ M was found
to be linear (tan OL= 1.67x Cu) with a linear regressioncorrelation coefficient of
0.998.
The partial orders in the different variables, obtained from log-log plots of rate
data for the catalyzed decomposition of hydrogen peroxide, are summarized in
Table 1. From the results obtained the kinetic equation is proposed for concen-
trations of hydrogen peroxide lower than 8.20 x lo-’ M andbuffer concentrations
lower than 1.67 x 10-l M:
2 = k[U(VI)][H~0J3’4[Buffer]1’2[H~0+] - 1’4.
The conditional rate constant has been calculated to
k = (3.42 f 0.17) x IO4mol-’ . liter * s-l.
The investigated reaction was applied to the determination of uranium(V1) by
using the differential form of the tangent method, under the following optimum
experimental conditions: hydrogen peroxide, 8.20 x 10e5M; carbonate buffer,
1.67 x 10-l M; and pH 10.5. The results obtained are shown in Table 2, from
which it might be concluded that the method proposed can be used for the deter-
mination of uranium(V1) in concentrations from 0.84 to 6.37 pg/ml, with relative
standard deviation up to 3.7%.
The effect of foreign ions on the accuracy of the determination of uranium(V1)
hasbeeninvestigated. On the basisof the results shown in Table 3it might be seen
that, except for copper (II), which interferes with the determination when present
at a concentration twice that of uranium(VI), and cobalt(II), which interferes
TABLE 3
Tolerance Ratio for the Foreign Ions in the
Determination of 3.30 &ml Uranium(V1)
Ion
NH,(I)
Ca(I1)
Al(II1)
Fe(II1)
Cu(I1)
Ratio Ion
1OOO:l Co(I1)
50:1 Mo(V1)
1OO:l F-
5O:l po,3-
1:l so,*-
Ratio
2:l
5OO:l
50:1
15O:l
15O:l

5. KINETIC DETERMINATION OF URANIUM(V1)
TABLE 4
Results of the Determination of Uranium in Different Samples
267
Uranium content”
(pm)
Sample Kinetic method
Phosphoric acid 134.6 t 3.66
Phosphate ore from Togo 303.3 -+ 3.89
Phosphate ore from Maroco 316.8 2 7.3
LIMean of seven replications and standard deviation.
b For P = 0.05 the critical t value is 2.18.
Spectrophotometric
method
135.3 f. 2.62
305.0 f 4.01
317.8 f. 8.48
tb
1.01
1.97
0.58
already when present at an equivalent concentration, other ions exhibit no effect
even when present at concentrations 50-1000 times higher.
The method described has been applied for the determination of uranium in
crude phosphoric acid and phosphateores. The results obtained aregivenin Table
4. From this table it can be seenthat the results obtained by the proposed method
are in good agreement with the results obtained by the spectrophotometric
method with dibenzoylmethane, after prior separation of uranium by extraction
with tri-n-octylphosphine oxide (13).
Finally, it can be concluded that the proposed kinetic method can be success-
fully applied for simple and rapid determination of uranium in diverse samples
without previous separation by solvent extraction.
ACKNOWLEDGMENT
The authors are grateful to the Serbian Republic Fund for financial support.
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