This document describes kinetic methods for determining ultramicro quantities of manganese(II), molybdenum(VI), and tungsten(VI) based on their catalytic effect on the oxidation of Azorubin S by hydrogen peroxide. The methods were found to determine these elements in the concentration ranges of 5.5-33 ng/ml for manganese, 1.3-8.1 μg/ml for molybdenum, and 5.9-44.1 μg/ml for tungsten, with relative standard deviations less than 11%, 9%, and 5% respectively. The oxidation reactions were determined to be first order with respect to the catalyst and zero order with respect to Azor

1. Mikrochimica Acta [Wien] 1978 I, 297--304 MIKROCHIMICA
ACTA
9 by Springer-Verlag 1978
Chemical Institute, Faculty of Sciences, University of Belgrade, Belgrade,
Yugoslavia
A Kinetic Determination of Ultramicro Quantities of
Manganese(II), Molybdenum(VI) and Tungsten(VI) on
the Basis of Their Catalytic Action on the Oxidation
of Azorubin S by Hydrogen Peroxide*
By
M. A. Sekheta, G. A. Milovanovid, and T. J. Janjid
(Received June 24, 1977)
Continuing our investigations on a kinetic catalytic determina-
tion of ultramicro quantities of inorganic and organic substances 1-6
we have established in the present work that the oxidation of Azo-
rubin S by hydrogen peroxide in carbonate buffer is catalysed by
manganese(II), molybdenum(VI) and tungsten(VI). In order to de-
velop new kinetic methods for the determination of these elements,
as well as to find suitable indicator reactions for the determination
of ultramicro quantities of organic substances, in the present work
the kinetics of catalytic oxidation of Azorubin S by hydrogen per-
oxide has been studied, and methods for the determination of Mn(II),
Mo(VI) and W(VI) are described.
Numerous kinetic catalytic methods for the determination of
Mn(II), Mo(VI) and W(VI) have been reported in the literature.
Many of them have been listed in the monograph by Yatsimirskii v
Besides these methods, many others requiring the use of more or
less complex and expensive apparatus have been described. Morgan
et al. s, Mottola and Harrison 9, Dolmanova et al. 1~ Janjid et al. 1,
Abe and Takahashi la and Abe et aI. 14 have worked on the deter-
mination of Mn(II), while Mo(VI) has been determined by Fuge 15,
* This work was presented in part at the 20th Annual Meeting of
the Serbian Chemical Society, Belgrade, January 17, 1977.
0026-3672/78/7801/0297/$ 01.60

2. 298 M.A. Sekheta et al.:
Hadjiioannou 16, Kambara et aI. 17, Weisz et al. 18, Pavlova and Yat-
simirskiil% Pelczar and Boleslaw ~~ Wilson 21, Jamane et al. 2~, Weisz
and Pantel z3, Weisz and Rothmaier 24, Kuroda and Tarni 25, and
Bradfield and Stickland 26. The determination of W(VI) has been
studied by Hadjiioannou and Valkana 27 and Omarova et al. 2s. In
the works quoted, manganese(II), molybdenum(VI) and tungsten(VI)
were determined in concentration ranges of 8 • • 10 -1,
2 • 10-4--3 • 10-1 and 6 • 10-3--7 #g/ml, respectively.
Results and Discussion
The kinetic investigations showed that in all the cases observed
the solution absorbance was a linear function of time. This means
that irrespective of the catalyst used the reaction is of zero order
with respect to Azorubin S. We determined the kinetics of oxidation
of Azorubin S by hydrogen peroxide in the presence of Mn(II),
Mo(VI) and W(VI) as catalyst in the usual way, i.e. by following
the variation of tan c~ with concentration of the component in-
vestigated, at constant concentration of all other components taking
part in the reaction. Since in all the cases observed a linear rela-
tionship was found between tan c~ and catalyst concentration, it
may be concluded that all the reactions investigated are of first
order with respect to catalyst.
Study of Reaction Kinetics and Determination
Manganese(H). Investigating the buffer component of the system,
we established that the reaction rate is considerably higher when
a solution of ammonium carbonate is used than for a carbonate
buffer of the same concentration and pH-value prepared from sodium
carbonate and bicarbonate. Therefore all subsequent experiments
were carried out with an ammonium carbonate medium, which re-
presents a combined buffer: NH4+/NH3 and HCO~-/CO32-. The
reaction studied is first order with respect to the concentration of
ammonium carbonate, while the reaction order with respect to
hydrogen peroxide is complex: at peroxide concentrations lower
than 4 • 10-2 M the reaction is of first order with respect to hydro-
gen peroxide, whereas at higher concentrations it is of zero order.
Thus we arrive, for concentrations of hydrogen peroxide lower than
4 • 10 -2 M, at the equation:
dc
dt =k~ [Mn] [H202] [(NH4)zCO3]; /~= (36 + 3) x 10 -z

3. A Kinetic Determination of Ultramicro Quantities 299
From this kinetic expression it may be concluded that the reac-
tion rate (and hence the sensitivity of the method) may be increased
by increasing the concentration of hydrogen peroxide up to
4 x 10 .2 M, as well as by increasing the buffer concentration. In
accordance with this the initial concentrations of the components
in determining manganese(II) were chosen to be Azorubin S,
4 x 10 -v M; hydrogen peroxide, 4 x 10 -2 M and ammonium carbon-
ate 0.8 M at pH 9.35. The equation for the calibration curve was
--tane=2.69x10-4XCMn+2.43x10 4, where CMn is in ng/ml.
The results obtained are given in Table I. It is seen that Mn(II)
Table I. A Kinetic Determination of Ultramicro Quantities of Some Elements
by Catalytic Oxidation of Azorubin S by Hydrogen Peroxide
Element
Taken Found Number of Relative
determi- standard
nations deviation,%
ng/ml ng/ml
Mn(II)* 5.5 5.3 • 0.6 3 11
22.0 22.2_+0.6 3 3
33.0 33.9• 1.7 3 5
#g/ml ,ug/ml
Mo(VI)** 1.3 1.2• 0.1 3 8
4.0 3.9_+0.2 3 5
8.1 8.2• 3 2
/*g/ml ,ug/ml
W(VI)*** 5.9 5.8 _+0.3 3 5
22.1 21.7_+0.6 3 3
44.1 43.6_+0.9 3 2
* The determination is not hindered by 1000times as much Cu(II) and
V(V) or 100 times as much Fe(III) and Cr(III) as Mn(II).
** The determination is not hindered by 104 times as much V(V), 1000
times as much Fe(III) and Cr(III) or 10 times as much Cu(II) and W(VI) as Mo(VI).
*** The determination is not hindered by 10 times as much V(V) or an
equal amount of Fe(III) or Cr(III), but is prevented by an equal amount of
Cu(II) or Mo(VI).
was determined at concentrations ranging from 5.5 to 33.0 ng/ml
with a relative standard deviation lower than 11%. It is also seen
that Cu(II), V(V), Fe(III) and Cr(III), if present at considerably higher
concentration than that of manganese, do not interfere.
Molybdenum(W). The reaction rate was found to be the same
for both ammonium carbonate and sodium carbonate/bicarbonate
buffers of the same concentration and pH-value. Therefore we

4. 300 M.A. Sekheta et al.:
decided to use the latter buffer, since it makes it possible to vary
the pH-value of the solution over a definite range. The reaction
is found to be of zero order with respect to buffer concentration.
The dependence of reaction rate on pH is complex: at pH-values
lower than 9.5 the reaction order with respect to hydrogen ion con-
centration is -0.25, whereas at higher pH-values it is of zero order.
It has also been established that the reaction is of first order with
respect to hydrogen peroxide over the entire range investigated (up
to 6 x 10 -~ M). At pH-values lower than 9.5, the following kinetic
equation is valid:
dc
d t =k~ [Mo] [H20~] [H30+] 0.~a; /~= (19 _+1) x 10 7
From this expression it is seen that the reaction rate may be
increased by increasing the hydrogen peroxide concentration and
the pH (up to 9.5). Accordingly we chose the following initial
concentrations of the components: Azorubin S, 4 x 10 -7 M; hydro-
gen peroxide, 5.9 x 10 ~M; 8 x10 -~ M buffer at pH 10.3. The
equation for the calibration curve was - tan ~ = 6.41 x 10-4 • CM ~ +
5.43 x 10 -4, where CMo is in #g/ml. The results obtained are also
given in Table I. It is seen that Mo(VI) was determined at concen-
trations varying between 1.3 and 8.1/~g/ml, with a relative standard
deviation less than 9%. V(V), Fe(III), Cr(III), Cu(II) and W(VI) do
not interfere even when present in concentrations substantially
higher than that of Mo(VI).
Tungsten(VI),The reaction rate is again higher in ammonium
carbonate medium. The reaction order with respect to ammonium
carbonate is 0.25 (the ammonium carbonate concentration in these
experiments was varied between 0.08 M and 0.8 M). The dependence
of reaction order on the concentration of hydrogen peroxide is
complex: at peroxide concentrations lower than 4 x 10 -2 M the
reaction is of first order with respect to hydrogen peroxide, while
at higher concentrations it is of zero order. The following kinetic
expression is valid for hydrogen peroxide concentrations lower than
4x10 2M:
dc
dt -~ [W] [H202] [(NH4)2CO8]~ /~ = (4.5 _+0.2) x 10-4
We therefore chose the following initial concentrations of the
components: Azorubin S, 4 x 10 -7 M; hydrogen peroxide, 4 x 10 2M
and ammonium carbonate 0.8 M (pH 9.35). The equation for the
calibration curve was -tan e = 1.51 x 10 -4 x Cw +5.94 x 10 4, where

5. A Kinetic Determination of Ultramicro Quantities 301
Cw is in #g/ml. The results obtained are again presented in Table I.
W(VI) was determined at concentrations ranging from 5.9 to
44.1#g/ml with a relative standard deviation less than 5%. V(V),
Fe(III), Cr(III), Cu(II) and Mo(VI), if present in small concentrations,
do not interfere.
The methods proposed are very simple to use, since there is a
linear relation between the solution absorbance and time. The un-
catalysed reaction causes a small intercept on the ordinate of the
plot of tan a vs metal concentration.
Also an ordinary photoelectric colorimeter can be used. The
catalytic oxidation of Azorubin S by hydrogen peroxide in the
presence of Mo(VI) as catalyst appears to be of special interest,
since, in contrast to other methods described in the literature, it is
carried out in alkaline medium. This makes it possible to have the
molybdenum coordinated to weak organic acids, which would be
impossible in acidic medium. Since the catalytic activity of a metal
is, as a rule, very different from that of its complexes, this means
that the oxidation of Azorubin S by hydrogen peroxide in the pres-
ence of Mo(VI) as catalyst would be applicable as an indicator reac-
tion for the determination of organic compounds of slightly acidic
character. An investigation of new kinetic catalytic methods for the
determination of ultramicro quantities of pharmacologically interest-
ing polyhydroxyl phenols is in progress.
Experimental
The rate of oxidation of Azorubin S by hydrogen peroxide was
followed photometrically on an Iskra (Kranj) MA 9501 colorimeter
fitted with a constant-temperature cell-holder. The reaction was
done as follows. In a reaction vessel with three compartments, the
solutions of Azorubin S and buffer were measured into one com-
partment, catalyst into the second, and hydrogen peroxide diluted
with water to 25 ml in the third compartment. The vessel was kept
for 15 min at a temperature of 25 _+0.1 o C. The reaction was then
initiated by vigorous mixing and the solution was transferred to
test-tubes. The absorbance of the solution was measured every
minute over a period of 10 rain from the onset of reaction, a blue
filter (2max at 540 rim) being used. The following relationship was
found to exist between the measured absorbance of the solution
(A) and the concentration (C) of Azorubin S:
A = 1.4 x 106 C.

6. 302 M.A. Sekheta et aI.:
All the solutions were prepared from analytical grade chemicals
and water redistilled in a quartz apparatus.
Carbonate buffer was prepared by mixing 0.2 M sodium car-
bonate and 0.2 M sodium bicarbonate; 2 M ammonium carbonate
was also used for preparing the buffer.
Standard solutions of Mn(II) were diluted with i x 10 -a M
hydrochloric acid, and those of Mo(VI) and W(VI) were diluted
with 1 x 10 -a M potassium nitrate to prevent adsorption on the
glass ware.
The vessels were cleaned with warm 6 M hydrochloric acid and
than rinsed successively with tap, distilled and redistilled water.
Acknowledgements
The authors are grateful to the Serbian Republic Research Fund
for financial support.
Summary
A Kinetic Determination o[ Ultramicro Quantities of Manganese(II),
Molybdenum(W) and Tungsten(W) on the Basis of Their Catalytic
Action on the Oxidation of Azorubin S by Hydrogen Peroxide
A study has been made of the kinetics of catalytic oxidation
of Azorubin S by hydrogen peroxide in the presence of Mn(II),
Mo(VI) and W(VI) in order to find optimal conditions for the
kinetic catalytic determination of these elements. Manganese(II),
molybdenum(VI) and tungsten(VI) were determined in the concen-
tration ranges 5.5--33.0 x 10 a, 1.3--8.1 and 5.9--44.1 #g/ml, re-
spectively. Standard deviations were less than 11%. The effect of
some foreign ions on these determinations was also investigated.
Zusammenfassung
Die Kinetik der katalytischen Oxydation von Azorubin S mit Wasser-
stoffperoxid in Anwesenheit yon Mn(II), Mo(VI) und W(VI) wurde unter-
sucht, um die optimalen Bedingungen fiir deren kinetisch-katalytische Be-
stimmung zu linden. Mn(II) wurde im Konzentrationsbereich yon 5,5--
33,0• -a, Mo(VI) yon 1,3--8,1 und W(VI) von 5,9--44,1#g/ml mit
Standardabweichungen <11% bestimmt. Autgerdem wurden einige der
m6glichen St6rungen untersucht.

7. A Kinetic Determination of Ultramicro Quantities 303
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