Kinetic±catalytic±spectrophotometric determination of lowconcentrations of molybdenum in white wines

Doina Bejan1

Tehnical University `̀ Gh.Asachi'' IASI, Faculty of Industrial Chemistry, Bdul Dimitrie Mangeron Nr. 71, Iasi 6600, Romania

Received 4 June 1998; received in revised form 1 February 1999; accepted 8 February 1999

Abstract

The paper describes the determination of the molybdenum content in white wines based on its catalytical action on the kalium

iodide oxidation by hydrogen peroxide in acid medium.

The optimum reaction conditions (the catalyst, KI and H2O2 concentrations, the pH value, the order of the reagent additions,

the temperature) have been found by studying the effect of the reaction variables.

The in¯uence of some metallic ions (Ca2�, Mg2�, Zn2�, Cd2�, Fe2� and Fe3�) and complexing anions (Fÿ, C2O2ÿ4 ,

EDTA4ÿ) on the catalyzed reaction rate was elucidated.

The molybdenum concentration was estimated by the tangent, ®xed-time and ®xed-absorbance method. The obtained

average values for molybdenum content in white wines are within the 1.77�10ÿ7±1.83�10ÿ7 mol lÿ1 range. # 1999 Elsevier

Science B.V. All rights reserved.

Keywords: Kinetic methods; Catalytic methods; Spectrophotometrical determination; Molybdenum trace; White wines

1. Introduction

Molybdenum is one of the essential mineral ele-

ments [1,2] having a particular biological importance.

Molybdenum is a component of sul®te-oxidase, an

enzyme in the human liver, and other organs of

animals and birds [2]. The enzyme oxidizes the toxic

SO2ÿ3 ion to non-toxic sulfate thus being an important

detoxifying agent. In the absence of sul®te-oxidase

human beings may suffer from neurological distur-

bances, mental retardation and even death [2].

It is known that wines, in particular white ones,

may be preserved by using sul®te. The molybdenum

in wine could favor the action of the sul®te-oxidase

in the consumer organism, determining the dimi-

nution of the harmful effect of the remnant SO2ÿ3 .

This is why the Mo concentration in white wines has to

be known.

The numerous methods for determining molybde-

num traces include the ¯ame atomic absorption spec-

trometry (FAAS) [3±6], atomic emission spectrometry

(AES) [7±9], spectrophotometry [10], and spectro-

¯uorometry [11,12].

The catalytic effect of Mo on the oxidation of

kalium iodide by H2O2 in acidic medium is the basis

of a sensitive method [13±15]. Mass spectrometry

Analytica Chimica Acta 390 (1999) 255±259

1Tel.: +40-32-147644; fax: +40-32-214024; e-mail:

dbejan@sigma.tuiasi.ro

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 1 7 4 - 9

[16,17] and neutron activation analysis [18,19] can

also applied.

In the present paper the kinetical±catalytical±spec-

trophotometrical method was chosen because of its

sensitivity and simplicity.

The oxidation reaction of iodide by H2O2 in acid

medium:

2Iÿ � H2O2 � 2H� � I2 � 2H2O

is known [20] to be catalyzed by several elements: Mo,

W, Ti, Nb, Ta, Zr, Hf, Fe, Th. Iron can interfere in the

determination of molybdenum since it is frequently

present in many drinks (beer [21], wine [22] and others

[23]). For this reason iron must be removed or masked

by adequate methods such as the masking by the

ascorbic acid [24], the retention by a cationite

[22,25], etc.

2. Experimental

2.1. Reagents

Analytical grade reagents were used and the solu-

tions were prepared with twice-distilled water.

The stock solutions of 10ÿ2 M Na2MoO4�2H2O,

Na2WO4�2H2O, FeCl3�6H2O, FeSO4�7H2O were

diluted with twice-distilled water for obtaining the

working solutions of the required concentrations.

The 1 M H2SO4 and 1 M HCl solutions were pre-

pared by the dilution of the corresponding concen-

trated acids and were standardized.

The 5�10ÿ2 M KI solution was standardized by the

Volhard method.

The 10ÿ2 M H2O2 solution was obtained by dilution

of the 31% (10.2 M) commercial solution and the

concentration was measured by the iodometrical

method.

The 0.2% starch solution was prepared before runs.

The 10ÿ3 M cation solutions (Ca2�, Mg2�, Cd2�,

Zn2�, Fe2�, Fe3�) and those of the complexing anions

(Fÿ, C2O2ÿ4 , EDTA4ÿ) were prepared from their ana-

lytical grade salts.

2.2. Equipment

A Spekol Carl Zeiss Yena Spectrophotometer with a

thermostated space for the cell was used for recording

the absorbance variation in time. The temperature was

controlled by a TIM-160 thermostat of the A type

(circulating thermostat bath). A glass cell of 1 cm was

used for the reaction. The solution was stirred in the

cell by means of a magnetic stirrer.

A Perkin Elmer-Atomic Absorption Spectrometer

3300 was used for the FAAS determinations of

Mo(VI) and Fe(III), Fe(II).

2.3. Kinetical measurements

Marked ¯asks of 50 ml were used in the determina-

tion. The components (the acidi®er, KI, catalyst,

starch, up to 35±40 ml twice-distilled water) were

mixed and thermostated at (25�0.1)8C. The H2O2

solution thermostated at the same temperature was

then added, quickly homogenized and placed in the

thermostated cell of the spectrophotometer. The

A�f(t) curve for the iodine±starch complex was drawn

at ��580 nm towards the blank. The kinetical study

on the catalytical reaction was performed by the

initial rate method, where the initial slope of the

reaction curve (dA/dt�tg �) was measured as

described in [26] and then taken as a measure of

the initial reaction rate.

The molybdenum concentration was estimated by

the tangent method, but determinations were also

made by the ®xed-time and ®xed-absorbance methods.

Alternative determinations of Mo(VI) content were

by FAAS in the recommended N2O±C2H2 reducing

(rich, red) ¯ame at the most sensitive 313.3 nm line

[27,28]. The interferences can be controlled by addi-

tion of NH4Cl or Na2SO4.

The matrix effects were assessed by the analysis of

synthetic samples with a composition similar to that of

wine samples by using the same methods. They were

negligible.

The wine samples were ®rst mineralized by the dry

way as recommended for food products [29].

The iron interference was overcome by its retention

on IR-120 Amberlite (H�) (pH�1.0±2.0).

The iron content in the eluate was determined

spectrophotometrically by the 1,10-phenanthroline

method (having reasonable high sensitivity:

��1.1�104 at 512 nm and ��2.2�104 at 553 nm)

and by FAAS at the most sensitive 248.3 nm line in

the recommended air±C2H2 oxidizing (lean, blue)

¯ame.

256 D. Bejan / Analytica Chimica Acta 390 (1999) 255±259

3. Results and discussion

3.1. Considerations on the indicator reaction

The following reaction was used as the indicator

reaction with the applied method:

2Iÿ � H2O2 � 2H� � I2 � 2H2O: (1)

This reaction proceeding in the presence of Mo(VI)

as catalyst may be described generally by the follow-

ing kinetical equation:

d�I2�=dt � kCaMoCb

H2O2Cd

Jÿ � k1Cb0H2O2

Cd0Jÿ ; (2)

where k1 is the rate constant of uncatalytic reaction, k

is the rate constant of catalytic reaction, a, b, d are the

coef®cients of kinetical equation indicating the reac-

tion order with respect to each reagent, and b0, d0 are

the coef®cients of uncatalyzed reaction.

Under our working conditions (the range of the

component concentrations and the reaction time), the

rate of the uncatalytic reaction is practically zero and

thus

d�I2�=dt � kCaMoCb

H2O2Cd

Jÿ : (3)

In order to ®nd the coef®cients a, b, d in Eq. (3) the

dependence of the initial reaction rate on the molyb-

denum, iodide and hydrogen peroxide concentrations

was studied. The runs were made at the pH and

temperature values accepted as optimal. The A�f(t)

curves for different cases were plotted, the corre-

sponding slope estimated and the tg ��f(c) curves

were drawn for each reagent.

3.2. Influence of the reaction variables

Experiments allowed setting of the optimal pH and

temperature values for catalytic reactions at pH�1.0

(HCl) and (25�0.1)8C, respectively.

The reaction was found to be of ®rst order with

respect to catalyst (a�1), to KI (d�1), but with respect

to H2O2 was of ®rst order (b�1) within the 0.5�10ÿ3±

1.5�10ÿ3 M range and of zero order (b�0) with

concentration higher than 1.5�10ÿ3 M.

By plotting the reaction rate versus H2O2 concen-

tration in coordinates 1/tg ��f(1/C(H2O2)) [30] a

straight line results, which con®rm the formation of

the peroxocomplex of Mo:H2O2�1:1. This fact

demonstrates the role of H2O2 in the oxidation pro-

cesses in acid medium when the maximum catalytic

activity is conferred by the catalyst peroxocomplex

[31].

The in¯uence of the addition order of the reagents

was followed with three different groups:

Group I : �H2O2 �Mo�VI� � HCl� � KI;

Group II : �Mo�VI� � HCl� H2O2� � KI;

Group III : �HCl� KI�Mo�VI�� � H2O2:

The last reagent was added immediately (0 min) or

after 10, 20, 30, 40 and 60 min. The curves A�f(t)

were drawn and tg � estimated. The obtained data

reveal that the adding order of the reagents does not

affect the reaction rate.

As regards the temperature in¯uence, the runs were

made within the 20±358C interval and the reaction rate

was found to increase with increasing temperature.

The temperature of (25�0.1)8C was chosen, because

this ensures a reaction rate that can be controlled.

3.3. Study of interferences

The in¯uence of some metallic ions (Ca2�, Mg2�,

Cd2�, Zn2�, Fe2�, Fe3�) on the initial reaction rate

was considered. The concentrations of the Ca2�,

Mg2�, Cd2�, Zn2� ions at 102±103 times higher con-

centrations than that of the catalyst (Mo) did not affect

the reaction rate. The Fe2� and Fe3� ions, showing a

catalytic action lower than that of Mo(VI) and W(VI),

interfere in the oxidation reaction of KI by H2O2

catalyzed by Mo(VI), even in concentration compar-

able to that of the catalyst (10ÿ7±10ÿ6 M). The fol-

lowing tg � values were obtained: 0.9325, 0.7813,

0.6248 and 0.5173 for W(VI), Mo(VI), Fe(III) and

Fe(II), respectively. For this reason iron was removed

from the samples before analysis by ion-exchange on

IR-120 Amberlite (H�). The retention was in fact

complete.

Molybdenum recovery under these conditions was

99.8%. The ef®ciency of the recovery was estimated

by sorption on the cationite of some wine samples with

known content of the added Mo(VI).

The in¯uence of some complexing anions (Fÿ,

C2O2ÿ4 , EDTA4ÿ) in concentration 1000 times higher

than the catalyst consisted of a decrease in the cata-

lytic reaction rate by 28%, 31% and 33% with Fÿ,

D. Bejan / Analytica Chimica Acta 390 (1999) 255±259 257

EDTA4ÿ and C2O2ÿ4 , respectively (C(Mo)�

1�10ÿ7 M; C(KI)�5�10ÿ4 M; C(H2O2)�1�10ÿ3 M; pH�1.0 (HCl); t�258C).

3.4. Determination of MO in white wines

The samples of white wines processed as described

in Section 2 were submitted to the kinetical±cataly-

tical±spectrophotometrical analysis under the opti-

mum reaction conditions: pH�1.0 (HCl);

t�(25�0.1)8C; C(KI)�5�10ÿ4 M; C(H2O2)�1�10ÿ3 M. The obtained average values are given in

Table 1 in comparison with those obtained by FAAS.

Analysis of foodstuff has been reviewed in [32] with

particular emphasis on AAS [33], hydride generation

AAS in wine and beverages [34], inductively coupled

plasma AES in wine and must [35].

Our results are according to the literature, the few

differences being caused by soil quality conditions

(soils usually contain variable quantities of molybde-

num, ranging from 1 ng gÿ1 up to 0.5 mg gÿ1).

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Table 1

The Mo content in white wines coming from Bucium and Cotnari vineyards

Number of sample Mo content

Kinetic±catalytic±spectrophotometry Flame AAS

n mol lÿ1 ng mlÿ1 n mol lÿ1 ng mlÿ1

1 178 17.10 176 16.90

2 179 17.18 178 17.10

3 177 17.00 177 17.00

4 180 17.28 179 17.18

5 178 17.10 177 17.00

6 182 17.47 183 17.57

7 180 17.28 182 17.47

8 182 17.47 183 17.57

9 181 17.38 182 17.47

10 183 17.57 183 17.57

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