Kinetic determination of traces of manganese in different materials by its catalytic effect on the methylene green-periodate reaction

Tafanru. Vol 33, No 2. pp 135-139, 1986 Printed III Great Bnta~n All rights reserved

0039-Y140:x6$3.00+ 0.00 Pergamcn Press Ltd

Copyright ‘f‘, 1986

KINETIC DETERMINATION OF TRACES OF MANGANESE TN DIFFERENT MATERIALS BY ITS CATALYTIC EFFECT ON THE METHYLENE GREEN-PERIODATE REACTION M. HERNANDEZ CORDOBA, P. V~RAS and C. SANCHEZ-PEDRE~SO* Department of Analytical Chemistry, Faculty of Chemistry, University of Muraa, Murcia, Spain (Recetved 14 May 1985. Accepted 25 September 1985) Summary-The oxidation of Methylene Green by sodium periodate is a slow process. A kinetic method based on the catalytic effect of manganese(l1) on this reaction in the presence of l,lO-phenanthroline as activator is described. The reaction is followed spectrophotometrically by measuring the decrease in the absorbance of the dye at 620 nm. Under the optimal experimental conditions [4 x 10e5M Methylene Green, 0.2M acetate buffer (pH 4), 2 x lO-‘M l,lO-phenanthroline, 2.5 x lo-‘M sodium periodate, 357, manganese(H) between 0.2 and 30 ng/ml is determined by the tangent method. The accuracy of the method and the influence of 44 foreign ions have been studied and an equation for the kinetics of the catalysed reaction is proposed. The procedure has been applied to the determmation of manganese in water, milk

and beer with excellent results.

The importance of being able to determine manganese at the nanogram level is widely recognized. Recently, several catalysed reactions for this purpose have been described.‘-‘4 Although catalytic reactions offer good sensitivity, several attempts have been made to decrease the detection limit for manganese(H) by using activator ligands. The activators most widely used have been 1,I 0-phenanthroline,~~19 2,2’-bipyridy1,19 ethylenediamine” and nitrilotriacetic acid.2’-23 The oxidation of numerous organic compounds by periodate is catalysed by Mn(II), but no report concerning the catalytic oxidation of thiazine dyes has been found. In the present study, the manganese-catalysed Methylene Green-periodate reaction in the presence of l,lO-phenanthroline as activator is described. A new and sensitive method for the determination of manganese with a detection limit of 0.2 ngjml is reported. The procedure has been applied to the analysis of beer, milk and tap water with excellent results. EXPERIMENTAL

Reagents All chemicals used were of analytical reagent grade and the solutions were prepared with doubly distilled water. Methyiene Green solution, 4 x 10e4M. Prepared from the commercial product (Fluka, C.I. 52020) without further purification, by dissolving 0.0365 g in 250 ml of water. Manganese(ll) standard solution, 0.01 M. Prepared from the sulphate (Merck) and standardized with EDTA. Working solutions were prepared by dilution just before use. Other reagent stock solutions were 1M acetate buffer (pH 4), 0.01 M l,iO-phenanthroline and 0.05N sodium periodate.

*To whom correspondence **I.

132-_(

should be addressed.

Apparatus A Pye Unicam SPS-100 double-beam spectrophotometer with l-cm cells and constant-temperature cell-holder was used for recording spectra and absorbance-time curves. General procedure In a IO-ml standard flask place I ml of 4 x IO-PM Methylene Green, 2 ml of In/r acetate buffer (pH 4). 2 ml of 0.01&f I ~lO-phenanthroline and an appropriate volume of diluted manganese(I1) solution to keep the final concentration of the cation between 0.2 and 30 ng/mI. Keep the flask in a thermostat at 35.0 + 0.5” for 15 min. then add 0.5 ml of 0.05M sodium periodate and dilute to volume with water. Turn on the recorder, mix the solution by vigorous shaking, transfer it to the spectrophotometer cell (kept at 35.0 k 0.5”) and record the absorbance-trme curve at 620 nm. Prepare a calibration graph by usmg the same procedure and obtaining tan Q = AA/At for the initial straightline portions of the absorbance zjs. time plots. Determination of manganese m samples Milk. Add 10 ml of concentrated nitric acid to 10 ml of milk in a loo-ml Erlenmeyer flask and heat to near dryness. Repeat the addition of nitric acid. Cool. add 5 ml of 2M sulphuric acid and heat until the nitric acid is evaporated and white fumes begin to appear. To complete the destructlon of the organic matter, add 2 ml of concentrated hydrogen peroxide and heat. Cool, adjust to pH 4 with sodium hydroxide, dilute to volume in a 50-ml standard flask, and analyse a suitable ahquot. Beer. Heat IO ml of beer m a IOO-mlErlenmeyer flask to near dryness. Then proceed as for milk. finally &lute to volume in a 15-ml standard flask, and analyse a suitable portion of this solution. RESULTS AND DISCUSSION

Preliminary experiments showed that the oxidation of Methylene Green (MG) by periodate is a slow process which is catalysed by Mn(II), and that I,lO-phenanthroline has an activating effect on the 135

M.

136

HERNANDEZ

et al.

CORDOBA

2 .-•

1

A

.-.

/ . 4

2

,I .

P x D

3

I . i 1

6

y-•\

/\

1

0

700

600

X,nm

Fig. 1. Catalytic effect of manganese(H) (activation by 1,IO-phenanthroline) on the Methylene Green-periodate reaction: 4 x 10m5M MG, 0.2M acetate buffer (PH 4) 2.5 x 10m3M NaIO,, 35”. (1) Uncatalysed reaction. (2) Catalysed reaction: 20 ng/ml Mn(II). (3) Activated reaction: 20 ng/ml Mn(II), 2 x lo-‘M l,lO-phenanthroline.

reaction. activating

Figure

1 shows

both

1 .~L---.

I

the

catalytic

and

effects at pH 4. All spectra were recorded 5 min after the start of the reaction. The reaction was followed spectrophotometrically by measuring the decrease in the absorbance of the Methylene Green at 620 nm. The slope of the absorbance vs. time graph was used as a measure of the reaction rate. Effects of reaction variables

The variation of the reaction rate with pH was studied over the range 2-11, with different Britton-Robinson buffer solutions. Figure 2 shows the results obtained in the presence and absence of manganese (20 ng/ml). The catalysed reaction rate

(,L 0

2

4

[phenl ,tOe3M

Fig. 3. Dependence of the reaction rate on the activator concentration: 4 x 10e5M MG, 0.2M acetate buffer (pH 4), 2.5 x 10m3M NaIO,, 25”. (1) Uncatalysed reaction, (2) 20 ng/ml Mn(I1).

does not depend on pH in the range 3.5-4.2. A pH of 4 was chosen because it leads to the maximum difference between the catalysed and uncatalysed reaction rates. As pointed out elsewhere,‘* activation is due to the formation of catalytically active complexes of manganese with phenanthroline. The theoretical formation curves for these complexes indicate that at pH 4 the 1: 1 complex predominates. The influence of several buffer solutions at pH 4 was tested. The best results were obtained with 0.05-0.4M acetate buffer. A 0.2M acetate buffer concentration was chosen. As can be seen in Fig. 3, the reaction rate increases linearly with l,lO-phenanthroline concentration up to lo-‘M for both the catalysed and uncatalysed reactions. Increasing the activator concentration above lo-‘M does not affect the rate of the catalysed

6-

2

4

6

Fig. 2. Variation of the reaction rate with pH: 4 x 10m5M buffer, 2 x IO-‘M 0.04M Britton-Robinson MG, l,lO-phenanthroline, 2.5 x lO-)M NaIO,, 25”. (I) Uncatalysed reaction, (2) 20 ng/ml Mn(II).

Fig. 4. Dependence of the reaction rate on the MG concentration: 0.2M acetate buffer (pH 4). 2 x IO-‘M l,lO-phenanthroline, 2.5 x 10mZM NaIO,, 25’. (I) Uncatalysed reaction, (2) 20 ng/ml Mn(II).

Kinetic determination

of man~nese

I37

Table 1. Dependence of the reaction rate on temperature K* x 102, min-’

2 *~.-....A-----..-.~

Temperature, “C

Uncatalysed reaction

Catalysed reaction

19.7 23.4 26.9 31.9 35.0 38.6

0.352 0.375 0.413 0.45 1 0.498 0.531

0.97, I .27, 1.75, 2.39, 2.71, 3.11,

Conditions: 4 x IO-‘A4 Methylene Green, 0.2M acetate

04 0

l’

,

I

I

5

10

15

buffer (pH 4), 2 x IO-jM I,lO-phenanthroline, 2.5 x IO-‘M NaIO,, IO ng/ml Mn(I1). K* = [log(A, /AZ)]/ (f, - fl ).

I

20

CNoIO, I, 10-~&f Fig. 5. Variation of the reaction rate with periodate concentration: 4 x IOB5M MG, 0.2M acetate buffer {pH 4), 2 x 10-AM I,lO-phenanthroline, 25”. (1) Uncatalysed reaction, (2) 20 ng/ml Mn(I1).

reaction. The maximum difference between the two rates is obtained with 2 x 10a3M I,lO-phenan-

throhne, so this was the concentration chosen as optimal. The theoretical curves for formation of the Mn(II)-phenanthroline complexes show that the catalyst is completely complexed with a 2 x 10-3M ligand concentration at pH 4, which strongly suggests that the activator complex is the catalytic agent. The variation of the reaction rate with MG concentration is shown in Fig. 4. Both the catalysed and uncatalysed reactions increase their rate with dye concentration in the range studied (8 x 10e6-4 x IOF5M). A 4 x IO-‘M MG concentration, which provides an adequate absorbance at 620 nm. was chosen. Figure 5 shows the effect of sodium periodate concentration in the range 2 x 10W4-2 x 10-rM. The rate of the catalysed reaction increases with periodate concentration up to 2.5 x 10e3M and then becomes constant. A 2.5 x 10e3M concentration was chosen because it gave maximum rate of the catalysed reaction and a low value for the blank reaction. The dependence of the reaction rate on temperature was studied between 20 and 40” for the uncatalysed reaction and in the presence of Mn(II) (10 ng/ml). In the absence of catalyst, the reaction

rate increases slightly with temperature. The temperature effect is more pronounced for the .catalysed reaction. Table 1 shows the overall pseudo first-order constants, K*, calculated as slopes of first-order plots at different temperatures. A temperature of 35’ was selected for further studies. The calculated activation energy is 12.9 f 0.2 kcal/mole for the catalysed reaction. Rate equation

The kinetic data obtained for the reaction variables are summarized in Table 2. The following kinetic equation is proposed for the manganese(H)-catalysed Methylene Green-periodate reaction in the presence of I,lO-phenanthrohne as activator: -F

This equation is useful for the proposed experimental conditions; K is the rate constant for the catalysed reaction. Calibration graph and efict

Methylene Gun-Reriodate

Uncatalysed reaction

H+ l,lO-Phenanthroline Methylene Green Periodate

Concentration range, M *O-“-lo-4 IO-‘-3 x IO-4 3 x IO-4-IO-’ 2 x 10~4-lo-’ IO-‘-S x IO-’ 8 x IO-h-4 x IO-5 5 x IO_‘-5 x IO-3 5 x 10-2-2 x 10-z

of foreign ions

The absorbanc~time curves at different manganese concentrations are shown in Fig. 6. Such curves were treated by different kinetic methods and the highest reproducibility was found for use of the tangent method. Under the optimal experimental conditions (see Fig. 6) a linear calibration graph was obtained for manganese between 0.2 and 30 ng/ml.

Table 2. Kinetic data for the Mn(II)-~talys~

Variable

= ~[MG][Mn~+]

Kinetic order -;;2 -l/4 --Y/2 + I +I 0

reactlon

Catalysed reaction Concentratton range, M IO-“-IO-4 10-4-3 x 10-d 3 x lo-4Plo-” 2 x IO-“-IO-’ IO--‘-5 x lo-’ 8 x 10-Q x lO-5 2.5 x 10-4-1.5 x lo-’ 1.5 x IO-?-2 x 10-z

Kmetic order + 312 0 - I/3 -El/2 0 +t + 114 0’

M. HERNANDEZ CORDOBAet al.

138

Table 4. Determination of manganese in milk and beer Manganese, Sample Milk

Beer

0.0 0

2 3 4 1 2

Kinetic method*

AAS

290 237 155 340 210 191

278 222 166 306 225 180

*In the presence of 0.02M H2P20:- to avoid the interference of Fe(II1).

ao

10

1

Fig. 6. Absorbance-time curves: 4 x IO-‘M MG, 0.2M acetate buffer (pH 4). 2 x IOm3M l,lO-phenanthroline, 2.5 x IO-)M NaIO,, 35”. [Mn(II)], rig/ml: (1) 0.0, (2) 2.5, (3) 5.1, (4) 10.2, (5) 15.2, (6) 25.4.

Table 3. The proposed method is quite selective. No ion interferes when present at less than IO-fold molar ratio to Mn(II). The strongest interference is caused by Fe(III), which also catalyses the MG-periodatephenanthroline reaction. However, the tolerance for Fe(II1) can be improved by using dihydrogen pyrophosphate as masking agent. In 0.02M HrPrO$medium, a 20-fold molar ratio of Fe(II1) to Mn(I1) is tolerated, but it is necessary to make a new calibration graph for Mn(I1) in the presence of the masking agent. Fe(II1) is only tolerated in molar ratio

The standard deviation of the method for IO-ng/ml manganese (10 determinations) was +O.S ng/ml, with a mean relative error of f3.8%. Interference by foreign ions in the system was studied with 1.5 x lo-‘M manganese solution containing the foreign ion. The results are summarized in

Table 3. Interference of foreign ions in the determination manganese(I1)

of 1.5 x IO-‘M

Limiting molar ratio.

[~onl/[MnWI 2000*

Foreign ion ClO,-, NO;, SO:-, HPO-, H,P,O;-, CO:-, Cl-, F-, IO;, ClO;, BrO; , VO; , citrate, tartrate, As(V), Ce(IV), Cd(II), Ca(II), WI), Ba(II), Mg(II), In(III), AI(III), TI(1) As(III)t, Cr(III), Th(IV), Zn(I1) Br-t, MOO:- t, WO:- t, oxalatet, Cu(II), Ni(I1) Cr,O:- t, Pb(II), Co(II), Bi(II1) Ce(III), Hg(I1) SCNEDTA, Fe(III)§ I-

1000 500 200 100 50 20 10

*Maximum ratio tested. tIons which increase the reaction rate. @Inthe presence of 0.02M H,P,O:- . Table 5. Determination of manganese in water, and recovery determined by the standard addition method

Mn, Sample Water I

wlml 6.7

Water 2

5.7

Water 3

3.3

ng/ml

Mn taken, ng/ml 2.0

1.7

I.0

Mn added,

Total Mn found,

nglml

nglml

2.0 4.0 6.0 2.0 4.0 6.0 2.0 4.0 6.0

2.1 4.2 5.8 8.0 1.7 3.5 5.9 7.8 I.0 3.1 4.9 6.8

Recovery, %

105 93 98 90 105 102 105 98 97

Kinetic determination [Fe(III)]/[Mn(II)] = 0.2 in the absence of H,P,O:- . Most of the ions tested decrease the reaction rate. The ions which increase the reaction rate are indicated in Table 3. Applications

The proposed method has been determination of manganese in milk results obtained for 3 determinations are given in Table 4 and shown

applied to the and beer. The of each sample

to be in good agreement with those obtained by AAS.

The method has also been applied to the determination of manganese in tap water and the results

obtained by the standard addition method are summarized

in Table 5. REFERENCES

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1977, 49, 414. 5. Idem, Talanta, 1977, 24, 270. 6. T. Yamane and T. Fukasawa, Bunseki Kagaku, 1977, 26, 300.

of manganese

139

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Analyst, 1977,