Spectrophotometric determination of iron with 2-oximinodimedone dithiosemicarbazone

MICROCHEMICAL

JOURNAL

35, 157- 163 (1987)

Spectrophotometric 2-Oximinodimedone

Determination of Iron with Dithiosemicarbazone

E SALINAS, J.C. JIMBNEZSANCHEZ,~AND J.M. Department

of Analyiical

Chemistry, Faculty of Sciences, 06071 Badajoz, Spain

LEMUS GALLEGO

University

of Extremadura,

Received April 17, 1986; accepted September 22, 1986 2-Oximinodimedone dithiosemicarbazone produces a violet complex with Fe(H) in strongly acid medium (molar absorptivity 9800 liters . mol-r . cm-t at 565 nm). The violet complex was used for the spectrophotometric determination of Fe(H)(determination limit 0.20 kg . ml-’ in homogeneous medium and 0.022 Fg . ml-’ by extraction with isoamyl alcohol. The methods were applied to the determination of iron in wines, foods, minerals, and water. 0 1987 Academic Press. Inc.

INTRODUCTION

In a previous paper (6) we described the synthesis and properties of 2-oximinodimedone dithiosemicarbazone (5,5-dimethyl-1,2,3-cyclohexanetrione-2-oxime1,3-dithiosemicarbazone) (ODDT) and its reactions with some cations and anions. We also proposed a spectrophotometric method for the determination of chromate with this reagent in strongly acid medium (4). In the present work ODDT was used for the determination of trace amounts of iron in a homogeneous medium and by extraction with isoamyl alcohol. We also proposed a spectrophotometric method for the determination of iron in a homogeneous medium with a similar reagent, 5,5-dimethyl-1,2,3-cyclohexanetrione-I ,2-dioxime-3-thiosemicarbazone (DCDT) (5), with comparable results being obtained. We can conclude that both reagents can be indistinctly used. In addition in this paper the extraction-photometric determination of iron in isoamyl alcohol with ODDT is proposed. This approach compared advantageously with the above-mentioned methods in homogeneous medium because the sensitivity is seven times greater. EXPERIMENTAL

Apparatus and Reagents A Beckman Model 25 spectrophotometer, equipped with I.O-cm glass cells, was used. All reagents were of analytical reagent grade. 2-Oximinodimedone dithiosemicarbazone was synthesized in our laboratory (1). Stock standard Fe(I1) solution, 1.002 g * liter- *, was standardized with K,Cr,O,. r To whom correspondence

should be addressed 157 0026-265X187 $1.50 Copynght 0 1987 by Academic Press. Inc. All rights of reproduction in any form reserved.

158

SALINAS,

JIMkNEZ

SANCHEZ,

AND

LEMUS

GALLEGO

Procedures Determination of iron without extraction. In a 25-ml volumetric flask, an aliquot of the sample solution containing up to 125 pg of iron, 3 ml of concentrated HCl, and 5 ml of 0.1% reagent solution in ethanol was transferred and diluted to the mark with deionized water. The absorbance was measured at 565 nm against deionized water in a l.O-cm cell. Determination of iron by extraction. Samples were prepared in lOO-ml separatory funnels by taking 50 ml of sample solution containing up to 5 p,g of iron, 24 ml of concentrated HCl, and 10 ml of 0.03% reagent solution in isoamyl alcohol. The mixture was shaken vigorously for 4-5 min. The phases were allowed to separate and the organic phase was transferred to a flask containing anhydrous sodium sulfate. The absorbance of this solution was measured at 565 nm against deionized water. Determination of iron in wines. In a 25-ml volumetric flask an aliquot of wine containing between 5 and 125 p.g of iron, 3 ml of concentrated HCl, and 5 ml of 0.1% reagent solution in ethanol was transferred and diluted to the mark with deionized water. The absorbance was measured at 565 nm against deionized water or against a blank of wine, if this present measurable color. Determination of iron in vegetables and verdures. The following method was applied to determine iron in spinach and lentils. About 20 g of spinach or lentils, after drying at 110°C was calcined and treated by heating with 10 ml of concentrated HCl. The solution was filtered and carefully poured into a IOO-ml volumetric flask. Aliquots of the solutions were used in the spectrophotometric determination of iron by the recommended procedure without extraction. Determination of iron in magnesites. About 0.5 g of sample were treated with 10 ml of concentrated HClO, by heating. The solution was cooled and diluted in order to obtain the required concentration of iron in the stock solution. To an aliquot of the solution containing between 5 and 125 pg of iron, in a 25-ml calibrated flask, 3 ml of concentrated HCl, 1 ml of ascorbic acid solution of 100 g/liter, and 5 ml of 0.1% reagent solution in ethanol were transferred and diluted to the mark with deionized water. The absorbance was measured at 565 nm against deionized water. Determination of iron in Portland cement. About 0.25 g of sample was mixed with 0.5 g of NH&l, heating in a water bath for 30 min; 20 ml of hot water was added and the solution was filtered and carefully washed into a 250-ml volumetric flask. Aliquots of the solution were used in the spectrophotometric determination of iron with ODDT by the recommended procedure without extraction. Determination ofiron in river water. River water was filtered immediately after sampling and acidified with 8 ml liter-’ of concentrated HCl and boiled for 10 min. Aliquots of the solution were used in the spectrophotometric determination of iron with ODDT by the recommended procedure by extraction. RESULTS

AND DISCUSSION

ODDT reacts with Fe(I1) and Fe(II1) ions in very acid medium to form a violet complex having A,, at 565 nm (Fig. 1). We proved that the Fe(I1) is the ion which

SPECTROPHOTOMETRIC

DETERMINATION

OF IRON

1.59

A 05 04 03

h

C

R

02 01

I

\

450

FIG. 1. Absorption

500

I 550

600

h(nm)

spectra of complex (C) and reagent (R).

forms the complex because this is obtained in the presence of ascorbic acid and the color does not appear in the presence of H,O,. Effect of the HCl Concentration The effect of the HCl concentration on the reaction of ODDT with Fe(I1) and Fe(II1) is shown in Fig. 2. A concentration 1.5 M of HCl was selected to prepare the samples for the formation of the Fe(D) complex. Effect of Concentration of Reagent and Order of Addition The color intensity remains constant above 7-fold molar excess of reagent; 5 ml of 0.1% reagent solution in a final volume of 25 ml was selected for the analytical procedure. The order of reagent addition was immaterial. Stability Stability is not satisfactory when the proportion of ethanol is less than 20%. Under the conditions of the recommended procedure, the complex of Fe(I1) is stable for 8 hr. Composition

of the Complex

The continuous

variation

and molar ratio methods were applied to determine

06 04

02 1

FIG. 2. Influence of HCl concentration Fe(H). (b) Reaction with Fe(III).

3

5

7 M HCI

on formation of Fe(U)-ODDT

complex. (a) Reaction with

160

SALINAS,

JIMENEZ

SANCHEZ,

AND LEMUS GALLEGO

the stoichiometry of the reactions of Fe(I1) and Fe(II1) with ODDT. Both methods show that the stoichiometry of the reactions were 3: 1 reagent to Fe(H) and 3S:l reagent to Fe(II1). If we start from Fe(III), the stoichiometry 3S:l is in accordance with the reduction of the Fe(II1) by the reagent, giving the same complex, because the general redox behavior of thiosemicarbazones and of the ODDT, in particular (4, 6), involves the exchange of two electrons per mole of reagent. The overall conditional formation constant of the Fe(I1) complex, by the method of Gonzalez et al. (2) was found to be 4 x 1013. Spectrophotometric

Characteristics

The molar absorptivity of Fe(II)-ODDT complex was 9800 liters . mol-’ * cm-l. The detection limit (3) was 0.06 pg . ml-’ of iron and the determination limit (I) was 0.2 kg . ml- l. The optimum range as evaluated by Ringbom’s method was l-5 pg . ml-r of iron. The relative error (11 determinations, 4 kg * ml-l of iron, 95% confidence level) was +0.36%. Effect of Foreign Ions In the determination of 4 p,g . ml-l of iron, foreign ions can be tolerated at the levels given in Table 1. It must be emphasized that the method is very selective because only Co(II), Se(IV), Te(IV), Pd(II), and Ag(1) interface significantly. Applications The results obtained in the determination of iron in wines, spinach, lentils, and minerals are shown in Tables 2 and 3. These results are compared with those obtained by atomic absorption spectrophotometry. TABLE 1 Tolerance of Foreign Ions in the Determination Minimum interfering foreign ion to Fe(B) ratio (w/w) co.5 0.5 1 2 5 10 25 50 75 100 >loo

of Iron without Extraction

Tested ions Co(B) Se(IV), Te(IV) PdUU, MU IO,, BrO, , NO, V(V), Cr(VI), CIO,, MnO; Cu(II), Ni(II) AI(III), Au(III), W(V1) Cr(III), Cr(VI)*, Sn(II), Ce(IV), Nb(V) v(v)* Pt(IV), Mo(V1) Bi(III), Cd(II), Sb(III), A~(111 and V), Ti(IV), Zn(II), Mn(II), Be(B), Ge(IV), Zr(IV), Th(IV), Ce(IV)*, Mg(II), Ca(II), Sr(II), Ba(II), MnOh*, CIO<*, BIG,*, IO,*, NOT*, F-, CO:-, BO;, PO:-, SO:-, Br-, I-, NO,, ClO;, EDTA, oxalate, tartrate, citrate, ascorbic acid

* In the presence of 1 ml of 10% ascorbic acid solution.

SPECTROPHOTOMETRIC Determination

DETERMINATION

161

OF IRON

TABLE 2 of Iron in Wines, Lentils, and Spinach Iron found“

Sample

ODDT method

AASb method

White wine Red wine 1 Red wine 2 Lentils Spinachs

15.4 ppm 20.4 ppm 7.6 ppm 0.0201% 0.0044%

15.5 ppm 20.0 ppm 7.8 ppm 0.0204% 0.0043%

a Average of three separate determinations. b Atomic absorption spectrophotometry.

Extraction of Fe(ZZ)-ODDT Complex The extraction of the Fe(II)-ODDT complex was tested in several organic solvents. The most favorable solvents were isoamyl alcohol, methylisobutyl ketone (with KClO,), and ethyl acetate (with KClO,). Effect of the HCl Concentration

in the Extraction

The reagent was dissolved in the organic phase (O.OS%>, in the case of ethyl acetate containing 1% DMF, because it was necessary to prevent cloudiness. To 40 kg of Fe(I1) in 10 ml of aqueous solution (2.5 M KClO, in the case of methylisobutyl ketone or ethyl acetate), 10 ml of the reagent in the organic solvent was added, after shaking for 5 min. The two phases were separated, and the absorTABLE 3 Determination of Iron in Minerals Standard sample Magnesite-chrome

High-purity

(BCSb No. 396)

magnesite (BCS No. 389)

Portland cement (BCS No. 372)

Certified composition (%)

Iron certified (%I

Iron found” (So)

Error (%I

SiO,, 1.37; TIO,, 0.25; A&O,, 5.73; CaO, 1.12; MgO, 64.6; Cr,O,, 15.6; MnO, 0.18; B,O,, 0.09 SiO,, 0.89; TiO,, 0.01; Al,O,, 0.23; Cr,O,, 0.28; Na,O, 0.03; K,O, 0.01; MnO, 0.008; B,O,, 0.029 SiO,, 21.3; TiO,, 0.33; AI,O,, 5.35; CaO, 65.8; Mn,O,, 0.06; MgO, 1.3; Na,O, 0.21; K,O, 0.62; SO,, 2.35; P,O,, 0.19

7.6

7.65

0.6

0.199

0.20

0.5

1.74

1.73

0.6

a Average of three separate determinations. b British Chemical Standard and European Certified Reference Materials.

SALINAS , JIMENEZ

162

SANCHEZ,

AND LEMUS

1

a

07

GALLEGO

05 0.3 I--b

8 0.7 5 g 05. Is C,o3.

D C

07 0.5 0.3

P i 1

3

5 mlHC112M

FIG. 3. Influence of HCl concentration on extraction of Fe(H)-ODDT alcohol. (b) In ethyl acetate. (c) In methylisobutyl ketone.

complex. (a) In isoamyl

bance was measured at 565 nm against the reagent blank. The results are plotted in Fig. 3. Extraction

of Fe(H)-ODDT

Complex

in Isoamyl

Alcohol

We chose the 1:7 ratio of isoamyl alcohol:H,O and a shaking time of 4-5 min as being satisfactory. We proved that 4.5 ml of 0.05% reagent solution is enough to obtain maximum absorbance. The absorbance at 565 nm was content for at least 2 days. The composition of the extracted complex was determined by the continuous variation method, modified accordingly for the application to a two-phase system. The stoichiometry found was 3: 1 (reagent:iron). TABLE 4 Tolerance of Foreign Ions in the Determination Minimum interfering foreign ion to Fe(H) ratio (w/w) lOO

of Iron by Extraction

Tested ions Co(II), V(V), ClO,, BrO, , NO,, Cr(V1) Pd(II), Au(III), Pt(IV) Se(IV), Te(IV), IO, Ni(II), Cu(II), MnO; WIV), Ag(I) Al(III), SO;-, F-, CO:Bi(III), A~(111 and V), Sn(II), Sb(III), Mo(VI), Cr(III), Cr(VI)*, V(V)*, Be(B), Ce(IV)*, Ti(IV); Th(IV), Zn(II), Mn(II), W(VI), Mg(II), Ca(II), Sr(II), Ba(II), Cloy*, BrO<*, IO?*, NO;*, MnO;*, Cl-, Br-, I-, SCN-, ClO; , NO,, BO,, PO:-, oxalate, EDTA, tartrate, citrate, ascorbic acid

* In the presence of 1 ml of 10% ascorbic acid solution.

SPECTROPHOTOMETRIC Determination

DETERMINATION

OF IRON

163

TABLE 5 of Iron in River Water Iron found (ppm)”

Sample

ODDT method

AASb method

Guadiana river (Badajoz) Gevora river (Badajoz)

0.16 0.10

0.17

0.10

a Average of three separate determinations. b Atomic absorption spectrophotometry.

Calibration and Precision The extracted complex obeyed Beer’s law in the range O-60 pg of iron. The detection limit (3) was 0.007 kg. ml-’ of iron, and the determination limit (I) was 0.022 kg * ml-’ of iron. The optimum range, as evaluated by Ringbom’s method, was 0.14-0.7 pg * ml-’ of iron. The relative error (11 determinations, 0.4 kg * ml-’ of iron, 95% confidence level) was ?0.3%. In the determination of 0.4 kg * ml-’ of iron foreign ions can be tolerated at the levels given in Table 4. Determination of Iron in River Water The recommended extraction spectrophotometric procedure was applied to the determination of iron in river water. The results are compared with those obtained by atomic absorption spectrophotometry (the water were preconcentrated). The results obtained are summarized in Table 5. ACKNOWLEDGMENTS We thank the Comision Asesora de Investigation Cientifica y Tecnica del Ministerio de Education y Ciencia for supporting this study (Project 2903-83).

REFERENCES 1.

2. 3. 4. 5. 6.

ACS Committee on Environmental Improvement, Subcommittee on Environmental Analytical Chemistry, Guidelines for data acquisition and data quality evaluation in environmental chemistry. And. Chem. 52, 2242 (1980). Gonzalez Garcia, D. V., Arrebola Ramirez, A., and Roman Ceba, M., A new graphic method for differentiating mononuclear and polynuclear complexes and for determining their stability constants. Talanta 26, 215 (1979). IUPAC Analytical Chemistry Division, Nomenclature, symbols, units and their usage in spectrochemical analysis. II. Data interpretation. Pure Appl. Chem. 45(2), 99-103 (1976). Salinas, F., Jimtnez Sanchez, J. C., and Galeano Diaz, T., Study by Cr(VI)-2-oximinodimedone dithiosemicarbazone reaction and simultaneous determination of Cr(V1) and Fe(II1). Mikrochim. Acta 1, 245-251 (1985). Salinas, F., Jimenez Sanchez, J. C., and Galeano Diaz, T., Spectrophotometric determination of iron in wines, foods and minerals with 5,5-dimethyl- 1,2,3-cyclohexanetrione I ,2-dioxime-3thiosemicarbazone. Ana!. Chem. 58, 824-827 (1986). Salinas, F., Jimenez Sanchez, J. C., and Lemus Gallego, J. M., Analytical properties of 2-oximinodimedone dithiosemicarbazone. Talunfa 32, 1074-1075 (1985).