Spectrophotometric investigation of the Fe(III) disulphonated hydroquinone complex

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1 J. Serb. Chem. Soc. 70 (4) (2005) UDC :543.4/.5 JSCS 3298 Original scientific paper Spectrophotometric investigation of the Fe(III) disulphonated hydroquinone complex M. V. OBRADOVI], S. S. MITI], S. B. TO[I]* and A. N. PAVLOVI] Faculty of Sciences and Mathematics, Department of Chemistry, University of Ni{, Vi{egradska 33, P. O. Box 224, Ni{, Serbia and Montenegro ( (Revised 20 May, revised 16 July 2004) Abstract: Iron(III) form an indigo-blue complex with the disulphonated product of hydroquinone (K 2 S 2 Hy) in acid media with an absorption maxima at 600 nm. The time stability of the complex, dependence of the complex absorbance on ph and the influence of temperature and solvent were followed on the basis of spectrophotometric measurements. Using the Job, mole ratio and Henry Franck Ostwald methods, the composition and relative stability constant of this complex, in 80 vol.% ethanol as solvent, were determined (1:1; log 293 = 3.37). A new spectrophotometric method for the determination of iron has been developed and the calibration curve is linear in the concentrationrangefrom0.65to6.45 gcm -3. The effects of foreign ions on the determination of iron were investigated in order to assess the selectivity of the method. The method was applied for the determination of Fe(III) in the natural juice of beet. Keywords: spectrophotometry, iron(iii), disulphonated product of hydroquinone, complex, determination. INTRODUCTION Literature data show that many metal ions form coloured complexes with many aromatic hydroxy compounds and their sulphonated products, which can be used for the development of spectrophotometric methods for the determination of microamounts of these ions in solutions. The spectrophotometric determination of iron(iii) is mostly carried out on the basis of the formation of coloured complexes with phenolate type reagents. 1 5 There are some data in the literature about complex between Fe(III) and some sulphonated phenols. Fe(III) forms a blue complex with 1,2-dihydroxybenzene-3,5-disulphonic acid (tiron) at ph < 5.6 (composition 1:1); a purple complex at 5.7 < ph > 6.9 (1:2; log = 13.12) and red complex at ph > 7 (1:3; log = 8.85). 6 Tiron has also been used as the chromogenic reagent for sequential injection analysis for the quantitative discrimination of the two iron species, Fe(II) and Fe(III). 7,8 Pink complex with an absorption maximum at 504 nm forms between Fe(III) and disulpho-1,3-dihydroxybenzene (sulphonated res- 651

2 652 OBRADOVI] et al. orcinol). The composition of this complex is 1:1 with log = The monosulphonated product of 1,2,3-trihydroxybenzene (sulphonated phlorglucinol) forms a dark-red complex with an absorption maximum at 490 nm (1:1 with log = 3.84) 10 while the monosulphonated product of 1,3,5-trihydroxybenzene (sulphonated pyrogallol) forms a green complex with an absorption maximum at nm in a water ethanol mixture (composition 2:1), which was used for the development of a new method for the determination of Fe(III) in the concentration range from0.56to3.36 gcm 3, with a relative error between %. The method was applied for the determination of iron in mineral water. 11 The aim of this work was to establish the composition and stability constant of the Fe(III)-disulphonated 1,4-dihydroxybenzene (sulphonated hydroquinone) complex and develop an analytical method for the determination of Fe(III) ions in solution. EXPERIMENTAL Apparatus A spectrophotometer UV/VIS Perkin-Elmer Lambda 15 with 10 cm cells connected to thermocirculating bath was used. A 716 DMS Titrino was used to measure the ph values of the solutions and Sigma buffers in aqueous solutions, ph and ph were used for the calibration of the ph-meter. Soccorex automatic macropippetes ( cm 3 and cm 3 )wereused to measure the exact volume of the solutions. Reagents All reagents (potassium salt of hydroquinone disulphonic acid, iron(iii) chloride, sodium perchlorate, perchloric acid, ethanol were of p.a. quality from Merck except ethanol which was from Zorka. A solution of the potassium salt of hydroquinone disulphonic acid was prepared by dissolving the appropriate amount of the substance in a 0.1 mol dm -3 solution of HClO 4, at the beginning of eash working day. The stock Fe(III) solution ( mol dm -3 ) was also prepared by dissolving FeCl 3 6H 2 Oin0.1moldm -3 HClO 4. The working solution was obtained by diluting the stock Fe(III) solution in 0.1 mol dm -3 HClO 4. The solutions of metal and ligand were prepared in HClO 4 because they are more stable in this medium and hence the reproducibility was improved. The ionic strength was kept constant at 0.1 by adding appropriate amounts of NaClO 4 solution (1.5 mol dm -3 ). The ph of the solutions was adjusted with HClO 4 solution (0.1 mol dm -3 ). All the glassware used was washed with aqueous HCl (1:1) and then thoroughly rinsed with tap, distilled and finally deionised water. Procedure After addition of metal, ligand and electrolyte for the ionic strength, the total volume of 5 cm 3 of the aqueous part of the mixture was completed the of 0.1 mol dm -3 HClO 4, to enhance the reproducibility of the measurements, and the 25 cm 3 volumetric flask was filled with 95 % ethanol. The volumetric flask was thermostated at the working temperature of 20 C and after a minimum of 15 min (after which the absorption maximum and absorbance did not change), the spectra were recorded using 80 vol% ethanol as the reference. The absorption spectra of FeCl 3 6H 2 O and the potassium salt of hydroquinone disulphonic acid were recorded in water as solvent, which was also used as the reference. RESULTS AND DISUCSSION Absorption spectra and optimal reaction conditions The adsorption spectra of FeCl 3 6H 2 O(c = mol dm 3 ), potassium salt of hydroquinone disulphonic acid (c =6 3 mol dm 3 ) and their mixture (c Fe(III)

3 Fe(III) DISULPHONATED HYDROQUINONE COMPLEX 653 Fig. 1. Absorption spectra: 1. Fe(III), c = mol dm -3,2.K 2 S 2 Hy, c = mol dm -3, 3. Fe(III) + K 2 S 2 Hy, c Fe(III) = c K2S 2Hy = mol dm -3 in water, 3. Fe(III) + K 2 S 2 Hy, c Fe(III) = c K2S 2Hy = mol dm -3 in water after a few minutes, 4. Fe(III) +K 2 S 2 Hy, c Fe(III) = c K2S 2Hy = mol dm -3 in 80 vol.% of ethanol; I =0.1moldm -3 ; t =20 C. = c K2S2Hy = mol dm 3 in water as solvent) were recorded (Fig. 1 curves 1, 2 and 3, respectively). A new absorption maximum at about 600 nm appeared which indicated the formation of the complex. In this wavelength range, the constituents of the complex do not absorb. A maximum at 450 nm also developed which was the result oxidation of the ligand and its intensity increased with time while the absorbance of the complex decreased (Fig. 1 curve 3 ). Literature data show that phenols and their derivates can be oxidized by air oxygen in basic media or by some metal ions in acid media. 12 Because of this, absorption spectra of the complex were recorded in different water ethanol media with the aim of determining the optimal solvent. The maximum of oxidation product disappeared in 80 vol.% of ethanol (c Fe(III) = c K2S2Hy = mol dm 3 ) (Fig. 1 curve 4) which means that all the added quantities of metal and ligand take part in the complex formation. It can be seen that the complex in ethanolic media has a higher absorbance at lower concentrations of the constituents, which is important for the sensitivity of the developed analytical method. In the first few minutes, the absorbance at 600 nm decreases and after then it remains constant in another for at lest 60 min. All of the spectra of the solutions wee recorder after waiting for at least 15 min. Fig. 2. Dependence of the absorbance of the Fe(III) S 2 Hy complex on the ph of the solution: c Fe(III) = c K2S 2Hy = 2.4 4, I =0.1moldm 3, t =20 C. The dependence of the absorbance of the complex on ph was also investigated, the ph of the solutions being adjusted by the addition of HClO 4 and NaOH.

4 654 OBRADOVI] et al. Fig. 3. Dependence of the absorbance of the Fe(III) S 2 Hy complex on temperature: c Fe(III) = c K2S 2Hy = , I =0.1moldm -3. This dependence has a maximum at a ph of about 3 which was the ph of the basic solution (Fig. 2). However, because of the chosen experimental procedure, the working ph was lower (ph 2.60). The influence of temperature on the absorbance of the complex is shown in Fig. 3. It can be seen that the absorbance of the complex decreases with increasing temperature, which means that the complexing reaction is exothermic. Composition and stability constant of the complex The stoichiometric ratio of Fe(III) and K 2 S 2 Hy in the complex was determined using the Job method. 13 Solutions of FeCl 3 2 OandK 2 S 2 Hy of the same concentration ( mol dm 3 ) were prepared and then mixed in the volume ratio from 1:9 to 9:1. The Job curve of this system at ph 2.60 and I = 0.1 is shown in Fig. 4. The maximum at X L = 0.5 indicates the formation of the complex in which the metal:ligand ratio is 1:1. Fig. 4. Determination of the stoichiometry of the Fe(III) S 2 Hy complex by the Job method of continuous variations: c M = c L = , I =0.1 mol dm -3, ph 2.60; t =20 C.

5 Fe(III) DISULPHONATED HYDROQUINONE COMPLEX 655 Fig. 5. Determination of the stoichiometry of the Fe(III) S 2 Hy complex by the mole ratio method: c M = mol dm -3 ; c L = ( ) mol dm -3 ; I =0.1mol dm -3, ph 2.60; t =20 C. The composition of the complex was also determined by applying the mole ratio method. 14 A series of solutions were prepared with a constant concentration of FeCl 3 6H 2 O( mol dm 3 ) and variable K 2 S 2 Hy concentrations ( mol dm 3 ). It can be seen (Fig. 5) that the metal:ligand ratio in the complex is 1:1, which agrees with result obtained by the Job method. The form of this curve (after ratio 1:1, a continual increase of the absorbance of the complex with increasing ligand concentration) also indicates that a complex of lower stability was formed. The causine stability constant log was determined using the Henry Franck Ostwald method. 15 A series of solutions with a constant concentration of FeCl 3 6H 2 O( mol dm 3 ) and variable concentations of K 2 S 2 Hy ( mol dm 3 ) was prepared and recorded. A straight line was obtained for c L c M /A as a function of c L +c M (Fig. 6) from Henry Franck Ostwald equation: cmcl 1 A a c M c 1 ( L) a Fig. 6. Determination of the stoichiometry and the relative stability constant of the Fe(III) S 2 Hy complex by the Henry Franck Ostwald method: c M = mol dm 3 ; c L = ( ) mol dm -3 ; I =0.1moldm -3 ; ph 2.60; t =20 C. (1)

6 656 OBRADOVI] et al. which is valid for complexes where the metal:ligand ratio is 1:1. In Eq. (1), a is the molar absorption coefficient of the complex, which can be determine from the slope of the straight line and is the relative stability constant p,r = [M p L r ]/[M] p [L] r, calculated from the intercept. The obtained straight line confirms the previous results that a complex having a 1:1 composition was formed. The calculated stability constant is log 293 = Calibration curve A calibration curve (Fig. 7) was constructed using the data obtained by the absorbance measurements of solutions with a constant concentration of K 2 S 2 Hy ( mol dm 3 ) and variable Fe(III) concentrations. At higher concentrations of ligand, a precipitate formed which defined the maximum concentration ratio and hence the sensitivity of the developed method. The concentration of the ligand was also limited by its solubility. The linear dependence of the absorbance on the iron(iii) concentration obeys the equation: A = c Fe(III) ( g cm 3 ) (2) for an iron concentration of gcm 3 and t =20 C. The statistical data obtained using three different concentrations and five repetitions are shown in Table I. The relative error of the method ranges from 0.46 to 6.69 % for the concentration range used. TABLE I. Accuracy and precision of the iron determination Fig. 7. Calibration curve: c L = mol dm -3 ; I =0.1moldm -3 ;ph 2.60, t =20 C. Taken/ g cm -3 Found ( x ) a / g cm -3 RSD b /% (x m)/m 100 c /% a Mean of five measurements, b relative error, c accuracy of the method, real value

7 Fe(III) DISULPHONATED HYDROQUINONE COMPLEX 657 The interference effect of many cations and anions on the determination of iron (c Fe =2.42 gcm 3 ) and the tolerance limits of the interfering ions are given in Table II. It can be seen that Al 3+,Cr 2 O 7 2 (significant decrease of absorbance) and Ba 2+ (some deposit occurred) interfere the complex formation reaction. TABLE II. Tolerance levels of interferents in the determination of iron(iii) Tolerance level c ion /c Fe(III) Ion added 10 2 Ni 2+ ;Co 2+ ;Cu 2+ ;Na + ;K + ;Cd 2+ ;Mn 2+ ;NO - 3 ; Br - ;CH 3 COO - 10 Ca 2+ ;Zn 2+ SCN -,CO 3 2- F - 1 SO 4 2- ;PO 4 3- ;J - ;Pb 2+ Interfere Al 3+ ;Cr 2 O 7 2- ;Ba 2+ Application of the method The method was applied to the determination of iron(iii) in natural juice of beet. A sample was prepared as follows: 50 g of juice (squeezed from beet) was dried in a water bath and subsequently pyrolyzed at temperature T = K for 24 h. The cold residue was put into a glass and 1 2 cm 3 of deionizated water were added. Then 1 cm 3 of concentrated HNO 3 was added and the sample was boiled dryagainanddissolvedin2cm 3 of concentrated HNO 3 anddilutedupto50cm 3 with daionizated water. 16 The procedure for the determination of iron(iii) in the prepared sample using the method of standard addition was as follows: 0.8 cm 3 Fe(III) c = mol dm 3 (as the standard addition), 1 cm 3 K 2 S 2 Hy c = mol dm 3,1.7cm 3 NaClO 4 c =1.5moldm 3,1cm 3 HClO 4 c =0.1mol dm 3,0.5cm 3 ofsampleand20cm 3 95 % ethanol were poured into a volumetric flask and thermostated at 20 C. The ph value of this mixture was about , which was the ph of the solutions from which the calibration curve was constructed. After 15 min, the absorption spectra were recorded and the concentration of iron was calculated from Eq. (2). The AAS method 17 of standard addition was used as the standard method and the obtained results are given in Table III. The values for the natural juice of beet sample are in good agreement with those obtained by the AAS method. From these results, the method seems to be applicable for the determination of soluble iron. TABLE III. Determination of Fe(III) in beet juice Fe(III) as standard addition/ g cm -3 Fe(III) found in 0.5 cm 3 of the sample/ g Fe(III) in the sample x / g cm -3 AAS method/ g cm -3 (x m)/m 100/% Real value obtained by AAS method

8 658 OBRADOVI] et al. CONCLUSION Fe(III) ions form an indigo-blue complex with disulphonated hydroquinone in water ethanol media with max = 600 nm at ph The composition and relative stability constant were determined (1:1, log 293 = 3.37). The formed complex is less stable than other sulphonated hydroxybenzene complexes. Nevertheless, using the optimum conditions for complex formation, a new spectrophotometric method was developed for the determination of iron(iii) in the concentration range of gcm 3, which has a good sensitivity for this type of analytical methods. Acknowledgement: This research was supported by grant number 1211 from the Ministry of Science, Serbia. The authors are grateful for the financial support provided by the Ministry of Science. IZVOD SPEKTROFOTOMETRIJSKO ISPITIVAWE KOMPLEKSA Fe(III) DISULFONOVANI HIDROHINON M. V. OBRADOVI], S. S. MITI], S. B. TO[I] i A. N. PAVLOVI] Prirodno-matemati~ki fakultet, Odsek za hemiju, Univerzitet u Ni{u, Vi{egradska 33, Ni{ Fe(III) gradi inidgo-plavi kompleks sa disulfonovanim produktom hidrohinona (K 2 S 2 Hy) u kiseloj sredini sa apsorpcionim maksimumom na 600 nm. Na bazi spektrofotometrijskih merewa ispitana je vremenska stabilnost kompleksa, zavisnost apsorbance kompleksa od ph sredine, uticaj temperature i rastvara~a. Metodom Job-a, molskih odnosa i Henri-Frenk-Ostvald-ovom metodom je odre en sastav (1:1) i uslovna konstanta stabilnosti kompleksa u 80 % etanolu kao rastvara~u (log 293 =3.37). Razvijena je spektrofotometrijska metoda za odre ivawe Fe(III) u rastvoru i dobijena kalibraciona prava je linearna u intervalu koncentracije Fe(III) od 0,65 do 6,45 g cm 3. Ispitan je uticaj stranih jona na odre ivawe gvo` a u ciqu odre ivawa selektivnosti metode. Metoda je primewena za odre ivawe gvo` a u prirodnom soku cvekle. (Primqeno 20 maja, revidirano 16. jula 2004) REFERENCES 1.P.H.Gore,P.J.Newman, Anal. Chim. Acta 31 (1964) H. Broumand, J. H. Smith, J. Am. Chem. Soc. 74 (1952) A. K. Mukherjee, Anal. Chim. Acta 13 (1955) A. Agren, Acta. Chim. Scand. 9 (1955) Lj. Milovanovi}, Z. Kori~anac, Z. Dugan~i}, M. Arch. Pharm. 3 4 (1978) D. L. Harvey, J. Manning, Am. Chem. Soc. 72 (1950) L. V. Mulaudzi, J. F. van Staden, R. I. Stefan, Analytica chimica Acta 467 (2002) M. Kass, A. Ivaska, Talanta 58 (2002) M. Obradovi}, N. Miladinovi}, S. To{i}, 4 th International Conference on Fundamental and Aplied Aspects of Physical Chemistry, Beograd (1998) M. V. Obradovi}, S. B. To{i}, M. Z. Grahovac, 2 nd International Conference of the Chemical Societes of the South-Eastern Counties, Greece (2000) PO211, S. S. Miti}, M. V. Obradovi}, D. S. Veselinovi}, D. C. Naskovi}, Latvijas kimijas `urnals 2 (1998) 55

9 Fe(III) DISULPHONATED HYDROQUINONE COMPLEX G. D. Kharlampovich, Yu. Churkin, Fenoly, Mir, Moskva, P. Job, Ann. Chim. Annal. 10 (1928) J. H. Yoe, A. L. Jones, Ind. Eng. Chem. Anal. Ed., 16, III, (1944) 15.H.S.Franck,R.S.Ostwald,J. Am. Chem. Soc. 69 (1947) F. T. H. Roos, W. F. Price, Analysis of fruit juice by AAS II-sci, Fs agric, 20 (1969) W. J. Price, Analytical Atomic Absorption Spectrometry, Pye Unicam Ltd., Cambride, 1978.

The temperature dependence of the disproportionation reaction of iodous acid in aqueous sulfuric acid solutions

J. Serb. Chem. Soc. 67(5)347 351(2002) UDC 542.9:546.155+535.243:536.5 JSCS-2955 Original scientific paper The temperature dependence of the disproportionation reaction of iodous acid in aqueous sulfuric