Analyst, September, 1973, Vol. 98,@. 659-662 659 Spectrophotometric and Chelatometric Determination of Iron(II1) with 3 =Hydroxypyridine--2=thiol BY MOHAN KATYAL, (St. Stephen's College, Delhi-7, India) MISS VEENA KUSHWAHA AND R. P. SINGH (Department of Chemistry, Delhi University, Delhi-7, India) 3-Hydroxypyridine-2-thiol can be satisfactorily used in the spectrophoto- metric and chelatometric determination of iron(II1). With the prescribed methods the determination can be made over a wide pH range without interference from many anions and cations. Structures are suggested for the complexes formed. IN continuation of studies on hydroxypyridines as analytical reagent~,l-~ it has been found that the ligand 3-hydroxypyridine-2-thiol (I) forms coloured complexes with iron(II1). The reaction is selective and sensitive.The colour of the complex formed is discharged by the addition of EDTA. In the Dresent DaDer. use has been made of this comDlex and its subseauent decomposition by EDTA in the siectrophotometric and chelatom&-ic determinati6n of iron (111). n I EXPERIMENTAL REAGENTS- Standard iron(ll1) solutions-Standard solutions of iron( 111) were prepared by dissolving precipitated iron( 111) hydroxide in the appropriate acid (perchloric, hydrochloric or sulphuric acid). The solutions obtained were then standardised gravimetrically for iron content and titrimetrically for acid content. Bufer solutions-Buff er solutions were prepared by mixing 0.1 N potassium hydrogen phthalate solution with 0.1 N hydrochloric acid or 0.1 N sodium hydroxide solution as required.3-Hydroxypyridine-2-t~ioZ yeagent solution, 0.01 M-The solution of 3-hydroxypyridine-2- thiol (Fluka, Switzerland) was prepared by dissolving the appropriate amount of the reagent in methanol. The reagent solution can be kept for several weeks in an amber-glass bottle or in a refrigerator without appreciable decomposition occurring. All other solutions were prepared by dissolving analytical-reagent grade chemicals in double-distilled water. APPARATUS- Metrohm E350 pH meter was used for pH measurements. Absorbance readings were taken with a Unicam SP600 spectrophotometer, and a SPECTROPHOTOMETRIC DETERMINATION When a few drops of a methanolic solution of 3-hydroxypyridine-2-thiol are added to an iron(II1) solution, a green complex is instantly formed, the green colour of the complex changing with increase in pH first to blue and finally to red.Iron(I1) appears to form similar species with this reagent but these have not been studied in detail. The system is reversible and can be represented as follows: OH- OH- H+ H+ Green complex + Blue complex + Red complex @ SAC and the authors.660 [Analyst, Vol. 98 3-Hydroxypyridine-2-thiol shows maximum absorption a t wavelengths 275 and 360 nm with corresponding molar absorptivities of 9.1 x lo3 and 1.1 x lo*. The spectra of the solutions obtained by mixing iron(II1) solution with appropriate amounts of reagent solution were recorded under various pH conditions. The green complex formed is stable in the range of acidity 0.2 to 0.01 N perchloric acid medium with maximum absorption at 675 nm, the blue complex is stable a t pH between 3-5 and 11.0 with maximum absorption at 600 nm and the red complex is stable at pH above 11.0 with maximum absorption at 520 nm.The blue complex is the most stable of these three complexes with respect to time and permits the use of a wide pH range for the determination of iron(II1). A 50 per cent. methanolic medium was used throughout this study. The system adheres to Beer's law for concentrations of iron up to 3.36 pg ml-l. The sensitivity of the reaction is 0.012 pg of iron for 0.001 absorbance unit in a 1-cm cell. The molar extinction coefficient and apparent stability constant of the complex in a 50 per cent. methanolic medium at pH 5-0 and at 20 "C are 4.7 x lo3 and 3.82 x los, respectively. EFFECT OF pH- The effect of pH on the 3-hydroxypyridine-2-thiol reagent and the blue complex that it forms with iron(II1) was studied.The reagent itself does not absorb at the wavelength of maximum absorption of its iron complex. The absorption of the blue complex remains unaltered between pH 4.2 and 5-6, and solutions were therefore maintained at pH 5.0 in the subsequent work. EFFECT OF REAGENT CONCENTRATION- MI, in- creasing amounts of reagent solution were added while maintaining a 50 per cent. concen- tration of methanol and the pH at 5.0. It was found that a twenty-fold molar excess of 3-hydroxypyridine-2-thiol was sufficient for full colour development ; the excess of reagent did not interfere. COMPOSITION OF THE COMPLEXES- Job's curves were plotted for the complexes: green (at pH 2.0 only), blue (at pH 3.5 to 11.0) and red (at pH above 11-0), at different wavelengths ranging from 450 to 700 nm.It is concluded that in each instance iron(II1) and 3-hydroxypyridine-2-thiol combine in a 1 : 2 molar ratio. Taking into account earlier work,ls3 the following tentative structures are proposed for the species: green complex (11), where X is Clop, C1- or SO,2-; blue complex (111); and red complex (IV): KATYAL et al. : SPECTROPHOTOMETRIC AND CHELATOhlBTRIC To a fixed volume of iron(II1) solution [final concentration of iron(II1) 5 x r- 1- RECOMMENDED PROCEDURE- To a solution containing up to 30 pg of iron, add 5.0 ml of 0.01 M solution of 3-hydroxy- pyridine-2-thiol in methanol and a volume of buffer solution such that the pH of the final solution is about 5.0 when the volume is made up to a total of 10.0 rnl (or 25 ml or more if necessary) in a calibrated flask.Measure the absorbance of the complex at 600 nm against a reagent blank and from the standard graph calculate the amount of iron in the unknown solution. INTERFERENCES- In the determination of 1 pg ml-1 of iron by this method, 200 pg ml-l of each of the species CH3COO-, NO2-, NO3-, C1-, Br-, I-, B033-, citrate, tartrate, thiourea, zinc(II), cadmiumfII), barium(II), strontium(II), aluminium(III), lead(II), arsenic(II1) and man- ganese(II), and 100 pg ml-1 of each of F-, C,O,2- and PO,3- did not interfere. Also, theSeptember, 19731 DETERMINATION OF IRON(II1) WITH 3-HYDROXYPYRIDINE-2-THIOL 661 following ions can be tolerated at the levels (pgml-l) given in parentheses: silver (25), copper (15), bismuth (50), thorium (25), uranyl (50), molybdate (lo), thiocyanate (50), cyanide (120) and sulphide (250).Silver, bismuth, thorium and barium gave precipitates that were removed by centrifugation before taking the reading. Platinum metals caused serious interference. The determination can be accomplished at low pH (about 2-0) with more selectivity by making use of the green complex. In this instance the readings should be taken within the 30-minute period during which the absorbance remains steady. In all of the above tests the ions examined were present before the reagent was added. CHELATOMETRIC DETERMINATION As previously indicated, the colour of the iron(II1) - 3-hydroxypyridine-2-thiol complex varies from green to red, depending on the level of acidity.Thus, if a titration with EDTA solution is carried out at a pH below 3-5, the colour of the solution changes from green to colourless (or yellowish with large amounts of iron). If the pH of the iron solution to be analysed is above 3.5 and below 11.0, thus giving a blue complex with the reagent, the end-point is shown by the disappearance of the blue colour. The optimum conditions for titrations with EDTA solution have been established. EFFECT OF pH- Titrations of iron(II1) against EDTA solution were carried out a t different pH values. The metal-ion and indicator (3-hydroxypyridine-2-thiol) concentrations were kept constant and the pH was adjusted by adding potassium hydrogen phthalate buffer solution.The results are shown in Table I. TABLE I TITRATIONS OF IRON(III) AT DIFFERENT pH VALUES Volume of iron(II1) solution taken, 2-0 ml of 0.01 M concentration ( ~ 1 . 1 1 7 mg of the metal) ; volume of buffer solution added, 10.0 ml; amount of 0-5 per cent. methanolic indicator solution added, 5 drops ; concentration of EDTA solution used, 0.01 M ; total volume made up to approximately 20 ml Volume of EDTA pfI solution used/ml 0-8 2.10 1.0 2.08 1.2 2.02 1.5 2.00 1.8 2.00 2.0 2-00 Volume of EDTA PH solution used/rnl 2.4 2-00 2.8 2.00 3.2 2.00 3.6 2.00 4.0 2.00 4.4 2.00 Volume of EDTA PH solution used/ml 4.5 2.00 4.6 2.01 4-8 2.02 5.0 2.04 5.2 2.08 5.5 2-12 It is evident that satisfactory results can be obtained in the pH range 1.5 to 4.5.REQUIREMENTS FOR CONCENTRATION OF IRON AND INDICATOR- Titrations with different amounts of iron (1 to 15 mg in 20 ml of solution) were carried out at various pH values from 1-5 to 4.5. It was found that when iron is present in amounts greater than 3 mg, the end-point is given by the appearance of the pronounced yellow colour of the iron(II1) - EDTA complex. Iron solutions (containing 1.117 mg of the metal) a t different pH values (1.5 to 4.5) were titrated by using different volumes of 0.5 per cent. solution of the indicator in methanol. It was found that the presence of 2 to 5 drops of the indicator produced a sharp change of colour. However, the use of slightly larger amounts of the indicator did not affect the results. EFFECT OF TEMPERATURE- Titrations were performed in the temperature range 15 to 70 "C; accurate results were obtained betwen 20 and 45 "C.At lower temperatures the reaction is slow, while at higher temperatures a false end-point is reached, presumably because of the dissociation of the iron( 111) - 3-hydroxypyridine-2-thiol complex.662 KATYAL, KUSHWAHA AND SINGH RECOMMENDED PROCEDURE- Adjust the pH of a solution containing 1 to 15 mg of iron(II1) to between 1-5 and 4.5 by adding 10.0 ml of potassium hydrogen phthalate buffer, add 5 to 10 drops of 0.5 per cent. methanolic indicator solution and dilute the solution to about 20 ml. Titrate the solution slowly with standard 0.01 M EDTA solution, occasionally shaking it until the green colour (pH below 3.5) or blue colour (pH between 3.5 and 4.5) completely disappears.EFFECT OF DIVERSE IONS- 1-5 to 2.0. The results are incorporated in Table 11. The effect of diverse ions was examined in the determination of iron(II1) in the pH range TABLE I1 EFFECT OF DIVERSE IONS Amount of iron(III), 1.117 mg ( ~ 2 . 0 ml of 0.01 M EDTA solution) Amount of foreign ion EDTA solution Volume of 0.01 M Foreign ion* added/mg usedlml Masking agent CH3COO- . . .. .. .. .. 700 2.00 - NO,-, C1-, Br-, I-, SO,2-, citrate, tartrate.. 100 2.00 - NO2- .. .. . . .. . . .. 100 1.98 - .. . . .. .. 100 2.01 - 1 3 0 ~ 3 - . . .. .. .. .. 60 1.98 - so32- . . .. . . . . .. . . 50 1.97 - F- . . .. .. .. .. .. 50 1.98 - ~ 0 ~ 3 - . . .. . . CNS- .. .. . . .. .. .. 30 2.02 - . . .. . . 25 2.00 - CN- .. .. .. .. . . . . .. 10 1.97 - c,o*2- .. .. .. .. .. .. 50 2.00 - S2- .. . . Silver(1) . . .. .. .. .. .. 50 2.00 c1- .. 10 1.98 - Barium(I1) . . .. .. * . .. .. . . 10 1-97 - Calcium(I1) . . * . Bismuth(II1) . . .. . . .. .. 10 2.00 c1- Lead(I1) . . .. .. .. .. . . 10 2.02 CH3COO- Mercury(I1) . . .. .. .. .. 6 2.02 CN- Zinc(I1) . . .. .. .. .. .. 5 2.01 CN- Cadmium(I1) . . .. .. .. .. 5 2.01 CN- Aluminium( 111) .. .. .. .. 5 2.01 CH3COO- Manganese(I1) . . .. .. .. .. 4 2.01 Tartrate Cobalt(I1) . , .. .. .. .. 2 2.02 CN- Copper( 11) .. .. .. .. 1.3 2-01 CN- Molybdenum(V1j ‘ . . .. .. . . 1-2 2.03 CNS- * These ions were present before the 3-hydroxypyridine-2-thiol reagent was added. DISCUSSION The use of 3-hydroxypyridine-2-thiol as a reagent for iron(II1) compares favourably in respect of selectivity and sensitivity with that of its precursors, viz., 2,3-dihydro~ypyridinel-~ and 2-hydroxy-6-methylpyridine-3-carboxylic acid.4 The only disadvantage in using the green complex formed at low pH (below 3.5) for the spectrophotometric determination of iron(II1) is that the absorbance readings require to be taken within half an hour, after which time the absorbance does not remain steady. The complexometric titrations can be carried out over a much wider pH range (between 1.5 and 4.5). Even though the intensity of the colour of the green complex may decrease slightly with time, the end-point is not affected when the solution is titrated against EDTA solution. The University Grants Commission (India) is gratefully acknowledged for financial assis- tance in the award of the scheme “Water Pollution.” REFERENCES 1 . 2. 3. 4. Katyal, M., Goel, D. P., and Singh, R. P., Talanta, 1968, 15, 711. Goel, D. P., and Singh, R. P., Analyst, 1971, 96, 123. Kushwaha, V., Singh, R. P., and Katyal, M., Mikrochim. Acta, 1972, 807. -,-,- , Talanta, 1973, 20, 431. Received December 8th. 1972 Accepted March 16th, 1973