Spectrophotometric Determination of Iron with 2-PyridyI-2’- Hydroxymethane SuIfonic Acid E. E. H. Pitt and V. M. Stanway’ Department of Chemistry and MetalIurgy, Lanchester College of Technology, Coventry, EngIand
COMPLEXES of copper, zinc, nickel, and cobalt with 2-pyridyl2‘-hydroxymethane sulfonic acid have been reported ( I ) . The ligand exhibits properties of 8-hydroxyquinoline in that the acid dissociation constants, of the pyridinium proton and the hydroxyl group, have similar values ( I ) . Relative positions of the nitrogen atom and the hydroxyl group are also similar. O n the basis of this evidence, it was thought possible that Z-pyridyl-2’-hydroxymethane sulfonic acid would form metal complexes, analogous to those of 8-hydroxyquinoline, which might be colored. A qualitative survey showed that colored complexes were formed with cobalt, nickel, copper, zinc, vanadium, and iron(II), the latter reacting quantitatively at the 0.1 to 4 ppm level to give a wine red color. Aspects of the chemistry of the reaction with iron have been investigated and a procedure for the spectrophotometric determination of this element is proposed. EXPERIMENTAL
Apparatus. Absorbance measurements were made with a Unicam SP 600 spectrophotometer. A Cambridge pH meter was used to determine pH. Reagents. REDUCING SOLUTION.A solution was prepared by dissolving 50 grams of hydroxylamine hydrochloride in one liter of distilled water. LIGANDSOLUTION.A solution was prepared by dissolving 0.2 gram of 2-pyridyl-Z’-hydroxyrnethanesulfonic acid in a small amount, about 20 drops, of 4N ammonia and diluting to 100 ml with distilled water. This solution was stable for several days but was discarded when it attained a pale yellow color. BUFFERSOLUTION.A buffer solution of pH 10 was prepared by dissolving 70 grams of ammonium chloride in a minimum of distilled water and adding 570 ml of 0.880 ammonia. This solution was diluted to one liter with distilled water. STANDARD IRONSOLUTION.A stock solution containing 200 ppm iron was prepared by dissolving 0.7025 gram of ferrous ammonium sulfate in about 50 ml of 1N sulfuric acid and diluting to 500 ml with distilled water. A 20 ppm iron solution was prepared by dilution with distilled water. Procedure. Pipet an aliquot of the sample containing 2.5-100 pg of iron into a 25-ml graduated flask. Add approximately 5 ml each of the reducing solution, the ligand solution, and the pH 10 buffer solution. Make up to volume with distilled water and mix well. Prepare a blank and standard in a similar manner using distilled water and 5 ml of the standard iron solution in place of the sample. After one hour, measure the absorbance of the sample and standard, at 525 mp, against the reagent blank using 1-cm glass cells. The concentration of iron in the sample is then calculated from the ratio of absorbances of sample and standard and the concentration of iron in the standard. The calibration graph is linear in the range 0-4 ppm of iron. 1 Present address, Chemical Analysis Section, Nelson Research Laboratories, Beaconhill, Stafford, England.
( 1) L. Banford
and W. J. Geary,J. Chem. Soc., 1964,378.
Figure 1. 2-Pyridyl-2’-hydroxymethane sulfonic acid RESULTS AND DISCUSSION
Preliminary qualitative tests showed that under specific conditions the ligand (Figure 1) gave a wine-red color with micro amounts of iron. Under alkaline reducing conditions the absorption spectra of the complex in both ammonia and sodium hydroxide, measured over the 400-600 mp wavelength range, were found to be identical with the color more intense and of greater stability when the solvent was ammonia. A single rather broad peak was observed which had a maximum at about 520 mp, the exact position of the maximum being dependent on the final pH of the system. The chemistry of the reaction was investigated by a systematic study of the effects of time, pH, reducing conditions, and
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