An improved spectrophotometric method using o-phenanthroline for the determination of iron content i

Introduction

In industrial production, iron-based vessels and pipelines are prone to iron corrosion and the formation of scale, which seriously compromises operational safety and quality assurance. Consequently, the determination of iron content in water samples has become a routine monitoring parameter in the petrochemical and other industrial sectors, and the accurate measurement of iron content in laboratories is of particular importance. Currently, methods for determining iron content include spectrophotometry, atomic absorption spectroscopy, fluorescence spectroscopy and catalytic kinetic photometry. Among these, the o-phenanthroline spectrophotometric method has long been widely adopted due to its advantages of high selectivity, good accuracy, low interference, good reproducibility, stable complex formation and the fact that it does not require the large, expensive instruments needed by other methods.

In current laboratory procedures for determining iron content, direct heating of water samples in an electric furnace is prone to causing violent boiling, resulting in iron contamination and loss, which affects the accuracy of the determination. Furthermore, the reducing agent used, ammonium hydroxide, is toxic and irritating to the skin, and is therefore incompatible with the principles of a green laboratory. In this study, the heating method was modified to use a water bath, whilst employing non-toxic ascorbic acid for reduction, to investigate the optimal dosage of reducing agent, reaction time and acidity conditions. The experiments demonstrated that the results of the improved iron content determination were more accurate, with precision and accuracy slightly superior to the methods currently in use in the laboratory, and more conducive to the establishment of a green laboratory.

1. Experiment

1.1 Apparatus

Spectrophotometer, electric water bath, AE200 electronic balance, 10 mm cuvettes; 150 mL conical flasks; volumetric flasks.

1.2 Reagents

Iron standard stock solution (containing 100 mg/L Fe²⁺): Accurately weigh 0.7020 g of ferrous ammonium sulphate ((NH₄)₂Fe(SO₄)₂·6H₂O) (analytical grade), dissolve in 50 mL of 1:1 sulphuric acid, transfer to a 1000 mL volumetric flask, make up to the mark with deionised water, and mix thoroughly; Iron standard working solution (containing 25 mg/L Fe²⁺): Transfer 25 mL of the above solution to a 100 mL volumetric flask, add deionised water to the mark, and shake to mix; Hydrochloric acid: (1:3); O-phenanthroline solution: 0.5%, with a few drops of hydrochloric acid added to aid dissolution; Ascorbic acid solution: 50 g/L, prepared fresh and stored in a brown bottle; usable for 10 days; Acetic acid–ammonium acetate buffer solution: buffer solutions of different pH values (pH: 3.5/4.5/5.5), commercially available; saturated sodium acetate solution; precision pH paper; boiler water samples used in the experiment were collected from different sampling points within the plant.

1.3 Optimisation via Orthogonal Experimentation

Based on preliminary experimental results, a three-factor, three-level orthogonal experiment was conducted using ascorbic acid concentration, reaction time and system acidity as factors. Samples were collected from a specific sampling point within the plant for testing, with three parallel determinations (designated as No. 1, No. 2 and No. 3). Ascorbic acid solutions were added in volumes of 2 mL, 4 mL and 6 mL, respectively, with reaction times of 10 min, 15 min and 20 min, and system pH levels of 3.5, 4.5 and 5.5, respectively. A 50 mL water sample was placed in a 150 mL conical flask, to which 1 mL of (1:3) hydrochloric acid was added. The mixture was heated in a constant-temperature water bath to approximately 15 mL, then cooled to room temperature before adding varying amounts of ascorbic acid solution. The mixture was then transferred to a 50 mL stoppered colorimetric tube. While slowly adding saturated sodium acetate solution, determine the pH using precision pH paper; add 5 mL of the corresponding buffer solution and 2 mL of o-phenanthroline solution, dilute to the mark, shake well, and measure the absorbance after allowing the mixture to stand for different durations. Use a 10 mm cuvette, with the reagent blank as the reference, and measure the absorbance of the corresponding sample at 510 nm.

1.4 Preparation of Iron Standard Solutions and Plotting of the Standard Curve

Take 0, 0.20, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.00 mL of iron standard working solution (containing 25 mg/L iron) and place them in 150 mL conical flasks. Add 1 mL of (1+3) hydrochloric acid, heat in a constant-temperature water bath until the volume reaches approximately 15 mL, cool to room temperature, then add 4 mL of ascorbic acid solution, and transfer to a 50 mL stoppered colorimetric tube. While slowly adding saturated sodium acetate solution, determine the pH using precision pH paper until it reaches 3.5; add 5 mL of the corresponding buffer solution and 2 mL of o-phenanthroline solution, dilute to the mark, shake well, and measure the absorbance after standing for 10 minutes. Using a 10 mm cuvette, measure the absorbance of the corresponding sample at 510 nm against a reagent blank, plot a curve of absorbance against iron content, and simultaneously perform the determination using the laboratory’s current testing method.

1.5 Preparation and Determination of the Test Solution

Place 50.00 mL of the water sample in a 150 mL conical flask, add 1 mL of (1+3) hydrochloric acid, heat in a constant-temperature water bath to 150 mL, cool to room temperature, and filter to remove any precipitate. Then add 4 mL of ascorbic acid solution and transfer to a 50 mL stoppered cuvette. While slowly adding saturated sodium acetate solution, determine the pH using precision pH paper until it reaches 3.5. Add 5 mL of the corresponding buffer solution and 2 mL of o-phenanthroline solution, dilute to the mark, shake well, and measure the absorbance after standing for 10 minutes. Use a 10 mm cuvette, with the reagent blank as the reference, and measure the absorbance of the corresponding sample at 510 nm.

1.6 Verification of Precision and Accuracy

Water samples collected from different sampling points were analysed using both the laboratory’s current method and the modified method. The water samples were designated as A, B and C. Five parallel determinations were performed for each set of samples, and the corresponding absorbance values were recorded. The iron content and relative standard deviation for each water sample were calculated using the respective standard curves.

Spiked recovery tests were conducted on the water samples from groups A, B and C using both the laboratory’s current method and the improved method. 2 mL of iron standard working solution was added to each water sample. Sample preparation and measurement procedures were the same as in 1.5. Each set of water samples was analysed in triplicate, and the corresponding absorbance values were recorded. The iron content and spiked recovery rates for the different water samples were determined using the standard curves.