Determining Total Iron in Water by Spectrophotometry

The determination of total iron in water by spectrophotometry relies on the reduction of all iron species to the ferrous state, followed by complexation with a chromogenic reagent and measurement of absorbance at a characteristic wavelength. The method described here uses the 1,10-phenanthroline system, which remains a standard approach for its stability and selectivity.

Reagents should be prepared with deionized water. A standard iron stock solution is diluted to a working concentration suitable for the calibration range. Hydroxylamine hydrochloride solution is used as the reducing agent. A buffer solution, typically ammonium acetate or sodium acetate-acetic acid, maintains the pH in the range required for complex formation. The 1,10-phenanthroline solution is prepared and stored in the dark. Glassware must be cleaned with dilute nitric acid and rinsed thoroughly to avoid iron contamination, especially when low concentrations are expected.

Sample pretreatment is the step that distinguishes total iron from dissolved iron. For total iron, an appropriate volume of well-mixed sample is transferred to a flask. Hydrochloric acid and hydroxylamine hydrochloride are added, and the mixture is gently boiled to dissolve suspended and colloidal iron and to reduce ferric iron to ferrous iron. 

The solution is then cooled to room temperature. If only dissolved iron is to be measured, the sample is filtered through a 0.45-micron membrane and acidified without boiling. The boiling step is essential for total iron because particulate and complexed forms must be released into solution before they can react with the color reagent.

For calibration, a series of standard solutions is prepared in volumetric flasks by diluting the working standard to cover the expected concentration range. The same volumes of hydroxylamine, buffer, and phenanthroline solutions are added to both standards and samples in identical order. After dilution to the final volume, the solutions are mixed and allowed to stand for approximately ten to fifteen minutes for full color development. 

The pH during color development should be maintained in the weakly acidic range, where the ferrous-phenanthroline complex forms quantitatively. Absorbance is measured against a reagent blank at the specified wavelength, typically 510 nanometers, using a spectrophotometer with a 1-cm or 2-cm cell.

The iron concentration in the sample is calculated from the calibration curve, which is constructed by plotting absorbance against the mass of iron in the standards. The result is expressed as milligrams of iron per liter of sample.