To determine the concentration of iron ions in a given water sample using spectrophotometry, you can follow these steps:1. Sample preparation: Collect a representative water sample and filter it to remove any suspended particles. This will ensure that the spectrophotometric measurements are not affected by light scattering from particulate matter.2. Iron complexation: Iron ions in the water sample can be complexed with a suitable reagent to form a colored complex. A common reagent used for this purpose is 1,10-phenanthroline, which forms a red-orange complex with Fe II ions. If the sample contains Fe III ions, they must be reduced to Fe II ions before complexation. This can be done by adding a reducing agent like hydroxylamine hydrochloride to the sample.3. Calibration curve: Prepare a series of standard solutions containing known concentrations of iron ions. Treat these solutions with the same reagents as the water sample to form the colored complexes. Measure the absorbance of each standard solution at the maximum absorption wavelength usually around 510 nm for the iron-phenanthroline complex using a spectrophotometer. Plot the absorbance values against the corresponding iron concentrations to obtain a calibration curve.4. Sample measurement: Measure the absorbance of the treated water sample at the same wavelength as the standards. Compare the absorbance value with the calibration curve to determine the concentration of iron ions in the sample.The practical range of absorption coefficients for iron ions depends on the specific complex formed and the spectrophotometer used. For the iron-phenanthroline complex, the molar absorptivity is typically around 11,000 L mol cm. The sensitivity and precision of spectrophotometric measurements in iron determinations are influenced by several factors, including:1. Absorption coefficient: A higher absorption coefficient results in a higher sensitivity, as it allows for the detection of lower concentrations of iron ions. However, it may also lead to a lower precision at high concentrations due to the potential for absorbance values to exceed the linear range of the spectrophotometer.2. Pathlength: The sensitivity and precision can be improved by using a longer pathlength cuvette, as it increases the absorbance values for a given concentration. However, this may also require a more dilute sample to avoid exceeding the linear range of the spectrophotometer.3. Instrumental factors: The quality of the spectrophotometer, including its wavelength accuracy, stray light, and baseline stability, can affect the sensitivity and precision of the measurements.4. Sample matrix: The presence of other absorbing species or light-scattering particles in the sample can lead to interferences and affect the accuracy of the iron determination.By optimizing these factors and using appropriate calibration and sample preparation techniques, spectrophotometry can provide a reliable and accurate method for determining the concentration of iron ions in water samples.