Why is an Fe3+ solution instead of an Fe2+ solution used in calibrating the spectrophotometer?
Note: hydroxylamine hydroxide is added to the Fe3+ solution then 1,10 phenanthroline is added to the solution after the addition of hydroxylamine hydroxide. After these additions, a buffer (NaOAC-HOac) was added then the solution is diluted to 50 mL.
The choice to use an Fe3+ solution instead of an Fe2+ solution in calibrating a spectrophotometer can be attributed to a few reasons.
First, Fe3+ ions have a more stable color and exhibit greater absorbance in the visible region of the electromagnetic spectrum compared to Fe2+ ions. This greater absorbance allows for a more accurate and precise calibration of the spectrophotometer.
To understand why Fe3+ ions are preferred in this calibration process, let's break down the steps involved and analyze the role of each reagent added.
1. Hydroxylamine hydroxide (NH2OH): This first step involves adding hydroxylamine hydroxide to the Fe3+ solution. Hydroxylamine hydroxide acts as a reducing agent, converting Fe3+ ions to Fe2+ ions.
2. 1,10-phenanthroline: Next, 1,10-phenanthroline is added to the solution after the reduction of Fe3+ to Fe2+ ions with hydroxylamine hydroxide. 1,10-phenanthroline forms a colored complex, known as a chelate, with Fe2+ ions. This complex has a distinct absorption peak in the visible region, facilitating accurate spectrophotometric measurements.
3. Buffer (NaOAc-HOAc): A buffer is added to maintain a constant pH during the calibration process. Keeping the pH stable ensures consistent chelation between 1,10-phenanthroline and Fe2+ ions, reducing variability in the spectrophotometric readings.
4. Dilution: Finally, the solution is diluted to a fixed volume of 50 mL. Dilution allows for a standardized concentration of Fe2+ ions in the solution, which is crucial for accurate calibration.
Considering these steps, the conversion of Fe3+ to Fe2+ ions, chelation with 1,10-phenanthroline, pH stabilization with a buffer, and proper dilution enable precise and reliable calibration of the spectrophotometer using an Fe3+ solution.