Determination of trace iron ions in water by UV-1200 spectrophotometer - Master's thesis - Dissertation

Determination of Trace Iron Ions in Water by Spectrophotometer

Key words: spectrophotometer; trace iron ion; mass spectrometer; V-1200; o-phenanthroline, absorbance, hydroxylamine hydrochloride. With the rapid development of urbanization and industrialization, people's living standards have significantly improved. However, environmental issues have become increasingly severe. Among these problems, water pollution stands out as a critical concern. This has led to a growing need for accurate methods to monitor and manage water quality, especially for trace contaminants like iron ions. In this experiment, we focus on determining the presence of trace iron ions using a spectrophotometric method. Iron typically exists in the Fe³⁺ state, but it can be reduced to Fe²⁺ using hydroxylamine hydrochloride. The reaction is as follows: 2Fe³⁺ + 2NH₂OH·HCl → 2Fe²⁺ + N₂ + 4H⁺ + 2H₂O + 2Cl⁻ Once reduced, Fe²⁺ forms a stable orange-red complex with o-phenanthroline at a pH range of 2–9. The maximum absorption occurs at 508 nm, with a molar absorptivity (ε) of 101,104 L/(mol·cm) and a stability constant (lgK) of 21.3. To ensure accurate results, the solution’s pH should be carefully controlled around 5. If the acidity is too high, the reaction rate slows down, while if it's too low, the ions may hydrolyze, affecting the color formation. The absorbance (A) is calculated using the formula: A = ε × L × C where ε is the molar absorptivity, L is the path length (in cm), and C is the concentration of iron (in mol/L). The relationship between absorbance and concentration is linear within the tested range, allowing for precise quantification. The experimental setup involves several key steps: preparing standard solutions, measuring the absorption spectrum, testing the effect of pH, determining the optimal reagent dosage, and evaluating the color development time. These steps help establish the most effective conditions for detecting trace iron ions. For instance, the absorption curve showed that the maximum absorbance occurred at 510 nm, making it the ideal wavelength for measurement. The acidity test revealed that a pH of approximately 5.4 yielded the highest absorbance. The optimal amount of o-phenanthroline was found to be 0.6 mL when 1 mL of iron solution was used, with a complex ratio of about 1:3. Additionally, the color development time was determined to be 10 minutes, as the absorbance reached its peak at this point. Using a standard curve, the iron content in the sample was calculated. The results showed that the average iron concentration in the drinking water was 0.050 μg/mL, which is well below the national limit of 0.3 mg/L set by GB 5749-2006. This confirms that the water meets the required safety standards. Overall, this experiment demonstrates the effectiveness of spectrophotometry in detecting trace metal ions in water samples. It highlights the importance of controlling experimental variables such as pH, reagent concentration, and reaction time to achieve accurate and reliable results.