Analytical Chem istry - DePauw University

580 Analytical Chemistry 2.0in any quantitative method are the effect of potential interferents and establishingan appropriate blank.The best way to appreciate the theoreticaland practical details discussed in this sectionis to carefully examine a typical analyticalmethod. Although each method isunique, the following description of thedetermination of iron in water and wastewaterprovides an instructive example of atypical procedure. The description here isbased on Method 3500- Fe B as publishedin Standard Methods for the Examination ofWater and Wastewater, 20th Ed., AmericanPublic Health Association: Washington,D. C., 1998.Figure 10.18 shows the visible spectrumfor Fe(phen) 3 2+ .Representative Method 10.1Determination of Iron in Water and WastewaterDescription o f Me t h o dIron in the +2 oxidation state reacts with o-phenanthroline to form theorange-red Fe(phen) 3 2+ complex. The intensity of the complex’s color isindependent of solution acidity between a pH of 3 and 9. Because thecomplex forms more rapidly at lower pH levels, the reaction is usuallycarried out within a pH range of 3.0–3.5. Any iron present in the +3oxidation state is reduced with hydroxylamine before adding o-phenanthroline.The most important interferents are strong oxidizing agents,polyphosphates, and metal ions such as Cu 2+ , Zn 2+ , Ni 2+ , and Cd 2+ . Aninterference from oxidizing agents is minimized by adding an excess ofhydroxylamine, and an interference from polyphosphate is minimizedby boiling the sample in the presence of acid. The absorbance of samplesand standards are measured at a wavelength of 510 nm using a 1-cm cell(longer pathlength cells also may be used). Beer’s law is obeyed for concentrationsof within the range of 0.2–4.0 mg Fe/LNNo-phenanthrolinePr o c e d u r eFor samples containing less than 2 mg Fe/L, directly transfer a 50-mLportion to a 125-mL Erlenmeyer flask. Samples containing more than 2mg Fe/L must be diluted before acquiring the 50-mL portion. Add 2 mLof concentrated HCl and 1 mL of hydroxylamine to the sample. Heatthe solution to boiling and continue boiling until the solution’s volumeis reduced to between 15 and 20 mL. After cooling to room temperature,transfer the solution to a 50-mL volumetric flask, add 10 mL of an ammoniumacetate buffer, 2 mL of a 1000 ppm solution of o-phenanthroline,and dilute to volume. Allow 10–15 minutes for color development beforemeasuring the absorbance, using distilled water to set 100% T. Calibrationstandards, including a blank, are prepared by the same procedureusing a stock solution containing a known concentration of Fe 2+ .Qu e s t i o n s1. Explain why strong oxidizing agents are interferents, and why anexcess of hydroxylamine prevents the interference.