Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods647working solution into separate 50-mL volumetric flasks, each containing5 mL of thioglycolic acid, 2 mL of 20% w/v ammonium citrate, and5 mL of 0.22 M NH 3 . After diluting to volume and mixing, the absorbancesof the external standards are measured against an appropriateblank. Samples are prepared for analysis by taking a portion known tocontain approximately 0.1 g of Fe 3+ , dissolving in a minimum amountof HNO 3 , and diluting to volume in a 1-L volumetric flask. A 1.00-mLaliquot of this solution is transferred to a 50-mL volumetric flask, alongwith 5 mL of thioglycolic acid, 2 mL of 20% w/v ammonium citrate,and 5 mL of 0.22 M NH 3 and diluted to volume. The absorbanceof this solution is used to determine the concentration of Fe 3+ in thesample.(a) What is an appropriate blank for this procedure?(b) Ammonium citrate is added to prevent the precipitation of Al 3+ .What is the effect on the reported concentration of iron in thesample if there is a trace impurity of Fe 3+ in the ammonium citrate?(c) Why does the procedure specify that the sample contain approximately0.1 g of Fe 3+ ?(d) Unbeknownst to the analyst, the 100-mL volumetric flask used toprepare the 10.00 ppm working standard of Fe 3+ has a volume thatis significantly smaller than 100.0 mL. What effect will this haveon the reported concentration of iron in the sample?12. A spectrophotometric method for the analysis of iron has a linear calibrationcurve for standards of 0.00, 5.00, 10.00, 15.00, and 20.00mg Fe/L. An iron ore sample that is 40–60% w/w is to be analyzedby this method. An approximately 0.5-g sample is taken, dissolved ina minimum of concentrated HCl, and diluted to 1 L in a volumetricflask using distilled water. A 5.00 mL aliquot is removed with a pipet.To what volume—10, 25, 50, 100, 250, 500, or 1000 mL—should itbe diluted to minimize the uncertainty in the analysis? Explain.13. Lozano-Calero and colleagues describe a method for the quantitativeanalysis of phosphorous in cola beverages based on the formationof the intensely blue-colored phosphomolybdate complex,(NH 4 ) 3 [PO 4 (MoO 3 ) 12 ]. 21 The complex is formed by adding(NH 4 ) 6 Mo 7 O 24 to the sample in the presence of a reducing agent, suchas ascorbic acid. The concentration of the complex is determined spectrophotometricallyat a wavelength of 830 nm, using a normal calibrationcurve as a method of standardization.21 Lozano-Calero, D.; Martín-Palomeque, P.; Madueño-Loriguillo, S. J. Chem. Educ. 1996, 73,1173–1174.