Question

Can you answer question 4 & 5? The experiment and porcedure is posted below

CHM3120C Analytical Chemistry Lab Lab 7. Spectrophotometric Determination of Iron in Mohr's Salt with o-Phenanthroline Required Reagents and Supplies: 50 mL of 0.0005M -0.0006 M of Mohr's solution (unknown), 10 g of Hydroxylamine hydrochloride (NH2OH, HCI), 500 mL of o-phenanthroline solution (contains 0.5 g of o-phenanthroline monohydrate), 83 g of sodium acetate (NaOAc. 3H20) in 500 mL of water (for 1.2 M of pH 5 buffer soln), minimum 3 g of 99.9 % Mohr's salt (ferrous ammonium sulfate hexahydrate, FeSO4,(NH4)2S04.6H20), 20 mL of conc, sulfuric acid, Ocean Optics, Kimwipe and parafilm. I. Introduction: The bidentate ligand, o-phenanthroline (or 1, 10-phenanthroline), reacts quantitatively with iron (II) to form a very stable, intensely red-colored complex ion. The complex with iron (III) is weaker color. In order to quantitate the total amount of iron in a sample by spectrophotometric means, it is therefore essential that all iron be present in the ferrous state (Fe). Fe(II) + 3 phenH → Fe(phen)32+ + 3H+ (Red, max = 508 nm) Spectrophotometric measurements will be made on an Ocean Optics spectrometer. Each group prepares a set of standard solutions of iron (solutions containing an exactly known amount of iron). II. Procedure: Start from step "d". a. 10% (w/v) Hydroxylamine hydrochloride (sufficient for about 80 to 90 measurements): Dissolve 10 g of NH2OH, HCl in about 100 mL of distilled water. b. 0-phenanthroline solution (sufficient for 80 to 90 measurements): Dissolve 0.5 g of - phenanthroline monohydrate in about 500 mL of water. Warm slightly if necessary. Each milliliter is sufficient for no more than about 0.09 mg of Fe. Must be fresh, so do not prepare more than needed; it darkens on standing and must then be discarded. c. Sodium acetate buffer, 1.2 M (sufficient for 80 to 90 measurements): Dissolve 166 g of NaoAc 3H20 in 1 L of distilled water. d. Standard iron solution, 0.01 mg/mL. Please follow the procedures below. d-1. Preparation of stock standard: Obtain a sample of Mohr's salt, ferrous ammonium sulfate hexahydrate. [FeSO4.(NH4)2SO4.6H20; MW=392.16 g/mol] DO NOT DRY Mohr's salt! Using the weigh-by-difference technique, accurately weigh out about 0.07 -0.08 g of the salt into a l L volumetric flask. Dissolve in 50 mL of distilled water that contains 1-2 ml of concentrated sulfuric acid, dilute to the mark with distilled water to the mark, seal with parafilm, and mix well. . Do not add too much H2SO4, or the buffering capacity of the sodium acetate, to be added later, may be exceeded. • d-2. Preparation of working standards, blank, and unknown solutions: Prepare all of the following solutions at least 30 min before spectrophotometric measurements are performed c. The six working standards are prepared by volumetrically delivering the indicated volumes of the "dilute stock solution" to 100 ml. volumetric flasks, using a burette. Again, make sure that the 51

Question 4 & 5

Answer 1

Answer 4- blank reagent means a solution having all component as sample solution except the analyte to be determined. The compounds in the solution may have the impurity of the analyte to be determined and may absorb the radiation of same wavelength. Most of time blank reagent is used to caliberate the instrument to ensure that the measurement device gives a zero signal when there is no sample present so signal of blank can be substracted to get the signal of desired analyte

Answer 5- Fe2+ + 2H2O = Fe(OH)2 + H+ This reaction would be a problem in determination of Iron because of the absence of charge transfer specially Ligand metal charge transfer as in case of iron-phenanthroline complex. Due to absence of charge transfer and d-d transfer Fe(OH)2 is colourless and can not be determined under photospectroscopy technique and exact amount of Fe can not be determined. This can be prevented by maintaining a pH of 2-3. According to le chatelier principle more H+ concentration shifts this reaction towards back.