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Unformatted text preview: 8-1 Ch. 8Redox Determination of IronAssigned Reading: R.B. Fischer and D.G. Peters, Quantitative Chemical Analysis, Saunders, Philadelphia, 1968, pp.12-18, 51-65,536-546; this reference is on reserve in the Chemistry Library. Introduction Iron is one of the most frequently encountered elements in industrial analyses. Therefore, its accurate determination is of great practical importance. A volumetric determination of iron consists of these steps: 1.dissolution of the sample 2.quantitative conversion of iron (III) to iron (II) with a suitable reducing agent 3.removal of the excess reducing agent 4.addition of special reagents to aid in detection of the endpoint and to ensure that the proper reaction occurs during the subsequent titration 5.titration of iron (II) to iron (III) with a suitable oxidizing agent Principles Dissolution of the Sample Major components of an iron sample are often iron (II) and iron (III) compounds that are insoluble in water. These can be dissolved in hot concentrated hydrochloric acid. Silica, which is present in most natural iron samples, does not dissolve in hot concentrated hydrochloric acid and may be observed as a white solid material floating on the surface of the solution; silica will not interfere with the iron determination. Reduction of Iron (III) to Iron (II) After the sample is dissolved, the iron will exist mainly as iron (III). Titration with a standard oxidizing agent requires that all the iron exist as iron (II). A quantitative reduction of iron (III) to iron (II) can be performed by addition of tin (II) to the hot hydrochloric acid solution of the sample: 2Fe3++ SnCl42-+ 2Cl-2 Fe2++ SnCl62-Chapter Eight8-2 Tin (II) possesses these advantages as a reductant in this system: 1. It reduces iron (III) to iron (II) quantitatively and rapidly 2. It possesses a certain degree of selectivity for iron (III), because it is not so strong a reducing agent that it reduces other species in solution 3. An excess is easily destroyed with HgCl2so that tin (II) will not interfere with the redox titration Due to the presence of chloride complexes of iron (III), the sample solution is yellow, whereas iron (II) species are colorless. Therefore, tin (II) solution should be added dropwise to the hot sample solution until the yellow color just disappears. One drop excess should be added to ensure complete reduction of iron (III). Removal of Excess Reducing Agent Excess tin (II) must be destroyed as it will interfere with the redox titration. This is accomplished by addition of a HgCl2solution: SnCl42-+ 2HgCl42-SnCl62-+ Hg2Cl2(s) + 4Cl-If the amount of tin (II) added is in great excess, the following reaction will also occur: SnCl42- + HgCl42- SnCl62-+ Hg (l) + 2Cl-Elemental mercury is produced in a colloidal form and will react with the standard oxidizing agent, thereby ruining the determination. Mercurous chloride (Hg2Cl2) is a white precipitate that will not interfere with the titration. If the precipitate is gray or black, elemental mercury is present, and the sample must be discarded. is gray or black, elemental mercury is present, and the sample must be discarded....
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- Fall '11