Preparation and Standardization of Sodium Thiosulfate

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The purpose of this experiment is to determine the concentration of sodium thiosulfate. This is done by preparing and standardizing sodium thiosulfate solution against potassium iodate. The primary standard solution will have an excess of Iodine that reduces the analyte which “produce a stoichiometrically equivalent amount of iodine” (Skoog, 563). When the liberated Iodine is titrated and reacts sodium thiosulfate solution, it converts to tetrathionate and iodide ions. When it reaches the end point, the solution should be colorless, meaning it reached equilibrium. In this experiment we will determine the amount of thiosulfate that needs to be titrated against the prepared solution of excess iodine.


The key to this experiment is the standardization of sodium thiosulfate by titrating against excess iodine. A process usually called back titration (but not back titrating in this experiment) when it exceeds the equivalence point to determine the amount of the sample. In this experiment it is necessary to add an excess of potassium iodide (KI) in standard iodate solution to measure the amount iodine in the solution, a reaction known has oxidation-reduction. The excess potassium iodide is added to an acidic solution to liberate I_2_ which produces a brownish color. It produced iodine because iodate and iodine reacts in an acidic solution. This reduction of the analyte gives the known amount of potassium iodate by producing a stoichiometric equivalent amount of iodine (Skoog, 563). Then the concentration measurement of the potassium iodate is possible. When the two solutions are prepared, concentration of sodium thiosulfate is calculated by the amount of sodium thiosulfate titrated. The reaction between iodine and thiosulfate is fast and the equilibrium leans towards the product side.


Equipment Chemicals

Graduated Cylinder 12.5 g of Na_2_S_2_O_3_ *5 H_2_O

Hot plate 0.050 g of Na_2_CO_3_

250/500 mL volumetric flask 0.55 g of...

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