EXPERIMENT A. Iodometric Analysis: Determination of Iodate Ion. Chem 2301 Iodometric Analysis: Determination of Iodate Ion Experiment A

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1 EXPERIMENT A Iodometric Analysis: Determination of Iodate Ion THEORY Iodometric analysis is an indirect method in which an oxidizing analyte is treated with excess I (aq). The I 2 (aq) produced (present as I 3 (aq)) is determined by titration with standard sodium thiosulfate solution, Na 2 S 2 O 3 (aq). Starch indicator, which forms an intense blue black complex with I 3, is added just before the equivalence point giving a sharp endpoint signal. The three equations representing the sequence of reactions involved in the procedure in both parts of this experiment are: IO 3 (aq) + 5 I (aq) + 6 H + (aq) 3 I 2 (aq) + 3 H 2 O(l) 3 I 2 (aq) + 3 I (aq) 3 I 3 (aq) 3 I 3 (aq) + 6 S 2 O 3 (aq) 3 S 4 O 6 (aq) + 9 I (aq) Combining these, the net ionic equation for the effective reaction between IO 3 (aq) and S 2 O 3 (aq) is: IO 3 (aq) + 6 S 2 O 3 (aq) + 6 H + (aq) I (aq) + 3 H 2 O(l) + 3 S 4 O 6 (aq) Purpose: To determine the concentration of iodate ion (IO 3 ) in a provided unknown solution. METHOD (OUTLINE) Part 1. Standardization of the provided solution of Na 2 S 2 O 3 by titrating it with the I 3 (aq) produced by the reaction of dissolved samples of primary standard KIO 3 (s) with excess I (aq). (Note The molar mass of KIO 3 is g mol 1.) Part 2. Determination of the [IO 3 ] in the unknown by titrating (with the Na 2 S 2 O 3 ) the I 3 (aq) generated by the reaction of pipetted samples of the unknown with excess I (aq). PROCEDURE STANDARD The primary standard KIO 3 has been dried, cooled and stored in a desiccator. Approximately 0.8 g will be provided, in a weighing bottle, at your bench. UNKNOWN Unknown sample solution is provided at your bench. 1 of 5

2 PROVIDED SOLUTIONS - Approximately 0.15 M sodium thiosulfate solution. (Collect ~ 300 ml in a pre-rinsed 500 ml Erlenmeyer flask.) - 1M potassium iodide solution (KI(aq)) (Collect 100 ml) - 1M hydrochloric acid solution (HCl(aq)) (Collect 100 ml) - Starch indicator (in dropper bottles). Part 1 Preparation of the KIO 3 (Standard) Samples Using an analytical balance weigh accurately and successively, by difference, four samples of g each of KIO 3 (s), carefully tipping each sample into a clean, identified 250 ml Erlenmeyer flask using a funnel to aid in the transfer. Rinse the funnel with deionized water so that any solid adhering to it is transferred to the flask below. Add deionized water to a total of about 30 ml and swirl gently to dissolve the KIO 3. Cover each flask with Parafilm. Part 2. Preparation of the Unknown Samples Clean a ml pipette, rinse it (three times) with small portions of the unknown solution and then pipette four aliquots into clean 250 ml labeled Erlenmeyer flasks. Parts 1 and 2 Titrations of Standard and Unknown Samples Clean your burette, then rinse and fill it with your Na 2 S 2 O 3 (aq). To the first standard sample, add, using a graduated cylinder, 10 ml of the HCl(aq) and 10 ml of the KI(aq). Titrate* with the Na 2 S 2 O 3 solution to a color between pale orange and yellow. Add 1-2 ml of starch indicator and continue the titration until the solution becomes colorless. Using the same procedure titrate a total of three each of standard and unknown samples, one at a time, in alternation. Note: Do not add the HCl(aq) and KI(aq) to each mixture until you are ready to start titrating it. *Do not forget to record (to two decimal places) the initial and final burette readings. Note Regarding Precision For the unknown samples (replicates) you are expected to obtain three titration results with a range no greater than 0.10 ml. If this criterion has not been met with the first three samples titrate the fourth sample. For the standards complete the calculations of concentration of Na 2 S 2 O 3 (outlined below). If the range of the concentration values is less than M the precision is acceptable. If not titrate the fourth standard. CALCULATIONS (Note Unless otherwise advised, use the three most precise results in each part.) Part 1. Concentration of Na 2 S 2 O 3 For each trial (separately) calculate the moles of KIO 3 used, the moles of S 2 O 3 (aq) required (based on the stoichiometry) and the [S 2 O 3 ] (and hence [Na 2 S 2 O 3 ]) from the volume of titrant required. Calculate the mean concentration and the average deviation from the mean. 2 of 5

3 Part 2. Concentration of IO 3 (aq) in the Unknown Use the mean titration volume and the calculated [Na 2 S 2 O 3 ] (from Part 1) to calculate the mean moles of S 2 O 3 required per titration, the mean moles of IO 3 in each sample (from the stoichiometry) and then the mean concentration of IO 3 in your unknown sample. Also calculate the overall average deviation in the mean [IO 3 ]. Report your data and results on the summary sheets provided. Raw data should be recorded in your lab notebook. REPORT Submit both a formal report and the summary sheets which are due one week after you complete the experiment. 3 of 5

4 EXPERIMENT A Determination of Iodate Ion UNKNOWN SAMPLE CODE: STUDENT S NAME: LAB SLOT: BENCH NUMBER: Part 1. Standardization of the Provided Na 2 S 2 O 3 Solution Titrations of accurately weighed samples of primary standard KIO 3 (s) with Na 2 S 2 O 3 (aq) Trial # * Mass of KIO 3 / g Final burette reading / ml Initial burette reading / ml Volume of Na 2 S 2 O 3 required / ml Results Trial # * Moles of KIO 3 / mol Moles of S 2 O 3 / mol Concentration of Na 2 S 2 O 3 / mol L 1 * Report the data/results for the fourth trial only if it is used in the calculations. Mean molar concentration of Na 2 S 2 O 3 = mol L 1 Average deviation # of the mean = mol L 1 # For a series of nn measurements of some quantity xx the average (or mean) deviation, dd, is given by dd = nn xx xx 4 of 5

5 Part 2. Titration of ml samples of the unknown solution with Na 2 S 2 O 3 (aq) Titration # * Final burette reading / ml Initial burette reading / ml Volume of Na 2 S 2 O 3 required / ml *Report the data/results for the fourth trial only if it is used in the calculations. Mean volume of Na 2 S 2 O 3 required = ml Average deviation of the mean volume = ml Results Mean moles of S 2 O 3 required = mol S 2 O 3 Mean moles of IO 3 per ml sample = mol IO 3 Mean concentration of IO 3 in the unknown = mol L 1 Overall average deviation # in the [IO 3 ] = mol L 1 # Overall average deviation = + IO 5 of 5