Table Salt for KI Content Theory and Background CHEM 222 Presenter: Seungsup Shin

Background Iodine Deficiency causes illness known as “Goiter” This became rare after Iodized salt is introduced in the US To test how much iodide in the iodized salt Goiter is the disease of enlargement of thyroid gland. The cause of this disease is not exactly known but commonly deficiency of iodine causes the disease. The purpose of this experiment is to find out how much iodide is in the iodized salt.

Experiment Using titration to find the amount of Iodide How to make 0.004M sodium thiosulfate solution The Formula mass of the sodium thiosulfate(Na2S2O3·5H2O): Na = 22.989770 ± 0.000002 g/mol S = 32.065 ± 0.005 g/mol O = 15.9994 ± 0.0003 g/mol H = 1.00794 ± 0.00007 g/mol Therefore, Na2S2O3·5H2O = 248.18 ± 0.01 g/mol Dissolve approx. 2.48g of sodium thiosulfate in H2O, Dilute to the 250mL mark  Approx. 0.04M Dilute 10mL of this solution to 100mL  Approx. 0.004M This method of making sodium thiosulfate solution could be inaccurate. Even though sodium thiosulfate can be obtained as highly pure solid, it could have been contaminated due to the high volume of the transaction between students.

Experiment Contd Use both regular salt and iodized salt. Regular salt would be our control group. Add bromine water into sample I- + 3Br2 + 3H2O  IO3- + 6Br- + 6H+ Eliminate Br and add excess KI – reduce iodate ion IO3- +6H+ + 8I-  3I3- + 3H2O Titration I3- + 2S2O32-  3I- + S4O62- The ratio of S2O32- and I- = 6 : 1 Titration ending point = First clear after add starch indicator First equation is to make IO3- ion out of the possible iodide in our sample. After we get iodate ion out of iodide, we have to eliminate Br before reduce the ion. To eliminate Br, add formate(COOH-) buffer. This will reduce excess Br in the solution Eqn) Br2 +HCOOH  2Br- + CO2 + 2H+ We get 3 tri-iodide ion out of 1 iodide ion. 2 mol of sodium thiosulfate will be consumed to titrate 1 mol of tri-iodide ion. Overall ratio is 6:1.

Experimental Error Standardization of sodium thiosulfate solution Cross-Contamination As I mentioned earlier, standardization of the sodium thiosulfate solution will minimize the experimental error. Many student observed too much iodide in their regular salt sample. Since all our samples were accessible to all student (back of the lab), regular salt may be cross-contaminated. If stock room provided the sample with non-contaminated sample, it would be much better to prevent any unnecessary experimental error.

Test Question The experiment has potentials for experimental errors. To minimize the error, we may standardize our sodium thiosulfate solution. How can we standardize this? Answer: Use KIO3 (FM=214.00). The treating process of KIO3 is similar with this experiment. (Reduce to I3- in acidic solution and titrate.) From known mass of KIO3, we can calculate the molarity of sodium thiosulfate solution.

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