Sodium Thiosulfate Lab

Prior to begin the experiment, preparation for some solutions is required to enhance the lab performance. The solutions needed are standardized potassium bromate, standardized sodium thiosulfate, and starch indicator. First, to prepare a starch indicator, pulverize 1g of soluble starch and add 15mL of water to make it as a paste-like substance. Dilute it with about 500mL of boiling water, and continue to heat until the mixture becomes clear. Cool it to room temperature and save it into a stoppered bottle.
To prepare a 0.015M KBrO3 (potassium bromate) solution, place about 1.5g of reagent-grade KBrO3 into a weighing bottle, and dry it for at least an hour at 110 C. After the drying process, place the bottle in the desiccator to cool down its temperature. Weigh about 1.3g of potassium bromate, with its measurement nearest to 0.1mg, and place
Mix it thoroughly until the solids are well dissolved. After Na2S2O3 solution is made, standardize the solution against KBrO3. 25.00-mL aliquots of KBrO3 solution is pipetted into three 250-mL Erlenmeyer flasks. Place about 2-3g of KI (potassium iodide) in each flask and 5mL of 3M H2SO4 (sulfuric acid) as following. Titrate with Na2S2O3 right away until the solution changes its color to pale yellow. The expression to this reaction is presented as BrO3-(aq) + 6I-(aq) + 6H+(aq) → Br-(aq) + 3I2(aq) + 3H2O and due to the excess amount of I-, I2 reacts with I- to form I3- (I- (aq) + I2 (aq) ↔ I3-). Add 5mL of starch indicator. The starch indicator with react with I3- in the solution and turn its color to blue. The I3- formed then reacts with S2O32- (thiosulfate) and produces S4O62- (tetrathionate) and three I1-. Titrate the solution until the blue color disappears. Then calculate the concentration of the thiosulfate