Sample report of reactions and computations involved in the standardization of sodium thiosulfate and iodine solutions
The standardization procedures in part II should be referred to when analyzing the reactions and computations in part I of this article.
I. Reactions Involved and Calculations
1.1. Reactions involved:
KBrO3 + 6KI +3H2SO4 → 3I2 + KBr + 3K2SO4 + 3H2O
I2 + 2Na2S2O3 → 2NaI + Na2S4O6
Note: it should be understood that the number at the right side of an element should appear as a subscript of that element since the online text editor does not support subscript formatting of the chemical formulas
1.2 Titration Results:
A. Sodium Thiosulfate solution
after heating additional volume
Final Reading = 25.00 ml. F. R. = 25.00 ml F. R. = 6.78 ml
Initial Reading = 0.00 ml I. R. = 0.00 ml
25.00 ml sodium thiosulfate + 25.00 ml Na2S2O3 + 6.78 ml
Total volume = ml sodium thiosulfate
F. R. = 25.00 ml F. R. = 15.00 ml
I. R. = 0.00 ml I. R. = 0.00 ml
25.00 ml sodium thiosulfate + 15.00 ml = ml sodium thiosulfate
Normality of sodium thiosulfate:
Normality of sodium thiosulfate = Weight of potassium bromate/Volume of sodium thiosulfate x Milliequivalent Weight of potassium bromate
B. Iodine Solution
Trial I Trial II
F. R. = 20.00 ml F. R. = 16.20 ml
I. R. = 0.00 ml I. R. = 0.00 ml
20.00 ml Iodine solution 16.20 ml Iodine solution
Formula: Normality of sodium thiosulfate x Volume of sodium thiosulfate = Normality of Iodine solution x Volume of Iodine solution
Normality of sodiumthiosulfate =
Volume of sodium thiosulfate = 20.00 ml
Volume of Iodine solution = 16.20 ml
Normality of Iodine = Normality of sodium thiosulfate x Volume of sodium thiosulfate/Volume of Iodine solution
= (0.1808 x 20.00)/16.20
= 0.2232 Normal
II. Standardization Procedures
Reagents needed: Solutions of KBrO3, KI, 6N-H2SO4, Iodine, thiosulfate, starch indicator
A. Standardization of thiosulfate solution
1. Weigh accurately in the analytical balance 2 samples of 0.1000 g. each of potassium bromated by the indirect method into their corresponding flasks.
2. Dissolve the first sample in 50 ml distilled water, add 10 for 3 minutes away from strong light and then dilute with 50 ml distilled water,
3. Titrate with sodium thiosulfate solution from the rubber-stoppered buret until a yellow color is seen in the solution.
4. Add 5.00 ml starch indicator and continue titrating with sodium thiosulfate until the disappearance of the blue iodo-starch color (until colorless). Record I. R. and F. R. of all thiosulfate used.
5. Repeat from No. 2 using the second sample.
6. Calculate the normality of the thiosulfate solution for the 1st and 2nd trials and get the average.
B. Standardization of Iodine Solution
1. Fill the glass-stoppered buret with Iodine solution and the rubber-stoppered buret with thiosulfate solution. Record I. R. for both.
2. Run out 20 ml of the thiosulfate solution into the flask No. 1. Dilute with 50 ml distilled water and add 5.00 ml starch indicator.
3. Titrate with iodine solution until the appearance of a permanent blue iodo-starch color.
4. Repeat for the 2nd trial.
5. Calculate the normality of iodine solution for the 1st and 2nd trials and get their average.
Formula weight of Potassium Bromate = 167.01
Reference: Quantitative Chemistry classroom laboartory workbook