Lab F report

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Lab F: Standardization of a Sodium Thiosulfate Solution Jayden Podmoroff 17122912 Partner: Josh Thomson Lab performed: Nov 2, 2018 Chem 121 L08 TA: Nitin Lab Submitted: Nov 16, 2018 1

Introduction: The purpose of this experiment was to find the standardize a provided sodium thiosulphate solution through titration using a dilute primary standard solution of potassium iodate. The standardized solution obtained from this experiment can be used as a secondary standard solution in future experiments. A primary standard solution is defined by a solution having been prepared by direct measurements of the mass of solute and the volume of solution, whereas a secondary standard solution is defined by a solution whose concentration cannot be determined directly from weight of solute and volume of solution. 1 Primary standard solutions are generally used to standardize solutions, such as sodium thiosulphate, and secondary solutions are generally used in more specific analytical experiments. This experiment involves two separate chemical reactions leading up to and during titration. Before titrating, a known quantity of iodine is produced from chemical reaction (1), where a known amount of iodate reacts with excess iodide in acid solution. 1 The iodine is then titrated with the sodium thiosulphate while using a starch indicator to mark the end point (reaction (2)). The corresponding reactions are given below. 1 KIO 3 + 5KI + 3H 2 SO 4  3I 2 + 3K 2 SO 4 + 3H 2 O (1) 2Na 2 S 2 O 3 + I 2  2NaI + Na 2 S 4 O 6 (2) 2

Procedure: The procedure of Lab F given by the First Year Chemistry Lab Manual 2 was followed as described below. In order to make the concentrated KIO 3 primary standard solution, 1.0126 g of potassium iodate was weighed using an analytical balance and then transferred into a 150 mL beaker. The potassium iodate was dissolved in the beaker and then transferred into a 250 mL volumetric flask and then inverted about 15 times to ensure proper mixture. The dilute KIO 3 primary standard solution was made by first pipetting 25.00 mL of the concentrated KIO 3 solution from a separate dry beaker into a 250 mL volumetric flask. The flask was then filled with deionized water up to the mark in order to dilute the solution and then inverted about 15 times to ensure proper mixture. Roughly 100 mL of the provided sodium thiosulphate solution was obtained in a beaker and used to fill a burette. 25.00 mL of the dilute KIO 3 standard solution was pipetted into a 125 mL Erlenmeyer flask. 0.2 g of potassium iodate was obtained using a scoop and mixed with the solution in the flask. 20 drops of 1 M H 2 SO 4 were then added to the solution under the fume hood, forming a brownish-yellow iodine solution. The solution obtained from the previous step was slowly titrated with the provided sodium thiosulphate solution until the iodine solution was a much lighter yellow. 5 drops of starch 3

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solution were added to the iodine solution using a plastic transfer pipette, turning the solution into a dark blue colour. Sodium thiosulphate was added extremely slowly until the iodine solution had lost its colour, marking the end of the titration. The final burette reading was recorded to obtain the volume used during titration. This titration was performed a total of 3 times until 2 results agreed within  0.1 mL. Observations: When titrating the dilute standard iodate solution with the thiosulphate, the colour of the solution gradually became lighter at a very slow rate. The solution then became a dark blue colour due to the addition of the starch indicator. After the starch indicator was added, the iodate solution’s change in colour was rapid in comparison to the first part of the titration. It took very little of the thiosulphate to reach the end point of the titration after the starch indicator was added. Data and Calculations: See the attached Lab F report sheets for Data and Calculations. 4

Discussion: The molar concentration of a sodium thiosulphate solution was determined through the technique of titration using a dilute primary standard solution. The average volume of sodium thiosulphate used in the titrations was 27.275  0.04 mL. Knowing this amount along with the quantity of iodine that reacted with it, the concentration of the thiosulphate can be determined through a series of stoichiometric and molarity calculations. Out of a total of 3 performed titrations, 2 agreed within the required  0.1 mL. The volumes of the 3 titrations were 25.70 mL, 27.30 mL, and 27.25 mL all with uncertainty of  0.04 mL. The average was taken from the 2 agreeing titrations and the outlier of 25.70 mL was excluded. The overall experimental uncertainty of the concentration of the sodium thiosulphate solution only was about 0.5%. The uncertainty takes is the sum of all of the weights and volumes of the various flasks, burettes, and pipettes used in the experiment. The fact that the uncertainty was <1% means that the method and technique used for the lab yielded quite accurate results. Major sources of uncertainty came from the many different types of glassware used throughout the experiment. The 25 mL pipette, burette, volumetric flask, and analytical balance all have associated uncertainties that contribute to the total experimental uncertainty. Because of the burette requiring initial and final readings during a titration makes it a larger source of uncertainty than the other glassware. Another source of uncertainty came from the mixing of 5

components while making the concentrated primary standard solution and the dilute primary standard solution. When multiple transfers are being required in order to make the solutions, there is the possibility that some trace amount of substance is left behind after each transfer (eg. Potassium iodate left in weighing boat, small amount of solution left inside beaker, etc.) Conclusion: The molar concentration of Na 2 S 2 O 3 solution was calculated to be 0.01041  0.00005 M. The uncertainty of this measurement is less than 1%, meaning that the experimental method was quite accurate. References: [1] First Year Chemistry Lab Manual: Chem 111/113 & Chem 121/123 , University of British Columbia: Kelowna, BC, 2018-19; p 39 [2] First Year Chemistry Lab Manual: Chem 111/113 & Chem 121/123 , University of British Columbia: Kelowna, BC, 2018-19; p 40-41 6

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