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Put on a lab coat, splash-proof safety glasses, and nitrile gloves. Obtain a waste container for aqueous sodium thiosulfate in advance. Note: HCl is toxic and highly corrosive, and sodium thiosulfate is an irritant, so the solutions for this lab must be prepared in a fume hood.
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Prepare 750 mL of 0.2 M sodium thiosulfate. Weigh 37.227 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) on a top-loading balance and pour it into a 1-L Erlenmeyer or volumetric flask through a powder funnel.
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Rinse with water to transfer residual material into the flask. Then, use a graduated cylinder to measure out and add 600 mL of deionized water to the flask.
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Place a magnetic stir bar in the flask and stir the solution on a large stir plate until the solution appears homogeneous.
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Use a graduated cylinder to add another 150 mL of deionized water to the flask and continue stirring.
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While the solution stirs, label a 750-mL bottle as ‘0.2 M sodium thiosulfate’.
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Once the solution again appears homogeneous, turn off the stir motor. Retrieve the stir bar and transfer the solution to the bottle. Cap the bottle and store it in a central fume hood.
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Prepare 350 mL of 6 M HCl. Measure 174 mL of 12.1 M HCl using a graduated cylinder and pour it into a 500-mL Erlenmeyer flask via a funnel.
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Use a different graduated cylinder to measure 176 mL of deionized water and slowly add the water to the flask. Place another large magnetic stir bar in the flask and stir the solution until it appears homogeneous.
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While the mixture stirs, label a 500-mL bottle as ‘6 M HCl’. Once the solution appears well mixed, turn off the stir motor and retrieve the stir bar.
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Carefully pour the 6 M HCl solution into the bottle via a funnel, cap the bottle, and store it in the instructor's hood. Put away the concentrated HCl and the sodium thiosulfate.
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Neutralize any acid waste and clean your glassware according to your lab's standard policies.