In the Winkler method for dissolved oxygen, each mole of O2 in the sample ultimately liberates 2 mol of I2. A 200 mL water sample requires 11.8 mL of 0.0100 M Na2S2O3 for the liberated iodine. The dissolved oxygen in mg/L (O = 16) is:

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In the Winkler method for dissolved oxygen, each mole of $O X_{2}$ in the sample ultimately liberates 2 mol of $I X_{2}$ . A 200 mL water sample requires 11.8 mL of 0.0100 M $Na X_{2} S X_{2} O X_{3}$ for the liberated iodine. The dissolved oxygen in mg/L (O = 16) is:

2 mol

I X_{2}

per

O X_{2}

means 4 mol thiosulfate per

O X_{2}

. Thiosulfate = 0.0100

\times

11.8 = 0.118 mmol, so

O X_{2}

= 0.0295 mmol = 0.944 mg (32 g/mol). Concentration = 0.944 mg/0.200 L = 4.72 mg/L.