Division 5.4 Sodium thiosulphate, 0,01 mol/l (equivalent to 0,01 N) accurately standardised aqueous solution, standardised just before use.F1

Prepare daily the 0,01 mol/l sodium thiosulfate solution freshly from a 0,1 mol/l sodium thiosulfate standard solution before use, or determine the exact molarity. As experience shows, the stability is limited and depends upon the pH value and the content of free carbon dioxide. Use only freshly boiled water for the dilution, possibly purged with nitrogen.

The following procedure is recommended to determine the exact molarity of the sodium thiosulfate solution:

Weigh, to the nearest 0,001 g, 0,27 g to 0,33 g of potassium iodate (m _KIO3 ) into a volumetric flask (250 ml or 500 ml) and dilute to the mark with freshly boiled water (V ₂ ), cooled down to room temperature. By means of a pipette, transfer 5 ml or 10 ml of this potassium iodate solution (V ₁ ) into a 250 ml Erlenmeyer flask. Add 60 ml of freshly boiled water, 5 ml of 4 mol/l hydrochloric acid, and 25 mg to 50 mg of potassium iodide or 0,5 ml of the saturated potassium iodide solution. Titrate this solution with the sodium thiosulfate solution (V ₃ ) to determine the exact molarity of the sodium thiosulfate solution.

$T=\frac{m_{\mathrm{KIO}3}\times V_1\times 6\times 10\times w_{\mathrm{KIO}3}}{M_{\mathrm{KIO}3}\times V_2\times V_3}$

Where

• m _KIO3 is the mass of potassium iodate, in grams

• V ₁ is the volume of the potassium iodate solution, in millilitres (5 ml or 10 ml)

• V ₂ is the total volume of potassium iodate solution, in millilitres (250 ml or 500 ml)

• V ₃ is the volume of the sodium thiosulfate solution, in millilitres

• w _KIO3 is the purity of potassium iodate in g/100 g

• M _KIO3 is the molecular mass of potassium iodate (214 g/mol)

• T is the exact molarity of the sodium thiosulphate solution (mol/l).