3 ml glass scoop.

Flasks, with ground necks and stoppers, of about 250 ml capacity, dried beforehand and filled with a pure, dry inert gas (nitrogen or, preferably, carbon dioxide).

Burette of 5-ml, 10-ml or 25-ml capacity, graduated in at least 0,05 ml, preferably with automatic zero adjustment, or equivalent automatic burette.

Analytical balance.

Chloroform, analytical reagent quality, freed from oxygen by bubbling a current of pure, dry inert gas through it.

Glacial acetic acid, analytical reagent quality, freed from oxygen by bubbling a current of pure, dry inert gas through it.

Potassium iodide, saturated aqueous solution, recently prepared, free from iodine and iodates. Dissolve approximately 14 g of potassium iodide in approximately 10 ml of water at room temperature.

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 (mKIO3) into a volumetric flask (250 ml or 500 ml) and dilute to the mark with freshly boiled water (V2), cooled down to room temperature. By means of a pipette, transfer 5 ml or 10 ml of this potassium iodate solution (V1) 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 (V3) 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

mKIO3 is the mass of potassium iodate, in grams V1 is the volume of the potassium iodate solution, in millilitres (5 ml or 10 ml) V2 is the total volume of potassium iodate solution, in millilitres (250 ml or 500 ml) V3 is the volume of the sodium thiosulfate solution, in millilitres wKIO3 is the purity of potassium iodate in g/100 g MKIO3 is the molecular mass of potassium iodate (214 g/mol) T is the exact molarity of the sodium thiosulphate solution (mol/l).

Starch solution, 10 g/l aqueous dispersion, recently prepared from natural soluble starch. Equivalent reagents may also be used.