Preparation of a standard solution for free chlorine

Reagents:
Cat.No. 100316 Hydrochloric acid 25 % for analysis
Cat.No. 105614 Sodium hypochlorite solution (6 - 14 % active chlorine)
Cat.No. 109147 Sodium thiosulfate solution 0.1 mol/l 0.1 N solution
Cat.No. 105043 Potassium iodide for analysis
Cat.No. 105445 Zinc iodide starch solution for analysis
Cat.No. 116754 Water for analysis

Preparation of a stock solution of free chlorine:
First prepare a 1:10 dilution using a sodium hypochlorite solution containing approximately 6 - 14 % of active chlorine. For this pipette 10 mL of sodium hypochlorite solution into a calibrated or conformity-checked 100-mL volumetric flask and then make up to the mark with water for analysis.

Precise assay of the stock solution for free chlorine:
Pipette 10.0 mL of the stock solution into a 250-mL ground-glass-stoppered conical flask containing 60 mL of water for analysis. Subsequently add to this solution 5 mL of hydrochloric acid 25 %. And 3 g of potassium iodide. Close the conical flask with the ground-glass stopper, mix thoroughly, and leave to stand for 5 min. Titrate the eliminated iodine with sodium thiosulfate solution 0.1 mol/l until a weakly yellow color emerges. Add 2 mL of zinc iodide starch solution and titrate from blue to colorless.

Calculation:
1 mL sodium thiosulfate solution = 3.55 mg free chlorine
Further investigational concentrations may be prepared from the stock solution prepared according to the procedure described above by diluting accordingly.

Stability:
When stored in a cool place (refrigerator), the stock solution remains stable for approximately one week. The diluted investigational solutions are stable for approximately 2 hours.

Materials
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