Analytical Method: Analytical Quality Assurance, Standard for Sulfide

Reagents:
Cat.No. 109099 Iodine solution 0.05 mol I₂/l 0.1 N solution
Sodium sulfide nonahydrate for analysis
Cat.No. 109147 Sodium thiosulfate solution 0.1 mol/l 0.1 N solution
Cat.No. 100716 Sulfuric acid 25% for analysis
Cat.No. 105445 Zinc iodide-starch solution for analysis
Cat.No. 116754 Water for analysis

Preparation of a stock solution:
Dissolve 7.5 g of glass-clear, if necessary washed crystals of sodium sulfide nonahydrate with distilled water in a calibrated or conformity-checked 1000-mL volumetric flask and make up to the mark with distilled water. The stock solution prepared according to this procedure has a concentration of approx. 1000 mg/l sulfide.

Precise assay of the standard solution (stock solution) for sulfide:
Place 100 mL of distilled water and 5 mL (full pipette) of sulfuric acid 25 % in a 500-mL ground-glass conical flask. To this solution add 25.0 mL (full pipette) of the sulfide stock solution of approx. 1000 mg/l and 25.0 mL (full pipette) of iodine solution 0.05 mol/l. Shake the contents of the flask thoroughly for about one minute, subsequently titrate with sodium thiosulfate solution 0.1 mol/l until the yellow iodine color has disappeared, add 1 mL of zinc iodide-starch solution, and continue to titrate until a milky, pure white color emerges.

Calculation of the exact content of the sulfide solution:
C1 = consumption of sodium thiosulfate 0.1 mol/l
C2 = quantity of iodine solution 0.05 mol/l (25.0 mL)
mg/l sulfide = (C2 - C1) x 64,13
Further investigational concentrations may be prepared from the stock solution exactly determined according to the procedure described above by diluting accordingly.

Stability:
When stored in a cool place (refrigerator), the stock solution remains stable for at most one day. The diluted investigational solutions must be used immediately.