Stock Ammonium Sulfide Solution

Such solutions can be obtained commercially with no loss in yield. 

A 250-mL Erlenmeyer flask is charged with 66 mL of a 30% aqueous NH3 and 134 mL H2O. This solution is saturated with H2S (Caution—toxicl) by passing the gas vigorously through the solution. The solution is initially warmed to 50°C. After 1.5 h the solution is cooled to room temperature, at which time the H2S flow is discontinued and the flask is stoppered. The yellow solution should be stored in a screwcap bottle (no metal) in a well-ventilated area. This solution darkens on prolonged storage but remains active. 

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Stock solutions Nos. 3 and 4 contain those elements whose sulfides or hydroxides are precipitated from neutral or slightly ammoniacal solution. Either the high-purity oxide or sulfate of each element is used, except for chromium, where a dichromate salt is satisfactory, and titanium, where the most convenient compound is ammonium titanyl oxalate. The titanium stock solution is stable for at least 30 days. 

Cyanide and sulfide ions interfere with the test for halides. If such ions are present, they must be removed. To accomplish this, acidify 2 mL of the stock solution prepared above with dilute nitric acid and boil it for about 2 minutes. This will drive off any HCN or H2S that is formed. When the solution cools, add a few drops of a 5% silver nitrate solution. A volumitwus precipitate indicates a halide. A faint turbidity does not mean a positive test. Silver chloride is white. Silver bromide is off-white. Silver iodide is yellow. Silver chloride will readily dissolve in concentrated ammonium hydroxide, whereas silver bromide is only slightly soluble. 

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