Arrange the following procedural steps in the correct chronological order for determining the dissolved oxygen concentration of an aquatic sample using the Winkler titration method.
- 1Collect the aquatic sample in a narrow-necked bottle beneath the surface without agitation or trapping air bubbles.
- 2Add manganous sulfate solution followed by alkaline iodide-azide reagent beneath the liquid surface to fix the oxygen.
- 3Stopper the bottle carefully, invert it repeatedly to mix, and allow the formed precipitate to settle completely.
- 4Acidify the mixture with concentrated sulfuric acid to dissolve the precipitate and release free iodine proportional to oxygen content.
- 5Titrate the liberated iodine solution against standardized sodium thiosulfate using starch indicator until the blue-black color disappears.
Answer
The correct procedural order for Winkler titration of dissolved oxygen is: 1) Collect water sample without air bubbles, 2) Add manganous sulfate and alkaline iodide-azide below surface, 3) Stopper, invert to mix, and allow precipitate to settle, 4) Add concentrated sulfuric acid to dissolve precipitate and release iodine, 5) Titrate liberated iodine against sodium thiosulfate with starch indicator until colorless.
The correct protocol begins with bubble-free sampling to avoid aeration error. Next, chemical fixation reagents (manganous sulfate and alkaline iodide-azide) are added to form a brown precipitate. After inverting and allowing settling, concentrated sulfuric acid is introduced to dissolve the precipitate and release free iodine proportional to oxygen concentration. Finally, titrating against sodium thiosulfate with a starch indicator yields a colorless end-point, accurately quantifying aquatic dissolved oxygen.
Step-by-Step Solution
Key Concept
Winkler Method for Dissolved Oxygen Quantification