Add reagent until the endpoint: the flask turns from pale yellow to pink and the colour stays. Record the reading then. If it turns dark magenta, you've overshot and the sample is ruined.
Regulatory limit: 12.0 mg/L. Three batches, three separate aliquots.
| Batch | V sample | V used | C |
|---|
Three batches have arrived from the factory, and you need to say which one is over the limit. There's no instrument for this: you measure drop by drop, until the flask changes colour, and you record how much reagent it took. Overshoot by a single drop and you ruin the sample — and each batch only has three aliquots.
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