4. To determine nitrates, a 5 ml sample of wastewater was diluted in a 100 ml volumetric flask. To 25 ml of the prepared solution, 2.0 ml of phenol disulfonic acid and 5 ml of 12 N sodium hydroxide solution were added and the solution was brought to 100 ml. Colorimetric at λ=410 nm. The nitrate content in the sample according to the schedule was 0.2 mg/l. Calculate the nitrate content in the sample.

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