Experiment 22 Determination of Nitrate Nitrogen in Water

Nitrate, N03, is reacted with phenoldisulfonic acid to give a yellow color with an absorption maximum at 410 nm. Chloride interference is removed by precipitating the chloride. Nitrite, N02, levels in excess of 0.2 mg/L cause positive interference, but these concentrations rarely occur in surface waters. 

Neutralize the chloride-free prepared sample from above or a 100-mL fresh sample if already chloride free to about pH 7 with dilute NaOH. Transfer to a casserole and evaporate to dryness. Mix the residue with 2.0 mL phenoldi-sulfonic acid reagent, using a glass rod to help dissolve the solids heat on a hot-water bath if necessary to aid dissolution. Dilute with 20 mL distilled water and then add 6 to 7 mL ammonia until maximum color is developed. If a floc-culent hydroxide forms, dissolve it by adding the EDTA reagent dropwise with stirring. (Alternatively, the sample may be filtered.) Transfer the clear solution to a 50-mL volumetric flask and dilute to volmne with distilled water. 

Prepare standards in the same manner, using the same volumes of reagents, by evaporating 10, 25, and 50 mL of the standard nitrate solution, respectively these represent 0.10, 0.25, and 0.50 mg N, respectively. Omit the chloride precipitation step. Prepare a blank using the same volumes of reagents. 

Read the absorbance of the solution at 410 nm, correct for the blank, prepare a calibration curve, and calculate the concentration of nitrate nitrogen in your sample in mg/L. 

As little as 1 /U.g nitrate nitrogen can be detected, representing 0.01 mg/L in a 100-mL sample. The nitrate concentration in drinking water usually falls below 10 mg/L. If concentrations are high, measurements can be extended sixfold by measuring at 480 nm, or twofold by diluting prepared samples to 100 mL instead of 50 mL. 

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