Determination of Nitrate Nitrogen

To determine the nitrate nitrogen of the given sample of water.

The reaction with the nitrate and brucine produces yellow colour that can be used for the colorimetric estimation of nitrate. The intensity of colour is measured at 410 nm. The method is recommended only for concentration of 0.1– 2.0 mg/L NO-3—N. All strong oxidising and reducing agent interfere. Sodium arsenite is used to eliminate interference by residual chlorine; sulphanilic acid eliminates the interferences by NO-2-N and chloride interference is masked by addition of excess NaCl. High concentration of organic matter also may interfere in the determination.

  1. Spectrophotometer
  2. Water bath
  3. Reaction tubes
  4. Cool water bath

Reagents (» click to check the preparation of reagents)
  1. Stock nitrate solution
  2. Standard nitrate solution
  3. Sodium arsenite solution
  4. Brucine-sulphanilic acid solution
  5. Sulphuric acid solution
  6. Sodium chloride solution

  1. Nitrate standards are prepared in the range 0.1-1.0 mg/LN diluting 1.00, 2.00, 4.00, 7.00 and 10.0 mL standard nitrate solution to 10 mL with distilled water.
  2. If residual chlorine is present 1 drop of sodium arsenite solution is added for each 0.1 mg Cl2 and mixed.
  3. Set up a series of reaction tubes in test tube stand. Add 10 mL sample or a portion diluted to 10 mL to the reaction tubes.
  4. Place the stand in a cool water bath and add 2 mL NaCl solution and mix well.
  5. Add 10 mL H2SO4 solution and again mix well and allow cooling.
  6. The stand is then placed in a cool water bath and add 0.5 ml brucine-sulphanilic acid reagent. Swirl the tubes and mix well and place the tubes in boiling water bath at temperature 95°C.
  7. After 20 minutes, remove the samples and immerse in cool water bath.
  8. The sample are then poured into the dry tubes of spectrophotometer and read the standards and sample against the reagent blank at 410 nm.
  9. Prepare a standard curve for absorbance value of standards (minus the blank) against the concentration of NO-3N.
  10. Read the concentration of NO-3N in the sample from the known value of absorbance.

Nitrate N in mg/L = µg NO3- – N
mL of sample

NO3 in mg/L = mg/L nitrate N � 4.43.

The observation are presented in Tables A and B respectively.

Table A: Observation for calibration

Table B: