Nitrite-API colorimetric assay

Colorimetric assay for measuring nitrite levels using API test kit. Range: 0-2ppm

Nitrite is measured with the standard Greiss test. In this test, sulfanilamide reagent is converted to a diazonium salt by nitrite (NO2) in the water sample. The diazonium salt is then reacted with NED (N-1-napthylethylene diamine dihydrochloride) reagent to form an azo dye which has a purple/magenta color that is measured at 520-550 nm (suitable for the colorimeter 528 nm green LED).

Absorbance spectrum

  • Reported values in literature around 540nm
  • Measured absorbance peak at 530nm
  • Open Colorimeter measurement LED:  520nm


Prepare nitrite calibration solutions: 0-2ppm

  • Prepare 100 ppm NO2: 3.05 mL of 3,280 ppm standard in 100 mL volumetric flask. Fill to line with distilled water.
  • Prepare 2.0 ppm NO2: 2.0 mL of the 100 ppm working stock in 100 mL distilled water in volumetric flask. Fill to the line with distilled water.
  • Dilute the 2 ppm working stock as outlined below
HTML Table Generator
Nitrite Conc. (ppm) 2ppm Nitrite (mL) Dist. water (mL)
 0.0  0
 0.4   1
 0.8  2
 1.2  3
 1.6  4
 2.0  5


  • To each 5 mL sample, add 5 drops of API Nitrite Test Solution
  • Cap the tube and invert several times to mix.
  • Wait 5-10 mins for the color to fully develop.
  • Make sure the Open Colorimeter has the 520nm LED installed
  • Turn on the Open Colorimeter and select absorbance. Blank the colorimeter with the 0 ppm nitrite solution and measure absorbance of all of the solutions.

Calibration data

From the data, we fit a curve to the concentration as a function of absorbance. 
  "Nitrite": {
    "units": "ppm",
    "led": "520",
    "fit_type": "polynomial",
    "fit_coef": [0.1311, 1.2591, 0.0],
    "range": {"min": 0.0, "max": 1.4}
Nitrite calibration data in Open Colorimeter JSON calibration file format