Ammonia-API colorimetric assay

Ammonia in water is commonly measured by the salicylate colorimetric assay. In this assay, the first reaction step involves the conversion of ammonia to monochloroamine by the addition of chlorine. The monochloroamine then reacts with salicylate to form 5-aminosalicylate. Finally, the 5-aminosalicylate is oxidized in the presence of sodium nitroferricyanide (a catalyst) to form a blue-green colored dye that absorbs light at 650 nm.

Absorbance spectrum

• Reported values in literature around 620nm
• Measured absorbance peak at 620nm
• Open Colorimeter measurement LED:  630nm

Materials

• Open Colorimeter with 630nm LED board
• API ammonia test kit
• 1,220 ppm as NH₃ (1,000 ppm as N) standard, Labchem Cat # LC17940-7
• Distilled water
• Cuvettes
• 6 x 5 mL capped tubes
• 1 x 100 mL volumetric flask
• 1 mL adjustable pipette

Prepare ammonia calibration solutions: 0-8ppm

• Prepare 8 ppm NH₃: 656 μL of 1,220 ppm standard in 100 mL volumetric flask. Fill to line with distilled water.
• Dilute the 8 ppm working stock as outlined below

Method

• To each 5 mL sample, add 8 drops of API Ammonia Test Solution, Bottle #1. Cap the tube and invert to mix.
• Add 8 drops of API Ammonia Test Solution, Bottle # 2. Cap the tube and shake thoroughly to mix.
• Wait 5-10 mins for the color to fully develop.
• Make sure the Open Colorimeter has the 630nm LED installed
• Turn on the Open Colorimeter and select absorbance. Blank the colorimeter with the 0 ppm ammonia solution and measure absorbance of all of the solutions.

Calibration data

{
  "Ammonia": {
    "units": "ppm",
    "led": "630",
    "fit_type": "polynomial",
    "fit_coef": [1.0129, 1.8909, 0.0],
    "range": {"min": 0.0, "max": 2.0}
  }
}