Determination of Ammonia Nitrogen in Water by Standard Dilution Method

The standard dilution method for determining ammonia nitrogen in water is based on Nessler's reagent spectrophotometry, specified in China's national environmental protection standard HJ 535-2009. This method is applicable to surface water, groundwater, domestic sewage, and industrial wastewater. When a 50 mL sample volume and a 20 mm cuvette are used, the method detection limit is 0.025 mg/L, with a lower determination limit of 0.10 mg/L and an upper limit of 2.0 mg/L.

The principle is straightforward: ammonia nitrogen present as free ammonia or ammonium ions reacts with Nessler's reagent to form a yellow-brown complex. The color intensity is directly proportional to the ammonia nitrogen concentration, and absorbance is measured at 420 nm. Nessler's reagent is an alkaline solution of potassium mercuric iodide, which provides high sensitivity for ammonia quantification.

Sample Pretreatment

The first step is selecting an appropriate pretreatment based on sample characteristics. Suspended solids, residual chlorine, calcium and magnesium ions, sulfide, and organic matter all interfere with the determination and must be eliminated. For turbid samples or those containing metal ions, potassium sodium tartrate solution is added to complex calcium and magnesium, preventing hydroxide precipitation under alkaline conditions. 

For samples with high color and turbidity, zinc sulfate and alkali are used in a flocculation step to adsorb and remove interfering substances. For heavily contaminated industrial wastewater, pre-distillation is required to separate ammonia as ammonia gas, which is then collected in the distillate. 

Dilution serves two purposes: adjusting the ammonia nitrogen content into the calibration curve's linear range and reducing interference from color, turbidity, and coexisting ions.

Calibration Curve and Measurement

A calibration curve is prepared by adding varying volumes of ammonia nitrogen standard solution to 50 mL colorimetric tubes, diluting to volume with ammonia-free water, and adding 1.0 mL of potassium sodium tartrate solution followed by 1.0 mL of Nessler's reagent. After mixing and allowing 10 minutes for color development in the dark at room temperature, absorbance is measured at 420 nm against a zero-concentration reference. The regression equation is calculated with ammonia nitrogen concentration as the independent variable and corrected absorbance as the dependent variable.

For sample measurement, an appropriate volume of pretreated water sample is transferred to a 50 mL colorimetric tube, diluted to volume with ammonia-free water, and treated identically to the calibration standards. If absorbance exceeds the calibration range, the sample must be diluted and re-measured.