Three Methods to Eliminate Chloride Interference in Ammonia‑Nitrogen Detection

Chloride ions (Cl⁻) commonly found in seawater, high‑salinity industrial wastewater, and some domestic sewage can severely interfere with the determination of ammonia‑nitrogen (NH₃‑N). Chloride may react with ammonia to form chloramines, alter the reaction medium, or cause spectral interference. To obtain accurate results, three effective methods are widely used to eliminate or suppress chloride interference.

1. Chemical Masking

Chemical masking is the simplest and most convenient approach. It involves adding a masking agent to the water sample to bind or neutralise the interfering species.

For residual chlorine: Sodium thiosulfate (Na₂S₂O₃) solution (0.35 %) is added. It reduces free chlorine, preventing chloramine formation. A starch‑iodide test paper can confirm the complete removal of residual chlorine.

For high chloride concentrations: Mercuric sulfate (HgSO₄) is commonly used. Mercury ions form stable chloro‑complexes with Cl⁻, thereby preventing chloride from interfering with the colour‑forming reaction (e.g., with Nessler’s reagent or salicylate). This method is effective when Cl⁻ is above 1000 mg/L, but care must be taken not to over‑add mercury, which may introduce new interference.

Chemical masking is rapid, requires no special equipment, and is suitable for routine batch analysis. However, it may not be sufficient for extremely high chloride levels (e.g., >20 g/L in seawater).

2. Distillation Pretreatment

Distillation is the most reliable method for samples with very high chloride concentrations or complex matrices. The principle is straightforward: under alkaline conditions (adding sodium hydroxide or borate buffer), ammonia is converted to volatile NH₃ gas. Heating the solution drives off NH₃, which is then trapped in a boric acid or dilute acid absorbing solution. Chloride and other non‑volatile anions remain in the distillation flask.

The distillate contains only ammonia (as ammonium ion after absorption), free from chloride interference. It can then be analysed by Nessler’s method, salicylate method, or ion chromatography. Studies have shown that distillation pretreatment achieves spike recoveries of 93–97 % even when Cl⁻ is as high as 128 000 mg/L. The main drawbacks are longer analysis time, the need for a distillation apparatus, and the risk of ammonia carry‑over between samples.