Eliminating Chloride Interference in Total Nitrogen Determination

Chloride ions (Cl⁻) commonly present in wastewater, brackish water, and seawater severely interfere with total nitrogen (TN) determination by alkaline potassium persulfate digestion‑UV spectrophotometry. Cl⁻ can consume oxidant, generate chlorine or hypochlorite, and produce spectral interferences, leading to low or erratic TN results. Several effective methods exist to remove or suppress this interference.

1. Dilution

Simple and rapid. Diluting the sample reduces Cl⁻ concentration below the interference threshold (typically <500 mg/L). However, when TN is near the detection limit, dilution may cause under‑detection.

2. Chemical Masking

Adding sodium thiosulfate (Na₂S₂O₃) or sulfamic acid prior to digestion consumes active chlorine species and protects the oxidant. Typical dosage: 0.5–1.0 times the Cl⁻ mass concentration. Excessive masking agents may introduce additional nitrogen, requiring a blank correction.

3. Silver Salt Precipitation

Adding silver sulfate (Ag₂SO₄) or silver nitrate (AgNO₃) precipitates Cl⁻ as AgCl. After centrifugation or filtration, the supernatant is digested. This method eliminates interference thoroughly but needs careful control of excess Ag⁺ (removable by adding NaCl or HCl) and possible adsorption of nitrogen‑containing compounds.

4. Ion‑Exchange Pretreatment

Passing the sample through a cation‑exchange column or a silver‑type pretreatment cartridge selectively removes Cl⁻. The treated solution is directly digested. This approach is highly reliable and suitable for batch samples, though it increases cost and time.

5. Modified Digestion Conditions

Increasing alkalinity (e.g., from 0.075 mol/L to 0.15 mol/L NaOH) and doubling persulfate dosage raises chloride tolerance to about 2000 mg/L. This method is convenient but may cause baseline drift in UV absorbance and glassware corrosion.

For samples with Cl⁻ < 2000 mg/L, chemical masking or modified digestion is preferred. For high‑chloride matrices (e.g., seawater), silver precipitation or ion‑exchange pretreatment is recommended. Always perform spike recovery tests to validate the chosen method. Proper elimination of chloride interference ensures accurate TN measurement and reliable water quality assessment.