Determination of Total Nitrogen in Water: Chinese National Standard (GB/HJ) Methods, Procedures, and

Total nitrogen (TN) is one of the most closely regulated eutrophication indicators in water quality management, and its measurement in China is governed by national and industry standard methods such as GB 11894-89 and its successor HJ 636-2012. This article provides a comprehensive technical review of the Chinese national standards for the determination of total nitrogen in water, covering the alkaline potassium persulfate digestion–UV spectrophotometric method, automated flow-based alternatives, sample requirements, quality control, and typical applications in environmental compliance monitoring.

Technical Overview

Total nitrogen (TN) is defined as the sum of all inorganic and organic nitrogen species dissolved or suspended in a water sample, including nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, and organic nitrogen. Excess nitrogen discharged into rivers, lakes, reservoirs, and coastal waters is a primary driver of eutrophication and algal blooms, which is why TN is a mandatory parameter in Chinese surface water quality standards, sewage discharge standards, and pollutant discharge permit programs.

In China, the determination of total nitrogen in water is standardized through the following national (GB) and environmental protection industry (HJ) standards:

• GB 11894-89 — Water quality — Determination of total nitrogen — Alkaline potassium persulfate digestion UV spectrophotometric method (the original national standard).
• HJ 636-2012 — Water quality — Determination of total nitrogen — Alkaline potassium persulfate digestion UV spectrophotometric method (the updated standard now applied in environmental monitoring).
• HJ 667-2013 — Water quality — Determination of total nitrogen — Continuous flow analysis (CFA) with N-(1-naphthyl)ethylene diamine dihydrochloride spectrophotometry.
• HJ 668-2013 — Water quality — Determination of total nitrogen — Flow injection analysis (FIA) with N-(1-naphthyl)ethylene diamine dihydrochloride spectrophotometry.
• HJ/T 199-2005 — Water quality — Determination of total nitrogen — Gas-phase molecular absorption spectrometry.

The core reference method is based on alkaline potassium persulfate oxidation. Under heating at 120–124 °C in a pressure digestion vessel (autoclave), potassium persulfate in alkaline medium oxidizes ammonia nitrogen, nitrite nitrogen, and organic nitrogen compounds quantitatively to nitrate. The nitrate is subsequently measured by ultraviolet spectrophotometry at 220 nm, with a parallel measurement at 275 nm to correct for interference from dissolved organic matter. The corrected absorbance is calculated as A = A220 − 2 × A275, and the nitrogen concentration is obtained from a calibration curve prepared with potassium nitrate standards.

For HJ 636-2012, the method detection limit is 0.05 mg/L and the lower determination limit is 0.10 mg/L when a 10 mL sample volume is used. These standards are technically aligned with international counterparts such as ISO 11905-1 and US EPA Method 353.2, while providing detailed procedural requirements adapted to Chinese regulatory conditions.

Testing Samples

Surface Water

Rivers, lakes, reservoirs, and estuarine waters represent the most common sample matrices for TN monitoring. Surface water TN concentrations are typically low (often below 5 mg/L), requiring careful control of blank values and reagent purity. Results are compared against the environmental quality thresholds in GB 3838-2002, which specify TN limits of 0.2, 0.5, 1.0, 1.5, and 2.0 mg/L for water quality Classes I through V.

Groundwater and Drinking Water Sources

Groundwater monitoring programs routinely include TN to track nitrate contamination from agricultural runoff, landfill leachate, and septic systems. Sampling and preservation follow HJ 164-2020 for groundwater technical specifications, and low-level samples benefit from the high sensitivity of the flow-based or gas-phase molecular absorption methods.

Municipal Wastewater

Influent and effluent samples from municipal wastewater treatment plants contain high concentrations of nitrogen species and suspended solids. Homogenization of the sample is essential before subsampling. Effluent results are evaluated against GB 18918-2002, which sets a TN limit of 15 mg/L for the strictest Class IA discharge category.

Industrial Wastewater

Effluents from petrochemical, chemical fertilizer, food processing, pharmaceutical, coking, and pulp and paper industries may contain elevated TN along with interfering substances such as chromium(VI), iron(III), iodide, bromide, and high alkalinity. Such samples frequently require dilution or pretreatment to bring the nitrogen concentration within the calibration range and to eliminate matrix interferences.

Sample Collection and Preservation

Samples are collected in clean glass or polyethylene containers following HJ 493-2009 and related sampling specifications. Ideally, samples should be analyzed promptly; when storage is unavoidable, they should be kept refrigerated at approximately 4 °C and analyzed as soon as possible within the holding time specified by the applicable sampling standard. Homogeneous mixing before subsampling is critical because particulate-bound nitrogen contributes to the total nitrogen value.

Testing Items

The primary testing item under these standards is total nitrogen expressed as N (mg/L). Depending on the monitoring program, the determination of TN is frequently combined with the analysis of individual nitrogen species to construct a complete nitrogen mass balance:

• Total nitrogen (TN) — the sum of all oxidizable nitrogen forms, per HJ 636-2012 or equivalent methods.
• Ammonia nitrogen (NH3-N) — determined separately for evaluating nitrification performance and discharge compliance.
• Nitrate nitrogen (NO3−-N) and nitrite nitrogen (NO2−-N) — inorganic oxidation products relevant to process monitoring.
• Total Kjeldahl nitrogen (TKN) — organic nitrogen plus ammonia, allowing the organic nitrogen fraction to be estimated by difference (TN minus nitrate, nitrite, and ammonia).
• Dissolved total nitrogen — obtained by filtering the sample through a 0.45 μm membrane before digestion.

Testing Methods

Alkaline Potassium Persulfate Digestion UV Spectrophotometry (HJ 636-2012 / GB 11894-89)

This is the benchmark national method and proceeds through the following key steps:

• Preparation of reagents: alkaline potassium persulfate solution (potassium persulfate dissolved in sodium hydroxide solution), potassium nitrate standard stock solution prepared from potassium nitrate dried to constant weight at 105–110 °C, and dilute hydrochloric acid solution.
• Calibration: a multi-point calibration curve is constructed from potassium nitrate working standards covering the expected concentration range, carried through the full digestion procedure.
• Digestion: a measured sample portion (typically 10 mL) is mixed with alkaline potassium persulfate solution in a quartz or borosilicate digestion tube, sealed, and heated at 120–124 °C in a pressure steam sterilizer for 30 minutes, then cooled to room temperature.
• Measurement: after digestion, a small volume of hydrochloric acid solution is added and the digestate is diluted to volume with ammonia-free water. Absorbance is measured at 220 nm and 275 nm against a reagent blank, and the corrected absorbance A220 − 2A275 is used to compute the concentration.

Known interferences include chromium(VI) and iron(III) at elevated concentrations, iodide and bromide, and carbonate or bicarbonate alkalinity that consumes oxidant. Turbidity and UV-absorbing organic matter must be addressed through proper blanking, dilution, or extended oxidation as appropriate.

Continuous Flow Analysis (HJ 667-2013)

This automated method performs online alkaline persulfate digestion followed by cadmium-column reduction of nitrate to nitrite, diazotization with sulfanilamide, and color development with N-(1-naphthyl)ethylene diamine dihydrochloride measured around 540 nm. It offers high throughput for large monitoring campaigns and reduces operator variability.

Flow Injection Analysis (HJ 668-2013)

Flow injection analysis implements the same persulfate digestion and color-forming chemistry within an automated injection manifold, achieving excellent reproducibility and detection limits in the low microgram-per-liter range. It is well suited to laboratories processing high sample volumes for surface water and groundwater networks.

Gas-Phase Molecular Absorption Spectrometry (HJ/T 199-2005)

In this technique, the digested sample is treated so that nitrate nitrogen is converted to nitrogen monoxide gas, which is swept into a measurement cell and quantified by UV molecular absorption. The method provides very low detection limits and is resistant to color and turbidity interference in the original sample.

Quality Control and Data Integrity

All methods require a documented quality control system, including method blank control (blank contamination from ammonium impurities in potassium persulfate is a classic source of error and demands high-purity reagents or re-crystallized persulfate), multi-point calibration with verification standards, duplicate analysis, matrix spikes, certified reference materials, control charts, and participation in interlaboratory proficiency testing by accredited third-party laboratories.

Applications

Determination of total nitrogen according to the Chinese national standards supports a wide range of regulatory, operational, and scientific objectives:

• Compliance monitoring of municipal and industrial discharges against integrated and sector-specific discharge standards and pollutant discharge permits.
• Surface water quality classification of rivers, lakes, and reservoirs under GB 3838-2002 for national and regional monitoring networks.
• Eutrophication assessment and nutrient load reduction programs for lakes, reservoirs, and coastal receiving waters.
• Wastewater treatment plant process control, including evaluation of nitrification and denitrification efficiency and optimization of carbon dosing and recycle ratios.
• Environmental impact assessments, pollution source apportionment studies, and total pollutant load cap management.
• Groundwater quality surveys and source water protection for drinking water supplies.
• Academic research and international data comparability, since the GB/HJ methods are harmonized with ISO and US EPA approaches.

FAQ

What is the difference between GB 11894-89 and HJ 636-2012?

Both standards describe the alkaline potassium persulfate digestion UV spectrophotometric method. GB 11894-89 was the original national standard, while HJ 636-2012 is the updated environmental protection industry standard that refines the reagent specifications, procedural details, and quality control requirements. For environmental monitoring purposes in China, HJ 636-2012 is the currently applicable reference method.

Why are absorbance measurements taken at both 220 nm and 275 nm?

Nitrate absorbs strongly at 220 nm, but dissolved organic matter also absorbs at this wavelength. Because organic matter shows characteristic absorbance at 275 nm where nitrate does not absorb, measuring both wavelengths allows the correction A = A220 − 2 × A275 to subtract the organic contribution and improve the accuracy of the nitrate determination.

What are the detection limit and working range of the standard method?

For HJ 636-2012, the method detection limit is 0.05 mg/L and the lower determination limit is 0.10 mg/L using a 10 mL sample volume. Samples exceeding the upper calibration range must be diluted with ammonia-free water and re-digested.

What are the most common sources of error in total nitrogen testing?

The most frequent problems are high reagent blanks caused by nitrogen impurities in potassium persulfate or laboratory water, incomplete digestion due to autoclave temperature deviations from 120–124 °C, interference from carbonate alkalinity that consumes oxidant, spectral interference from chromium(VI), iron(III), iodide, and bromide, and non-homogeneous subsampling of samples containing particulate nitrogen.

How should samples be preserved before analysis?

Samples should be collected in clean glass or polyethylene containers, kept refrigerated at approximately 4 °C, protected from contamination, and analyzed as soon as possible in accordance with the holding times specified in the applicable sampling standards such as HJ 493-2009. Samples must be thoroughly homogenized before the aliquot for digestion is taken.

How is total nitrogen related to ammonia nitrogen and TKN?

Total nitrogen equals the sum of organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. Total Kjeldahl nitrogen (TKN) covers only organic nitrogen plus ammonia nitrogen. Therefore, organic nitrogen can be estimated as TN minus TKN, nitrate nitrogen, and nitrite nitrogen, which makes combined determination valuable for process diagnostics and nutrient mass balances.