Determination of Total Nitrogen in Water by UV Spectrophotometry: Principles, Methods, and Applicati

Total nitrogen (TN) is one of the most important indicators of nutrient pollution and eutrophication risk in aquatic environments. The alkaline potassium persulfate digestion–UV spectrophotometric method enables accurate, sensitive, and cost-effective determination of total nitrogen in surface water, groundwater, and wastewater, and is widely adopted in environmental monitoring, regulatory compliance, and laboratory analysis worldwide.

Technical Overview

Total nitrogen refers to the sum of all nitrogen species present in a water sample, including nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), ammonia nitrogen (NH3-N), and organic nitrogen. Elevated total nitrogen levels promote excessive algal growth, oxygen depletion, and degradation of aquatic ecosystems, making TN a routine parameter in water quality legislation and discharge permitting.

The UV spectrophotometric determination of total nitrogen is typically performed in accordance with recognized standards such as HJ 636-2012 (Water quality — Determination of total nitrogen — Alkaline potassium persulfate digestion UV spectrophotometric method), which superseded GB 11894-89, and is technically aligned with international approaches including ISO 11905-1. The method is based on two key steps:

• Alkaline digestion: The sample is digested with alkaline potassium persulfate at 120–124°C in an autoclave or pressure steam sterilizer for approximately 30 minutes. Under these conditions, organic nitrogen, ammonia, and nitrite are quantitatively oxidized to nitrate.
• UV spectrophotometric measurement: Nitrate ion exhibits strong absorption at 220 nm, while many dissolved organic compounds that interfere at 220 nm also absorb at 275 nm. Corrected absorbance is calculated as A = A220 − 2 × A275, which effectively compensates for organic interference and yields a linear response proportional to nitrate concentration.

The method offers a practical working range of approximately 0.10–10 mg/L (extendable by dilution), a method detection limit of about 0.05 mg/L, and excellent reproducibility, making it suitable for both trace-level environmental monitoring and high-strength wastewater analysis. The technique can be executed manually or automated on discrete and flow-injection analyzers for high-throughput laboratories.

Testing Samples

Total nitrogen analysis by UV spectrophotometry is applicable to a broad range of aqueous matrices. Typical sample categories submitted to accredited third-party laboratories include:

Surface Water

Rivers, streams, lakes, reservoirs, and ponds, where total nitrogen data support eutrophication assessment, water body classification, and total maximum daily load programs.

Groundwater

Well water and aquifer samples used for drinking water source evaluation, contamination investigations, and agricultural non-point source studies.

Municipal Wastewater

Influent and effluent samples from wastewater treatment plants, including biological nutrient removal (BNR) process streams, used to verify nitrogen removal efficiency and discharge compliance.

Industrial Wastewater

Effluents from industries such as food processing, chemical manufacturing, pharmaceuticals, fertilizers, textiles, and pulp and paper, which may contain elevated organic or inorganic nitrogen loads.

Marine and Estuarine Water

Coastal and brackish waters, where nitrogen loading is a primary driver of harmful algal blooms; special consideration is given to bromide and chloride interference.

Drinking Water and Process Water

Raw water intakes and treated water used to confirm nitrogen levels remain within acceptable limits.

Testing Items

The core testing parameter is the total nitrogen concentration of the water sample, expressed in mg/L. Depending on project requirements, related nitrogen species may be determined alongside TN to provide a complete nitrogen profile:

• Total nitrogen (TN) — sum of all oxidizable and oxidized nitrogen forms
• Ammonia nitrogen (NH3-N)
• Nitrate nitrogen (NO3-N)
• Nitrite nitrogen (NO2-N)
• Total Kjeldahl nitrogen (TKN), where required for regulatory or process purposes
• Calculated organic nitrogen (TN minus inorganic nitrogen fractions)

Reporting typically includes the measured concentration, the applicable detection limit, dilution factors, recovery results, and an evaluation against the relevant regulatory limit, such as GB 3838 environmental quality standards for surface water or GB 18918 discharge standards for municipal wastewater.

Testing Methods

Sample Collection and Preservation

Samples are collected in clean glass or polyethylene bottles. For total nitrogen determination, samples are acidified with sulfuric acid to pH < 2 and stored at 2–5°C. Analysis should be completed as soon as practicable, generally within 7 days of collection. Particulate-laden samples must be homogenized before subsampling, since total nitrogen includes both dissolved and particulate nitrogen.

Digestion Procedure

• A measured aliquot of sample is mixed with alkaline potassium persulfate reagent in a quartz digestion vessel.
• The sealed vessels are heated in a pressure steam sterilizer at 120–124°C for 30 minutes, ensuring complete oxidation of all nitrogen species to nitrate.
• After digestion, vessels are cooled naturally, depressurized, and the digestate is brought to a defined volume.

Spectrophotometric Determination

• Calibration standards are prepared from potassium nitrate stock solution and digested identically to samples.
• Absorbance of standards and samples is measured at 220 nm and 275 nm using a UV spectrophotometer with 10 mm quartz cuvettes.
• Corrected absorbance is computed as A = A220 − 2 × A275, and concentrations are obtained from the calibration curve.
• A reagent blank is processed through the complete procedure and subtracted from all results.

Quality Control and Quality Assurance

Reliable results depend on rigorous quality control, including:

• Method blanks and digestion blanks to monitor reagent and contamination contributions
• Calibration verification with an independent quality control standard
• Matrix spike recoveries, typically acceptable within 90–110%
• Duplicate analyses with controlled relative deviation
• Certified reference materials (CRMs) for total nitrogen to verify method accuracy
• Quartz vessels and chloride-free reagents to minimize background absorbance

Interferences and Mitigation

Known interferences include high concentrations of chloride and bromide (significant in seawater, where persulfate is consumed and free halogens absorb in the UV region), elevated concentrations of certain organic compounds, and turbidity. Mitigation strategies include sample dilution, extended digestion with increased reagent dosage where validated, and homogenization of suspended solids. Automated segmented-flow or discrete analyzers apply the same chemistry with blank correction and validated matrices.

Applications

UV spectrophotometric total nitrogen determination supports a wide spectrum of regulatory, operational, and scientific objectives:

• Environmental compliance monitoring of municipal and industrial discharge permits
• Surface water quality classification and eutrophication risk assessment for lakes, reservoirs, and coastal zones
• Performance evaluation of wastewater treatment plants, including nitrification and denitrification process optimization
• Drinking water source protection and watershed management programs
• Environmental impact assessments and pollution source apportionment studies
• Academic research and long-term water quality trend analysis
• Verification of nutrient removal technologies such as biological nutrient removal, membrane bioreactors, and advanced oxidation systems
• Third-party data packages for regulatory submission, litigation support, and environmental site assessments

FAQ

What is the detection limit and working range of the method?

Under standard conditions per HJ 636-2012, the method detection limit is approximately 0.05 mg/L when using a 10 mm path length, with a practical quantification range of roughly 0.10–10 mg/L. Samples exceeding the upper range are diluted and re-digested to remain within the calibration curve.

Why are absorbances measured at both 220 nm and 275 nm?

Nitrate absorbs strongly at 220 nm, but dissolved organic matter also absorbs at this wavelength. Because these organics absorb at 275 nm while nitrate does not, the corrected absorbance A = A220 − 2 × A275 removes the organic contribution, improving accuracy across diverse water matrices.

How is total nitrogen different from TKN?

Total nitrogen includes nitrate and nitrite plus organic and ammonia nitrogen, whereas TKN (Total Kjeldahl Nitrogen) measures only organic and ammonia nitrogen. TN = TKN + NO3-N + NO2-N, allowing each fraction to be cross-verified.

What sample holding time applies?

Acidified samples stored at 2–5°C should be analyzed promptly, and generally no later than 7 days after collection. Field filtration is not performed for total nitrogen unless dissolved total nitrogen is specifically required, since particulate nitrogen is part of the total.

Are there interferences in saline waters?

Yes. High chloride and bromide concentrations can consume persulfate and generate UV-absorbing halogen species. Saline samples are typically diluted, digested with adjusted reagent quantities, and confirmed by spike recovery or alternative validated techniques such as high-temperature catalytic combustion-chemiluminescence methods.

Can the method be automated?

Yes. The alkaline persulfate digestion UV detection chemistry is readily implemented on discrete analyzers, segmented flow analyzers, and flow injection systems, enabling throughput of hundreds of samples per day with automated calibration and quality control, which is ideal for accredited third-party laboratories.