Total nitrogen (TN) is one of the most critical indicators of nutrient pollution and eutrophication risk in surface water, groundwater, and wastewater. The alkaline potassium persulfate digestion–UV spectrophotometric method, standardized in GB 11894-89, HJ 636-2012, and ISO 11905-1, converts all nitrogen species to nitrate and quantifies them by dual-wavelength UV absorbance measurement. This article explains the measurement principle, reagents, step-by-step procedure, quality control requirements, typical sample types, and frequently asked questions for the UV spectrophotometric determination of total nitrogen in water.
Technical Overview
Total nitrogen (TN) is defined as the sum of all nitrogen forms dissolved or suspended in a water sample, including organic nitrogen, ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and nitrite nitrogen (NO2-N). Elevated TN levels accelerate algal growth and eutrophication in lakes, reservoirs, and slow-moving rivers, making TN a mandatory parameter in most environmental monitoring programs and discharge regulations.
The UV spectrophotometric method with alkaline potassium persulfate digestion is the dominant technique for TN determination because it combines broad applicability, high sensitivity, and inexpensive instrumentation. In this method, all nitrogen species are oxidized to nitrate under heated alkaline persulfate conditions, and the resulting nitrate is quantified by ultraviolet absorbance measurement. Accredited third-party laboratories, environmental monitoring stations, and wastewater treatment plants routinely apply this technique for compliance testing, process control, and research.
- Method detection limit of approximately 0.05 mg/L for a standard 10 mL sample aliquot
- Working range typically extending to several mg/L, with dilution available for higher concentrations
- Dual-wavelength measurement (220 nm and 275 nm) automatically corrects for dissolved organic matter interference
- Applicable to drinking water, surface water, groundwater, and most domestic and industrial wastewaters
Method Principle
Alkaline Potassium Persulfate Digestion
A measured aliquot of sample is mixed with an alkaline potassium persulfate solution in a sealed borosilicate colorimetric tube and heated in a pressure steam sterilizer at 120–124 °C for 30 minutes. Under these conditions, persulfate decomposes into highly reactive sulfate radicals that oxidize organic nitrogen, ammonia, and nitrite quantitatively to nitrate. The alkaline environment (provided by sodium hydroxide) prevents loss of nitrogen during digestion and suppresses interconversion to volatile species.
Dual-Wavelength UV Measurement
After digestion and cooling, the solution is acidified with hydrochloric acid and diluted to volume. Nitrate ions exhibit strong absorbance at 220 nm, but dissolved organic matter also absorbs at this wavelength. Because organic matter additionally absorbs at 275 nm while nitrate does not, the corrected absorbance is calculated as:
A = A220 − 2 × A275
This dual-wavelength correction effectively eliminates the interference of most coexisting organic compounds and yields the net nitrate signal attributable to the digested nitrogen.
Calibration and Quantification
A series of potassium nitrate standards is prepared and subjected to the identical digestion and measurement procedure. The corrected absorbances are plotted against the corresponding nitrogen masses to construct a calibration curve. The total nitrogen concentration of the sample is calculated by dividing the nitrogen mass read from the curve by the original sample volume, and is reported in mg/L as N.
Testing Samples
Surface Water
Rivers, lakes, reservoirs, and estuarine waters are the most common matrices for TN testing. National surface water quality standards such as GB 3838-2002 assign TN limits ranging from 0.2 mg/L (Class I) to 2.0 mg/L (Class V), which fall comfortably within the working range of the method.
Drinking Water and Source Water
Raw water intakes, treatment plant influent and effluent, and distribution system samples can be analyzed to track nitrogen levels, although finished drinking water is usually regulated through nitrate rather than TN.
Municipal and Industrial Wastewater
Domestic sewage, food processing effluent, fertilizer plant discharge, and livestock wastewater frequently contain TN concentrations of 10–100 mg/L. These samples are typically diluted with ammonia-free water before digestion so that the final absorbance falls within the calibrated range. Discharge standards such as GB 18918-2002 set TN limits of 15 mg/L (Class 1A) and 20 mg/L (Class 1B) for municipal treatment plants.
Groundwater and Special Matrices
Groundwater, aquaculture water, cooling water, and landfill leachate can also be tested. High-salinity samples such as seawater require special attention because elevated chloride can consume persulfate and bias results low; dilution or an alternative digestion technique is recommended for such matrices.
Sample Collection and Preservation
Samples are collected in clean glass or polyethylene containers. Because TN includes particulate nitrogen, samples should be homogenized by shaking before subsampling; if dissolved nitrogen is required, filtration through a 0.45 µm membrane should be performed before preservation. Samples are typically stored at approximately 4 °C and analyzed as soon as practicable, generally within 7 days. Residual chlorine, if present, should be quenched with a small amount of sodium thiosulfate at the time of collection.
Testing Items
The primary testing item is the total nitrogen concentration of the water sample, expressed in mg/L as N. The result represents the aggregate of all oxidizable nitrogen species, from which related parameters can be derived or corroborated when individual species are measured separately:
- Total nitrogen (TN) — the sum of organic N, NH3-N, NO3-N, and NO2-N
- Total Kjeldahl nitrogen (TKN) — calculable as TN minus (NO3-N + NO2-N) when combined with ion-specific measurements
- Dissolved total nitrogen — determined on 0.45 µm filtered subsamples
- Particulate nitrogen — estimated as the difference between unfiltered and filtered TN results
- Method-defined quality control items — blank values, calibration verification, duplicate precision, and matrix spike recovery
Testing Methods
Applicable Standards
- GB 11894-89: Water quality — Determination of total nitrogen — Alkaline potassium persulfate digestion-UV spectrophotometric method
- HJ 636-2012: The current Chinese environmental protection industry standard for the same technique, which supersedes GB 11894-89 for routine monitoring
- ISO 11905-1:1997: Water quality — Determination of nitrogen — Part 1: Method using oxidative digestion with peroxodisulfate
Reagents and Equipment
- Alkaline potassium persulfate digestion solution (typically 40 g/L K2S2O8 with 15 g/L NaOH, freshly prepared)
- Potassium nitrate stock standard solution (100 mg/L as N) and working standard (10 mg/L as N)
- Hydrochloric acid solution (1+9) for post-digestion acidification
- Ammonia-free deionized water for reagent preparation and dilution
- UV-Visible spectrophotometer equipped with 10 mm quartz cuvettes
- Pressure steam sterilizer (autoclave) capable of stable operation at 120–124 °C
- 25 mL ground-glass-stoppered colorimetric tubes, analytical balance, pipettes, and volumetric ware
Step-by-Step Procedure
- Pipette 10.00 mL of the well-mixed sample (or diluted sample) into a stoppered colorimetric tube; prepare reagent blank and calibration standards in parallel
- Add 5.0 mL of alkaline potassium persulfate solution to each tube
- Seal the tubes tightly, secure the stoppers, and place them in the autoclave
- Digest at 120–124 °C for 30 minutes, starting the timer once the target temperature is reached, then allow natural cooling to room temperature
- Add 1.0 mL of hydrochloric acid solution (1+9) to each tube and dilute to the 25 mL mark with ammonia-free water; mix thoroughly
- Measure the absorbance of each solution at 220 nm and 275 nm against ammonia-free water using a 10 mm quartz cuvette
- Calculate the corrected absorbance (A220 − 2A275) and read the nitrogen mass from the calibration curve
- Report the TN concentration in mg/L as N, accounting for any dilution factor
Quality Control Requirements
- Reagent blank must be low and stable; an elevated blank indicates contaminated reagents or water and invalidates the batch
- Calibration curve should exhibit a correlation coefficient of at least 0.999 and be verified with a mid-range check standard (typically within ±10%)
- Duplicate analyses should agree within predefined relative deviation limits, commonly 10% or less at typical environmental concentrations
- Matrix spike recoveries are generally expected to fall between 90% and 110% to confirm digestion completeness
- Certified reference materials or inter-laboratory comparison samples are analyzed periodically to verify ongoing method accuracy
Interferences and Mitigation
- High chloride concentrations (seawater, brackish water, certain industrial effluents) can consume persulfate and cause low-biased results; diluting the sample or using an alternative digestion method is recommended
- Strongly UV-absorbing organic compounds may overload the 275 nm correction; dilution usually resolves this
- High carbonate alkalinity can neutralize the alkaline reagent and impair oxidation; reducing sample volume or increasing reagent strength compensates
- Residual chlorine oxidizes nitrogen species inconsistently and must be neutralized at collection
- Strict temperature control is essential — below 120 °C the oxidation of organic nitrogen is incomplete, while excessive heating wastes persulfate
Method Performance
Using a 10 mL sample aliquot, the method achieves a detection limit of approximately 0.05 mg/L and a practical determination lower limit of about 0.20 mg/L. Samples above the calibrated range are diluted and reanalyzed. Typical within-laboratory precision for natural waters is better than 10% relative standard deviation, and recoveries from fortified samples routinely fall within 90–110%.
Applications
Environmental Compliance Monitoring
Regulatory agencies and accredited third-party laboratories use the method to verify compliance of surface water bodies with quality classifications under GB 3838-2002 and to confirm that municipal and industrial discharges meet TN limits under GB 8978-1996 and GB 18918-2002.
Eutrophication Assessment
Together with total phosphorus and chlorophyll-a, TN is a core parameter in trophic status evaluation of lakes and reservoirs. Long-term TN datasets support nutrient loading calculations and watershed management decisions.
Wastewater Treatment Process Control
Plant operators track TN across aeration basins, anoxic zones, and final effluent to monitor nitrification and denitrification performance, optimize carbon dosing and rates, and ensure stable compliance with discharge permits.
Scientific Research and Engineering Studies
The method supports academic and engineering investigations including nutrient budget modeling, constructed wetland performance evaluation, aquaculture water quality management, and ecological restoration projects.
FAQ
Why is absorbance measured at both 220 nm and 275 nm?
Nitrate absorbs strongly at 220 nm, but many dissolved organic compounds also absorb there. These organics additionally absorb at 275 nm, where nitrate absorption is negligible. Applying the correction formula A = A220 − 2 × A275 removes the organic contribution and yields the true nitrate signal.
Why is alkaline digestion necessary?
The alkaline environment ensures that ammonia and organic nitrogen are fully oxidized to nitrate rather than being lost to the gas phase or converted to unmeasured intermediates. If the sample is not sufficiently alkaline throughout the digestion, results will be biased low.
What is the difference between total nitrogen and TKN?
Total nitrogen includes organic nitrogen, ammonia, nitrate, and nitrite. TKN (Total Kjeldahl Nitrogen) covers only organic nitrogen plus ammonia. Consequently, TN equals TKN plus nitrate nitrogen plus nitrite nitrogen for the same sample.
What is the detection limit and working range of the method?
With a standard 10 mL sample, the method detection limit is approximately 0.05 mg/L and the practical lower determination limit is about 0.20 mg/L. Concentrations up to several mg/L can be measured directly, and higher levels are handled by dilution before digestion.
Can the method be used for seawater?
High chloride content interferes with persulfate digestion by consuming the oxidant, which typically causes negative bias. Dilution can reduce the effect for moderately saline samples, but for full-strength seawater an alternative digestion or instrumental total nitrogen technique is preferable.
How should samples be stored before analysis?
Samples should be collected in clean glass or polyethylene bottles, kept refrigerated at approximately 4 °C, protected from contamination, and analyzed as soon as possible — generally within 7 days. Residual chlorine must be neutralized at the time of sampling.
Why are quartz cuvettes required?
Ordinary borosilicate glass strongly absorbs ultraviolet light below approximately 320 nm. Only quartz cuvettes provide sufficient transmittance at the 220 nm and 275 nm measurement wavelengths, so their use is mandatory for accurate results.
How do analysts verify that digestion is complete?
Laboratories confirm digestion efficiency through matrix spike recovery (accepted range typically 90–110%), analysis of certified reference materials, and consistent performance of low-level check standards. Persistent low recoveries usually indicate insufficient digestion temperature, exhausted persulfate reagent, or chloride interference.