Total nitrogen (TN) determination is one of the most important analytical experiments in water quality assessment, providing a comprehensive measure of organic and inorganic nitrogen compounds that drive eutrophication in aquatic ecosystems. This technical article explains the principles, standard methods, sample handling requirements, and step-by-step experimental procedures for accurately measuring total nitrogen in various water matrices, referencing internationally recognized standards such as HJ 636-2012, ISO 11905-1, and ASTM D3590.
Technical Overview
Total nitrogen (TN) refers to the sum of all nitrogen-containing compounds in a water sample, including organic nitrogen, ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and nitrite nitrogen (NO2-N). Excessive nitrogen discharged into lakes, rivers, and coastal waters promotes algal blooms and eutrophication, making TN a regulated parameter in environmental protection legislation worldwide. The determination experiment typically involves converting all nitrogen species into a single measurable form—usually nitrate—followed by spectrophotometric quantification.
The most widely adopted technique is the alkaline potassium persulfate digestion ultraviolet spectrophotometric method, specified in the Chinese national and industry standards HJ 636-2012 and GB 11894-89, as well as the international standard ISO 11905-1. Under controlled heating at 120–124°C in an autoclave or dedicated digestion instrument, alkaline potassium persulfate oxidizes ammonia, nitrite, and organic nitrogen compounds quantitatively to nitrate. The resulting nitrate is then measured by UV absorbance at 220 nm with interference correction at 275 nm, where aromatic organic compounds and surfactants exhibit absorption.
Alternative approaches include continuous flow analysis, segmented flow injection analysis, persulfate digestion with cadmium reduction, and instrumental combustion-chemiluminescence detection using total nitrogen analyzers. Method selection depends on the expected concentration range, sample matrix complexity, required detection limit, and laboratory throughput.
Testing Samples
Surface Water
Rivers, lakes, reservoirs, and estuaries are routinely monitored for TN to evaluate trophic status and compliance with environmental quality standards. TN concentrations in surface water typically range from below 0.1 mg/L in pristine systems to several mg/L in polluted receiving waters.
Groundwater
Groundwater samples are analyzed to detect contamination from agricultural leaching, septic systems, and landfill infiltration. Careful sampling through dedicated monitoring wells is required to avoid oxidation state changes during collection.
Domestic and Municipal Sewage
Influent and effluent samples from wastewater treatment plants are tested to verify treatment performance and discharge compliance. These samples contain high levels of organic matter and suspended solids and often require homogenization and dilution before digestion.
Industrial Wastewater
Effluents from industries such as chemical manufacturing, food processing, fertilizer production, and pharmaceuticals are monitored for nitrogen loading. Complex matrices may demand pretreatment to eliminate interfering chromophores and turbidity.
Drinking Water and Recycled Water
Treated potable water and reclaimed water are analyzed to confirm that nitrogen concentrations remain within acceptable limits and to monitor biological stability in distribution systems.
Testing Items
The core analytical deliverable of the experiment is the total nitrogen concentration expressed in mg/L, calculated from the difference between corrected absorbance values at 220 nm and 275 nm. Related quantitative and qualitative outputs include:
- Total nitrogen concentration of the sample and any diluted aliquots
- Calibration curve parameters, including slope, intercept, and correlation coefficient (typically required to be ≥ 0.999)
- Blank correction values from reagent blanks and digestion blanks
- Method detection limit and quantification limit verification
- Quality control results, including duplicate precision, matrix spike recovery, and certified reference material checks
- Speciation support data, such as ammonia nitrogen, nitrate, and nitrite measured separately, allowing calculation of organic nitrogen by difference
Testing Methods
Alkaline Potassium Persulfate Digestion UV Spectrophotometric Method
This is the reference method described in HJ 636-2012 and ISO 11905-1. A measured water aliquot is mixed with an alkaline potassium persulfate reagent in a quartz digestion vessel, sealed, and heated at 120–124°C for 30 to 60 minutes. After cooling, the digest is acidified with hydrochloric acid, and absorbance is measured at 220 nm and 275 nm using a 10 mm or 40 mm quartz cuvette against a blank. The corrected absorbance A = A220 − 2 × A275 is compared against a potassium nitrate calibration curve spanning approximately 0 to 10 mg/L. When a 10 mm cuvette is used, the method detection limit is approximately 0.05 mg/L.
Continuous Flow and Segmented Flow Analysis
Automated flow-based systems combine online persulfate digestion with UV detection or cadmium-column reduction followed by colorimetric detection of nitrite. These methods, covered by standards such as HJ 667-2013, offer high throughput for laboratories processing large numbers of environmental samples, with typical analytical rates of 20 to 40 samples per hour.
Combustion and Chemiluminescence Method
A small sample volume is injected into a high-temperature catalytic combustion tube where nitrogen compounds are converted to nitrogen oxide (NO), which reacts with ozone to generate chemiluminescence proportional to the total nitrogen content. This instrumental approach requires minimal sample pretreatment and delivers excellent precision across a wide concentration range.
Kjeldahl-Based Determination
For comparison and speciation studies, total Kjeldahl nitrogen (TKN) can be determined following ASTM D3590 or equivalent procedures, and organic nitrogen can be estimated by subtracting ammonia nitrogen from TKN. Total nitrogen equals TKN plus nitrate plus nitrite.
Key Experimental Precautions
- Collect samples in glass or polyethylene bottles and analyze promptly, or acidify to pH below 2 with sulfuric acid and store at 2–5°C for up to 7 days
- Use ammonia-free water throughout reagent preparation, glassware cleaning, and dilution
- Employ quartz cuvettes and digestion vessels, as ordinary glass absorbs in the ultraviolet region
- Verify that the persulfate reagent is freshly prepared and stored away from light to prevent decomposition
- Confirm complete sealing of digestion vessels to prevent loss of pressure and incomplete oxidation
- Correct for turbidity and dissolved organic matter interference using the 275 nm measurement and appropriate blank subtraction
- Run at least 10% duplicates and matrix spikes to demonstrate analytical control
Applications
Total nitrogen determination experiments support a broad spectrum of environmental, industrial, and scientific objectives:
- Regulatory compliance monitoring of wastewater discharge permits and receiving water quality objectives
- Eutrophication risk assessment and nutrient load allocation for lakes, reservoirs, and coastal zones
- Performance evaluation and process optimization of biological nutrient removal systems in wastewater treatment
- Verification of drinking water treatment efficiency and distribution system stability
- Agricultural non-point source pollution studies and fertilizer management research
- Ecological and limnological research on nitrogen cycling in natural waters
- Data generation for environmental impact assessments and pollution remediation projects
- Quality control in accredited third-party laboratories reporting to national monitoring networks
FAQ
What is the difference between total nitrogen and total Kjeldahl nitrogen?
Total nitrogen includes all nitrogen forms—organic nitrogen, ammonia, nitrate, and nitrite—whereas total Kjeldahl nitrogen (TKN) measures only organic nitrogen and ammonia. Total nitrogen can therefore be calculated as TKN plus nitrate nitrogen plus nitrite nitrogen when all three measurements are available.
Why are absorbance readings taken at both 220 nm and 275 nm?
Nitrate absorbs strongly at 220 nm, but dissolved organic matter and certain surfactants also absorb at this wavelength. Because these interfering compounds absorb proportionally at 275 nm while nitrate does not, a corrected absorbance is calculated as A220 minus two times A275, effectively removing the organic interference.
What is the detection limit of the persulfate digestion UV method?
According to HJ 636-2012, the method detection limit is approximately 0.05 mg/L when a 10 mm quartz cuvette is used, and the lower limit of determination is about 0.20 mg/L. Using a longer 40 mm cuvette improves sensitivity for clean water samples.
How should samples be preserved if immediate analysis is not possible?
Samples should be acidified with concentrated sulfuric acid to pH below 2 and stored at 2–5°C for no more than 7 days. Before digestion, the acidity is neutralized with sodium hydroxide solution. Freezing is also acceptable for longer storage in research contexts.
What are the main sources of interference and error?
Common interferences include high concentrations of dissolved organic matter, turbidity, residual chlorine, bromide and iodide in saline waters, and carbonate alkalinity above certain thresholds. Incomplete digestion due to low temperature, expired persulfate reagent, or leaking vessels leads to systematic underestimation. Contamination from laboratory air, ammonia-containing detergents, and inadequately rinsed glassware causes elevated blanks and false positives.
Can seawater samples be analyzed by the standard method?
High chloride and bromide content in seawater can interfere with persulfate oxidation and UV measurement. Dilution of the sample, use of matrix-matched calibration standards, or selection of an instrumental combustion-chemiluminescence method is recommended for estuarine and marine samples.