Total nitrogen (TN) determination is a core water quality experiment that quantifies the combined concentration of nitrate, nitrite, ammonia, and organic nitrogen in environmental and treated water samples. This technical article explains the alkaline potassium persulfate digestion–UV spectrophotometric method in accordance with standards such as GB 11894-89, HJ 636-2012, and ISO 11905-1, covering sample handling, reagents, procedure, quality control, and typical applications in environmental monitoring and wastewater compliance.
Technical Overview
Total nitrogen (TN) is defined as the sum of all nitrogen-containing species in a water sample, including nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), ammonia nitrogen (NH3-N), and organic nitrogen. Because nitrogen is a primary driver of eutrophication and a regulated pollutant in most discharge permits, accurate TN measurement is essential for environmental monitoring, wastewater treatment optimization, and regulatory reporting.
The most widely applied determination technique is the alkaline potassium persulfate digestion–UV spectrophotometric method. Under alkaline conditions at 120–124 °C in a pressure steam sterilizer, potassium persulfate oxidizes ammonia nitrogen, nitrite nitrogen, and organic nitrogen to nitrate. The nitrate is then quantified by ultraviolet spectrophotometry at 220 nm, with a correction reading at 275 nm to compensate for absorption by dissolved organic matter and suspended solids. The corrected absorbance is calculated as A = A220 − 2 × A275, and the concentration is read from a potassium nitrate calibration curve.
The essential laboratory equipment for this experiment includes:
- UV-visible spectrophotometer capable of measurement at 220 nm and 275 nm, with 10 mm quartz cuvettes
- Pressure steam sterilizer (autoclave) with temperature control at 120–124 °C and an accurate pressure gauge
- 25 mL ground-stopper colorimetric (colorimetric) digestion tubes made of borosilicate glass
- Analytical balance, fixed-volume pipettes, and ammonia-free laboratory water system
Testing Samples
The persulfate digestion–UV method is applicable to a wide range of aqueous matrices. Sample types routinely accepted by accredited third-party laboratories include the following:
Surface Water
River, lake, and reservoir water typically contains TN in the range of 0.1–5 mg/L. Low-level measurement requires strict blank control, freshly prepared persulfate reagent, and a well-established calibration curve at the low concentration end. Results are commonly evaluated against the lake and reservoir TN criteria in GB 3838-2002 (for example, 0.5 mg/L for Class II and 1.0 mg/L for Class III).
Groundwater
Groundwater samples are usually low in nitrogen species but may show elevated nitrate due to agricultural runoff or leaching. Because the target concentrations are often near the detection limit, care must be taken to avoid cross-contamination and to verify the method detection limit periodically.
Drinking Water and Source Water
Source water and treated drinking water are tested to track nutrient loading and verify that nitrogen levels remain consistent with subsequent treatment stages and applicable hygiene requirements.
Municipal and Domestic Wastewater
Influent and effluent from wastewater treatment plants frequently contain 10–60 mg/L TN. These samples usually require dilution to fall within the calibration range. Effluent results are compared against the discharge limits of GB 18918-2002, for example 15 mg/L for Class 1A and 20 mg/L for Class 1B.
Industrial Wastewater
Effluents from fertilizer, chemical, food processing, fermentation, and pharmaceutical industries often contain high organic nitrogen and complex matrices. Matrix spike recovery tests are recommended to verify digestion efficiency and analytical accuracy.
Seawater, Estuarine, and Brackish Water
High chloride content can consume the oxidant and suppress persulfate digestion efficiency. For saline samples, dilution, extended oxidation, or an alternative combustion-based technique such as ASTM D5176 or EN 12260 may be more appropriate.
Sampling and Preservation
Samples are collected in clean glass or polyethylene containers. For a holding time of more than 24 hours, samples should be acidified with sulfuric acid to pH ≤ 2 and stored at approximately 4 °C, with analysis completed within 7 days. Samples containing residual chlorine should be dechlorinated with sodium thiosulfate at the time of collection.
Testing Items
The primary testing item is the total nitrogen concentration, reported in mg/L. Related nitrogen parameters are frequently tested together to build a complete nitrogen profile of the sample:
- Total nitrogen (TN) – the sum of all oxidizable nitrogen species
- Ammonia nitrogen (NH3-N) – tested by Nessler or salicylate spectrophotometry
- Nitrate nitrogen (NO3-N) and nitrite nitrogen (NO2-N) – tested by ion chromatography or spectrophotometry
- Total Kjeldahl nitrogen (TKN) – the sum of organic nitrogen and ammonia nitrogen
- Organic nitrogen – obtained by difference (TN minus nitrate, nitrite, and ammonia)
- Method performance metrics – method detection limit, quantification limit, spike recovery, and repeatability
Testing Methods
Reference Standards
The experiment is performed in accordance with recognized standards, including GB 11894-89 (water quality – determination of total nitrogen – alkaline potassium persulfate digestion–UV spectrophotometric method), HJ 636-2012 (the current environmental protection industry method of the same principle), ISO 11905-1 (oxidative digestion with peroxodisulfate), and, for alternative techniques, EN 12260 and ASTM D5176.
Reagents
- Alkaline potassium persulfate solution: dissolve potassium persulfate and sodium hydroxide in water (typically 40 g/L K2S2O8 in 15 g/L NaOH); store protected from light and prepare fresh if blank absorbance rises
- Potassium nitrate standard stock solution (100 mg/L as N): dissolve 0.7218 g of potassium nitrate, dried at 105–110 °C, and dilute to 1000 mL; prepare a 10 mg/L working solution by tenfold dilution
- Hydrochloric acid solution (1+9) for post-digestion acidification
- Ammonia-free distilled or deionized water for all solution preparation and dilutions
Digestion and Measurement Procedure
Prepare a calibration series by transferring measured volumes of the 10 mg/L nitrate working solution (for example 0 to 8.00 mL in increments) into 25 mL ground-stopper colorimetric tubes and diluting to 10.00 mL with water. Pipette 10.00 mL of the homogeneous sample (or a suitable dilution) into an identical tube. Add 5.00 mL of alkaline potassium persulfate solution to each tube, stopper tightly, and wrap the stoppers.
Place the tubes in the pressure steam sterilizer, bring the temperature to 120–124 °C, and maintain digestion for 30 minutes. Allow natural cooling to room temperature and release the pressure slowly before opening the vessel. Add 1.00 mL of hydrochloric acid solution (1+9) to each tube, dilute to the 25 mL mark, and mix thoroughly.
Measure the absorbance of standards, blanks, and samples at 220 nm and 275 nm using 10 mm quartz cuvettes against water or reagent blank. Apply the correction A = A220 − 2 × A275, construct a linear calibration curve (correlation coefficient typically required to be ≥ 0.999), and calculate the sample concentration, multiplying by any dilution factor.
Interferences and Remedies
- High carbonate and bicarbonate alkalinity can affect digestion efficiency; neutralization or dilution is recommended
- Chloride at seawater levels consumes oxidant; dilute the sample or select a combustion-chemiluminescence method
- Organic matter absorbing strongly at 220 nm is partially compensated by the 275 nm correction; heavily colored samples should be diluted or verified by spike recovery
- Turbidity and suspended solids must be addressed, since nitrate UV measurement assumes a clear solution after digestion
- Iodide and reducing agents interfere with persulfate oxidation and must be removed or evaluated case by case
Quality Control
Each analytical batch should include a method blank, a calibration verification standard, a certified reference material or laboratory control sample, a matrix spike, and duplicate samples. Typical acceptance criteria used by accredited third-party laboratories include spike recovery of 90–110 percent (80–120 percent for difficult matrices), relative percent difference of duplicates within 10 percent, and continuing calibration verification within ±10 percent of the true value. The method detection limit is verified periodically from replicate low-level analyses; for a 10 mL sample volume, a detection limit of approximately 0.05 mg/L is achievable, with a working range up to about 10 mg/L after dilution.
Alternative Techniques
Combustion-based total bound nitrogen (TNb) analyzers, conforming to EN 12260 or ASTM D5176, inject the sample into a high-temperature oxidative furnace and detect the resulting nitric oxide by chemiluminescence. These systems offer high throughput, low detection limits, and better tolerance to salinity. Automated continuous flow and discrete analyzers that perform online persulfate digestion followed by UV detection are also widely used for large monitoring programs, provided they are validated as equivalent to the reference method. Kjeldahl digestion (ISO 11905-2) measures only organic plus ammonia nitrogen (TKN) and does not replace TN determination where nitrate and nitrite contribute significantly.
Applications
Total nitrogen determination supports a broad range of regulatory, operational, and scientific purposes:
- Compliance monitoring of municipal and industrial wastewater discharge against permit limits and GB 18918-2002
- Surface water quality classification of rivers, lakes, and reservoirs under GB 3838-2002
- Process control of nitrification and denitrification stages in wastewater treatment plants
- Eutrophication and nutrient loading assessment for lakes, reservoirs, and coastal receiving waters
- Environmental impact assessments, pollution source apportionment, and total pollutant load programs
- Acceptance testing of reclaimed and reused water, and verification of denitrification performance
- Academic and industrial research on nutrient cycling and treatment technology development
FAQ
What is total nitrogen and why is it measured?
Total nitrogen is the sum of nitrate, nitrite, ammonia, and organic nitrogen in water. It is measured because excess nitrogen accelerates eutrophication, depletes dissolved oxygen, harms aquatic ecosystems, and is strictly limited in discharge regulations.
How does TN differ from TKN?
TKN (total Kjeldahl nitrogen) covers only organic nitrogen and ammonia nitrogen, while TN additionally includes nitrate and nitrite. In waters with significant nitrate content, TN is always higher than TKN.
Why are absorbance readings taken at both 220 nm and 275 nm?
Nitrate absorbs strongly at 220 nm, but dissolved organic matter also absorbs there. A reading at 275 nm estimates this background absorption, and applying the formula A = A220 − 2 × A275 corrects the nitrate signal for organic interference.
What are the detection limit and working range of the method?
With a 10 mL sample volume, the alkaline persulfate digestion–UV method typically achieves a detection limit near 0.05 mg/L. The working range commonly extends to several mg/L directly and higher after dilution.
How should samples be preserved before analysis?
Collect samples in glass or polyethylene containers. If analysis cannot be completed within 24 hours, acidify to pH ≤ 2 with sulfuric acid and refrigerate at about 4 °C; analyze within 7 days. Dechlorinate chlorinated samples with sodium thiosulfate at sampling.
What quality control data should a valid report include?
A defensible result set includes a method blank, calibration curve statistics (r ≥ 0.999), calibration verification, certified reference material or spike recovery results, and duplicate precision, demonstrating that the digestion and measurement system was in control during the batch.
Can high-salinity water be tested by this method?
Seawater and brackish water can suppress persulfate oxidation due to chloride interference. Dilution helps at moderate salinity; for high salinity, a combustion–chemiluminescence TNb method per EN 12260 or ASTM D5176 is generally preferred.