Total Nitrogen Determination by Persulfate Digestion UV Method

The alkaline potassium persulfate digestion ultraviolet spectrophotometric method is the standard technique for determining total nitrogen in surface water, groundwater, industrial wastewater, and domestic sewage. 

The method is based on a straightforward principle: all nitrogen compounds in a sample are converted to nitrate through oxidative digestion, and the resulting nitrate is then quantified by ultraviolet spectrophotometry. This approach eliminates the need to measure different nitrogen species separately, offering a unified and efficient analytical pathway.

Principle of the Method

The method relies on the strong oxidising power of potassium persulfate. Under alkaline conditions at elevated temperature, persulfate decomposes to release active oxygen, which oxidises ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, and most organic nitrogen compounds to nitrate. 

Organic matter present in the sample is also oxidised during this process, removing potential interference from organic substances. After digestion, all nitrogen species exist in the sample as nitrate, and the determination of total nitrogen becomes a measurement of nitrate concentration.

Digestion Procedure

A measured volume of the well‑mixed water sample is placed in a stoppered digestion tube. An alkaline persulfate solution is added, the tube is sealed, and it is heated in an autoclave or pressure cooker at 120 to 124 degrees Celsius for 30 minutes. After digestion, the sample is cooled to room temperature. At this point, all nitrogen compounds have been converted to nitrate, and the solution is ready for spectrophotometric measurement.

Dual‑Wavelength Measurement and Interference Correction

Nitrate ions exhibit a characteristic absorption peak at a wavelength of 220 nanometres. The absorbance at this wavelength is directly proportional to nitrate concentration. However, dissolved organic matter remaining in the sample also absorbs at 220 nanometres, causing a positive interference that would overestimate the nitrate concentration. 

To correct for this, a second measurement is made at 275 nanometres, where nitrate does not absorb but organic matter still shows some absorbance. The net absorbance attributable to nitrate is obtained by subtracting twice the 275‑nanometre absorbance from the 220‑nanometre absorbance, according to the formula A = A220 − 2A275. This empirical correction effectively eliminates interference from most dissolved organic compounds.

Reagent Quality and Blank Control

The quality of the potassium persulfate reagent is critical to the accuracy of the method. Persulfate itself absorbs strongly at 220 nanometres, and if any residual persulfate remains after digestion, it will cause a positive bias. The standard method requires that the total nitrogen content of the persulfate be below 0.0005 percent. Every new batch of reagent must be tested through a blank experiment before use. In addition, the water used for reagent preparation and sample dilution must be ammonia‑free, and the laboratory environment should be protected from ammonia contamination. All glassware must be cleaned with dilute acid and rinsed with deionised water.