Water by Persulfate Digestion-Ultraviolet Spectrophotometry

The persulfate digestion-ultraviolet spectrophotometric method is a standard technique for determining nitrogen content in water quality analysis. Although it is primarily designed for total nitrogen (TN) measurement, it effectively quantifies ammonia nitrogen by converting it into nitrate through oxidative digestion, followed by UV spectrophotometric detection. This paper outlines the principles, procedures, and quality control measures of this method.

1. Principle of the Method

The method is based on the strong oxidizing property of potassium persulfate (K₂S₂O₈) under alkaline conditions. In an alkaline medium at 120–124°C, potassium persulfate decomposes to generate atomic oxygen, which oxidizes various nitrogen species—including ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, and most organic nitrogen compounds—into nitrate. 

During this process, most organic matter in the sample is also oxidized and decomposed, thereby reducing potential interference from organic substances. After digestion, all forms of nitrogen in the sample exist exclusively as nitrate.

2. Digestion Procedure

A measured volume of the water sample (diluted if necessary for high nitrogen content) is placed in a stoppered colorimetric tube. Alkaline potassium persulfate solution is added, the tube is securely capped, and it is heated in an autoclave at 120–124°C for 30 minutes. After digestion, the sample is allowed to cool naturally to room temperature.

3. UV Spectrophotometric Determination

Following digestion, the nitrate ions in the sample are quantified by UV spectrophotometry. Nitrate exhibits a characteristic absorption peak at 220 nm, where absorbance is proportional to nitrate concentration. The absorbance of the digested sample is measured at both 220 nm and 275 nm.

4. Interference Correction

A critical challenge is that residual dissolved organic matter in the sample also absorbs at 220 nm, causing positive interference. To correct for this, a dual-wavelength correction technique is employed: absorbance at 275 nm is measured as a background reference, since nitrate shows negligible absorption at this wavelength while organic matter still absorbs. The corrected absorbance (A) is calculated as:A = A₂₂₀ − 2A₂₇₅

This empirical correction effectively eliminates interference from dissolved organic substances.

5. Quantification and Quality Control

The corrected absorbance is then substituted into a calibration curve prepared from standard nitrate solutions to determine the total nitrogen concentration. Since all nitrogen species have been converted to nitrate, the total nitrogen value reflects the sum of ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, and organic nitrogen in the original sample.

Several quality control measures are essential. The potassium persulfate reagent must have a total nitrogen content below 0.0005%, as even trace residues can cause significant interference due to its strong absorption at 220 nm. Ultrapure ammonia-free water should be used throughout, and glassware must be acid-washed and rinsed with deionized water. For turbid samples containing suspended solids, filtration through a 0.45 μm membrane filter is recommended after digestion.