1.1 Substance determined
This part of ISO 7890 specifies a method for the determination of nitrate ion in water.
1.2 Type of sample
The method is suitable for application to raw and potable water samples.
Up to a nitrate concentration ϱN = 0,2 mg/l using the maximum test portion volume of 25 ml. The range can be extended upwards by taking smaller test portions.
1.4 Limit of detection
Using cells of optical path length 40 mm and a 25 ml test portion volume the limit of detection lies within the range ϱN = 0,003 mg/l to 0,013 mg/l.
A nitrate nitrogen concentration ϱN = 0,2 mg/l gives an absorbance of about 0,68 unit, using a 25 ml test portion and cells of optical path length 40 mm.
A range of substances often encountered in water samples has been tested for possible interference with this method. Full details are given in annex A. The main potential interferents are chloride, orthophosphate, magnesium and manganese(II), as shown in annex A.
Other tests have shown that this method will tolerate a sample colour of up to 150 mg/l Pt providing the test portion absorption correction procedure is followed. (See 6.5.)