The Selectivity Coefficient (SC) represents the approximate apparent increase in the measured concentration caused by 1 unit of the interferent. Effect (% error) = (interferent concentration × SC / analyte concentration) × 100.
| Interfering Ion | Selectivity Coefficient | Notes |
|---|---|---|
| Chloride (Cl⁻) | 0.006 | Can be removed using special ISAB: 10.5g K₂SO₄ + 3.11g Ag₂SO₄ + 25ml 0.1M H₂SO₄ per litre, added 1:1 |
| Bicarbonate (HCO₃⁻) | 0.005 | Removed by the Ag₂SO₄ ISAB |
| Nitrite (NO₂⁻) | 0.001 | — |
| Acetate (CH₃COO⁻) | 0.0005 | — |
| Fluoride (F⁻) | 0.0001 | — |
| Sulphate (SO₄²⁻) | 0.00001 | — |
Apparatus Required
- Ion-Selective Electrode: ELIT 8021
- Reference electrode: Double junction (ELIT 003)
- Dual electrode head (ELIT 201)
- ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter
- 150 ml polypropylene beakers, 100 ml volumetric flask, 1, 2, 5, 10 ml pipettes
Calibration
Calibrate with 1000, 100, 10, 1 ppm NO₃ solutions. For high ionic strength samples (> 0.01 M), add 2 ml ISAB to each 100 ml standard. Cl and HCO₃ up to 10× the nitrate concentration cause less than 10% error.
Sample Preparation & Measurement
Low ionic strength: immerse electrodes directly in 50–100 ml sample. High ionic strength: add 2 ml ISAB to 100 ml sample. Best results are obtained in still (unstirred) solutions.
Results
Results are displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples, figures will not need adjusting as all are affected by the same dilution factor. Allow 2–3 minutes stabilisation after electrode immersion. Wash and dry electrodes between samples to avoid cross-contamination.
Nitrate in Soils
Weigh ~50g air-dried soil into 100 ml deionised water containing biological inhibitor (0.1g phenylmercuric acetate in 20 ml dioxane + 100 ml water, at 1 ml/L). Stand for at least 1 hour with occasional stirring. Let residue settle, take 50 ml of solution and add 1 ml of 2M (NH₄)₂SO₄. Concentration result: multiply ppm × 100 ÷ sample weight = µg/g in soil.
Nitrate in Plant Material
Shake 1g dried ground material with 250 ml distilled water (+ biological inhibitor) for 10 minutes. Remove interference using Dowex 50-X8 resin doped with Al₂(SO₄)₃ (bicarbonate/citrate interference) or AgNO₃ (chloride interference). Filter, take 100 ml and add 2 ml ISAB. Result: ppm × extraction volume ÷ sample weight = µg/g.
Samples with High Chloride Content
Use special ISAB (Ag₂SO₄/K₂SO₄): dissolve 10.5g K₂SO₄ and 3.11g Ag₂SO₄ in 800 ml deionised water, add 25 ml 0.1M H₂SO₄, make to 1 litre. Add to all standards and samples in equal volumes (1:1). This also minimises bicarbonate, cyanide, bromide, sulphide and nitrite interference.
Biological Contamination
Aqueous solutions are prone to biological degradation of nitrate. For samples that must be stored or transported, add a few drops of biological inhibitor immediately after collection (1g phenylmercuric acetate in 20 ml dioxane, made up to 100 ml with deionised water, stored in a glass bottle).
- Measure in still (unstirred) solutions for best results.
- Chloride and bicarbonate can be tolerated up to 10× the nitrate concentration before significant error.