Nitrate Electrode Method

Nitrate Electrode Method 

1. Principle: The NO₃^- ion electrode is a selective sensor that develops a potential across a thin, porous, inert membrane that holds in place a water-immiscible liquid ion exchanger. The electrode responds to NO₃^- ion activity between about 10-5 and 10-1 M (0.14 to 1400 mg NO₃^- N/L). The lower limit of detection is determined by the small but finite solubility of the liquid ion exchanger.
2. Interferences: Chloride and bicarbonate ions interfere when their weight ratios to NO₃- -N are >10 or >5, respectively. Ions that are potential interferences but do not normally occur at significant levels in potable waters are NO₂-, CN-, S²-, Br-, I-, CIO₃-, and CIO₄-. Although the electrodes function satisfactorily in buffers over the range pH 3 to 9, erratic responses have been noted where pH is not held constant. Because the electrode responds to NO₃- activity rather than concentration, ionic strength must be constant in all samples and standards. Minimize these problems by using a buffer solution containing Ag₂SO₄ to remove Cl-, Br-, I-, S²-, and CN-, sulfuric acid to remove NO₂-, a buffer at pH 3 to eliminate HCO₃- and to maintain a constant pH and ionic strength, and Al₂ (SO₄) ₃ to complex organic acids.

2. Apparatus

1. pH meter, expanded-scale or digital, capable of 0.1 mV resolution.
2. Nitrate combination ion selective electrode: Carefully follow manufacturer’s instructions regarding care and storage.
3. Magnetic Stirrer: TFE-coated stirring bar.

3. Reagents

1. Nitrate-free water: Use re-distilled, distilled, or deionized water of highest purity to prepare all solutions and dilutions.
2. Stock nitrate solution: Dry potassium nitrate (KNO₃) in an oven at 105°C for 24 hours. Dissolve 0.72118g in water and dilute to 1000 ml; 1.00 ml= 100mg NO₃- -N. Preserve with 2 ml CHCl₃/L. This solution is stable for at least 6 months.
3. Standard nitrate solutions: Dilute 1.0, 10, and 50 ml stock nitrate solution to obtain standard solutions of 1.0, 10, and 50 mg NO₃- -N/L, respectively.
4. Buffer solution: Dissolve 17.32g Al₂ (SO₄)₃-18H₂O, 3.43g Ag₂SO₄, 1.28g H₂BO₃, and 2.52g sulfuric acid (H₂NSO₃H) in about 800 ml water. Adjust to pH 3.0 by slowly adding 0.10N NaOH. Dilute to 1000 ml and store in a dark glass bottle.
5. Sodium Hydroxide, NaOH, 0.1N.

4. Procedure

1. Preparation of calibration curve: Transfer 10 ml of 1 mg NO₃- -N/L standard to a 50-ml beaker, add 10 ml buffer, and stir with a magnetic stirrer. Immerse tips of electrodes and record mV reading when stable (after about 1 min.). Remove electrodes, rinse, and blot dry. Repeat for 10-mg NO₃- -N/L standards. Plot potential measurements against NO₃- -N concentration on semi-logarithmic graph paper, with NO₃- -N concentration on the logarithmic axis (abscissa) and potential (in mV) on the linear axis (ordinate). A straight line with a slope of + 57 ±3 mV/decade at 25°C should result. Recalibrate electrodes several times daily by checking potential reading of the 10 mg NO₃- -N standard and adjusting the calibration until the reading plotted on the calibration curve is displayed again.
2. Measurement of sample: Transfer 10 ml sample to a 50-ml beaker, add 10 ml buffer solution, and stir with a magnetic stirrer. Measure standards and samples at about the same temperature. Immerse electrode tips in sample and record potential reading when stable (after about 1 min.). Read concentration from calibration curve.

5. Precision

Over the range of the method, precision of ±0.4 mV, corresponding to 2.5% in concentration, is expected.