Method 55a Determination of nitrate by selective ion electrode

Reagents.

• Calcium sulphate solution, 0.01 M - dissolve 1.72 g CaSO4.2H2O in water and make up to 1 l.

• Nitrate stock standard solution, 0.2 mg ml-1 nitrate-N - dissolve 0.722 g of potassium nitrate in 0.01 M CaSO4 solution, dilute to 500 ml and store in a refrigerator.

• Nitrate working standard solution, 1-100 pg ml-1 nitrate-N - pipette 0.5, 2, 5, 10, 25 and 50 ml of the stock standard into 100-ml volumetric flasks and make up to the mark with 0.01 M CaSO4 solution and mix.

Procedure. Transfer a 20-ml level scoop of air-dry soil sieved to ^2 mm to a 125-ml wide-mouth HDPE screw-cap bottle, add 50 ml 0.01 M calcium sulphate solution, and shake for 15 min on a reciprocating shaker (MAFF/ADAS use 275 strokes per minute, Northeastern US use 200 oscillations per minute). Filter through a 125-mm Whatman No. 2 filter paper into a sample bottle, discarding the first few millilitres, which may contain a trace of nitrate from the paper. Filter a solution of extractant in the same manner to provide a blank.

Set up the meter and first measure the lowest standard, allowing sufficient time for a steady reading to be obtained. Record the indicated millivolts, and proceed to analyse the remaining standards and sample solutions. Ensure standards and sample solutions are at the same temperature, which should be noted for reference. The standard graph is prepared on semi-logarithmic paper by comparing mV readings on the linear axis to pg ml-1 nitrateN on the logarithmic axis. Values of the order of 140 mV for 1 pg ml-1, and 30 for 100 pg ml-1 nitrate-N may be expected.

Calculation. There were 20 ml soil in 50 ml of extractant, therefore sample concentration values from the standard graph should be multiplied by 2.5, the results being expressed as mg l-1 nitrate-N calcium sulphate extractable in air-dry soil. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

A separate bulk-density determination enables results to be expressed in terms of mg kg-1 soil.

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