Method for determining the concentration of NITRATE (NO3^-)
in Aqueous Solutions

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Click on the links below for extra details for various substrates:

SOIL | PLANTS | Samples With High Cl Content | Samples with biological contamination |

Ion-Selective Electrode for nitrate ion (ELIT 8021 PVC membrane)

Reference electrode: double junction lithium acetate (ELIT 003n)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm NO₃ as NaNO₃

Buffer solution (ISAB): 2 Molar (NH₄)₂SO₄.

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10, 25ml pipettes.

Before use, the electrodes must be calibrated by measuring a series of known standard solutions, made by serial dilution of the 1000ppm standard solution. For a full calibration, prepare 100ml of solutions containing 1000, 100, 10, 1ppm NO₃. If the approximate range of concentrations of the samples is known, and this is within the linear range of the ISE, then it is only necessary to make two solutions (preferably one decade apart)which span this range.

NB: If the samples to be measured are expected to have a total ionic strength of greater than 0.01 Molar, then add 2ml of buffer solution to each standard and mix thoroughly to compensate for different activity coefficients between samples and standards.

Follow the instructions in the General Operating Instructions to measure these standard solutions and prepare a calibration graph.

For low ionic strength samples, no sample preparation is necessary. Simply take approximately 50 to 100mls of sample in a plastic beaker, or even immerse the electrodes directly in a lake or river (but take care to avoid losing the electrodes!). For high ionic strength samples, take 100mls of sample and add 2mls of buffer solution and stir well before measurement.

Follow the instructions in the electrode operating instructions to measure a series of samples and record the results. Briefly, it is important to note that, the electrodes must be washed and dried between each sample, to avoid cross contamination, and sufficient time must be allowed (2 or 3 minutes), before taking a reading after immersion, to permit the electrode signal to reach a stable value. For the highest precision, frequent recalibration is recommended (see operating instructions).

The concentration in the measured solution will be displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor.

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Extra details for determining NITRATE (NO₃^-) in SOILS

Sample Preparation:

Weigh accurately about 50g of air-dried soil and disperse in 100 ml of de-ionised water containing 1 ml per litre of biological inhibitor (0.1g phenylmercuric acetate dissolved in 20ml of dioxane and diluted to 100ml with water) and allow to stand for at least one hour, stirring occasionally. Allow the insoluble residue to settle, then take 50mls of the solution and add 1ml of 2M (NH4)2SO4 buffer solution.

The concentration in the measured solution will be displayed as ppm and mol/l. Since buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor.

The calculated sample concentrations must be multiplied by 100 and divided by the sample weight to give the concentration (microg./g)in the soil sample.

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Extra details for determining NITRATE (NO₃^-) in PlANT MATERIAL

Nitrate is extracted from plant tissue by shaking a known weight (say 1g) of the dried, ground material with a known volume (say 250ml) of distilled water (containing 1 ml per litre of biological inhibitor - 0.1g phenylmercuric acetate dissolved in 20ml of dioxane and diluted to 100ml with water) for at least 10 minutes. Interference from bicarbonate and citrate can be reduced by adding a few mls of ion exchange resin during the extraction process. This is prepared by mixing Dowex 50-X8 (50-100) with Aluminium sulphate in the ratio 10g resin to 2.2g Al₂(SO4)₃ and making a slurry with de-ionised water and allowing to stand for a few minutes. After equilibration, excess aluminium salt must be removed by repeated washing and filtering (or decanting) until the resulting liquid does not show any precipitate when dilute BaCl₂ solution is added. Only then is the resin suitable for adding to the sample extract. Similarly, chloride interference can also be removed by using more resin, this time doped with 7g of AgNO₃ per 10 g of resin and washed using NaCl as the test reagent. (Reference: Nitrate Determinations in Plant Extracts by the Nitrate Electrode.Paul, J.L. and Carison, R.M., J. Ag. Food Chem. 1968, 16(5), 766.)

The residue is then filtered off and 100ml of the resultant liquid must be mixed with 2ml of buffer solution before analysis.

The concentration in the measured solution will be displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor. The concentration in the solution (microg/ml) must be multiplied by the initial volume used to dissolve the sample and divided by the sample weight to obtain the concentration (microg/g) in the original plant material.

Nitrate with Chloride

The selectivity coefficient of the nitrate electrode for chloride is about 0.0056. This means that chloride can be present in concentrations up to 10 times the nitrate before producing a significant error. Nevertheless, Chloride is a common component in many natural waters and can often be present in sufficient concentration to cause a significant error in the nitrate measurement.

The standard method for removing chloride is by adding an excess of a soluble silver salt and precipitating the sparingly soluble AgCl. This can be achieved by using a special ISAB made with 10.5 g potassium sulphate, 3.1 g of silver sulphate and 25 ml of 0.1 Molar sulphuric acid per litre of solution and diluting all samples and standards 50% by volume (1:1). This ISAB will also minimise other minor interferences such as bicarbonate, cyanide, bromide, sulphide and nitrite.

Nitrate with bugs.

Aqueous solutions are prone to biological degradation of the nitrate ion. Therefore if samples have to be stored or transported, or subject to prolonged sample preparation, then they must be preserved by adding a few drops of biological inhibitor immediately after collection. This is prepared by dissolving 1 g of phenylmercuric acetate in 20 ml of dioxane and making up to 100 ml with de-ionised water, then storing in a glass bottle.

Technical Specifications
for the Nitrate Ion-Selective Electrode (ELIT 8021)

Click here to download a printer-friendly (pdf) Specification Sheet.