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Click on the links below for extra details for various substrates:
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.
Extra details for determining NITRATE (NO3-) in SOILS
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.
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.
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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.
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.
Preconditioning / Standard solution : Normally 1000 ppm (NO3) as NaNO3
(But see General Operating Instructions)
Preconditioning time : at least 5 minutes
Optimum pH range : pH 2 to pH 11
Temperature range : 0 to 50°C
Recommended ISAB : 2M (NH4)2SO4 (Add 2% v/v)
Recommended reference electrode : Double Junction (ELIT 003)
Reference electrode outer filling solution : 0.1M CH3COOLi
Electrode slope at 25°C 54 ± 5 mV/decade
Concentration range : 0.3 to 6,200 ppm (5x10-6 to 0.1 Molar)
Response time : < 10 seconds
Defined as time to complete 90% of the change in potential after immersion in the new solution.
Time for stable reading after immersion : < 1 to > 5 minutes
Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.
Potential drift (in 1000 ppm) < 3 mV/day (8 hours)
Measured at constant temperature and with ISE and Reference Electrode continually immersed.
The following ions interfere with the nitrate electrode (selectivity coefficients (SC) in brackets).
Chloride (0.006) Bicarbonate (0.005), Nitrite
(0.001), Acetate (0.0005), Fluoride (0.0001), Sulphate (0.00001).
This means that chloride and bicarbonate can be present in concentrations up to 10 times the nitrate before producing a significant error. If these ions are greater than this then the interference can be removed by using a special ISAB made by dissolving 10.5g Potassium Sulphate (A.R. grade) and 3.11g of silver sulphate in 800ml de-ionised water. The add 25ml of 0.1M sulphuric acid and make up to 1 litre. This is then added to standards and samples in equal volumes (1:1) instead of the normal ISAB.
Return to top Last Update: CCR 11 Feb 2015