Method for determining the concentration of
NITRITE (NO2-) in WATER
Links to relevant publications on the WWW
for the Nitrite Ion-Selective Electrode (ELIT 8071)
to download a printer-friendly (pdf) Specification Sheet.
The Nitrite Ion-Selective Electrode has a solid-state PVC polymer matrix membrane and is designed for the detection of nitrite ions (NO2- ) in
aqueous solutions. It is suitable for use in both field and laboratory
applications. However, it must be noted that, because of the ease of
oxidation of nitrite, it may be necessary to soak the electrode in the
pre-conditioning solution overnight or longer before use - depending on
the length of time since last use; may need several days when new or if unused for
The Nitrite Ion is a monovalent anion.
One mole of
( NO2- ) has a mass of 46.006 grams; 1000 ppm is 0.022 M
Dissolve 1.500g anhydrous sodium nitrite (NaNO2) in 1 litre water.
Length of body
excluding gold contact = 130 mm
Length of body including gold contact =
Diameter of body = 8 mm
DC resistance at 25°C < 2.5
Minimum feasible sample volume = 5 ml
Chemical / Operational
Preconditioning /Standard solution : Normally 1000 ppm (NO2-) as NaNO2
(But see General Operating Instructions)
Preconditioning time : at least 5
Optimum pH range : pH 4.5 to pH 8
Temperature range : 0 to
Recommended ISAB : special buffer BS-1 OR BS-2 (mixed 1:1 with standards and samples).
BS-1 (pH=3.8) is nomally used for meat extracts and consists of 3.26g sodium acetate and 10ml glacial acetic acid dissolved in 1000ml water.
BS-2 (pH=3.2) is used for natural water samples and is made by dissolving 14.32g Disodium Phosphate, and 15.37g Citric acid in 1000ml water.
Note that the quoted optimum pH range
(4.5 to 8) is the range over which there is no change in measured mV in a
graph of pH versus mV for a constant NO2 concentration. i.e, if samples
have variable pH within this range then the pH will not affect the
measured mV. In practice, however, the BS-1 and BS-2 buffers have been
found to be a convenient way of controlling the pH if all standards and
samples are buffered to a constant pH value.
reference electrode : Single junction (ELIT 001)
Electrode slope at
25°C : 54± mV/decade
Concentration range : 0.5 to 500ppm
(1x10-5 to 0.01 Molar)
Response time : < 10 seconds
Defined as time to complete 90%
of the change in potential after immersion in the new
Time for stable reading after immersion : < 1 to > 5 minutes
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
The following ions interfere with the nitrite electrode (selectivity coefficients
(SC) in brackets):
Acetate (0.001), Fluoride (0.0008), Chloride
(0.00005), Nitrate (0.00001), Sulphate (0.00001).
The SC is the approximate apparent increase in the measured
concentration caused by 1 unit of the interferent. Thus the likely effect of any
interfering ion (% increase) can be calculated as follows:
((expected concentration) x (SC) / (expected NO2 concentration)) x
Thus these would only cause a significant error if they were present in concentrations several times that of the nitrite. For example, if the Chloride ion were present in equal concentration to the nitrite then it would contribute only about 0.005% to the nitrite concentration measurement, so it would have to be 1000 times more concentrated to cause a 5% error.
Cyanide has a
very high interference and must be absent, or only present in very low
concentrations compared to the nitrite.
Note low concentration range (about 0.5 to 500 ppm) and low pH tolerance (4.5 to
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Procedure for determining the concentration of NITRITE
(NO2-) in WATER
Ion-Selective Electrode for nitrite ion (ELIT 8071 PVC membrane)
Reference electrode: single junction silver chloride (ELIT 001)
Dual electrode head (ELIT 201)
Standard solution: 1000 ppm NO2 as NaNO2
(Note relatively short shelf-life due to oxidation)
Buffer solution (ISAB): Special buffer BS-2
This is a solution containing
14.32g Bisodium phosphate, and 15.37g Citric acid dissolved in 1000ml water.
All standards and samples must be mixed with this in a ratio of 1:1.
ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.
100 or 150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10,
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 500, 100, 10, 1,
and 0.5ppm NO2. If the approximate range of concentrations of the
samples is known, and this is within the specified linear range of the ISE, then
it is only necessary to make two solutions which span this range: e.g. if
samples are known to lie between, say, 3 and 13ppm then you could use standards
of 1 and 20ppm or even 2 and 15ppm.
NB: Nitrite solutions are prone to oxidation when exposed to the air, and the
nitrite electrode has a relatively low pH range, therefore it is necessary to
mix all standards and samples with an equal volume of special buffer solution
before analysis. For the same reason it is best to make up the standard NaNO2 solution fresh before each analytical session.
Follow the instructions in the General Operating Instructions to measure these
standard solutions and prepare a calibration graph.
Mix 25ml of sample with 25ml of buffer solution before analysis.
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 results 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.Return to top