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Method for determining the concentration of NITRITE (NO2-)
in Aqueous Solutions

Electrode Specifications       Links to publications on the WWW     Go to Ion Selective Electrodes page

NB: because of the ease of oxidation of Nitrite it is recommended that NO2 electrodes and standard solutions are kept in a refrigerator when not in use but they must be removed to allow sufficient time to equilibrate to room temp before use. Also for this reason it is best to make calibration standards fresh before each analytical session and refresh the bulk solution frequently. Measuring solutions with a temperature above about 30°C is not recommended.

Apparatus Required:

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.

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

Calibration:

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 200, 20, 2, and 0.2 ppm NO2. 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 a decade apart) which span this range.

NB: Nitrite solutions are prone to oxidation when exposed to the air, and the nitrite electrode has a relatively low pH range, therefore, for the most accurate work, it is advisable to mix all standards and samples with an equal volume of special buffer solution before analysis.

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

Sample Preparation:

Mix 25ml of sample with 25ml of buffer solution before analysis.

Sample 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).

Results:

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.

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Technical Specifications
for the Nitrite Ion-Selective Electrode (ELIT 8071)

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

Introduction
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 several months.
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.

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25°C < 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning /Standard solution : Normally 1000 ppm (NO2-) as NaNO2
(But see General Operating Instructions)
Preconditioning time : at least 5 minutes
Optimum pH range : pH 4.5 to pH 8
Temperature range : 0 to 50°C
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.

Recommended 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 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.

Interference:
Cyanide has a very high interference and must be absent.
The following ions have a small interference (Selectivity Coefficients 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 100.

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.

Note low concentration range (about 0.5 to 500 ppm) and low pH tolerance (4.5 to 8)

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