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Method for determining the concentration of SODIUM (Na+) in WATER.

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Technical Specifications
for the Sodium Ion-Selective Electrode
(ELIT 8230)

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

Introduction
The Sodium Ion-Selective Electrode has a solid-state PVC polymer matrix membrane. The electrode is designed for the detection of sodium ions (Na+ ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Sodium Ion is a monovalent cation.
One mole of ( Na+ ) has a mass of 22.99 grams; 1000 ppm is 0.0435 M
Dissolve 2.542g anhydrous sodium chloride (NaCl) 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 25C = < 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000 ppm Na+ as NaCl
(But see General Operating Instructions )
Preconditioning time : at least 5 minutes
Optimum pH range : pH 3 to pH 10
Temperature range : 0 to 50C
Recommended ISAB : NO BUFFER SUITABLE
Samples with Ionic Strength above about 0.01Molar should be analysed using the
Standard Addition method , or diluted before analysis to bring the ionic strength below this value; providing that the Na content will not be reduced below about 0.2ppm by this dilution.

Recommended reference electrode : Single Junction AgCl (ELIT 001)
OR Double Junction Lithium Acetate (ELIT 003).
Electrode slope at 25 C : 54 5 mV/ decade
Concentration range : 0.05 to 2,300 ppm (2x10-6 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.

Interference:
Virtually all cations interfere with the sodium electrode to some extent! Thus it is best used for measuring pure Na solutions or solutions where Na is far more concentrated than other components. This factor also means that it is difficult to choose a satisfactory ISAB for this electrode.
Known Selectivity Coefficients are: K+ ~ 0.6, NH4+ ~ 0.2, Ca++ ~ 0.02, Mg++ ~ 0.03.

Where 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 Na concentration)) x 100.

 




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Procedure for determining the concentration of SODIUM (Na+) in WATER

Apparatus Required:

Ion-Selective Electrode for sodium ion (ELIT 8230 PVC membrane)

Reference electrode: double junction lithium acetate (ELIT 003)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm Na as NaCl

Buffer solution (ISAB): None suitable.
Samples with Ionic Strength above about 0.01Molar should be analysed using the Standard Addition method, or diluted before analysis to bring the ionic strength below this value; providing that the Na content will not be reduced below about 0.2ppm by this dilution.

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

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

Analysis of low ionic strength samples using Direct Potentiometry

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 1000, 100, 10, 1, and 0.1ppm Na. 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, 30 and 130ppm then you could use standards of 10 and 200ppm or even 20 and 150ppm.

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

Sample Preparation:

No sample preparation necessary. Simply immerse the electrodes in a sample to a depth of a few millimetres.

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).


Analysis of high ionic strength samples(>0.01Molar) using Standard Addition Method.

The Standard Addition method involves measuring the electrode potential in a known volume of sample (say 100ml) and then adding a smaller volume of standard (say 2ml), mixing well and measuring a second potential. Details of this method are given in the general electrode operating instructions. Measurement and calculations are made very easily using the ELIT two-channel electrode-computer interface software. Click Here for more details

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