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Method for determining the concentration of SODIUM (Na+)
in Aqueous Solutions


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



Apparatus Required:

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

Reference electrode: single junction AgCl/KCl (ELIT 001)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm Na as NaCl

Buffer solution (ISAB): None suitable.
Samples with Ionic Strength above about 0.01 Molar 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.

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 linear range of the ISE, then it is only necessary to make two solutions (preferably one decade apart) which span this range.

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 samples with Ionic Strength >0.01Molar.

Because the Na electrode has several interferences from other cations which are commonly found in Ionic Strength Buffer Solutions there is no buffer soutions suitable. Thus it is better to use the Standard Addition Method.

This 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. Since standard and sample are both measured at the same time and in the same solution there is no need for compensation for differences in IS , pH or Temperature. Measurement and calculations are made very easily using the ELIT two-channel electrode-computer interface software.




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
Recommended reference electrode : Single Junction AgCl (ELIT 001)
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:

Although the Na PVC membrane has interference from several other cations most can be ignored if Na is the main component of the sample. The main significant interferences and their 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.

Thus these ions will not cause a significant error if Na/K>20. Na/NH4>10 and Na/Mg or Ca >1.
If samples contain significant amounts of any interfering ion then a glass-bodied, glass-membrane electrode is available which has much less interference.


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