This is a method for determining the concentration of sodium in water samples between 0.1 ppm- 1000 ppm concentrations. Applicable to ground, drinking, and wastewater.
- pH/ION analyzer or pH meter with millivolt scale
- Sodium combination ion selective electrode
- Ionic strength adjustment buffer (ISAB), NH4Cl/ NH4OH
- Standard sodium solution
- Electrode rinse solution NH4HF2
Electrode Rinse Solution
Dissolve 5.7g of NH4HF2 in 100 ml of distilled water. Store in a plastic bottle.
Dissolve 2.54g of ammonium chloride (NH4Cl) into 100 ml of 1M ammonia solution. This produces 4M NH4Cl/ 1M NH4OH solution.
Dissolve 2.54g of sodium chloride into distilled water in a volumetric flask and dilute to 1000 ml.
Prepare 0.1, 1, 10, 100 ppm standards by serial dilution of the 1000 ppm stock.
Reference Electrode Filling Solution
Dissolve 11.1g of calcium chloride into distilled water in a volumetric flask and dilute to 100 ml-0.1M CaCl2.
To 100 ml of each of the standard solutions add 2 ml of the ISAB and stir thoroughly. Immerse the electrode in the 0.1 ppm standard solution and record the mV reading. Repeat the operation for the 100 ml of 1,10,100 and 1000 ppm standards. Plot the calibration curve on semi-log paper.
To 100 ml of sample add 2 ml of ISAB and immerse electrodes and record mV value. Plot sodium concentration directly from the calibration curve.
As both sample and standards have been diluted by the same amount, the sodium concentrations are directly read off the calibration curve.
The following common ions cause a 10% error in a 10 ppm solution:
Ag+ 0.014 ppm
Li+ 151 ppm
K+ 1,701 ppm
NH4 as N 18,277 ppm
Tl+ 44,448 ppm
Rb+ 111,532 ppm
These levels of interference are scaled directly, e.g. In a 1 ppm sodium solution, 828 ppm is the tolerable NH4+ level.
Plastic ware is preferred to glass, especially at low levels of sodium.
After prolonged use, a hydrated layer forms on the surface of sodium sensitive glass. This gives rise to increased response times. This layer may be removed by briefly exposing the electrode to a dilute ammonia bifluoride solution. A dilution of 10 is required for the rinse solution. A new calibration curve would be required after this operation.