Determination of Sodium in Water

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.

Equipment Required

1. pH/ION analyzer or pH meter with millivolt scale
2. Sodium combination ion selective electrode

 

Reagents Required

1. Ionic strength adjustment buffer (ISAB), NH₄Cl/ NH₄OH
2. Standard sodium solution
3. Electrode rinse solution NH₄HF₂

Standard Preparation:

Electrode Rinse Solution

Dissolve 5.7g of NH₄HF₂ in 100 ml of distilled water. Store in a plastic bottle.

ISAB

Dissolve 2.54g of ammonium chloride (NH₄Cl) into 100 ml of 1M ammonia solution. This produces 4M NH₄Cl/ 1M NH₄OH solution.

Sodium Standard

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.

 

Method

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.

 

Calculation

As both sample and standards have been diluted by the same amount, the sodium concentrations are directly read off the calibration curve.

 

Interferences

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.

Notes

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.