Introduction

It is desirable to measure sodium in drinking water because some persons have health conditions requiring the restriction of sodium intake. The Orion 8611 ROSSTM Sodium electrode allows sodium in aqueous solutions to be measured quickly, accurately and economically. With its unique redox internal system, the ROSS electrode provides faster response, greater stability and accuracy.

Recommended Equipment

1. Direct concentrations readout ISE meter. Other suitable meters with millivolt accuracy to 0.1 mV may be used; calibration curves will be required. Some of Orion’s compatible meters are EA 940, 920A, 920Aplus, 720 or 720Aplus.
2. Orion Sodium Electrode, Orion 86-11 (ROSS Sure-FlowÒ Combination Electrode). No reference electrode is required.
3. Volumetric glassware and plastic beakers are recommended for measurements.
4. Magnetic Stirrer and stir bars.

Required Solutions

1. Orion Catalogue No. 941105, Sodium Standard, 10ppm Sodium as Na+.
2. Orion Catalogue No. 941107, Sodium Standard, 100ppm Sodium as Na+.
3. Orion Catalogue No. 900010/ 900012, Reference Filling Solution, ROSS reference filling solution.
4. Orion Catalogue No. 841111, Ionic Strength Adjustor for Sodium Electrode.
5. Electrode Rinse Solution: Dilute 20 ml of Ionic Strength Adjustor, Orion Catalogue No. 841111 to a 1-liter volumetric flask. Dilute to the mark with distilled or deionized water. Swirl to mix.
6. Orion Catalogue NO. 841101, Sodium Electrode Storage Solution.
7. Distilled or Deionized water.

Set Up

1. Assemble the combination Sodium electrode and check the electrode operation (Slope) as described in the electrode instruction manuals.
2. Prepare a 10ppm Sodium standard: Use the standard directly without any dilution, Orion Catalogue No. 941105.
3. Prepare a 100ppm Sodium standard: Use the standard directly without any dilution, Orion Catalogue No. 941107.
4. Connect the Sodium combination electrode to the meter as specified in the meter instruction manual.

Calibration Standard Preparation

1. Using the standards prepared above, Pour 100 ml of the 10ppm prepared standard in a 150 ml beaker. Add 2 ml of ISA to the beaker. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
2. Using the standards prepared above, Pour 100 ml of the 100ppm prepared standard in a 150 ml beaker. Add 2 ml of ISA to the beaker. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
3. These are the standards used in actual calibration.

Sample Preparation

1. Store about 1 liter of fresh water just prior to the analysis into a plastic beaker to maintain the consistency of the sodium content in that particular “batch” of water.
1. Using a graduated cylinder, measure 100 ml of the sample into a 150 ml beaker. Add 2 ml of the ISA into the sample. Add a magnetic stir bar to the beaker and cover the beaker with a watch glass.
2. Repeat this procedure three to five times to obtain five readings of the same for consistent results.

Analysis

For instruments with direct concentration readout capability, consult meter manual for correct direct measurement procedures. Brief procedures for all meter types are found in the section How Ion-Selective Electrode Measurements are made.

1. Allow all the standards and the samples to attain room temperature for precise measurements since the electrodes are temperature sensitive.
2. Calibrate the meter using prepared standards, stirring the standards at a uniform rate.
3. Rinse electrodes with distilled or deionized water between measurements. Shake after rinsing to prevent solution carry-over. Do not wipe or rub the sensing element.
4. Measure samples, stirring at the same uniform rate used for calibrating the standards, and rinsing electrodes in distilled or deionized water between measurements.

Results

SAMPLE NUMBER PH
Sample # 1: 38.1
Sample # 2: 38.1
Sample # 3: 38.3
Sample # 4: 38.0
Sample # 5: 38.3

Mean:   38.160
Standard Deviation: 0.1342
%CV:   0.3516

Storage

For brief storage periods, store the electrode in Orion electrode Storage Solution, Orion 841101. Immerse both the tip of the electrode and the reference junction. Replace the storage solution weekly or sooner if crystallization or contamination occurs. For longer storage periods, refer to the instructions manual provided with the electrode.

Maintenance

Press the electrode cap and let filling solution drain from the electrode carrying away any contamination. Refill the electrode with fresh filling solution. For particularly dirty electrodes, the junction can be held open and flushed with Sodium Rinse Solution. Refill the electrode with fresh filling solution.