The FC300B sodium ion selective electrode is a combination electrode that has a silver/silver chloride (Ag/AgCl) reference and a sodium ion selective electrode. Both are housed within a single glass body design that uses an ammonium chloride/silver chloride electrolyte for a fill solution.

The sodium sensitive membrane should be conditioned before performing a calibration. The conditioning of the electrode is a two-step process in which the glass is etched then rehydrated.  The first step is etching. This process removes a thin layer of sensitive glass from the sodium membrane, exposing a sodium-containing fresh surface. The second step is rehydration in which the probe is placed in storage solution to rehydrate the glass membrane. Allowing the glass to rehydrate after etching provides optimum conditions for the exchange of sodium ions between the membrane and solution.

For the FC300B to measure sodium accurately, it is important that the ionic strength of the standards and sample be adjusted to a high and constant value with the sodium ionic strength adjuster (ISA) solution. The sodium ISA is important for two reasons. One is that the ISA buffers the pH of the standard and sample to above pH 9.8. This removes any interference from hydrogen ions that occur at lower pH’s. The second purpose of the ISA is  to ensure that the activity coefficient is constant so the concentration of free sodium ions can be measured. Having solutions with a constant ionic strength reduces the margin of error between measurements of different samples at different concentrations.

The FC300B sodium glass ISE can measure from 10µM (0.23 mg/L) to 1M (22,990 mg/L).

Conversion Table for Na+ and NaCl