PASCO Specialty & Mfg. CI-6734 User Manual download pdf (Page 5)

012–06615A Sodium Ion Selective Electrode

Introduction

The PASCO scientific Sodium Ion Selective Electrode is used to quickly, simply, accurately, and economically

measure sodium ions in aqueous solutions.

Theory

The Sodium Ion Selective Electrode is composed of a sodium-selective glass membrane bonded to a glass body.

When the membrane is in contact with a solution containing sodium ions, an electrode potential develops across

the membrane. This electrode potential is measured against a constant reference potential, using an ISE Amplifier

and a ScienceWorkshop computer interface. The level of sodium ions, corresponding to the measured potential, is

described by the Nernst equation.

log

where:

= measuredelectrodepotential

= referencepotential (a constant)

S = electrodeslope ( ≈ 59

decade

)

= level o

sodium ions in solution

The activity, X, represents the effective concentration of free sodium ions in the solution. The activity is related to

the free ion concentration, C

, by the activity coefficient,

by:

Activity coefficients may vary, depending on the total ionic strength, I, determined as:

I =

ΣC

where:

= concentratio

ion

= chargeof ion X

Σ = sum of all of the types of ions in the solution

In the case of high and constant ionic strength relative to the sensed ion concentration, the activity coefficient,

constant and the activity, X, is directly proportional to the concentration.

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PASCO Specialty & Mfg. CI-6734 User Manual Page 5