Fig. 7 Difference In State Of Solution Depending On Ion; About Activity - Horiba Scientific 6560S-10C Instruction Manual

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About activity

The ion concentration can be accurately measured by performing
calibration and measurement using an ISA.
The following describes the relationship between the ion concentration and
activity and the reason for adding an ISA.
Fig. 7 Difference in state of solution depending on ion concentration
If specific ions exist in the sample solution, the ion selective membrane of
the ion selective electrode generates a potential that corresponds to the
concentration of those ions. The generated potential is measured as the
potential difference on the basis of the reference electrode by using an ion
meter or meter with an ion measurement function. A proportional
relationship exists between the measured potential difference and the
logarithm of the concentration of ions to be measured in the sample
solution which can be represented by the Nernst equation (
According to the above equation, if the potential gradient is determined by
the calibration using the standard solutions, the concentration of the
targeted ions can be measured by obtaining electrode potential E of the ion
selective electrode in the sample solution.
The ion concentration is getting increased, however, the movement of the
ion is getting restricted. As a result, the ions, which can not reach the ion
selective membrane, do not contribute to the reaction to measure the
potential.
Accordingly, "the amount of ions capable of actually contributing to the
reaction of potential measurement in the solution" needs to be represented
as the effective concentration. This "effective concentration" is known as
the activity.
60
Distance
The concentration
increases
Solvent Ion
molecule
Distance
The number of solvent
molecules surrounding
the ion decreases
causing the distance
between ions to reduce.
page 58).