Horiba Scientific 6561S-10C, Instruction Manual

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65
Reference
The activity is a concentration of ions in the solution that is corrected using
a coefficient called the activity coefficient, and the relationship between
them can be expressed with the following relational expression.
Activity (a) = Activity coefficient (

) · Ion concentration (C)
Table 4 shows the relationship between the ion concentration and the
activity coefficient.
From this table, you can see that when the ion concentration becomes
lower than 1

10
-3
mol/L, the activity coefficient becomes a value close to 1,
and as it becomes higher than 1

10
-3
mol/L, the activity coefficient
becomes lower than 1.
If the activity coefficient becomes lower than 1 (activity < ion
concentration), an error occurs in the potential measurement. A TISAB is
used to reduce this error. An TISAB is generally selected so that it does not
react with the ions to be measured and does not affect the measured
potential.
The measurement error can be reduced because adding the TISAB
enables to maintain the ion strength constant and to adjust the activity
coefficients of the sample solution and the standard solution almost at the
same value.
Table 4 (Example) Activity coefficient for monovalent ion concentration
Ion concentration (C) mol/L Activity coefficient (

)
1

10
-5
0.998
1

10
-4
0.988
1

10
-3
0.961
1

10
-2
0.901
1

10
-1
0.751