Radiometer Analytical TitraLab 870, Reference Manual

Page: 95 / 286

Titra Lab ® TitraLab 870 Reference Manual
Page 95
Direct ISE
measurement
method -
definition Measurement method using a selective electrode (ISE) of the ion you
want to determine the concentration.
In a Direct ISE measurement method, you must calibrate the ISE
electrode using 1 to 9 standard solutions of known concentration.
Refer to "Electrode calibration (ISE)", page 109 .
If a calibration with 3 to 9 standards is carried out, E
0
, S and C
0
are
determined by non linear regression using the following equation:
E = E
0
+ S
25
x T/T
25
(-log (C+C
0
)) where:
• E = potential measured in the sample,
• E
0
= electrode standard potential,
• S
25
= electrode response slope (sensitivity) at 25°C,
• T = temperature of the solution in K,
• T
25
= 298.16 K,
• C = concentration of sample,
• C
0
= detection limit concentration. It is the “experimental detection
limit of the electrode regarding the species under study“.
If a 1-point calibration is performed, only E
0
is calculated. The TIM870
takes the S
25
sensitivity from the last calibration done or takes the the-
oretical value which depends on the ion valency (for example:
-59.16 mV for a positive monovalent ion). C
0
is equal to 0.
If a 2-point calibration is performed, E
0
and S
25
are calculated using
the same equation as above but with C
0
= 0. It is recommended to
perform a 2-point calibration in the linear response zone of the ISE
electrodre.
Refer to "Direct ISE measurement method - notes", page 96 .
E (mV)
log (C)
C
0
0
E
90
60
30
0
-30
-90
-120
-150
-180
-1 -2 -3 -4
-5 -6 -7
S
25