Radiometer Analytical TitraLab 870, Reference Manual

Page: 142 / 286

Titra Lab ® TitraLab 870 Reference Manual
Page 142
ISE Standard
addition method
- definition Measurement method using an selective electrode (ISE) of the ion
you want to determine the concentration.
In this method, a sample measurement is performed then followed by
1 to 9 additions of a standard of known volume and concentration.
When working in the linear response zone of the ISE electrode, this
standard addition measurement method allows to determine the
sample concentration using the following equation (*):
With:
• Csmp = sample concentration (unknown, to be calculated),
• Vsmp = sample intial volume,
• Ca = standard concentration,
• Va = volume of standard added,
• E1 = potential measured after the addition,
• E = potential measured before the first addition,
• S = ISE response slope at the sample temperature. This slope is
recalculated after each addition. If only one addition is performed,
the slope must be determined by calibrating the ISE electrode.
Refer to "Electrode calibration (ISE)", page 109 .
(*) This equation does not incude the dilution factor due to the addition of a
supporting electrlolyte. Refer to "6. ISE measurements - ISE standard addi-
tion method", page 271 for a description of all calculations done.
In a standard addition method, the total volume of standard added
must be small in comparison with the sample volume. The accuracy of
the method depends on the following:
• No variation of the ionic strength (add a supporting electrolyte),
• No interfering ions present in the sample,
• No variation of the reference electrode junction potential,
• The number of additions performed. Higher this number is, higher
the measurements accuray will be.
With the TIM870, the additions can be:
• programmed: the user enters the volume of one addition and the
number of additions to be performed,
• automatic: the user enters the potential jump due to all additions
and the number of additions to be performed. The TIM870
determines automatically the volume to be added.
How to edit an ISE standard addition method?
see "ISE Standard addition method - programmation", page 143 .
How to run an ISE standard addition method?
Refer to "Running a method", page 216 .
Csmp
Ca Va
×
Csmp Va
× ( )
E1 E –
( )
S
⁄
×
10 Vsmp –
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