Hach-Lange ISENa38103 - Manual (Page 7)

4. Put the probe in the standard solution and push Read. The display will show
"Stabilizing" and a progress bar as the reading stabilizes. The display shows the
value of the check standard and either Check Standard Passed or Check Standard
Failed.
5. If the display shows Check Standard Passed, the check standard measurement is
within the accepted limits set by the administrative user. Select Done to continue with
the sample measurement.
6. If the display shows Check Standard Failed, the measurement is outside of
accepted limits set by the administrative user and a recalibration is recommended. If
the acceptance criteria is set to Cal Expires on Failure: Yes, the display shows the
calibration icon and a question mark until the probe is recalibrated. To correct the
probe calibration and status indicator, calibrate the probe (refer to
Calibration
on page 3).
Interferences
The glass membrane responds to sodium as well as other ions. Typically, probe response
to another ion increases the potential, and causes a positive error. The response to other
ions can be semi-quantitatively determined through the Nikolsky equation, an extended
Nernst equation:
E = E° + (RT/(zF)) ln [aN
a
+ KN
a
x × ax]
Where:
• ax = the activity of the interfering ion
• KN
a
x = the selectivity coefficient for the interfering ion relative to sodium
If the probe is exposed to high levels of interferences, soak the probe in 1 M sodium
chloride to help remove the absorbed ions from the glass membrane. The major
interferences are silver and hydrogen ions. Hydrogen ion concentration is decreased by
the ISA, which raises the pH.
If the samples are highly acidic, or have a high buffer capacity, check that the sample pH
is above 9 after adding ISA. If necessary, add ammonium hydroxide (NH
4
OH) to the
calibration standards and samples in equal proportions to raise the pH. The ammonium
hydroxide (NH
4
OH) will not affect the measurement due to the low selectivity coefficient
of NH
4
+ ion.
The ions that interfere in sodium determinations are given for molar concentrations of all
ions. The smaller the value of the selectivity coefficient, the lower the interference.
Approximate values of selectivity constants (K) are ordered from highest to lowest in
Table 1 .
Table 1 Interferences
Interference Selectivity coefficient
Ag+ (>1000) H+ (20) - reduced by ISA addition
Li+ (0.01) K+ (0.001)
Ti+ (0.0002) —
Advanced operation
Parameter-specific settings can be changed through the Full Access Options menu.
Details about menu navigation, available options and how to change them are given in
the screens, tables and procedures throughout this section.
7