Hach-Lange HQD - Sodium (Na+) Ion Selective Electrode ISENa - Manual (Page 7)

DOC272.53.80027 7
Interferences
The glass membrane responds to sodium as well as other ions, so interferences exist.
Typically, electrode response to another ion increases the potential, causing a positive
error. The response to other ions can be semi-quantitatively determined through the
Nicholsky equation, an extended Nernst equation:
Where
• ax = the activity of the interfering ion
• KN
a
x = the selectivity coefficient for the interfering ion relative to sodium
If the electrode is exposed to high levels of interferences, soak the electrode 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 reduced by the
ISA which raises the pH. If the samples are extremely acidic, or have a high buffer
capacity, check that the sample pH is above 9 after adding ISA. If necessary, ammonium
hydroxide (NH
4
OH) can be added 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 will interfere in sodium determinations are given for molar concentrations of
all ions. The smaller the value of the selectivity coefficient results in lower interference.
Approximate values of selectivity constants (K) are ordered from highest to lowest:
Maintenance
Storage
Note: For optimal performance, the reference junction must not dry out.
Short-term storage
For short term storage, store the electrode in 25 mL of 100 mg/L Na
+ standard solution
with one Sodium Ionic Strength Adjustor (ISA) powder pillow (0.4g).
Long-term storage (> 4 weeks)
1. Rinse the electrode with ISA rinse solution (refer to ISA rinse solution preparation ).
Blot the electrode with a lint-free cloth.
2. Fill the electrode soaker bottle halfway with 0.02M NH
4
Cl Electrode Filling Solution.
3. Install the electrode soaker bottle. Make sure the storage solution in the cap
completely covers the glass bulb.
4. Install the filling-hole cap.
Note: The electrode must be conditioned again in 25 mL of 100 mg/L Na + standard solution with one
Sodium Ionic Strength Adjustor pillow (0.4 g) for at least 8 hours prior to use.
Clean the probe
For general contaminants:
• Rinse the electrode with ISA rinse solution and blot dry with a lint-free cloth. Refer to
ISA rinse solution preparation .
• Soak the glass bulb for 12 to 16 hours in Electrode Cleaning Solution.
E E
° (RT zF ( ) ⁄ )
aNa KNax ax
×
+
[ ]
ln + =
Interferences
Ag+ (>1000) H+ (20) - reduced by ISA addition
Li+ (0.01) K+ (0.001)
Ti+ (0.0002) —