Ionode IJ pH V5.1 - Manual (Page 15)

Ionode IJ Instruction Manual 15
further details, please contact Ionode specifying
make and model of pH/mV meter or titrator.
Sodium Error
The selectivity of H
+
over Na
+
is extremely high,
but small errors become apparent at high pH
(i.e. low H
+
concentration) and high Na
+
concentration.
The pH glass used in the IJ44C has a very low
sodium error. Corrections to the measurement
can be made if the sodium concentration of the
sample is known. There are typical sodium
errors for a new IJ44 pH electrode (see
Graph
1 ). The sodium error increases with aging.
A measured pH can be corrected by locating it
on the x-axis and drawing a line straight up until
it intersects the sodium concentration line of the
sample (extrapolate between lines for sodium
concentrations not drawn). Draw a line from the
intersection directly over to the y-axis to
determine the error in the measurement. The
actual pH of the sample is determined by
adding the error to the measured pH. For
example, if a sample with 1M Na
+
concentration
measures pH 12.6, then the actual pH is 12.77.
The sodium error is larger in the
“A” glass pH
electrodes. As a consequence,
“A” glass pH
electrodes are best suited to pH measurements
below pH12. The
“A” glass would be preferred
for measurements in strong acids, titrations and
for measurement in lower conductivity samples.