pH 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 IJ44/46 electrodes
has a very low sodium error. It has been characterised at several sodium
concentrations so that corrections to the measurement can be made if the
sodium concentration of the sample is known. Graph 1 shows the typical
sodium error for a new IJ46/46 pH electrode. 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.
