YSI ADV6600 - Manual (Page 121)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 111
WARNING: Nitrate sensors can only be used at depths of less than 50 feet (15 meters). Use of the
sensors at greater depths is likely to permanently damage the sensor membrane.
The ADV6600 nitrate probe consists of a silver/silver chloride wire electrode in a custom filling
solution. The internal solution is separated from the sample medium by a polymer membrane,
which selectively interacts with nitrate ions. When the probe is immersed in water, a potential is
established across the membrane that depends on the relative amounts of nitrate in the sample and
the internal filling solution. This potential is read relative to the Ag/AgCl reference electrode of the
pH probe. As for all ISEs, the linear relationship between the logarithm of the nitrate activity (or
concentration in dilute solution) and the observed voltage, as predicted by the Nernst equation, is the
basis for the determination.
Under ideal conditions, the Nernst equation predicts a response of 59 mV for every 10-fold rise in
nitrate activity at 25°C. However, in practice, empirical calibration of the electrode is necessary to
establish the slope of the response. Typical slopes are 53-58 mV per decade for YSI sensors. This
slope value is determined by calibration with two solutions of known nitrate concentration (typically
1 mg/L and 100 mg/L NO
3
-N). The slope of the plot of log (nitrate) vs. voltage is also a function of
temperature, changing from its value at calibration by a factor of the ratio of the absolute
temperatures at calibration to that at measurement. The point where this new plot of log (nitrate) vs.
voltage intersects the calibration plot is called the isopotential point, that is, the nitrate concentration
at which changes in temperature cause no change in voltage. Our experience with ISEs indicates
that for best accuracy, the isopotential point should be determined empirically. To do so, the user
employs a third calibration point where the voltage of the lower concentration standard is
determined at a temperature at least 10°C different from the first two calibration points. The slope,
offset, and isopotential point drift slowly, and you should recalibrate the probe periodically.
All ion selective electrodes are subject to the interaction of species with the sensor membrane,
which are similar in nature to the analyte. For example, chloride ion binds in this way to the nitrate
membrane and produces positive nitrate readings even when no nitrate is present in the medium.
Fortunately, most fresh water does not usually contain significant quantities of ions that produce a
large interference on the nitrate reading, such as azide, perchlorate, and nitrite. It usually does
contain some chloride and carbonate ions, but the interference from these ions is relatively small.
For example, if the all of the ionic content of water with a conductivity of 1.2 mS/cm (Sal = 0.6)
were due to the presence of sodium chloride, the nitrate reading would be erroneously high by about
1.6 mg/L. If the conductivity in this sample were all due to sodium bicarbonate, the sensor output
would indicate the presence of only 0.2 mg/L of non-existent nitrate from the interference.
Even though the interference from chloride is relatively small and thus tolerable at low salinity, the
large quantity of this species in salt or brackish water creates interference so great as to make the
sensor unsuitable for these media.
Despite the potential problems with interference when using ISEs, it is important to remember that
almost all-interfering species produce an artificially high nitrate reading. Thus, if the sonde