YSI 600DW-B Sonde - Manual (Page 222)

Principles of Operation Section 5
quoted accuracy specification for water which has a pH of greater than 8.5 is only valid if the pH changes
less than 0.3 pH units from the pH at the time of calibration. In addition, users should note that it will not
be possible to make free chlorine measurements with the YSI free chlorine sensor under any conditions if
the pH exceeds ca. 9.3.
5.8 NITRATE
The sonde 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 sonde 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 indicates the presence
of only small quantities of nitrate, it is unlikely that the reading is erroneously low because of interference.
Unusually high nitrate readings (which could be due to interfering ions) should be confirmed by laboratory
analysis after collection of water samples.
Ion selective electrodes have the greatest tendency to exhibit calibration drift over time of all the sensors
available on the sonde. This drift should not be a major problem for sampling studies where the instrument
can be frequently calibrated. However, if a nitrate sensor is used in a longer-term deployment study with
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-9