YSI 6000UPG - Manual (Page 109)

6-9
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) If water-saturated air is used as the calibrating medium, make certain that both the DO reading
and the temperature have stabilized (10-15 minutes) before starting the calibration sequence. A wet
thermistor can indicate artificially low temperature readings due to evaporation and this situation
will result in poor temperature compensation and inaccurate readings.
(2) Insure that the calibration cup being used is vented or pressure released.
(3) Keep the probe moist when not in use, either by immersing in water or by placing a damp
sponge in the calibration vessel. If the membrane appears to be damaged or has dried out, be sure to
replace it prior to calibration and deployment.
(4) If possible, calibrate the Rapid Pulse system at a temperature as close as possible to that of the
sample being measured. If possible, immerse the calibration chamber (which contains either a
small amount of water or a wet sponge) into the body of water which is later to be measured. Do
not allow the sample water to seep into the calibration chamber. Monitor the readings; after
thermal equilibrium has been established, proceed with the calibration.
(5) Before you install a new membrane, make sure that the O-ring groove and the probe tip are
clean and smooth. If the KCl electrolyte solution leaks from the probe surface during monitoring
studies, the readings are likely to be less accurate in a shorter period of time.
6.8 NITRATE
The 6000 UPG nitrate probe consists of a sealed module containing of a silver/silver chloride wire
electrode immersed 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 which depends on the relative
amounts of nitrate in the sample and the internal filling solution (which is invariant). This potential
is read relative to the Ag/AgCl reference electrode of the 6000
UPG 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 empirically 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.