YSI ADV6600 - Manual (Page 122)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 112
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 ADV6600. 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 the sonde, the user should be aware that drift is almost certain to occur.
The extent of the drift will vary depending on the age of the probe, the flow rate at the site, and the
quality of the water. For all monitoring studies using ion selective electrodes, the user should
acquire a few “grab samples” during the course of the deployment for analysis in the laboratory by
chemical means or with another nitrate sensor which has been recently calibrated. Remember that
the typical accuracy specification for the sensor (+/- 10 % of the reading or 2 mg/L, whichever is
larger) refers to sampling applications where only minimal time has elapsed between calibration and
field use.
9-13. Ammonium and Ammonia
WARNING: Ammonium 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 ammonium probe employs a silver/silver chloride (Ag/AgCl) wire electrode in a
custom filling solution. Nonactin membrane separates the internal solution from the sample
medium and this membrane selectively interacts with ammonium ions. When the probe is immersed
in water, a potential is established across the membrane that depends on the relative amounts of
ammonium in the sample and the internal filling solution. This potential is read relative to the
reference electrode of the sonde pH probe. As for all ISEs, there is a linear relationship between the
logarithm of the ammonium activity (or concentration in dilute solution) and the observed voltage.
The Nernst equation describes this relationship.
Under ideal conditions, the Nernst equation predicts a response of 59 mV for every 10-fold rise in
ammonium activity at 25°C. In practice, however, empirical calibration of the electrode is
necessary to establish an accurate slope of the response. Typical empirical slopes are 53-58 mV per
decade for YSI sensors. This slope value is determined by calibration with two solutions of known
ammonium concentration (typically 1 mg/L and 100 mg/L NH
4
+
-N).
The slope of the plot of log (ammonium) vs. voltage is also a function of temperature. The slope
changes by a factor that is the ratio of the absolute temperature of calibration to absolute
temperature of measurement. The point where this new plot of log (ammonium) vs. voltage
intersects the calibration plot is called the isopotential point, that is, the ammonium 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, use a third
calibration point where the voltage of the lower concentration standard is determined at a