YSI 6000UPG - Manual (Page 111)

6-11
MEASUREMENT AND CALIBRATION PRECAUTIONS
(1) The temperature response of ion selective electrodes is not as predictable as that of pH
sensors. Therefore, be sure to carry out a 3 point calibration the first time you use the probe.
This will provide a default setting for the effect of temperature on your particular sensor. After
this initial calibration, you can use the less time-consuming 2 point and 1 point routines to update
the 3 point calibration. However, we strongly recommend a new 3 point calibration after each
deployment of 30 days or longer.
(2) Ion selective electrodes may not stabilize as rapidly as pH sensors. Be sure to allow plenty of
time for the readings to come to their final values during all calibration routines.
(3) Ion selective electrodes generally drift more than pH sensors. Be sure to check for this drift
by placing the sonde in one of your standards at the end of each deployment.
(4) Nitrogen standards are good growth media for a variety of organisms. This growth can
significantly reduce the nitrogen content of your standards - an effect which is particularly
important for the 1 mg/L solution. It is best to use new standards for each deployment, but if you
decide to save your solutions for reuse, we recommend refrigerated storage to minimize the
growth of these organisms.
(5) Remember that the nitrate sensor will take longer to stabilize after exposure to pH buffers.
To accelerate this process, soak the sensor in 100 mg/L standard for a few minutes after
performing a pH calibration. In addition, be particularly careful that readings are stable during
nitrate calibration after exposure to buffers.
CAUTION: The nitrate membrane module is for use only at depths less than 50 feet (15.2
meters). Use of the probe at greater depths is likely to permanently damage the sensor.
6.9 AMMONIUM AND AMMONIA
The 6000 UPG ammonium probe employs a sealed module containing a silver/silver chloride
(Ag/AgCl) wire electrode immersed 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 which depends on the relative amounts of ammonium in the sample and the internal
filling solution (which is invariant). This potential is read relative to the reference electrode of
the 6000
UPG 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