YSI 600DW-B Sonde - Manual (Page 225)

Principles of Operation Section 5
another ammonium 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.
CALIBRATION AND EFFECT OF TEMPERATURE
The ammonium sensor should be calibrated using solutions of known total ammonium-nitrogen content
according to the procedures detailed in Sections 2.6.1 and 2.9.2. If a two point calibration protocol is used,
the temperature of the standards should be as close as possible to that of the environmental medium to be
monitored. The recommended calibration procedure is one involving three solutions. Two of the solutions
should be at ambient temperature while the third should be at least 10 degrees Celsius different from
ambient temperature. This protocol minimizes the effects of taking readings at temperatures that are
significantly different ambient laboratory temperatures.
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. To check for this drift, place the sonde in
one of your standards at the end of each deployment.
(4) Ammonium standards are good growth media for a variety of organisms. This growth can significantly
reduce the nitrogen content of your standards, an effect that 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 ammonium sensor will take longer to stabilize after exposure to buffers in a sonde
pH calibration. 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 ammonium
calibration after exposure to buffers.
CAUTION: The ammonium 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.
5.10 CHLORIDE
The sonde chloride probe employs a solid state membrane attached to a conductive wire. When the probe
is immersed in water, a potential is established across the membrane that depends on the amount of
chloride in the medium. 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 chloride 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 chloride
activity at 25°C. However, in practice, empirical calibration of the electrode is necessary to establish the slope
of the response. Typical slopes are
45-55 mV per decade for YSI sensors. This slope value is determined by
calibration with two solutions of known chloride concentration (typically 10 mg/L and 1000 mg/L Cl - ).
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-12