YSI 600DW-B Sonde - Manual (Page 226)

Principles of Operation Section 5
The slope of the plot of log (chloride) 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 (chloride) vs. voltage intersects the calibration plot is called the isopotential
point, that is, the chloride 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 the probe should be recalibrated periodically.
All ion selective electrodes are subject to the interaction of species with the sensor membrane, which are
similar in nature to the analyte. These interfering species thus include other halide ions (fluoride, bromide,
and iodide) as well as other anions.
Despite the potential problems with interference when using ISEs, it is important to remember that almost
all interfering species produce an artificially high chloride reading. Thus, if the sonde indicates the
presence of only small quantities of chloride, it is unlikely that the reading is erroneously low because of
interference. Unusually high chloride readings (which could be due to interfering ions) should be
confirmed by laboratory analysis after collection of water samples.
Of all the sensors available on the sonde, ion selective electrodes have the greatest tendency to exhibit
calibration drift over time. This drift should not be a major problem for sampling studies where the
instrument can be frequently calibrated. However, if a chloride 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 chloride sensor
which has been recently calibrated. Remember that the typical accuracy specification for the sensor (+/- 15
% of the reading or 5 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 chloride sensor should be calibrated using solutions of known chloride 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.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-13