YSI ADV6600 - Manual (Page 124)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 114
whichever is larger) refers to sampling applications where only minimal time has elapsed between
calibration and field use.
9-14. Chloride
The ADV6600 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
-
).
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 ADV6600, 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 ADV6600, 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,