YSI 600DW-B Sonde - Manual (Page 224)

Principles of Operation Section 5
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 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 interference from ions, which are similar in nature to the analyte.
For example, sodium and potassium ions bind to the ammonium membrane and produce positive readings
even when no ammonium is present. Fortunately, fresh water does not usually contain enough interfering
ions to produce large errors. For example, a common conductivity for freshwater is about 1.2 mS/cm (Sal
= 0.6). Even if the ionic content were due to sodium chloride, the ammonium reading would be
erroneously high, about 0.4 mg/L.
However, brackish or seawater has enough sodium and potassium to cause interference so great as to
make the sensor unsuitable for these media.
The sensor used in the sonde detects only ammonium ions (NH
4
+
), the predominant form of total
ammonium nitrogen in most environmental samples. However, using the concurrently determined values
of pH, temperature, and conductivity, the sonde software can also provide the user with the concentration
of free ammonia (NH
3
) in the sample under investigation.
Ammonium ions and free ammonia are in equilibrium in any solution according to the following equation:
NH
4
+ NH
3
+ H +
The value of the equilibrium constant associated with this reaction, K = [NH
3
][H + ]/[NH
4
+ ], and its variation
with temperature and salinity, is well known. This information allows the free ammonia concentration
[NH
3
] to be automatically calculated by the sonde software and displayed if this parameter is activated.
Despite the potential problems with interference when using ISEs, it is important to remember that almost
all interfering species produce an artificially high ammonium reading. Thus, if the sonde indicates the
presence of only small quantities of ammonium, it is unlikely that the reading is erroneously low because of
interference. Unusually high ammonium 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 an ammonium 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
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-11