YSI ADV6600 - Manual (Page 123)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 113
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 ADV6600 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 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 an ammonium 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, 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 ammonium sensor which has been recently calibrated.
Remember that the typical accuracy specification for the sensor (+/- 10 % of the reading or 2 mg/L,