YSI 6000UPG - Manual (Page 112)

6-12
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 interferences 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 all of that ionic content were
due to sodium chloride, the ammonium reading would be erroneously high by about 0.4 mg/L.
However, brackish or sea water has enough sodium and potassium to cause interferences so
great as to make the sensor unsuitable for these media.
The sensor used in the 6000
UPG
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 6000
UPG 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:
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 6000 UPG sonde software and
displayed if this parameter is activated.
Despite the potential problems with interferences when using ISEs, it is important to remember
that almost all interfering species produce an artificially high ammonium reading. Thus, if the
6000
UPG 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.
Ion selective electrodes have the greatest tendency to exhibit calibration drift over time of all the
sensors available on the 6000
UPG . 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 6000
UPG
, the user should be aware that drift is almost
NH
4
+ NH
3
+ H +