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There are several conventional ways to calibrate fluorometers; those are explained in Calibrating Eijkelkamp’s Turner
Fluorometers. Eijkelkamp also has a new method for “standardizing” fluorometers if you do not use a conventional
calibration method; that is explained in Standardizing Eijkelkamp’s Turner Fluorometers. Both of these documents can be
found on the flash drive.
The maintenance procedure is pretty much the same for all fluorometers:
1 Cleaning the sensor: Rinse the chlorophyll sensor in fresh water following each deployment, ideally until it is
completely clean again. Do not let the chlorophyll sensor come in contact with any organic solvents, such as acetone
and methanol, or strong acids and bases.
2 Cleaning the optics: Visually inspect the optical window after each deployment following a soaking in fresh water.
Use optical tissue to clean the window with soapy water, if needed.
4.15 Ion-Selective Electrodes (ISE’s)
ISE’s are traditionally used in the laboratory at a constant, moderate
temperature, with ionic strength adjusters added to each sample so that
the sample and calibration solution have roughly the same ionic
strength. ISE’s can provide valuable information in the field, for instance
in watching short-term trends, but their calibration requirements,
stability, and accuracy are not nearly as simple and reliable as those for
DO, conductivity, turbidity, etc. For field use, Eijkelkamp makes several
theoretical and empirical corrections to elicit the best possible field
performance.
ISE’s operates much like a pH electrode except that the pH glass is replaced by a membrane that is selective for the
analyte of interest (ammonium, chloride, nitrate, sodium). The electrode’s filling solution contains a salt of the analyte,
and the difference between that salt’s concentration and the analyte concentration in your water produces a charge
separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably
with changes in the analyte concentration in the water adjacent the membrane.
It’s best not to let your ISE dry out, so place a small amount of tap water in the storage cup to ensure 100% humidity. The
sensing elements (tip) for nitrate and ammonium ISE’s have lifetimes of about 90 days. Then, you must replace the tip by
unscrewing it from the sensor body and screwing in a new tip.
Each sensor body is programmed for a specific ISE; the ISE is identified by a series of rings or dots. One ring or dot means
the sensor body is programmed for a chloride ISE; two mean calcium, three mean nitrate, four mean ammonium, 5 mean
bromide, and six mean sodium.

It’s a good idea to limit the submersion of ISE’s to about 10 meters. If you need to go deeper than that with the
other sensors, you can use a plug for the ISE electrode
.
Note that the ammonium ISE senses ammonium, but at pH’s higher than about 8 the ammonium (NH4+) is mostly
converted into ammonia gas (NH
3
). Eijkelkamp’s software uses the pH, conductivity, and temperature of the sample
water to calculate ammonia (as mg/L-N). You can also display total ammonia; the sum of ammonia and ammonium.
Note that ammonium and nitrate ISE’s suffer interference from positive ions, especially potassium and sodium, and
sodium ISE’s suffer interference from positive ions, especially potassium and ammonium. The chloride ISE does not
normally suffer from interfering ions.
Note that Eijkelkamp’s sodium ISE has a plastic membrane with a wider pH range (pH 3 – 10) and less pH interference
than the traditional sodium ISE’s (which are made with glass membranes). This sensor is specified to have a 10-second
response time and a range of 0.05 – 2,300 mg/L Na+. The sodium ISE can be immersed to 15 meters of water without
damage, but there may be a pressure effect on the reading. Although Eijkelkamp testing on a limited sample size showed
this error to be within our ±20% accuracy specification, we recommend that users check the performance of their
particular sensors under actual field conditions.
Note that that the sodium ISE has a slow response to changes in temperature and may take many minutes to reach a final
reading when the temperature changes significantly.
If your Scuba is equipped with more than one ISE, use care when replacing tips so that you don’t put a tip on the wrong
sensor (for example put a nitrate on the sodium sensor).