Xylem 6-Series Multiparameter - Manual (Page 242)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-4
MEASUREMENT AND CALIBRATION PRECAUTIONS
1 - Instructions for preparation of ORP calibrator solutions (including Zobell reagent) can be found in
Section 2580 A. of Standard Methods for the Examination of Water and Wastewater. Alternatively, the
Zobell solution is available from YSI.
2 – Reagants to confirm ORP sensitivity are available. Contact YSI Technical Support for ordering
information.
3 - ORP readings usually stabilize much more rapidly in standards (e.g., Zobell solution) than in most
environmental water. Be certain to consider this factor when determining ORP in field studies.
4 - Clean and store the pH/ORP sensor by following the instructions in Section 2.10, Care, Maintenance
and Storage of this manual.
5.5 pH
The sondes employ a field replaceable pH electrode for the determination of hydrogen ion concentration.
The probe is a combination electrode consisting of a proton selective glass reservoir filled with buffer at
approximately pH 7 and a Ag/AgCl reference electrode that utilizes electrolyte that is gelled. A silver wire
coated with AgCl is immersed in the buffer reservoir. Protons (H + ions) on both sides of the glass (media
and buffer reservoir) selectively interact with the glass, setting up a potential gradient across the glass
membrane. Since the hydrogen ion concentration in the internal buffer solution is invariant, this potential
difference, determined relative to the Ag/AgCl reference electrode, is proportional to the pH of the media.
The pH system of the 600R utilizes individual glass sensing and reference electrodes, but the operating
principle is the same.
YSI offers three styles of pH and pH/ORP relative to the glass portion of the probe – flat-, bulb-, and
hemisphere-type. The bulb-type sensors (6561, 6565, and 6566) use lower impedance glass and therefore
are more stable in laboratory applications and calibration when in the presence of static. The flat-type
sensors have higher impedance glass and so may show some jumpiness during laboratory operations,
including calibration, unless the user avoids touching the sonde and stays out of close proximity to the
instrument. The advantages of the flat-type glass sensors are that (a) they are much less susceptible to
breakage than their bulb-type analogs and (b) they are more readily cleaned in long term field studies when
using the auxiliary brush of the 6600EDS V2-2 sonde. Users should consider these factors when deciding
which YSI pH or pH/ORP sensor to purchase.
Our testing of the 6561, 6561FG, 6565, 6565FG, 6566 pH or pH/ORP sensors, 6569 pH/ORP, 6579 pH,
and the 600R/600QS pH/ORP system indicates that they should provide long life, good response time, and
accurate readings in most environmental waters, including freshwater of low ionic strength. Thus, no
special sensor is required (nor offered) for water of low conductivity.