YSI ADV6600 - Manual (Page 120)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 110
intercept at T
m
(temperature of measurement). Fortunately, the Nernst equation provides a basis for
this conversion.
According to the Nernst equation as shown above, the slope of the plot of pH vs. millivolts is
directly proportional to the absolute temperature in degrees Kelvin. Thus, if the slope of the plot is
experimentally determined to be 59 mv/pH unit at 298 K (25 C), then the slope of the plot at 313 K
(40 C) must be (313/298) * 59 = 62 mv/pH unit. At 283 K (10 C), the slope is calculated to be 56
mv/pH unit ((283/298) * 59). Determination of the slope of pH vs. mv plots at temperatures
different from T
c
is thus relatively simple. In order to establish the intercept of the new plot, the
point where plots of pH vs. mv at different temperatures intersect (the isopotential point) must be
known. Using standard pH determination protocol, the sonde software assigns the isopotential point
as the mv reading at pH 7 and then calculates the intercept using this assumption. Once the slope
and intercept to the plot of pH vs. mv are assigned at the new temperature, the calculation of pH
under the new temperature conditions is straightforward, and is automatically carried out by the
sonde software.
9-11. ORP
Procedure from Standard Methods for the Examination of Water and Wastewater: 2580
The ADV6600 determines the Oxidation-Reduction Potential (ORP) of the media by measuring the
difference in potential between an electrode which is relatively chemically inert and a reference
electrode. To measure ORP with an ADV6600, a combination pH/ORP probe must be in place in
the sonde bulkhead and ORP must be accessed via the ISE2 channel of the ADV6600. The ORP
sensor consists of a platinum button found on the tip of the probe. The potential associated with this
metal is read versus the Ag/AgCl reference electrode of the combination sensor that utilizes gelled
electrolyte. ORP values are presented in millivolts and are not compensated for temperature.
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.
9-11.1. Effect of Temperature
ORP readings for the same solution can vary up to 100 mv depending on the temperature. However,
no standard compensation algorithms exist for this parameter. Be sure to take this factor into
account when reporting ORP values and checking sensor calibration.
9-12. Nitrate