YSI ADV6600 - Manual (Page 119)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 109
9-10. pH
Procedure from Standard Methods for the Examination of Water and Wastewater: 4500-H+
The ADV6600 employs 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.
Our testing of the 6561 pH, 6565 and 6566 pH/ORP sensors 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.
9-10.1. Effect of Temperature
The software of the ADV6600 calculates pH from the established linear relationship between pH
and the millivolt output as defined by a variation of the Nernst equation:
E = E
o
+ 2.3RT * pH
nF
where E = millivolts output
E
o
= a constant associated with the reference electrode
T = temperature of measurement in degrees Kelvin
R, n, and F are invariant constants
Thus, in simplified y = mx + b form, it is (mv output) = (slope)x(pH) + (intercept). In order to
quantify this simple relationship, the instrument must be calibrated properly using commercially
available buffers of known pH values. In this procedure, the millivolt values for two standard buffer
solutions are experimentally established and used by the sonde software to calculate the slope and
intercept of the plot of millivolts vs. pH. Once this calibration procedure has been carried out, the
millivolt output of the probe in any media can readily be converted by the sonde software into a pH
value, as long as the calibration and the reading are carried out at the same temperature. This last
qualifier is almost never met in actual environmental measurements since temperatures can vary
several degrees during a deployment simply from a diurnal cycle. Thus, a mechanism must be in
place to compensate for temperature or, in other words, to accurately convert the slope and intercept
of the plot of pH vs. millivolts established at T
c
(temperature of calibration) into a slope and