YSI 6000UPG - Manual (Page 104)

6-4
6.4 pH
The 6000
UPG
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 which utilizes gelled
electrolyte. 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 6031 pH sensor currently-available for the 6000 UPG should provide long life, good response
time, and accurate readings in most environmental waters, and tests have shown that it functions
well in water of very low conductivity as well as higher conductivity media. However, YSI is
continually testing pH sensors for improved performance and ruggedness and in the future may
offer additional sensors which are more specific for various types of water.
CALIBRATION AND EFFECT OF TEMPERATURE
The software of the 6000 UPG 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 where E = millivolts output
nF
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 6000
UPG 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 6000
UPG
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 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