Xylem 6-Series Multiparameter - Manual (Page 243)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-5
CALIBRATION AND EFFECT OF TEMPERATURE
The software of the sondes 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 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 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.
MEASUREMENT AND CALIBRATION PRECAUTIONS
1- The 6561FG and 6565FG sensors are characterized by a flat glass pH sensor which is more rugged than
the glass in traditional bulb-type pH sensors. Users should note, however, that the flat glass sensors are
characterized by significantly higher impedance which makes them somewhat more susceptible to static
interference when users are handling or are in close proximity to the sonde during calibration or other
laboratory activities. The static interference can sometimes cause the pH readings observed during
Discrete Sample studies or sensor calibration to be very jumpy. If this jumpiness is observed during
calibration or use of a flat glass pH probe, users should simply avoid touching the sonde and should
maintain a distance away from the sonde of at least 2 feet until the calibration has been confirmed. The
static interference is particularly noticeable during winter when the air is drier.
2- When filling the calibration cup prior to performing the calibration procedure, make certain that the level
of calibrant buffers is high enough in the calibration/storage cup to cover at least ½ inch of the pH probe
and the temperature sensor of the 6560 probe.
3 - Rinse the sensors with deionized water between changes of calibration buffer solutions.