YSI 63 - Manual (Page 30)

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9. Principles of Operation
9.1 pH
The YSI Model 63 employs a field replaceable pH sensor for the determination of hydrogen ion
concentration. The sensor 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.
Our testing of the Model 63 pH sensor indicates that it should provide long life, good response
time and accurate readings in most environmental waters, including fresh water of low ionic
strength. No special sensor is required (nor offered) for water of low conductivity.
pH Calibration And Effect Of Temperature
The software of the YSI Model 63 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 buffers of
known pH values. In this procedure, the millivolt values for two standard buffer solutions are
experimentally established and used by the YSI Model 63
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 YSI Model 63
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, 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 YSI Model 63 software
assigns the isopotential point as the mv reading at pH 7 and then calculates the intercept using