Figure 11.
Figure 12.
6.4.2
Erroneous Values
An erroneous pH value could be obtained if the module has been stored dry without the protection of
pH/ORP Storage Solution for a long period of time. In this case, please fill the module cell with pH/ORP
Storage Solution to wet the electrode and allow it to soak for 30 minutes before use. If the pH/ORP Storage
Solution is not available, please use the sample water to hydrate the electrode for at least 30 minutes.
6.4.3
pH Measurement Principle
The SP-710 pH/ORP module uses the standard electrochemical cell for the pH measurement. The cell con-
sists of a glass electrode and an Ag/AgCl reference electrode. Potassium chloride (KCl) electrolyte filling gel
is sealed in the Ag/AgCl electrode. The amount of reference electrolyte in the pH/ORP module is signifi-
cantly larger than that used in a common laboratory pH electrode. This reduces the chance of the filling
solution being diluted or contaminated and increases the electrode life.
The pH value is calculated from the measured cell potential (
EM F
in mV):
pH
=
EM F
S
(
T
) +
pH
o
(2)
S
(
T
) = 0
.
1986(
T
+ 273
.
15)
(3)
S
(
T
)
in the above equation is the calibration slope, where
T
is temperature in degrees Celsius.
S
(
T
)
has
a theoretical value of 59.17 mV at 25 °C.
pH
o
is the calibration intercept. The calibration slope,
S
(
T
)
, at
the nominal temperature 25 °C and the intercept,
pH
o
, are determined in the two-point or three-point
calibration procedure.
pH
o
is determined as well in the singe-point pH 7.00 calibration. The temperature
value measured by the pH/ORP module is used in the above equation to calculate the pH value at the sample
temperature.
*
NOTE
*
The temperature compensation involved in the pH value calculation is quite dif-
ferent from that in the conductivity measurement. The temperature-compensated conduc-
tivity value is a would-be value at the reference temperature 25 °C, while the pH value
displayed by the SP-710 is the true pH value at the sample temperature.
SP-710 User Manual
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