LaMotte DHA 3000 Digital pH Meter - Manual (Page 2)

WHAT IS pH?
pH is one of the most common analyses in soil and water testing. An
indication of the sample’s acidity, pH is actually a measurement of the
activity of hydrogen ions in the sample.
pH measurements run on a scale from 0 to 14, with 7.0 considered neutral.
Those solutions with a pH below 7.0 are considered acids, and those
between 7.0 and 14.0 are designated bases. The pH scale is logarithmic, so
a one unit change in pH actually reflects a ten fold change in the acidity.
For instance, orange juice, pH 4, is ten times more acidic than cottage
cheese, which has a pH of 5.
Many industries rely heavily on pH for their process to work properly, or
to maintain expensive equipment. Breweries maintain the pH between
4.2 and 4.6 to keep infectious bacteria from breeding during the
fermentation process. In many industrial applications if the pH is too low
the water may corrode metal equipment, but if it is too high scaling may
result.
pH can be measured visually or electronically. Visual comparisons use pH
indicators where color changes reflect the pH, which are then matched to
color standards. pH meters, such as the DHA-3000, simplify the pH test.
An electrode is placed in the sample, and the pH read directly from the
meter.
While the meter is very easy to use, the electronics within the meter are
more complex. After the pH electrode measures the millivolts of potential
between the reference electrode and the pH electrode, the meter converts
this reading to pH units using the Nernst Equation:
E = Ex + 2.3RT
k
log(a
i
)
nF
where
E
x
= constant depending upon reference electrode
R = constant
T
k
= absolute temperature
n = charge of the ion (including sign)
F = constant
a
i
= activity of the ion
For actual use in converting pH readings to millivolts, this equation can
be simplified to:
(pH
- 7.00) x (-0.198) = mV
4