Environmental Engineering Reference
In-Depth Information
pH Meters
A pH meter measures the electric potential (millivolts) across an electrode
when immersed in water. This electric potential is a function of the hydro-
gen ion activity in the sample; therefore, pH meters can display results in
either millivolts (mV) or pH units. A pH meter consists of a
potentiometer
,
which measures electric potential where it meets the water sample; a refer-
ence electrode, which provides a constant electric potential; and a
temperature
compensating device
, which adjusts the readings according to the temperature
of the sample (because pH varies with temperature). The reference and glass
electrodes are frequently combined into a single probe called a
combination
electrode
. A wide variety of meters is available, but the most important part
of the pH meter is the electrode; thus, purchasing a good, reliable electrode
and following the manufacturer's instructions for proper maintenance are
important. Infrequently used or improperly maintained electrodes are sub-
ject to corrosion, which makes them highly inaccurate.
“Pocket Pals” and Color Comparators
pH “pocket pals” are electronic handheld “pens” that are dipped in the
water to obtain a digital readout of the pH. They can be calibrated to only
one pH buffer. (Lab meters, on the other hand, can be calibrated to two or
more buffer solutions and thus are more accurate over a wide range of pH
measurements.) Color comparators involve adding a reagent to the sample
that colors the sample water. The intensity of the color is proportional to the
pH of the sample and is matched against a standard color chart. The color
chart equates particular colors to associated pH values, which can be deter-
mined by matching the colors from the chart to the color of the sample. For
instructions on how to collect and analyze samples, consult the latest edition
of
Standard Methods
.
Tu r bid it y
Turbidity is a measure of water clarity—how much the material suspended
in water decreases the passage of light through the water. Suspended materi-
als include soil particles (clay, silt, and sand), algae, plankton, and microbes,
among other substances. These materials are typically in the size range
of 0.004 mm (clay) to 1.0 mm (sand). Turbidity can affect the color of the
water, and higher turbidity increases water temperature because suspended
particles absorb more heat. This in turn reduces the concentration of DO
because warm water holds less DO than cold. Higher turbidity also reduces
