Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 263
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 hydrogen 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 the electric potential where it meets the
water sample; a reference electrode, which provides a constant electric potential; and a temperature
compensating device, which adjusts the readings according to the temperature of the sample (since
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 are 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 is important. Infrequently used or improperly maintained electrodes are
subject to corrosion, which makes them highly inaccurate.
pH Pocket Pals and Color Comparators
pH pocket pals are electronic handheld pens that are dipped in the water, providing 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, which is then matched against a standard
color chart. The color chart equates particular colors to associated pH values, which can be
determined by matching the colors from the chart to the color of the sample.
For instructions on how to collect and analyze samples, refer to Standard Methods.
Turbidity Measurement
As previously described, turbidity is a measure of water clarity—how much the material suspended
in water decreases the passage of light through the water. Turbidity consists of suspended particles
in the water and may be caused by a number of materials, organic and inorganic. These particles
are typically in the size range of 0.004 mm (clay) to 1.0 mm (sand). The occurrence of turbid source
waters may be permanent or temporary. It can affect the color of the water.
Higher turbidity increases water temperatures 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 the amount of light penetrating the water, which reduces photosynthesis and
the production of DO. Suspended materials can clog fi sh gills, reducing resistance to disease in
fi sh, lowering growth rates, and affecting egg and larval development. As the particles settle, they
can blanket the stream bottom (especially in slower waters) and smother fi sh eggs and benthic
macroinvertebrates.
Turbidity also affects treatment plant operations. For example, turbidity hinders disinfection
by shielding microbes, some of them pathogens, from the disinfectant. Obviously, this is the most
signifi cant aspect of turbidity monitoring; the test for it is an indication of the effectiveness of fi ltration
of water supplies. It is important to note that turbidity removal is the principal reason for
chemical addition, settling, coagulation, and fi ltration in potable water treatment. Sources of turbidity
include:
1. Soil erosion
2. Waste discharge
3. Urban runoff
4. Eroding stream banks
5. Large numbers of bottom feeders (such as carp), which stir up bottom sediments
6. Excessive algal growth