pH
Measurement of Acid or Base Content of Water
Water (H2O) contains both hydrogen (H+) and hydroxide ions (OH-). The relative
concentrations or amounts of these ions determines whether a solution is acidic
or basic. In pure water at 20 C, the concentrations of hydrogen and hydroxide
ions are equal, so we refer to this as a neutral solution. In an acidic solution,
the concentration of hydrogen ions is greater than the concentration of hydroxide
ions. In a basic solution, the concentration of hydrogen ions is less than the
concentration of hydroxide ions.
The activity of hydrogen ions is expressed in pH units (pH = power of Hydrogen).
In most cases the concentration of hydrogen ions [H+] can be used as a very
close estimate of pH. pH is a measure of the intensity of the acidic or basic
character of a solution at a given temperature. It is reported as the negative
log of the hydrogen ion concentration (-log [H+]). The pH scale ranges from
1 (most acidic) to 14 (most basic), with pH 7.0 being neutral. If the solution
has more H+ ions than OH- ions, it is acidic and has a pH of less than 7. If
the sample contains more OH- ions than H+ ions, it is considered basic with
a pH greater than 7. It is important to remember that pH is measured on a logarithmic
scale, so a change of one pH unit means a ten-fold (times) change in [H+][OH-].
For example,
10-6 moles of [H+] = pH 6
10-7 moles of [H+] = pH 7
10-8 moles of [H+] = pH 8
The logarithmic scale is the same principle used in the Richter scale for
rating earthquake intensity.
Reasons to test water samples for pH
pH is a critical water quality component because it can affect fish egg production
and survival, aquatic insect survival and emergence, and influences the toxicity
(how poisonous something is) other pollutants, such as heavy metals and ammonia.
pH naturally varies both daily and seasonally. Photosynthesis by aquatic plants
is the primary cause of daily pH fluctuations. During daylight hours, plants
consume carbon dioxide (CO2), which forms an acid in water, resulting in increased
pH during the day (Because there is less CO2 in the water, less acid is formed,
and thus the pH rises.). Respiration by plants and animals produces CO2 (acid),
which results in pH during the night. Be aware that because there is a lot of
carbon dioxide in the atmosphere around us, that the pH of natural rainwater
(without pollution) is approximately 5.6.
How a pH meter works
The most accurate way to measure pH is with a calibrated pH meter and pH electrode.
the pH electrode measures the concentration of H+ ions in the water by developing
an electrical potential in the water surrounding the electrode. A reference
electrode completes an electrical circuit with the electrode and provides a
stable electrical reference by slowly leaking a salt solution or gel into the
sample through a small opening, or junction. The voltage developed at the electrode
is amplified by the pH meter, and then converted to the pH scale and displayed
on the readout. The pH meter and electrode are calibrated with solutions of
known pH called buffers. The three most common buffer concentrations are pH
4.0, 7.0, and 10.0 at a specific temperature (usually 25 C).
Temperature can impact the electrical potential of the pH probe, but using
a pH meter with automatic temperature compensation (ATC) eliminates this potential
error. Meters with ATC need to remain in the sample water for several minutes
in order for the internal thermometer to equilibrate with the temperature of
the water. The standard operating temperature range for pH meters is 0 to 50
C (32 to 122 F).
Summary
Aquatic organisms can be very sensitive to high or low pH values, particularly
those that are less than 6.5 or greater than 8.5. The reproductive portion of
the growth cycle is especially sensitive. Adult organisms may continue to live,
but young will not be produced! Ways that the pH in a natural body of water
may be changed: 1. Algal blooms may raise pH. In extreme cases, the pH may be
greater than 9. An algal bloom is a mass of algae that increases in size so
quickly that the water turns from clear to green in color. Thick growths of
matted algae may be visible in the water. The result is that these algae take
all of the CO2 out of the water, which then makes the water less acidic. 2.
Many industrial processes result in the release of acids or bases, thus raising
or lowering the pH. 3. Oxidation of sulfide-containing sediments can lower the
pH through the production of sulfuric acid. This occurs mainly in coastal bays
or estuaries.
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