The alkalinity of water refers to its capacity to neutralize acid, or rather, its ability to resist changes in pH that would make water more acidic.
Natural water contains different ions that can contribute to its
overall alkalinity. These primarily include hydroxide, carbonate
and bicarbonate anions, and to a lesser extent borates, phosphates and silicates. Because these ions are capable of combining
with and neutralizing excess hydrogen ions (H+) upon addition
of acid to a body of water, they can help maintain a relatively
constant hydrogen ion concentration, and therefore a consistent
pH. This is known as buffering; the greater the contribution from
these ions, the greater the alkalinity and buffering capacity.
The alkalinity of natural water is largely dependent on the
geology through which it passes and the carbon dioxide in the air.
Sedimentary rocks and soil rich in carbonates often contribute to
naturally high levels of surface water alkalinity. Limestone, for
example, contains high levels of calcium carbonate (CaCO3), and
water passing through this type of geology tends to have a greater
buffering capacity. Granite, on the other hand, contains low levels
of carbonate and bicarbonate ions, and generally contributes to
water with a lower alkalinity.
Importance for environmental protection
Given the natural variation in the buffering capacity of surface
waters, determining the alkalinity of waterways plays an important role in anticipating and preventing large changes in pH that
could be harmful to the environment. Many organisms, including freshwater animals and plants, have adapted to survive in a
relatively narrow pH range, and are generally unable to tolerate
sharp changes in the pH of their environment.
Alkalinity offers aquatic life natural protection against changes
in pH. However, if large amounts of acid find their way into
waterways with a low buffering capacity, pH values can fall to
harmful levels. Acid rain, the discharge of tannic water (slow
moving water rich in decaying vegetation) and the rapid melting
of acidic snow and ice can all result in substantial changes in pH.
Industrial waste and the excessive discharge of household goods
can also have a significant impact on water pH.
Determining the natural baseline alkalinity of rivers, streams
and lakes can help to identify those that are more sensitive to
changes in pH. As a result, monitoring and protection measures
can be put in place to minimize negative environmental effects.
The role of alkalinity monitoring in water treatment
Changes in the alkalinity of water can also have a significant
impact on a wide range of industrial processes, including those
necessary for the preparation of safe drinking water. Ensuring
water is of an appropriate alkalinity is essential to ensure each
treatment step operates effectively, from the raw water through
the final disinfection before distribution. Alkalinity plays a role
in water softening, disinfection, coagulation and oxidation steps.
To achieve reliable and expected results, a consistent level of
alkalinity must be maintained. Moreover, alkalinity monitoring
and correction are also necessary to minimize the corrosion of
pipework and equipment. Alkalinity and TOC levels affect the
levels of disinfectant byproducts that may arise during the water
treatment process. Some drinking water plants in the U.S. must
report alkalinity and TOC measurements to demonstrate compliance with the Environmental Protection Agency’s (EPA) Disinfectants and Disinfection Byproducts Rules.
Monitoring water alkalinity is also important to enable optimal
treatment of wastewater. Various processes require and consume
alkalinity, such as biological nitrification in aeration tanks, gas
chlorination for effluent disinfection, as well as the neutralization of acids generated from biomass in anaerobic digesters. For
example, if there is not enough alkalinity, nitrate levels in the
biological treatment process effluent may rise. In this case, it may
be necessary to supplement the alkalinity of the influent with
alkaline compounds. Maintaining a consistent buffering capacity,
and therefore alkalinity, is thus important for the correct functioning of wastewater treatment processes.
Water alkalinity testing methods
The traditional EPA-approved approach to determining the
alkalinity of test samples is by means of titration with a standard acid solution to a defined endpoint. This endpoint can be
measured using a pH electrode or by observing a color change
using indicators. By measuring the volume of a standard solution
of Alkalinity Testing and Obtaining
The importance of alkalinity testing for safeguarding water
treatment processes and the environment
by Gayle Gleichauf, Applications Lab Manager, Water and Laboratory Products, Thermo Fisher Scientific
ENVIRONMENTAL & FIELD TESTING