of acid (such as sulfuric acid or hydrochloric acid) that is required
to reach this endpoint, the alkalinity of the test solution can be
determined. Alkalinity is often expressed in units of milligrams
per liter of calcium carbonate (mg CaCO3/L).
Two endpoints are commonly used to determine alkalinity.
Phenolphthalein alkalinity, or P-alkalinity, involves the titration
of a sample with acid to pH 8. 3. This determines the extent of the
contributions from carbonate and hydroxyl ions, as well as part
of the bicarbonate contribution. An alternative measure of alkalinity, known as total alkalinity or M-alkalinity (derived from the
methyl orange indicator used to calculate it), can be determined
by titration with acid to pH 4. 5. At this pH, all of the hydroxyl,
carbonate, bicarbonate and other basic ions have been titrated.
The measurement of P- and M-alkalinity enables analysts to
determine the properties of water. For industrial water applications, for instance, a certain level of P-alkalinity is important to
minimize corrosion due to acid. However, excessive P-alkalinity
can result in the deposition of caustic substances on the inside of
boilers, for example, which can cause key components to become
brittle and fail. A P-alkalinity measurement of greater than half of
the M-alkalinity, means that the water is caustic due to contributions from hydroxide and carbonate alkalinity. A P-alkalinity of
zero, on the other hand, means that the water is free of hydroxide
and carbonate alkalinity.
pH electrodes are commonly used for alkalinity titration using
the preset endpoint technique. To ensure reliable pH measurements, calibrate the electrode prior to the analysis. Substances
such as detergents, oils, suspended solids and precipitates can
coat the glass surfaces of the electrode, causing slow measurement
responses. To collect accurate readings, it is important to regularly
clean the electrode and allow sufficient time between the addition
of titrant to enable the measurement to reach equilibrium.
Advantages of the titration method include the fact that color
and turbidity do not interfere with measurements and the testing
range covers the entire range of possible alkalinities, enabling
optimal confidence in results. Titration can be automated to offer
enhanced workflow efficiencies, while improving the consistency
and accuracy of results by reducing the potential for human error.
An alternative method of determining alkalinity is the colorimetric approach. Colorimetric alkalinity measurements are based
on a similar principle to conventional titration—however, a preset
volume of acid is added while the pH of the endpoint varies.
Using this method, various acids are added along with a pH-sensi-tive color indicator. As the acids react with the alkaline species in
the sample, the associated pH changes result in a color change of
the indicator that is proportional to the alkalinity of the sample.
This color change can be measured at a specific wavelength that is
dependent on the indicator. While this approach is not approved
by the EPA for water testing applications, it can provide a fast and
convenient means of determining alkalinity for research or preliminary testing purposes. It is important to note that water color
and turbidity can interfere with measurements, and measures to
minimize these effects must be taken.
Regardless of the chosen testing method, to drive the collection
of accurate and reliable results, proper sample collection and stor-
age is essential. In general, samples for alkalinity testing should
ideally be collected in polyethylene or borosilicate glass bottles
and stored at low temperatures. Bottles must be completely filled
and capped tightly. pH measurements should be taken at the
sample source. Alkalinity samples should be analyzed as soon as
possible; holding times are generally up to 14 days at 4 C.
The latest technological innovations
Recent advancements have resulted in the introduction of systems that have been designed to optimize accuracy, reproducibility
and reliability of results and boost overall laboratory productivity.
These instruments offer highly accurate burette and dispenser
capabilities to enable precise dosing of the titrant and ensure
consistency of results. The latest systems also offer the ability to
save methods to support repeatable titration procedures without
the need to repeat setup. This can be particularly beneficial to laboratories routinely performing titrations on similar samples that
require only minor method changes. Methods can also be shared
among multiple titrators by simply using a USB flash drive—for
example, when a system needs to undergo routine maintenance.
To meet laboratories’ traceability needs, modern systems are
also capable of saving large numbers of data sets with date/time
stamp, allowing users to maintain a history of sample measurements for future reference. Additionally, instruments have been
developed to promote usability, featuring simplified navigation, as
well as quick and easy reading of results, being ideal for laboratory technicians transitioning from manual to automated titrations.
Robust water alkalinity monitoring
Measurement of water alkalinity is important for a wide range
of applications, including environmental protection as well as
drinking water and wastewater treatment. Accurate determination of P- and M-alkalinity is essential to ensure that processes
operate safely and efficiently, pipes and equipment are protected
from the damaging effects of corrosion, and the harmful impact
of increased pH on the environment is minimized. Titration and
colorimetric methods offer a reliable means of measuring alkalinity, giving analysts the information they need to drive safe and
Ion concentration measurements using a Versa Star Pro meter and Orion ion
selective electrode. Photo: Thermo Fisher Scientific