Reserve Alkalinity
In areas of the
country where freezing
temperatures occur, glycols are
added to water-based heat
transfer fluids to provide
freeze protection. The addition
of glycols tend to make heat
transfer fluids more corrosive
than plain water. This is caused
by the degradation of glycols
into organic acids in the
presence of oxygen and heat.
Therefore, the fluid must
include an inhibitor formulation
to prevent corrosion by the
acids. The inhibitor formulation
should contain a buffering agent
and metal passivators. It
functions to buffer the acids as
they are formed, and passivate
metal surfaces to control
corrosion.
The specific term used to
describe this buffering is
reserve alkalinity. It is
measured as the number of
milliliters of N/10 hydrochloric
acid needed to titrate 10
milliliters of glycol to a pH of
5.5. It indicates how resistant
a fluid is to becoming acidic.
Reserve alkalinity is
important because it determines
how often the system should be
tested, and how the inhibitor
level should be adjusted. A
fluid with little or no reserve
alkalinity would require
constant testing and adjustment.
A reserve alkalinity of 10 to 12
is adequate in most situations.
An extremely high reserve
alkalinity, 20 to 25, may be
needed if the fluid is exposed
to very high heat or if the
installation is difficult to
monitor.
Reserve alkalinity should be
measured periodically by
laboratory analysis or by using
a test kit. In addition, the
metal passivators such as
molybdate or nitrite should be
maintained at proper levels. All
inhibitors will lose their
effectiveness over time.
Thermidaire Corporation can provide the
proper inhibitor in order to
maintain both reserve alkalinity
and the metal passivators.
TO DETERMINE RESERVE
ALKALINITY
- Using a glycol
refractometer, determine the
freeze point of the
glycol-based sample
- Obtain the percent glycol
from the Freezing Point of
Glycol Solutions Graph
- Obtain the number of
milliliters of glycol-based
sample required for the
reserve alkalinity
determination from the
Quantity of Glycol-based
Sample Determination Graph
- Transfer the required
amount of glycol-based
sample obtained in step #3
to a 100 mL graduated
cylinder. Dilute to 100 mL
with distilled water.
- Transfer contents of
graduated cylinder into a
250 mL Erlenmeyer flask
- Titrate using N/10
hydrochloric acid until a pH
of 5.5 is reached. Record
the volume of N/10
hydrochloric acid titrated
in milliliters as reserve
alkalinity
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