THE IMPORTANCE OF DENSITY Density
is
defined
as
mass
per
unit volume and is a commonly used measurement technique to determine
range, while precision without accuracy
ensure the concentration determination
will have your measurements tightly
of ethanol is accurate in the range of ±
grouped, but at the wrong temperature.
0.01 % v/v.
Depending on the liquid you are
Since
density
is
directly
related
characteristics.
sampling (or, more specifically, on the
to temperature, precisely measuring
Density measurement is an officially
liquid’s coefficient of expansion), the
temperature will also affect ethanol
recognized method to determine alcohol
measuring temperature has to be stable
concentration
concentration in alcoholic beverages
to within a narrow range in order to yield
take another look at the example on
that is used by the Alcohol and Tobacco
the required density accuracy. If the
the left. The density change as a result
Tax and Trade Bureau (TTB). Officially
measuring temperature is unstable, then
of temperature, when applied to an
recognized
density
the accuracy of the density measurement
ethanol table, will result in a change
measurement include the use of the
can be called into question, as well as any
in ethanol concentration. The 0.02°C
digital density meters.
other values (like ethanol concentration)
temperature change results in an ethanol
derived from the density.
concentration change of approximately
various
material
methods
for
DENSITY: A FUNCTION OF TEMPERATURE
For example, the chart on the left
The density of a sample is strongly dependent on the temperature at which the sample is measured. A change in temperature will influence the volume of the sample based on the sample’s coefficient of expansion, which is a measure of how the liquid will expand as a result of a temperature change. Since the mass of a sample does not change with temperature, a change in volume will influence the sample’s
determination.
Let’s
0.01 %v/v.
shows the measurement data of a 35
It is optimal to keep the measuring
volume percent (%/v/v) ethanol solution
temperature stable to within ± 0.01°C
around 20.00°C. If we look at the change
to allow for the ability to accurately
in density with respect to the measuring
measure alcohol concentration to less
temperature around the region of 20.00°
than ± 0.01 %v/v. If you were to expand
± 0.01 (from 19.99°C to 20.01°C),
the temperature uncertainty up to ±
we can see that a 0.02°C change in
0.03°C, this would result in a threefold
temperature will result in a density
loss of accuracy in the measurement of
change in the fifth decimal place. This
density and ethanol concentration.
may not seem like much but, as we will see, this will become significant.
TEMPERATURE AND THE BOTTOM LINE It is important to avoid a higher temperature inaccuracy otherwise you
relationship between temperature and
ACCURATE ETHANOL DETERMINATION WITH DENSITY MEASUREMENT
density. If temperature increases, the
The concentration of a two component
higher temperature inaccuracy can lead
volume increases, and density goes
mixture, like ethanol and water, can
to measurement errors, which can result
down. Conversely, when temperature
be determined by means of density
in such complications as claiming a
decreases, the volume decreases, and so
measurement. By measuring the density
lower proof and having to pay fines, or
density increases.
and using a density versus concentration
claiming a higher proof and then having
table,
to pay taxes on alcohol that is not there.
density. Generally, there is an inverse
This means that accurate and precise temperature
control
is
crucial
to
determining correct density values. High
the
concentration
of
both
components can be determined. Modern digital density meters can
run risks in your production process. A
In
the
highly
regulated
distilled
spirits market, density values have to be
accuracy is required to make sure that
automatically
measured
accurate and precise. In particular, the
the measured temperature is the same
density into ethanol concentration. The
ability to accurately and precisely control
as the temperature required to perform
concentration tables used will ideally
the measuring temperature should be
the
precision
comply with national standards such
given thorough consideration.
is required to make sure that that
as OIML, AOAC, IUPAC, KAEMPF, etc.
measured temperature can be repeated
These tables often refer to the density
time and time again. Accuracy without
of ethanol/water mixtures as a function
precision will have your temperature
of temperature and prescribe density
measurements spread out over a wide
accuracy in the fifth decimal place to
measurement.
High
convert
the
Siegfried Hold, Christine Bachler & Nicholas Groseclose represent Anton Paar GmbH of Graz, Austria & Anton Paar USA of Ashland, Virginia USA. For more info visit www.anton-paar.com or call (804) 550-1051.
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