Artisan Spirit: Summer 2014

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Page 67

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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