Water Temperature and Volumetric Glassware
Date: August 2008
Why is it necessary that water must attain room
temperature before it is used for an experiment which involves the
calibration of volumetric glassware?
The expectation is that the glassware will be used to measure liquids
at room temperature. Since liquids have a tendency to change volume
(at the level of precision of calibrated instruments), then you want
the liquid to be at the temperature you are most likely to use so that
the calibration will have the most accuracy.
Greg (Roberto Gregorius)
According to PV = nRT, the temperature of a liquid or gas is directly
proportional to its pressure and volume. In other words, as the
temperature of your water increases, its volume will increase. That
being said, most volumetric glassware is rated by Class (Class A being
the highest standard for quantitative use, while Class B is less accurate
and used for more qualitative work). The Class A glassware is typically
calibrated to contain (TC) a certain volume of water (with an indicated
level of error) at a certain temperature (often 20 °C). This 20 °C
temperature indicates that you may use the glassware at "room temperature"
for the stated volume.
On a side note, when intending to measure certain quantities of water at
temperatures other than "room temperature" (~20-25 °C); we often measure
the quantity of water "gravimetrically" or by weighing the water. Since
the weight of the water is independent of the temperature, if we target a
certain weight, we will always arrive at the same quantity of water
regardless of temperature...
Hope this helps,
Things (liquids, solids and gases) respond to changes in temperatures. For
instance, liquid water is at its most dense at 4 degrees C. If you then heat
that volume of water to just under the boiling point, the volume will expand
by around 4 & 1/2%. If measure out a volume of 100 milli-liters of water at
4C and I measure out 100 milli-liters of very hot water, we will have two
different amounts of water. To have the same amount, I will need 104.5 milli-
Because things such as temperature or pressure can change from place to place,
we need a "standard" by which to compare. One of the easiest for thermal
properties is "STP" or "standard temperature and pressure." Thus we will
agree if we are going to measure a certain quantity or volume of water then
we measure it at an agreed temperature and pressure. Most frequently that
standard is what we will call room temperature. For most applications that
is good enough for the accuracy we need. However, for very precise comparisons
we need to really specify what the value of room temperature actually is and
what elevation (pressure) we will use.
Taking things one additional step : We humans cannot really agree on things
very often and even "STP" is no exception, nor is it universal and "standard".
There are several different definitions and if you are going to be doing a
very precise experiment or comparison, you will have to be very careful to
make sure that you adjust and use a consistent standard. Some of the
"standards" are based on our best attempts and making universal measurements.
Others make sense only for extreme conditions. And others we have purely
from what seem to me to be historical or archaic use and we have just never
moved beyond them. The classic example is that the US largely uses old
"English" units while most of the rest of the world switched years ago to
the highly superior metric system.
Glass expands as the temperature increases, so a 1 liter volumetric flask is
slightly larger than 1 liter if the temperature is greater than the calibration
temperature (25 C.). This leads me to my "soap box". I am a proponent of using
weight titrations instead of volumetric titrations. There are several
significant advantages. 1. There is no need to be concerned about the
volumetric correction. 2. The precision of the titration is at least 10
times that of volumetric titrations. 3. Hypodermic syringes can be used as
vessels, so the size of the samples can be reduced to less than 100 cm^3
rather than the clumsy liter-sized volumetric flasks. 4. There are other
advantages, but the three above are adequate to illustrate the advantages
of weight titrations.
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Update: June 2012