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Name: Joshua
Status: other
Grade: other
Location: N/A
Date: August 2008


Question:
Why is it necessary that water must attain room temperature before it is used for an experiment which involves the calibration of volumetric glassware?



Replies:
Joshua,

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)


Hello Joshua,

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,

Joel Jadus


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- liters.

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.

cheers,

Michael Pierce


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.

Vince Calder



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