what is the purpose of heating the crucible to a constant mass

Heating a Crucible to Constant Weight

by Oliver Seely

This Web page and accompanying photos are in the public domain and may exist copied without restriction.

Your first exercise teaches you some skills on the proper use of the laboratory burner (in this case called a Tirill Burner), the adjustment of the flame and the proper placement of a crucible which is to be heated to constant weight.

You ought to make sure at the beginning that the crucibles yous utilize have indelible identifying marks on them. Sometimes at that place are indentations left during the manufacture of a crucible which volition allow you to distinguish one from another. More likely, there volition be a alphabetic character or number written into the side by a one-time student, such equally the one shown at the left. Annotation the "A" scratched into the sleeky surface. This tin be done with the diamond pencil available in the lab, as shown at the correct.

The hottest part of the flame is at the top of the bright cone of oxidation. The zone beneath the sharp point of the cone is filled with a rising mixture of cool gases non yet in the process of combustion. Although the rim around this zone is quite hot, the gases inside are substantially at room temperature. Run across that placement on the left. Such a placement is not recommended Note the right placement of the crucible in the photograph at the right.

The crucible may be placed right side up, as in the left photo, or on its side, as in the correct photo. Heating it on its side is preferred because the overall strain on the ceramic material at the high temperatures used is somewhat less.

The crucible ought to be heated to incandescence for five-10 minutes (left). At the end of this time, remove the flame and allow the crucible to cool on the iron triangle for a few minutes (correct)

When transferring a crucible to the dessicator tongs must be used. If the crucible has been allowed to absurd, you may use bare tongs (left), but if yous must transfer a hot crucible, make sure that you comprehend the ends of the tongs with aluminum foil (right) as the paint on the tongs might melt onto the crucible and change its weight. Brand certain you take hold of the crucible on the edge as shown. Don't try to selection up a crucible by squeezing the tongs on the outer rim both considering the crucible may slip from your grip owing to its slick walls. There is too the possibility of breaking a hot crucible that way.

Place the cool crucible into i of the holes in the plate in your dessicator and cover until it has reached room temperature. At that time you may remove it for weighing.

If a crucible is not at the temperature of the balance compartment information technology will likely evidence a weight different from that information technology would show were information technology at the same temperature. If it is hotter the weight volition exist slightly less, if colder slightly more than attributable to the production of convection currents which affect the apparent mass. Here are some photos to prove the point. (Please note that two separate analytical balances had to be used for this sequence of photos so as non to interfere with student utilize. The final agreement is within that which is routinely observed for two split balances.) A dry cold crucible is weighed to the nearest 0.0001 1000.

The crucible is then heated to incandescence for five minutes, as shown at the left. The fire is extinguished and the crucible is allowed to cool for five minutes.

Its apparent weight is less by more than than four milligrams.

It is immune to cool for an additional five minutes.

Its exhibited weight is nevertheless 0.0003 thousand less than that of the cold crucible, above.

Finally, after an boosted five minutes of cooling, the crucible exhibits a weight slightly greater than that of the cold crucible, to a higher place. The understanding here is inside that which is routinely observed for two separate balances. This cooling sequence took identify outside the dessicator. The amount of time necessary to achieve thermal equilibrium with the surroundings will likely be somewhat longer when you let the crucible cool in the dessicator.

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Source: http://www2.csudh.edu/oliver/demos/heatcruc/heatcruc.htm

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