what is the purpose of heating the crucible to a constant mass
Heating a Crucible to Abiding Weight
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As part of the procedure for the determination of the sulfate content of an unknown sample information technology volition be necessary to accurately determine the empty weights of a prepare of crucibles. This is accomplished by offset cleaning the crucible and so heating them in the flame of a Tirrel burner. They are and then cooled and weighed. This procedure of heating and cooling is repeated until successive weighings hold to within 0.2mg. The process volition, in improver, accustom the educatee with the use of the analytical balance and the proper apply of the desiccator. Be sure that you read the appendix section dealing with the apply of desiccator.
Procedure
Launder and rinse four x ml crucibles and their lids. Place each of the crucibles past means of existing visual differences. Carefully annotation in your lab notebook what these identifying marks are and assign a number to each crucible. Do non use marker pens or pencils to mark your crucibles. The loftier heat which volition be used to take them to constant weight volition obliterate such markings and more importantly volition affect their weight in an unpredictable manner. The lids to the crucibles do not need to be taken to constant weight, nonetheless, they should be clean.
Place a Tirrel burner nether the crucible and adjust the flame of the burner to give a not-luminous flame with a full gas supply. The tip of the flame cone should exist just below just non touching the crucible. Heat the crucible at cherry heat in this manner for 5-x minutes. Allow information technology to cool for a few minutes and then place information technology in the desiccator. When information technology has reached room temperature weigh the crucible accurately, to ± 0.one mg, and then put information technology back into the desiccator. Reheat and reweigh it until successive weighings concur to within 0.ii mg.
NOTES
- Both the hot and cool crucibles should only be handled with tongs. Information technology is therefore imperative that the tips of the tongs be very clean. If they appear dirty or rusty be sure that you clean them. Dipping them into some dilute HCl is helpful in removing rust. Never allow the tips to come up in contact with the desk summit.
- Never bear on a ruby hot crucible with the tongs. The high rut used volition soften the enamel enough so that some information technology will stick to the tongs and thereby lead to erratic weights.
- Crucibles should never exist placed directly on your desk. Obtain a piece of aluminum foil which tin can exist used to place the crucibles and tongs.
- Make certain that the plate in the desiccator is completely clean. As well cheque to come across that crucibles sitting in the holes of the plate do non touch on any role of the wire gauze which separates the desiccant from the rest of the desiccator.
Sit-in
Your first exercise teaches you some skills on the proper use of the laboratory burner (in this case called a Tirill Burner), the aligning of the flame and the proper placement of a crucible which is to be heated to constant weight.
Y'all ought to make sure at the showtime that the crucibles you use take indelible identifying marks on them. Sometimes there are indentations left during the industry of a crucible which will allow you to distinguish ane from some other. More than likely, at that place will be a letter or number written into the side by a former student, such as the one shown at the left. Note the "A" scratched into the glossy surface. This tin can be done with the diamond pencil available in the lab, every bit shown at the right .
The hottest part of the flame is at the elevation of the vivid cone of oxidation. The zone below the sharp point of the cone is filled with a rising mixture of cool gases not all the same in the process of combustion. Although the rim around this zone is quite hot, the gases inside are substantially at room temperature. See that placement on the left. Such a placement is not recommended Note the right placement of the crucible in the photograph at the correct.
The crucible may be placed right side upwards, equally in the left photograph, or on its side, as in the right photo. Heating information technology 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 exist heated to incandescence for 5-10 minutes (left). At the terminate of this time, remove the flame and allow the crucible to absurd on the iron triangle for a few minutes (right)
When transferring a crucible to the dessicator tongs must be used. If the crucible has been allowed to cool, you may use blank tongs (left), but if you must transfer a hot crucible, make sure that you embrace the ends of the tongs with aluminum foil (right) every bit the pigment on the tongs might melt onto the crucible and alter its weight. Make certain you take hold of the crucible on the edge as shown. Don't endeavour to pick upward a crucible past squeezing the tongs on the outer rim both because the crucible may slip from your grip owing to its slick walls. There is likewise the possibility of breaking a hot crucible that mode.
Place the cool crucible into 1 of the holes in the plate in your dessicator and cover until it has reached room temperature. At that time you lot may remove it for weighing.
If a crucible is non at the temperature of the balance compartment it will likely show a weight unlike from that it would show were it at the same temperature. If it is hotter the weight will be slightly less, if colder slightly more than owing to the production of convection currents which bear on the apparent mass. Here are some photos to prove the point. (Please note that 2 separate analytical balances had to be used for this sequence of photos so as non to interfere with pupil use. The final agreement is within that which is routinely observed for ii separate balances.) A dry common cold crucible is weighed to the nearest 0.0001 thou.
The crucible is then heated to incandescence for 5 minutes, equally shown at the left. The fire is extinguished and the crucible is allowed to cool for five minutes.
Its credible weight is less by more than than four milligrams.
Information technology is allowed to cool for an boosted 5 minutes.
Its exhibited weight is still 0.0003 g less than that of the cold crucible, above.
Finally, after an additional five minutes of cooling, the crucible exhibits a weight slightly greater than that of the cold crucible, to a higher place. The agreement here is within that which is routinely observed for two split balances. This cooling sequence took place outside the dessicator. The amount of time necessary to achieve thermal equilibrium with the environs will likely exist somewhat longer when you allow the crucible absurd in the dessicator.
Source: https://chem.libretexts.org/Ancillary_Materials/Demos_Techniques_and_Experiments/General_Lab_Techniques/Heating_a_Crucible_to_Constant_Weight#:~:text=As%20part%20of%20the%20procedure,of%20a%20set%20of%20crucibles.
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