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CR Scientific


CAUTION:  If you choose to attempt any of the experiments or procedures described on this site, you do so at your own risk. Working with fire and/or chemicals can be dangerous and should be attempted only by competent, trained persons.

Glass is fragile.  Broken glass can be extremely sharp!  Be very careful when cleaning up broken glass.

Information presented on this web site is not intended to substitute in any way for proper training.  Please review our Terms of Use.


Proper heating of Retorts and Flasks

Any lab instructor worth his or her "salt of phosphorus" will keep constant watch for improper heating of glassware.  Students occasionally forget a simple rule:  Do not heat glassware with a direct flameAny glassware can shatter if heated directly with a torch, a Bunsen burner, or even an alcohol lamp;  the heat where the flame is applied does not have the chance to diffuse into the rest of the flask.  This drastic temperature differential will result in dangerously uneven thermal expansion.  Even with borosilicate glass (which has a low thermal coefficient of expansion), this can be sufficient to shatter it.  
We have successfully shattered examples of the best-known brands of glassware by direct heating with a gas flame.  The breakage did not always happen right away;  sometimes the glass was observed to shatter after more than thirty minutes of seemingly problem-free heating with a direct torch flame.  Though a given piece of glassware may have survived direct-flame heating in the past, you are needlessly asking for trouble each time you do it again.
As mentioned before, wire gauze is the bare minimum precaution for heating with a flame.  However, even this setup can lead to glass breakage in some circumstances.  If the center of the gauze is heated so quickly or for so long that it glows, then there is a great excess of localized heat that could cause a problem.  Wire gauze to boiling flask is like tangent to circle;  it contacts the round-bottom glassware at only a small footprint;  thus, there is still an undesirable concentration of heat.
The best way to heat a flask, then, is to do so evenly and gradually.  This brings us to the sand bath, which can be a useful way to heat retorts, distilling flasks, and the like.  Sand, like water or mineral oil, conforms to the shape of the vessel.  The sand bath, however, is often preferable to the water bath or mineral-oil bath in that (1) there's no open container of liquid to worry about, and (2) sand won't evaporate or impose a particular temperature limit (though this could also work against you).  The sand bath is especially useful when something must be heated above 250 C (the approximate limit for an oil bath).
The sand bath is slow to bring up to the desired temperature and also slow to cool.  It can be heated with a burner flame or a hot plate.  The sand bath container should be iron or steel.  (Pure copper is acceptable, as long as the flame used to heat it doesn't approach copper's melting point of 1083 C).  Don't use aluminum, zinc, tin, pot metal, pewter, or unidentified  metals, though.  This author once used a container which on cursory examination appeared to be steel;  a simple test with a magnet would have proven otherwise.  The container melted through at the bottom after three twelve-hour heating sessions.  Fortunately, the only thing to escape was dry sand.  Hence, sand baths are desirable in more ways than one.

Supplies: 

Safety goggles
Electric hot plate
Clean, dry sand; 
Retort & receiving flask (or other distillation setup);  
Metal pot with flat bottom and having diameter somewhat larger than the bottom of the retort or distilling flask; 
Soft-plastic or rubber tubing (if using a condenser with water jacket)  with nearby access to a faucet where it can be connected;
Material to be distilled

Generalized methods:

The retort or boiling flask is placed in a metal container of dry silica sand (avoid types of sand that contain gypsum, fillers, colorants, etc).   This container, in turn, sits on a hot plate or a ring stand.  The retort itself should be supported with a clamp that's attached to a support stand.
The long spout of the retort ends up in a collection flask.  The fit must not be too tight, or the flask and retort will get stuck together and possibly cause breakage.
The collection flask might sit in an ice bath, as illustrated below:

Heating a Retort with a Sand Bath

A. Retort containing the substance to be heated.
B. Clean, dry sand in a metal container, preferably steel.
Between A and B, one may wish to have a thin layer of aluminum foil; this protects the glass from the abrasive action of the sand.
C. Hot plate or other laboratory heat source.
D. Receiving flask into which the spout of the retort will fit (not too tightly!)
E. Container of crushed ice.
F.  Steady support.  A lab scissor jack is acceptable if it is anchored in place so it can't be knocked over.
G. Clamp & Support Stand to steady the retort.  The hot plate and retort may need to be set  directly on the lab bench, not on the support stand base (depending on their size).
H. This joint is wrapped with polyolefin laboratory film (such as Parafilm), depending on what you're distilling.  Laboratory film can be used only if the  retort doesn't get too hot or the material being distilled doesn't attack the plastic.   If you're going to do use the polyolefin film, wrap it before placing the flask, D, into the crushed ice bath.

Make sure everything is stable;  the work area must be a hard surface that's free of wobble and clear of any papers and other clutter that could pose a fire hazard.
A laboratory thermometer can be inserted into the sand to monitor the temperature.  It must be a high-temperature type (either a mercury or an oven-type thermometer).  Do not use an alcohol thermometer.  The thermometer bulb should have a layer of aluminum foil protecting it from abrasion.  DO NOT leave the thermometer unattended.  DO NOT select a thermometer that isn't rated for the anticipated temperature.  If the temperature gets too high, the thermometer will burst!

As noted above, you can also use a layer of aluminum foil between the retort bottom and the sand (unless you're working with something that reacts strongly with aluminum).



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Be careful with cooling hoses...

If you're using a condenser with a water-cooled jacket (e.g., a Liebig condenser), make sure the water hoses are attached snugly.  They must not come loose at any time during heating.  The cooling water must never splash onto the hot glassware, not even in tiny droplets.

Also be careful that you  don't catch your sleeve, arm, hand, foot, or anything else on the cooling hoses.