Various experimental procedures requires the use of indirect heating via water, oil or sand

1. Handling of sand bath, water
bath, oil bath
By
Dr. S. PUGALENDHI, Emeritus Professor
Dr. P. VIJAYAKUMARY, Assistant Professor
Er. T. AYISHA NAZIBA, II-M.Tech.(REE)

2. Water bath with hotplate/stirrer
 A hot water bath is a very effective heat source when a temperature below 80°C is required. A
beaker (250 mL or 400 mL) is partially filled with water and heated on a hot plate.
 A thermometer is clamped into position in the water bath. You may need to cover the water bath
with aluminum foil to prevent evaporation, especially at higher temperatures.
 The water bath is illustrated in Figure. A mixture can be stirred with a magnetic stir bar.
 A hot water bath has some advantage over a heating mantle in that the temperature in the bath is
uniform.
 In addition, it is sometimes easier to establish a lower temperature with a water bath than with
other heating devices.
 Finally, the temperature of the reaction mixture will be closer to the temperature of the water, which
allows for more precise control of the reaction conditions.

4. Oil bath with hotplate/stirrer
 An oil bath can be used when one is carrying out a distillation or is heating a reaction mixture that needs
a temperature above 100°C.
 An oil bath can be heated most conveniently with a hotplate, and a heavy-wall beaker provides a suitable
container for the oil.
 A thermometer is clamped into position in the oil bath. In some laboratories the oil may be heated
electrically by an immersion coil.
 Because oil baths have a high heat capacity and heat slowly, it is advisable to heat the oil bath partially
before the actual time at which it is to be used.
 An oil bath with ordinary mineral oil cannot be used above 200 to 220°C.
 Above this temperature the oil bath may "flash," or suddenly burst into flame.
 A hot oil fire is not extinguished easily. If the oil starts smoking, it may be near its flash temperature;
discontinue heating.
 Old oil, which is dark, is more likely to flash than new oil is.
 Also, hot oil causes bad burns. Water should be kept away from a hot oil bath, since water in the oil will
cause it to splatter.

5.  Never use an oil bath when it is obvious that there is water in the oil. If there should be water present,
replace the oil before using the heating bath.
 An oil bath has only a finite lifetime. New oil is clear and colorless but, after extended use, becomes dark
brown and gummy from oxidation.
 Besides ordinary mineral oil, a variety of other types of oils can be used in an oil bath.
 Silicone oil does not begin to decompose at as low a temperature as does mineral oil.
 When silicone oil is heated high enough to decompose, however, its vapors are far more hazardous than
mineral oil vapors.
 The polyethylene glycols may be used in oil baths. They are water-soluble, which makes cleaning up after
using an oil bath much easier than with mineral oil.
 One may select any one of a variety of polymer sizes of polyethylene glycol, depending on the
temperature range required. The polymers of large molecular weight are often solid at room temperature.
Wax may also be used for higher temperatures, but this material also becomes solid at room
temperature.

7. Sand bath with hotplate/stirrer
 The sand bath is used in to heat organic mixtures. It can also be used as a heat source in some standard-
scale experiments. Sand provides a clean way of distributing heat to a reaction mixture.
 To prepare a sand bath, place about a 1cm depth of sand in a crystallizing dish and then set the dish on a
hotplate/stirrer unit. Clamp the thermometer into position in the sand bath.
 You should calibrate the sand bath in a manner similar to that used with the aluminum block. Because
sand heats more slowly than an aluminum block, you will need to begin heating the sand bath well
before using it.
 Do not heat the sand bath much above 200°C or you may break the dish. If you need to heat at very high
temperatures, you should use a heating mantle or an aluminum block rather than a sand bath.
 With sand baths, it may be necessary to cover the dish with aluminum foil to achieve a temperature near
200°C.
 Because of the relatively poor heat conductivity of sand, a temperature gradient is established within the
sand bath.

8.  It is warmer near the bottom of the sand
bath and cooler near the top for a given
setting on the hotplate. To make use of
this gradient, you may find it convenient
to bury the flask or vial in the sand to
heat a mixture more rapidly.
 Once the mixture is boiling, you can then
slow the rate of heating by raising the
flask or vial. These adjustments may be
made easily and do not require a change
in the setting on the hotplate.