Hot plates, electric

Electric hot plates may also be employed for heating. These should be of substantial construction and be provided with three-way switches. The diameter of the heavy cast-iron top may vary between 5 and 10". It is usually advisable to interpose a sheet of asbestos board between the metal top and the vessel to be heated, particularly if the contents of the latter are liable to bump. ... 

Many forms and sizes of magnetic stirring apparatus are available commercially. These include those fitted with an electric hot plate attached to the flat top the hot plate is controlled by an energy regulator or variable transformer (Variac). 

Fig. II, 37, 2 depicts the apparatus for dealing with comparatively small volumes of liquid. The essential feature is the special condenser with a take-off tube. A few small pieces of porous porcelain are introduced and the flask is heated either on a water bath or upon an electric hot plate. The assembly is also useful in the recrystaUisation of compounds which dissolve slowly excess of solvent may be employed, and the excess of solvent subsequently removed by distillation.

With some acids (e.g., succinic acid and sulplianilic acid) more satisfactory results are obtained by reversing the order of mixing, i.e., by adding the solution of the so um salt of the acid to the reagent. It should be pointed out that the melting points of the derivatives as determined on the electric hot plate (Fig. II, 11, 1) may differ by 2-3° from those obtained by the capillary tube method. In view of the proximity of the melting points of the derivatives of many acids, the mixed m.p. test (Section 1,17) should be applied. 

Azlactone of a-benzoylaminocinnamic acid. Place a mi.xture of 27 g. (26 ml.) of redistilled benzaldehyde, 45 g. of Mppuric acid (Section IV,54), 77 g. (71-5) ml. of acetic anhydride and 20-5 g. of anhydrous sodium acetate in a 500 ml. conical flask and heat on an electric hot plate with constant shaking. As soon as the mixture has liquefied completely, transfer the flask to a water bath and heat for 2 hours. Then add 100 ml. of alcohol slowly to the contents of the flask, allow the mixture to stand overnight, filter the crystalline product with suction, wash with two 25 ml. portions of ice-cold alcohol and then wash with two 25 ml. portions of boiling water dry at 100°. The yield of almost pure azlactone, m.p. 165-166°, is 40 g. Recrystallisation from benzene raises the m.p. to 167-168°. 

Acetylsalicylic acid decomposes when heated and does not possess a true, clearly-defined m.p. Decomposition points ranging from 128° to 135° have been recorded a value of 129-133° is obtained on an electric hot plate (Fig. II, 11, 1). Some decomposition may occur if the compound is recrystaUised from a solvent of high boiling point or if the boiling period during recrystallisation is unduly prolonged. 

Burning reagents. Turn out all gas burners and switch off all electric hot plates in the vicinity remove everything which may ignite. The control of the fire depends upon its size and kind. 

Various methods of heating are required in the analytical laboratory ranging from gas burners, electric hot plates and ovens to muffle furnaces. 

Hot plates. The electrically heated hot plate, preferably provided with three controls — Low , Medium and High — is of great value in the analytical laboratory. The heating elements and the internal wiring should be totally enclosed this protects them from fumes or spilled liquids. Electric hot plates with stepless controls are also marketed these permit a much greater selection of surface temperatures to be made. A combined electric hot plate and magnetic stirrer is also available. For some purposes a steam bath may be used. 

The advantages of the above electric hot plate method for the determination of melting points are obvious these include —... 

Preheat 3 L of buffer (see Note 7) to boiling in a stainless steel 5.5-L-capacity pressure cooker (Prestige) without sealing the lid, using an electric hot plate as the heat source. 

In a stainless-steel beaker of approximately 2-1. capacity (120 mm. by 165 mm.) equipped with an efficient mechanical Ni-chrome or Monel stirrer (Note 1) and heated by an electric hot plate are placed 178 g. (4.3 moles) of 97% sodium hydroxide pellets, 178 g. (2.7 moles) of 85% potassium hydroxide pellets (Note 2), and 50 ml. of water. The mixture is stirred and heated to 160°, at which temperature the hot plate is turned off. Twen-... 

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