Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

 
from : Edwin Edser. Heat for Advanced Students. Macmillan and Co., Limited. London (1911) pp. 188-190.
 
Copyright © 1998 Richard A. Paselk
 

 
 
Experimental Determination of Boiling Points.--From what has previously been said, it appears that the temperature of a liquid may, in certain circumstances, rise considerably above its normal boiling point. Consequently it is only in determining the boiling points of solutions that the thermometer is placed in the liquid, and some arrangement must then be made to promote free ebullition.
 
The usual method of determining the boiling point of a pure liquid is to place the bulb of the thermometer in the vapour given off during ebullition. The space containing the thermometer should be jacketed in the manner described in connection with the determination of the boiling point of a thermometer (p. 11)
 
The actual temperature of the vapour above a boiling solution is generally slightly lower than the temperature of the solution. Thus above a salt solution, the temperature of which is 110° C., the steam may reach a temperature, say, of 105° C. A thermometer placed in this steam will, however, indicate a temperature of 100° C. The reason of this is, that the steam condenses to pure water on the bulb and stem of the thermometer, and as this water has a large surface, any further heat communicated will cause part of the water to evaporate without a rise of temperature occurring.
 
On the other hand, if the temperature of the thermometer falls below 100° C., steam will be condensed on it, and the temperature will be raised by the latent heat rendered up. In order to provide a larger surface for evaporation, the bulb of the thermometer is often surrounded loosely with cotton wool.
 
Determination of the Boiling Point of a Solution.--As pointed out above, a thermometer when placed in the steam given off from a boiling aqueous solution of a salt, will indicate the boiling point of the water, and not that of the solution. A similar law applies to solutions in general. In order to determine the boiling point of a solution, the thermometer bulb must be completely immersed. Special precautions must be taken to prevent the occurrence of boiling by bumping, and the consequent rise of temperature of the solution above its boiling point.
Beckmann's Boiling Point Apparatus.--Fig. 85 represents diagrammatically the essential points of Beckmann's apparatus for determining the boiling points of solutions. A test tube A, provided with a side inlet tube U, is used to contain the solution. A piece of platinum wire P, is fused through the bottom of the test tube, and a number of glass beads, G, are also contained by it. The beads and platinum wire serve to promote free ebullition. The bulb of a sensitive thermometer T, similar to that described. on p. 15, dips into the solution. A spiral glass tube, K1, serves to condense the vapour given off; the condensed liquid runs back into the solution, so that the strength of the latter is maintained constant. The test tube containing the solution is surrounded by a glass vessel C, provided with double walls, forming a vapour jacket. A liquid, possessing a boiling point slightly higher than that of the solution in A, is placed in C; some pieces of porous earthenware, D, serve to promote free ebullition, whilst the condensing spiral K2, prevents loss of the liquid. The whole of the above arrangement is mounted on a stand made from asbestos mill-board. Two pointed Bunsen flames play on the wire gauze E, and the hot gases impinge directly on the lower surface of the vapour jacket C. Direct communication of heat to the vessel, A, is prevented by the double cylinder, F1, F2, of asbestos mill-board, and by a roll, H,of asbestos paper. Thus the solution in A receives heat only from the liquid and vapour in the vessel C.
 
The solution may be introduces into the vessel by way of the side inlet tube B, the condenser K1 being removed.
 


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Last modified 22 July 2000