The Beckmann thermometer

The Beckmann Thermometer.—In order that the determination of the molar weight shall be made with sufficient accuracy, it is necessary to be able to read the temperatures with an error not exceeding o ooi — o oo2° The thermometer should therefore be graduated to, at least, hundredths of a degree. Such a thermometer, however, if made in the ordinary way, would have only a very short range unless its length were made inconveniently great. It would be necessary, therefore, to have a number of these thermometers for use at different temperatures. To obviate this necessity, a thermometer was 

This regulation of the amount of mercury in the bulb is rendered possible by having at the upper end of the capillary a small reservoir, R, into which the excess of mercury can be driven, or from which a larger supply of mercury can be introduced into the bulb. 

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Before describing the chief methods by which the molecular weight of an organic compound in solution may be determined, a description is given of the Beckmann thermometer, and of the Tabloid Press, both of which are frequently used in the above determinations. 

Freezing-point Determination. — I n the example to be described, pure benzene (see p. 136) is used as the solvent. Carefully dry the inner tube. Fit it wuth a cork and weigh it together w ith the coik suspended by a wire to the aim of the balance. Intioduce sufficient benzene to cov er the bulb of the Beckmann thermometer when it is pushed nearly to the bottom of the tube. About 10 c.c. will be found to be sufficient. Insert... 

Beckmann, E. Chem. Ber. 1886, 89, 988. Ernst Otto Beckmann (1853—1923) was bom in Solingen, Germany. He studied chemistry and pharmacy at Leipzig. In addition to the Beckmann rearrangement of oximes to amides, his name is associated with the Beckmann thermometer, used to measure freezing and hoihng point depressions. Mazur, R. H. J. Org. Chem. 1961,26, 1289. 

BECKMANN, ERNST (1853-1923). Beckmann was a German chemist who discovered in 1886 the arrangement of oximes of ketones into acid amides or anilides, named the Beckmann molecular transformation. He was the inventor of two pieces of apparatus used in determining freezing and boiling points of solutions. The Beckmann thermometer is used for determining molecular weights in solutions. 

There arc several methods for measuring this depression In the Beckmann method the freezing point of pure solvent and that of solution is measured by a special type of thermometer, the "Beckmann thermometer. The solvent or solution is contained in a double-walled glass apparatus and placed in a freezing mixture not more than S°C below the freezing point of the solvent. By rapid stirring when the liquid has supercooled about i. crystallization is induced and the temperature rises to the freezing point. 

The Beckmann thermometer used with the bomb calorimeter should be calibrated for the normal depth of immersion with which it is used. To cover the normal range of laboratory temperatures, this calibration should be obtained for three settings of the zero on the scale, convenient values being 10, 15, and 20°C. Such a series of calibrations allows automatically for emergent stem corrections and variations in the value of the degree on the thermometer scale with different quantities of mercury in the bulb, in addition to those arising from inherent variations in the diameter of the capillary bore. 

Thermometer Scale Correction. An NBS thermometer was used for the adiabatic jacket and a Beckmann differential thermometer for the calorimeter water. The Beckmann thermometer was calibrated against the NBS thermometer. Corrections were made to the initial and final temperatures recorded on the Beckmann thermometer. 

Eventually, we annotate that the experimental technique of ebullioscopy was greatly improved by the invention of the Beckmann thermometer [23],... 

Very few modem calorimeters employ mercury-in-glass thermometers. The limit of accuracy of the most accurate instrument of this type, the Beckmann thermometer, is about 0.001 K it is easily broken, and subject to errors caused by exposed stem, pressure, sticking of the mercury column, and drift in calibration. 

Setting the Beckmann Thermometer.—Before using the Beckmann thermometer, it must be set, i.e. the amount of mercury in the bulb must be so regulated that at the particular temperature of the experiment, the end of the mercury thread is on the scale. This is done as follows —... 

Since the scale of the Beckmann thermometer does not extend upwards to the end of the capillary, the temperature of FlO. 43. this bath must be at least 2°—3° higher than the highest temperature to be met with in the experiment. 

In the case of the thermometer, which consists of glass and mercury, the weight of which cannot be determined separately, the water-equivalent is obtained by making use of the fact that the specific heat of equal volumes of glass and mercury is practically the same and equal to 0 47 per cubic centimetre. To obtain the volume, a beaker of water is counterpoised on the balance, and the thermometer then supported on a stand so that the bulb is immersed in the water. The weight which has now to be added in order to obtain equipoise gives the volume of the bulb. In the case of the Beckmann thermometer, the stem above the bulb is not solid, so that the external volume does not represent the volume of the glass and mercury. The external volume of... 

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