Thermometer cryoscopic

However, it is now recommended that thermometers be calibrated in °C. More recently, thermistors have been used instead of mercury thermometers. Cryoscopes based on dew point depression have also been approved for use. These latter instruments also use thermistors and are based on changes in osmotic pressure. Thermistor cryoscopes are now used more widely than Hortvet instruments. 

Camphor has a very high cryoscopic constant and its melting point is very greatly depressed by substances dissolved in it. The depression is about 8 times as great as that in benzene. Ebenzene = 5-1, Eoamphor = 40. This means that a molar solution in camphor melts 40° lower than the solvent, i.e. than camphor itself. Accordingly, even with relatively dilute camphor solutions the depressions obtained are so great that the sensitivity of an ordinary thermometer (which can be read to 0-25°) suffices completely for the determination.2... 

Figure 11.5 Schematic representation of a Hortvet cryoscope. 1,4, Inlet and outlet for air or vacuum supply 2, thermometer calibrated at 0.001 °C intervals 3, agitator 5, milk sample 6, glass tube 7, alcohol 8, ether cooled by evaporation 9, insulated jacket.

Fig. 1.28. Cryoscopy cell, (a) Typical dimensions of the lower section are 260-mm height and 20-mm diameter. The thermistor well is about 27 mm. (b) Cooling apparatus for the cryoscopy cell. The cryoscope slips into the jacketed tube. The jacket is evacuated or partially evacuated to control the cooling rate. Electrical readout is achieved with a digital multimeter or commercial electronics for thermistor thermometers.

Apparatus for cryoscopic determination of molar mass. The cryoscopic mercury thermometer can be replaced by a resistance thermometer or a calibrated thermistor. 

The apparatus used in this experiment is shown in Fig. 1. The thermometer is either a special cryoscopic mercury thermometer of appropriate range, with graduations every 0.01 or 0.02°C, a resistance thermometer with a resolution of 0.01°C, or a calibrated thermistor. In this experiment an aqueous solution of a weak or strong acid is mixed with crushed ice until equilibrium is attained. The temperature is recorded, and two or more aliquots of the liquid phase are withdrawn for titration to determine the equilibrium nominal concentration iiiq. The ice to be used should preferably be distilled-water ice. 

Wash about 300 mL of crushed, distilled-water ice with several small amounts of the chilled distilled water, then fill the Dewar flask about one-third full with this washed ice. Add about 100 mL of the chilled distilled water to the Dewar, assemble the apparatus, and stir the mixture well to achieve equilibrium. Record the temperature T. If a cryoscopic mercury thermometer is being used, tap it gently before reading and estimate to tenths of the smallest division. It should be realized that a cryoscopic thermometer may not read... 

Special thermometers are made for calorimetric and cryoscopic work, where it is desired to measure very accurately (to 0.01 or even 0.001 K) a temperatiue difference of the order of a few kelvin. For these thermometers the fineness of scale graduation has little to do with the accuracy with which the thermometer measures a single temperature. The scale may be in error by several tenths of a kelvin, but this error cancels out in taking differences. A typical thermometer for bomb caloiimetiy has a range of 19 to 35°C, with graduations of 0.02°C. For measuring freezing-point depressions with water or cyclohexane... 

The measurement of temperature is obviously of prime importance in all cryoscopic methods. The precise measurement of temperature and the use of thermometers is well documented. The choice of thermometer will depend on the precision required and thecryoscopic method adopted, which will be largely dictated by the solvent system under investigation. 

The temperature-resistance characteristics of thermistors depend upon their method of fabrication as well as their chemical composition. One consequence of this is that they are less stable than metal resistors. This instability is a serious disadvantage if a series of measurements is to be made over an extended period of time. Stable temperature characteristics are particularly desirable when making cryoscopic studies with organic solvent systems which are readily contaminated, because a determination of the freezing temperature is often the most convenient check on the solvent purity. This problem of long term stability can be alleviated by frequently checking the calibration against a thermometer which has stable characteristics, e.g. a platinum resistance thermometer. The temperature-resistance relationship for a thermistor follows the equation... 

Problems often arise in the calibration of temperature-measuring devices which have been built into specialised apparatus. However, in the case of aqueous solutions sufficiently reliable values of cryoscopic depressions are available in the literature from which to calibrate the thermometer within the apparatus. 

It is given by At = K(Cm, where Q is the molar concentration of dissolved solute and KfisA constant (the cryoscopic constant) for the solvent used. Measurements of freezing-point depression (using a Beckmann thermometer) can be used for finding relative molecular masses of unknown substances. 

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