Apparatus for determination of molecular weight by colligative methods

2 Apparatus for determination of molecular weight by colligative methods 

1 Vapour pressure osmometry VPO). At present commercially available apparatus for VPO is the Corona molecular-weight apparatus models 114 and 117 manufactured by the Corona Co., which have been produced in the past by Hitachi Co. Ltd Knauer Vapour Pressure Osmometer No. 731.110000 (Dr. H. Knauer Wissenschaftliche Cerate, AG) Jupiter Model 233 Vapour Pressure Osmometer (Jupiter Instrument Co. Inc.) and Gonotec model 070 (Gonotec Gesellschaft fiir MeB- und Regeltechnik m6H). A typical VPO apparatus consists of (1) two thermistors covered with glass, to which solution and solvent drops are attached (2) a cell, the interior of which is saturated with solvent vapour (3) a solvent vessel, placed at the lower part of the cell and (4) an electronic circuit. The instrument is usually thermostatted. 

Except for the Corona model 117, two carefully matched thermistors, on which the solution and solvent drops are placed, are installed in a chamber saturated with solvent vapour. The temperature difference (T-Tq) is detected as a change in the resistance of the thermistors. A large factor governing the accuracy of the temperature measurement of the drops by the thermistors in VPO is the unmatched resistance of the thermistor bead-temperature relations between two thermistors. Even if an aged and well-matched pair of thermistors are chosen at a specific temperature, these do not always match at a different temperature. That is, even if the temperature of the surroundings fluctuates only to a small extent, the conventional Wheatstone bridge is kept in a balanced state only when [14] 

The Corona model 117 is based on the principle established by Kamide et al. [14], who constructed a modified version of the sol vent-vapour chamber and thermistor assembly of Dohner et al. [15], and installed them in a Hitachi model 115 to obtain a new prototype VPO apparatus with increased sensitivity. With the conventional Wheatstone bridge circuit modified by introducing a matching circuit that has two precision variable resistors, the measurements can be made at any desired temperature without changing the thermistors. This enables a temperature difference of about 6x 10 °C to be detected. 

Eqn (5.14) indicates that the ratio KsIKe, which is a measure of the efficiency of the measurement, will become larger under the following conditions (1) large drop size (2) solvent with large A// (3) solvent with high vapour pressure (4) low temperature and (5) reduced total pressure. 

---