Thermistor average temperature

Thus the molecular weight of the solute can be determined in theory by measuring AT/c2 and extrapolating this ratio to zero C2- (Since AT is small in practice, T may be taken without serious error as the average temperature of the two thermistors or as the temperature of the vapor in the apparatus.)... 

Procedure Following the manufacturer s instructions, balance the osmometer to zero with o-dichlorobenzene on both thermistor beads, and establish the calibration constant, K, at 100°, using the four Calibration Standards. When the temperature within the osmometer has re-equilibrated to 100°, place an aliquot of the most concentrated Sample Preparation on the sample thermistor bead. After 4.0 min, balance the instrument to zero with the potentiometer, and record the AR value. Repeat this procedure with the same Sample Preparation two or three times, and average the AR values for that concentration. In a similar manner, obtain the average AR values for each of the other three concentrations of the Sample Preparation. Plot the four average AR values for the Sample Preparations as a function of AR/concentration, and extrapolate the line to zero to obtain the constant, Kjj, for the sample. Divide K by Kv to obtain the molecular weight of the sample tested. 

Vapor pressure osmometric (thermoelectric, vaporometric) measurements depend on the following principle A drop of a solution with a nonvolatile solute resides on a temperature sensor, i.e., a thermistor. The surrounding region is saturated with solvent vapor. Initially, the drop and vapor are at the same temperature. Since the vapor pressure of the solution is lower than that of the pure solvent, solvent vapor condenses on the solution drop. Because heat of condensation is released, the temperature of the drop rises until the difference in temperature A Tth between the drop and the solvent vapor again eliminates the difference in vapor pressure, so that the chemical potential of the solvent in both phases is equal. An analogous equation to that which applies in ebulliometry is applicable in this case to the relationship between the temperature difference A Tth and the number-average molar mass Mn)= Mioi the solute. 

The vapor pressure method uses two thermistor probes to measure the temperature difference between a drop of solvent placed on one probe and a drop of a solution of solute and solvent on the other probe. The difference in rates of vaporization at the two probes leads to a difference in temperature at the two probes. This difference in temperature can be related to the number average molecular weight. For more insight into the method and the magnitude of the temperature differences encountered (see Kumar and Gupta (1998)). 

Semiconductor thermometers can be built in many shapes. Frequently they are very small beads, so that their heat capacity and thermal lag are small. They may also be made in form of large disks, so that they can average the temperature over a larger object. Typical materials which are used in thermistor thermometers are iron oxide, magnesium chromate, magnesium aluminate or sintered mixtures of nickel oxide, manganese oxide and cobalt oxide. 

Vapor-phase osmometry is also a colligative method and is based on the decrease of the vapor pressure, due to the addition of a solute into a pure solvent (Figure 6.4). Like osmotic pressure this property depends exclusively on the number of molecules of solute introduced and thus gives access to the number average molar mass. The disadvantage of VPO is its lack of sensitivity, due to the very small variations of the vapor pressure exhibited by dilute polymer solutions. It is, in fact, well suited to the analysis of low molar mass samples (<2 x 10 g mol ) and is thus complementary to membrane osmometry. Because it is easier to measure small variations in temperature than those in vapor pressure, a thermoelectric device is used to transform the increase of vapor pressure in a VPO experiment into a variation of temperature. Thus, two thermistors are placed in a closed chamber containing a pure solvent at a given temperature. If a pure solvent drop is placed on each of the two thermistors, they will indicate the same temperature. On the other hand, if a drop of a dilute polymer solution is placed on one of the two thermistors, a variation in temperature will result, caused by the condensation of pure solvent on this thermistor due to the difference in chemical potential between the drops. 

For molecular weights less than about 20,000, another technique has been automated. In the so-caUed vapor pressure osmometer, there is no membrane [21]. A drop of solution and a drop of pure solvent are placed on adjacent thermistors. The difference in solvent activity brings about a distillation of solvent from the solvent bead to the solution. The temperature change that results from the differential evaporation and condensation can be calibrated in terms of the number-average molecular weight of the solute. The method is rapid, although multiple concentrations and extrapolation to infinite dilution are still required. 

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