Thermistors solvent

Technically, two matched thermistors are placed within a thermally insulated compartment with a saturated solvent atmosphere. A droplet of solvent is placed onto one, a droplet of solute onto the other thermistor (Figure 4). Solvent will condense into the solution droplet and raise its temperature until the solution has the same vapour pressure as the solvent. At this point, the temperature difference between the two droplets is read. Solvents with sufficient vapour pressure, such as toluene, tetrahydrofuran, or chloroform, are best suited for strong signals, but water has also been used successfully. 

The sample chamber of the osmometer shown in figure above consists of a foaminsulated thermal block containing solvent. The chamber is machined such that the syringes can be lowered in order to apply a drop of solution to one thermistor and a drop of solvent to the other without any need to open the system. The syringe tips and the thermistors can be viewed through the minar viewing path located on the side of the sample chamber. 

Vapour pressure osmometer is a variation of the isopiestic or of the isothermal distillation techniques by which a solvent and a solution in that solvent are placed side by side in a closed container. It measures the difference in temperature created by the condensation of solvent on a sensitive thermistor containing a solution of the solute whose Molecular weight is to be determined. 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

Another correction that must be made is to account for the decrease with time of the value of DR/C caused due to the change in concentration of the solution droplet resulting from the condensation of solvent on the sample thermistor. If we assume that the concentration time dependency is a linear... 

In the vapor phase osmometry (VPO) technique, drops of solvent and solution are placed in an insulated chamber close to thermistor probes. Since the solvent molecules evaporate more rapidly from the solvent than from the polymer solution, a temperature difference results that is related to the molarity of the polymer (M), which can be determined if the heat of vaporization per gram of solvent (A) is known using the following relationship ... 

Vapor Pressure Osmometry. VPO is a very practical method for determining Mn values in a wide range of solvents and temperatures. Recently, results obtained with classical pendant-drop instruments showed a significant dependence of the calibration constant upon the molecular weight of the standards (8,9). On the other hand, with an apparatus equipped with thermistors allowing the volume of the drops to be kept constant, this anomaly is not observed (10,11). 

The thermistors used in this study were of the latter type. As shown in Tables I and II, the corresponding K values appeared clearly to be a function of the solvent and temperature but not of the molecular weight of the standards. 

Adjust the thermistor probe in the cell lid so that one of the thermistors can be loaded only with solvent, while the second can receive either solvent or lignin solutions. 

Set the bridge to zero the zero balance is adjusted with solvent on both thermistors by means of a potentiometer. For good reproducibility, the drop size should remain as uniform as possible. 

The effect of several operating parameters must be emphasized. They are the size of the drops on the thermistor beads, the response time, and the purity of both solvents and lignin samples. Furthermore, it must be noted that conflicting results have been reported on the constancy of the calibration factor (Bersted 1973, Brzezinski et al. 1973, Kamide et al. 1976, Burge 1979, Marx-Figini and Figini 1980, Kim 1985, Froment and Pla, unpubl. results, 1988). 

The thermometer and stirrer should be inserted with the thermometer carefully mounted in such a way that the sensing element (bulb of a mercury thermometer, resistance coil of a platinum resistance thermometer, or calibrated thermistor) is about halfway between the bottom of the test tube and the upper surface of the liquid and concentric with the tube so that the stirrer can easily pass around it. If the thermometer is too close to the bottom, a bridge of frozen solvent can easily form, which will conduct heat away from the thermometer and result in low readings. 

In the vapor phase osmometer, two matched thermistors are located in a ther-mostatted chamber which is saturated with solvent vapor. A drop of solvent is placed on one thermistor and a drop of polymer solution of equal size on the other thermistor. The solution has a lower vapor pressure at the test temperature (Eq. 2-72), and so the solvent condenses on the solution thermistor until the latent heat of vaporization released by this process raises the temperature of the solution sufficiently to compensate for the lower solvent activity. At equilibrium, the solvent has the same vapor pressure on the two temperature sensors but is at different temperatures. 

Vapor phase osmometers differ in design details. The most reliable instruments appear to be those which incorporate platinum gauzes on the thermistors in order to ensure reproducible solvent and solution drop sizes. In any case, the highest purity solvents should be used with this technique, to ensure a reasonably fast approach to steady state conditions. 

As defined above, osmolality = n 122.4, where iz is measured in atmospheres. In one instrument, solution and solvent vapor pressures are measured by use of sensitive thermistors to detect the difference in temperature decrease caused by evaporation of solvent from a drop of pure solvent and from a drop of solution. Because the rate of evaporation (vapor pressure) of the solution is lower, the temperature change will be less and the vapor pressure difference can be calculated. 

Another method (ASTM D-2878) provides a procedure to calculate these properties from test data on evaporation. In the method, the sample is dissolved in an appropriate solvent. A drop each of this solution and the solvent are suspended on separate thermistors in a closed chamber saturated with solvent vapor. The solvent condenses on the sample drop and causes a temperature difference between the two drops. The resultant change in temperature is measured and used to determine the molecular weight of the sample by reference to a previously prepared calibration curve. This procedure is based on an older method (ASTM D-972) in which the sample can be partly evaporated at a temperature of 250-500°C (482-932°F), and fluids not stable in this temperature range may require special treatment. 

In the test method for molecular weight (ASTM D-2503) a small sample of wax is dissolved in a suitable solvent, and a droplet of the wax solution is placed on a thermistor in a closed chamber in close proximity to a suspended drop of the pure solvent on a second thermistor. The difference in vapor pressure between the two positions results in solvent transport and condensation onto the wax solution, with a resultant change in tempera-... 

The two thermistors on which the solution droplet and the solvent droplet are placed are arranged in an Wheatstone bridge circuit in such a way that the temperature rise can be measured very accurately as a function of the bridge imbalance output voltage, AV. The operating equation is... 

An example of a vapor pressure osmometer is shown in Figure 8.14. Two thermistors, connected to measure the difference, are suspended in a thermostated measuring cell filled with the saturated vapor of the solvent. The measuring probes, which are first covered with solvent droplets, adapt to the cell temperature. 

Practical Aspects There is no membrane in a vapor-pressure osmometer. Instead there are two matched thermistors in a thermostated chamber that is saturated with solvent vapor (Fig. 4.7). With a hypodermic syringe a drop of solution is placed on one thermistor and similarly a drop of solvent of equal size on the other thermistor. The solution has a lower vapor pressure than the solvent at the same temperature, and so the solvent vapor condenses on the solution droplet. The solution droplet, therefore, starts getting diluted as well as heated up by the latent heat of condensation of solvent condensing on it. In a steady state, the total rise in temperature AT can be related by an analog of Eq. (4.55) ... 

As measuring vapor pressure depression using VPO requires extreme sensitivity, a thermoelectric method is used based on the following principle. One drop of pure solvent is placed on one of two matched temperature-sensitive thermistors located in a chamber saturated with vapor solvent, at constant temperature. On the other thermistor, a drop of polymer solution is placed. The solvent condensation on the solution drop will heat it up until its vapor pressure matches that of the pure solvent drop [35]. 

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