DETERMINATION OF MOLECULAR WEIGHTS OF ORGANIC COMPOUNDS USING THERMISTORS

G. N. IVANOV, V. P. LOPATINSKY

(Presented by the scientific seminar of the organic chemistry section)

In practice, when working with organic substances, such methods for determining molecular weights as cryoscopic and ebullioscopic methods are usually used. Other methods are used less often because of their greater complexity. A drawback of both methods is their not very high accuracy, which is usually associated with inaccurate temperature measurement. Therefore it was proposed to measure temperature differences by means of semiconductor thermal resistors—thermistors, which are distinguished by high sensitivity. Such methods for determining molecular weights are described in the works of Müller [1], Neumeier [2], Kulkarni [3], and Tarakanov and Gusev [4]. The advantages of these methods are the speed of determination and fairly high accuracy. However, their instrumental arrangement is rather complicated; moreover, in works [2] and [3] the technique for determining molecular weights is not described.

In the present work a new variant of the method for determining molecular weights using thermistors as temperature-sensitive elements is presented. In contrast to the methods described in works [1], [2], [4] and based on measuring temperature differences arising during condensation on thermistors of vapors from the pure solvent and the solution under investigation, in our work we measured the temperature differences arising during evaporation of the solvent (from a pure sample of it and from the solution), which covered two thermistors with a thin film. A similar method was used by Kulkarni [3], with the difference that he applied drops of solvent and solution to the thermistors.

On the apparatus assembled by us, the molecular weights of carbazole derivatives and a number of other organic compounds with molecular weights up to 1500 were determined. The relative error in the determinations usually ranged from ±0.3% to ±2% and only in individual cases exceeded ±3%. Pure benzene and acetone were used as solvents, and their purification was carried out by generally accepted methods [5]. The advantages of the method are its accuracy, the simplicity of the determinations, and the speed with which they are performed. The main expenditure of time falls on establishing thermal equilibrium in the thermostats; the determination itself is carried out within a few minutes.

Apparatus and Technique of the Determinations

The apparatus of the installation consisted of the following units (Fig. 1): a water thermostat—\(D\), an air thermostat—\(C\), evaporating vessels \(A\) and \(B\), and a measuring circuit (Fig. 2). The evaporating vessel was a glass test tube with a ground-glass stopper, in which the thermistor \(T\) was fastened (Fig. 3). Pure solvent was poured into one vessel, and the solution under investigation into the other. The measuring circuit consisted of the following elements (Fig. 2): thermistors \(T_1\) and \(T_2\) (KMT-4), fixed resistances \(R_1\) and

Fig. 1.

Fig. 2.

\(R_2\) (150 kΩ), a wire rheostat \(R_3\), a microammeter \(G\) (M-95), and a dry battery \(B\) (15 V). The evaporating vessels were placed in a “Vobser” water thermostat И-8 with a temperature-control accuracy of \(\pm 0.02^\circ\text{C}\). The air thermostat was a glass–Plexiglas frame with a temperature-control accuracy of \(\pm 0.1^\circ\text{C}\).

Application of the solution to the thermistor in the evaporating chamber was carried out as follows. A volume of solution equal to approximately 1 ml filled a recess at the bottom of the chamber, made in the shape of the thermistor and with dimensions slightly larger than those of the thermistor. The thermistor entered approximately halfway into the recess.

A spiral of thin nickel wire was wound onto the body of the thermistor. The amount of solution was selected so that almost its entire volume was spent on wetting the spiral around the thermistor. The connecting leads between the thermistors and the bridge circuit were brought out through the side tube and were potted in it with epoxy resin.

Fig. 3. 1—connecting leads potted with epoxy resin; 2—recess for the thermistor.

The procedure for the determinations is as follows. Approximately 1 ml each of pure solvent and solution is poured into the evaporating vessels. The vessels are closed with stoppers and allowed to reach thermal equilibrium, after which the position of the galvanometer light pointer is adjusted with potentiometer \(R_3\). Then the stoppers are opened simultaneously and the readings are taken.

galvanometer reading. Before the next determination the vessels are washed and the same operations are repeated.

The instrument is calibrated in the coordinates: galvanometer readings—mole fraction of the dissolved substance. The molecular weight of the unknown substance is determined from the equation

\[ M_{\mathrm{B}}=\frac{q_{\mathrm{n}}\left(1-M_{\mathrm{d}}\right)} {M_{\mathrm{d}}-\dfrac{q_{\mathrm{r}}}{M_{\mathrm{r}}}}}\,; \]

where

\[ \begin{aligned} q_{\mathrm{n}}&\text{— sample weight of the substance under investigation;}\\ M_{\mathrm{d}}&\text{— mole fraction of the substance under investigation in the solution;}\\ q_{\mathrm{r}}&\text{— sample weight of the solvent;}\\ M_{\mathrm{r}}&\text{— molecular weight of the solvent.} \end{aligned} \]

The apparatus is calibrated with known solutions with \(M_{\mathrm{d}}\) equal to 0.0015, 0.0010, 0.0005. During operation it is necessary to maintain a constant external temperature.

Table 1 gives the results of determinations of the molecular weights of several low-molecular-weight organic compounds.

Table 1

Conclusions

A method has been developed for determining the molecular weights of organic compounds using thermistors, making it possible to determine rapidly molecular weights up to 1500 with an accuracy within the range from ±0.3 to ±2%.

References

1. R. Müller, H. Stolten. Anal. Chem., 25, 1103, 1953.
2. J. Neumayer. Anal. Chem. Acta., 20, 519, 1959.
3. S. Kulkarni. Nature, 171, 219, 1953.
4. O. G. Tarakanov, I. G. Gusev. Plasticheskie massy. No. 11, 47, 1961.
5. Yu. K. Yur’ev. Practical Work in Organic Chemistry. Issue 2, Moscow State University, 1957.