DETERMINATION OF MOLECULAR WEIGHTS OF ORGANIC COMPOUNDS USING ELECTRICAL MEASUREMENT CIRCUITS

G. N. IVANOV, V. A. BUDAEVA, V. P. LOPATINSKII

(Presented by the scientific seminar of the problem laboratories of the Faculty of Chemical Technology)

It was noted earlier [1] that most existing methods for determining molecular weights are insufficiently accurate, relatively labor-intensive, and time-consuming to carry out. This is often associated with technical difficulties in the design of the apparatus. In cases where the apparatus is not complex, the experiment is made more difficult by its duration, which sometimes leads to irreproducibility of the data obtained, especially when determining the molecular weights of high polymers [2, 3]. The present work describes a method for determining the molecular weights of organic compounds that is largely free of the above-mentioned shortcomings.

The measurements are based on the principle of cryoscopy. The depression of the freezing point of the solution was recorded by a thermistor connected into a bridge measuring circuit.

The apparatus consists of:

1) a measuring cell with a thermistor built into it (Fig. 1), of the KMT-4 type (45 kΩ). The volume of solution loaded into the cell was about 5 ml;

2) a Dewar vessel with a cooling mixture, into which the measuring cell was immersed;

3) a motor with stirrer;

4) an R-333 measuring bridge;

5) an M-95 microammeter and \(i = 10^{-9} a\).

Fig. 1.

The measuring cell is made of glass. All electrical leads are coated with polyvinyl alcohol and baked at \(T = 100^\circ C\). The general layout of the apparatus is shown in Fig. 2.

The measurement process was as follows.

An approximately one-percent solution of the substance in benzene was prepared. The working volume of the measuring cell was filled with the solution. The cell was placed in the cooling mixture, while the solution in it was continuously stirred by the stirrer.

As the solution cooled, the resistance of the thermistor decreased. This decrease in the temperature of the solution, and consequently in the resistance

Fig. 2.

Fig. 3.

Fig. 4.

can be calibrated directly in units of molecular weight; in this case the values of the weighed portions of the substance and solvent are entered into the thermistor circuit. As a rule, cooling of the solution occurs with supercooling. From the moment crystallization begins, the temperature (resistance) rises abruptly to some maximum value and has the form of a peak (Fig. 3), which is taken as the reading.

A definite value of the resistance in ohms of the thermistor corresponds to this peak. Taking a reading on the measuring bridge, from the dependence \(R — \Delta T\) we find the depression of the freezing temperature. This dependence, over a small interval \(\Delta T\), has the form of a straight line (Fig. 4).

The molecular weight is calculated by the formula:

\[ M_{\mathrm{v}} = \frac{g \cdot 5100}{\Delta T \cdot G}, \]

where \(g\) is the weighed portion of the dissolved substance,
\(\Delta T\) is the depression of the freezing temperature,
\(G\) is the weighed portion of the solvent.

Table 1

Table 1 gives data on the determination of \(M_{\mathrm{v}}\) of several substances by this method.

As can be seen from the table, the average error of the determinations lies within 1–3%.

An advantage of this circuit is that the process of measuring \(M_{\mathrm{v}}\) can be automated.

One such circuit is shown in Fig. 5.

The temperature sensor \((R_T)\), a KMT-4 thermistor (100 kΩ) (at the measured values of \(T^\circ\)C), is connected into a bridge circuit.

The signal obtained as a result of imbalance is fed to a cathode follower, which serves as a matching link between the high-resistance measuring circuit and the low-resistance recording instrument.

The purpose and ratings of the individual circuit elements are conventional and require no special explanation.

In the case where one has to work all the time with substances whose \(M_{\mathrm{v}}\) values lie within a small interval, the scale of the instrument

can be graduated directly in units of molecular weight; in this case the values of the weights of the substance and the solvent are introduced into the circuit before measurement by setting the potentiometers to the corresponding values \((R_2, R_4, R_7)\).

But such a solution to the problem is in most cases inconvenient; therefore, the reading should more often be made in the same quantities as in the manual version of the apparatus.

When the automatic version is used, the need for constant monitoring is eliminated, since all readings of the instrument are recorded on the chart tape.

Fig. 5.

Conclusions

1. A simple and convenient circuit has been developed for determining the molecular weights of organic compounds, using thermistors as thermosensitive elements.

2. The method is characterized by high reproducibility, rapidity \((15—20\ \text{min per determination})\), the absence of continuous manual operations, and accuracy (relative error \(1—3\%\)).

3. The method permits automation of the process of determining molecular weights; one of the circuits for this has been developed.

LITERATURE

1. G. N. Ivanov, V. P. Lopatinskii. Izvestiya Tomskogo politekhnicheskogo instituta, vol. 126, 1964.

2. Methods of Polymer Research, edited by Allen, Inostranlit, 1961.

3. Chiang Zhen-Yuan. Determination of Molecular Weights of Polymers, Goskhimizdat, 1962.