DEVICE FOR HIGH-FREQUENCY DETERMINATION OF CONCENTRATION

O. P. LOBAS, S. I. SMOLYANINOV

(Submitted by the scientific seminar of the Section of Organic Chemistry)

Complete automation of chemical enterprises is impossible without organizing automatic production control. Therefore, at the present time the task of creating methods and instruments for the analysis of raw materials and products in a stream is extremely urgent. For this purpose, the use of physical methods of analysis is especially promising. It is precisely physical methods, and especially the development of radio electronics, that make it possible to employ fast-acting instruments in analytical control, which, in addition to providing the conditions for automation, greatly increases the productivity of the personnel working in laboratories.

Recently, for the indicated purposes, high-frequency sensors have found ever wider application; they are used, in particular, to determine the concentration of various substances [1, 2, 3, 4, 5].

The present work is devoted to studying the possibility of applying a Q-meter circuit for determining the concentration of substances using a high-frequency inductive sensor, and also to creating an instrument more convenient in practice than a Q-meter.

With the aid of a Q-meter, as is known, the quality factor (i.e., the quantity characterizing losses) of an electromagnetic oscillatory circuit is determined.

When some substance is introduced into an oscillatory circuit, for example into an inductance coil, which thus serves as the sensor, the circuit becomes detuned, since an additional load is introduced into the generator.

Our investigations were carried out with an inductive sensor on KV-1 and UK-1 Q-meters. The inductive sensor was a hollow cylindrical frame made of dielectric material, onto which silver-plated wire was wound.

Initially, the quality factor of the inductive sensor is measured at a certain resonant frequency. The substance under investigation, placed in a glass test tube, is inserted inside the sensor frame. In this case, owing to the change in inductance, the circuit becomes detuned, and it must again be tuned to the resonant frequency by changing the capacitance. After retuning, the quality factor of the circuit is measured a second time. Transition to another resonant frequency of the sensor is carried out by changing the capacitance.

First of all, we investigated the effect of the frequency of the electromagnetic oscillations of the circuit with an inductive sensor on the quality factor of the latter when introducing sulfuric and oxalic acids of various concentrations.

The frequency was varied within the range from 7.5 to 30 MHz. The experiments showed (Fig. 1) that the quality factor of the sensor increases linearly with increasing

Fig. 1. Dependence of the quality factor of the circuit (a sulfuric-acid solution is placed in the sensor coil) on frequency.

frequency. This dependence can be expressed by the empirical formula

\[ \Delta Q = k \cdot \omega + v, \]

where \(\Delta Q\) is the difference between the quality factor of the sensor itself and the quality factor of the sensor with the substance introduced;

\(k\) is a constant for the given concentration of sulfuric acid;

\(\omega\) is the frequency;

\(v\) is a constant for the given substance.

For oxalic acid no linear dependence was found.

The circuit of the final version of the device is presented in Fig. 2. Measurement of the concentration of substances by means of a high-frequency sensor is based on the change in the quality factor of the circuit and, consequently, in the energy consumed. The oscillatory circuit is included in the anode circuit of the oscillator tube. Deviations of the current in the circuit are measured by means of a microammeter connected in the tube bridge.

With this device the effect of coupling on the sensitivity to changes in concentration was investigated. This dependence is expressed by the graph (Fig. 3). Thus, it proves possible to tune the sensor to maximum sensitivity.

Experiments also established that the most sensitive sensors are those with the highest quality factor, provided that the highest possible frequencies are used.

With the aid of this device it was possible to measure the concentration of sodium bicarbonate up to a content of 1600 mg/liter with an accuracy of up to 2 mg/liter at the maximum limit, while the determination time is measured in seconds. A typical graph of the dependence of the anode current on the concentration of a sodium carbonate solution is given in Fig. 4.

When using the device it is necessary that the vessels (test tubes) be identical in size. In addition, it should be taken into account that, when pro-

Fig. 2. Schematic circuit diagram of the generator.

Fig. 3. Dependence of the sensor sensitivity on the distance of the coupling coil and on the number of coupling turns (the zero point corresponds to the sensor current without solution): 1 — decinormal solution of oxalic acid; 2 — centinormal solution of oxalic acid; 3, 4 — the same, respectively, but with an increased number of coupling turns.

With prolonged residence of the solution under investigation in the sensor coil, its temperature may rise, which will distort the results. In general it should be noted that calibration curves obtained at different temperatures differ from one another at all points by a constant

Fig. 4. Calibration curve for measuring the concentration of a sodium carbonate solution.

quantity, and therefore the temperature correction can be easily calculated.

Conclusions

1. An instrument has been designed for high-frequency determination of concentration using an inductive sensor. The instrument is based on a Q-meter circuit.

2. It has been shown that the sensitivity of the instrument is affected by the characteristic of the coupling coil, the Q-factor of the circuit, and the frequency of the electromagnetic oscillations. Higher-frequency sensors with a higher Q-factor are more sensitive.

3. An empirical formula has been proposed for the dependence of the Q-factor of a circuit with a sulfuric acid solution on the frequency of electromagnetic oscillations.

References

1. O. A. Kaptsan, V. A. Teplyakov. High-frequency titration. Journal of Analytical Chemistry, issue 3, 1953.

2. V. A. Zarinskii, D. I. Koshkin. High-frequency titration. Journal of Analytical Chemistry, issue 1, 1954.

3. V. A. Zarinskii, D. I. Koshkin. High-frequency titration. Journal of Analytical Chemistry, issue 2, 1955.

4. V. A. Zarinskii, N. R. Pandel’berg. High-frequency titration. Factory Laboratory, No. 3, 1955.

5. V. Melkuzyan. High-frequency conductometers. Radio, No. 2, 1961.