PROCEEDINGS

OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR

Volume 257　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1973

COMBINED PHOTOELECTRIC METHOD FOR MEASURING CONCENTRATIONS OF SUBSTANCES

P. I. Goskov, V. L. Ivasenko, V. P. Lopatinskii,
V. T. Novikov, Yu. K. Shelkovnikov

(Presented by the scientific seminar of organic departments
and laboratories)

To determine the concentration of various substances, chemical and physicochemical methods are used. Among the latter, spectrophotometry, as well as refractometry, have become widespread.

The authors have developed a method based on the use of a refractometric cuvette and a differential photodetector (photocells or photoresistances), by means of which the total measurement of the refractive index and light absorption is ensured.

Figure 1 shows a block diagram of the setup for implementing the proposed method; Fig. 2 shows experimental curves.

Fig. 1. Block diagram of a setup for measuring the concentration of substances by the combined photoelectric method.

According to Fig. 1, the proposed method is implemented by means of a light-flux source 1, a refractometric cuvette 2, a differential photodetector 3, a balance circuit 4, and a recording device 6.

The method is carried out as follows. First, a reference substance with known optical density $D_{\text{et}}$ and known refractive index $n_{\text{et}}$ is placed in cuvette 2. In this case, photodetector 3 is set, by means of the appropriate mechanical adjustment device, so that the light flux passing through the cuvette falls exactly on the boundary between the two parts of the photodetector and so that their illumination is identical. In this case the output signal from balance circuit 4, to which photodetector 3 is connected, will be absent, i.e., the unbalance voltage is equal to zero.

After this, instead of the reference substance, the substance under investigation is placed in the cuvette; its optical density is unknown, but it is less than $D_{\text{et}}$, and the refractive index of this investigated substance $n$ is unknown, but it is not equal to $n_{\text{et}}$ (either $n > n_{\text{et}}$, or $n < n_{\text{et}}$). Since $n \ne n_{\text{et}}$, the light flux is displaced relative to the middle of the photodetector, and the sign of the displacement (upward or downward) is determined by the sign of $\Delta n = n_{\text{et}} - n$.

Since $D < D_{\text{et}}$, simultaneously with the displacement of the light flux its intensity increases, i.e., as a result the illumination of one half of the photodetector decreases sharply (in our case, the upper half, Fig. 1), while the illumination of the other half increases considerably both because of the displacement of the light flux and because of the increase in its intensity. Consequently, the magnitude of the unbalance voltage appearing at output 4 will be determined by the total change in both characteristics $n$ and $D$ relative to $n_{\text{et}}$ and $D_{\text{et}}$. In this case the sign of the unbalance voltage is determined by the sign of $\Delta n$, while the unbalance voltage itself is amplified and measured by recording device 5.

Thus, owing to the combined determination of $\Delta n$ and $\Delta D$, the output signal 4 increases correspondingly in comparison with the case when the change of only one optical characteristic ($n$ or $D$) is used. Fig. 2 illustrates this. Here, in Fig. 2a, the dependences of $\Delta D$ and $\Delta n$ on the concentration of $\mathrm{CuSO_4}$, measured with the aid of known instruments in the IR spectrum, are given; and in Fig. 2b the dependences of the unbalance signal on concentration are given when either only $\Delta D$, or only $\Delta n$, or only $(\Delta n + \Delta D)$ is recorded. The experiment was carried out with one and the same differential photodetector of the FS-K7 type in the visible region. In this case, for obtaining ...

dependence $\Delta R_{\mathrm{unbal.}}=\psi(N)$ on $\Delta D$, a photometric cuvette was used, and the light flux that had passed through it was projected onto only one half of the photodetector.

Fig. 2. Experimental curves

The dependence $\Delta R_{\mathrm{unbal.}}=\psi(n)$ on $(\Delta D+\Delta n)$ was obtained according to the scheme in Fig. 1, while the dependence $\Delta R_{\mathrm{unbal.}}=\psi(n)$ only on $\Delta n$, shown by the dashed line in Fig. 2b, was obtained as the difference of the two preceding dependences. In this case the lengths of the cuvettes and the distance between sources 1 and photodetector 3 were identical in both cases.

The results obtained show that the sensitivity, resolving power, and accuracy of the measurements increase correspondingly when $(\Delta n+\Delta D)$ is determined in total, in comparison with cases in which only one of the indicated characteristics is recorded.

In our case, for example, a real increase in sensitivity is observed when $(\Delta n+\Delta D)$ is recorded: by a factor of 2.2 in comparison with recording only $\Delta D$, and by a factor of 1.35 in comparison with recording only $\Delta n$.

The proposed method is especially promising for small changes in $\Delta n$ and $\Delta D$, when recording only $\Delta D$ or $\Delta n$ is difficult.

In addition, the proposed method can be used not only for measuring concentrations; for example, it is also very promising for recording residual changes in substances subjected to various types of electrophysical action, which is necessary when adjusting equipment for processing to an optimal regime, when clarifying the nature of these actions, etc.

The described method may also find application in monitoring separation in column chromatography with quantitative evaluation of the separated substances.