Effect of ?-Naphthol Adsorption on the Amalgam-Polarographic Behavior of Cadmium
Abstract Generated abstract
This study examines how adsorption of the surface-active substance beta-naphthol affects the amalgam-polarographic determination of cadmium. Using a mercury film electrode in 0.1 N sulfuric acid, the authors measured cadmium anodic peak areas and heights at varying beta-naphthol concentrations and electrolysis times, then calculated electrolysis and anodic-current constants. The results show that increasing beta-naphthol concentration decreases the electrolysis constant, peak area, and anodic dissolution current, indicating hindered delivery of cadmium ions to the electrode and lower amalgam concentration. At the same time, the anodic-current constant increases, supporting the view that adsorption also influences ion removal during amalgam dissolution.
Full Text
EFFECT OF ADSORPTION OF $\beta$-NAPHTHOL ON THE AMALGAM-POLAROGRAPHIC BEHAVIOR OF CADMIUM
V. E. Gorodovykh, T. V. Tomza, S. A. Akulova
(Presented by the scientific seminar of the Department of TIP and EKhP)
The need to study the effect of adsorption of surface-active substances (SAS) on the amalgam-polarographic behavior of metals is dictated by practical requirements. It is important to determine how the characteristics of anodic peaks change when SAS are introduced into solution, and to what extent their presence can interfere with the determination of microimpurities. Since the method of amalgam polarography with accumulation involves preliminary concentration of the metal being determined in the volume of a mercury stationary electrode by electrolysis and subsequent electro-dissolution of the amalgam obtained, it may be assumed that the presence of SAS in solution will affect both the first and the second stage. We have already considered this question for reversible processes [1], and criteria were proposed for the influence of SAS adsorption on individual stages of the process.
Difficulties in delivering the substance to the electrode, caused by the formation of an adsorbed film on its surface, will lead to a decrease in the electrolysis constant, as a result of which the concentration of the amalgam obtained will decrease. Correspondingly, the height of the anodic dissolution peak will also decrease; however, it may also be affected by a change in the anodic-current constant associated with the hindered removal of ions from the electrode surface during the process of amalgam dissolution.
The aim of the present work is the experimental verification of the considerations set forth above. Cadmium was chosen as the object of study, since in most supporting electrolytes it gives reversible peaks. Anod-
Cadmium peaks were recorded using an OH-101 recording polarograph (Hungary). The supporting electrolyte was 0.1 N sulfuric acid with various additions of $\beta$-naphthol. The work used an electrolyzer with insert cups and a mercury film electrode with a silvered platinum substrate [2]. To remove oxygen from the solution, preliminarily purified nitrogen was used.
When working with surface-active substances, purification of water, vessels, and reagents from them becomes of enormous importance. Since $\beta$-naphthol is readily soluble in ethyl alcohol [3], the electrode, the end of the salt bridge, and the cup were first washed with alcohol, then repeatedly with tridistilled water, steamed in nitric-acid vapors, and washed with concentrated sulfuric acid. Chemically pure cadmium sulfate was preliminarily ignited. $\beta$-Naphthol was subjected to double distillation with steam. This gave a product with a melting point of 121.5°C (literature value 122°C [4]).
The accumulation of cadmium in the amalgam (electrolysis potential $-0.9$ V vs. sat. cal. el.) and its electro-dissolution (peak potential $-0.4$ V vs. sat. cal. el.) occurred in the region of adsorption of $\beta$-naphthol, located on our background from $+0.15$ to $-1.2$ V (vs. sat. cal. el.). The concentration of cadmium in the amalgam was determined from the area under the peak by the weighing method. From the experimental results the values of the electrolysis constant $K_\mathrm{e}$ were calculated by the formula
$$ K_\mathrm{e}=\frac{q}{S_\mathrm{e} C_0 \tau}, $$
where $q$ is the area under the peak, coulombs; $S_\mathrm{e}$ is the electrode surface, cm$^2$; $C_0$ is the concentration of cadmium ions, g-ion/l; $\tau$ is the electrolysis time, s,
and the anodic-current constant
$$ K_a=zF\frac{hSl}{q}, $$
where $z$ is the number of electrons participating in the reaction; $F$ is the Faraday constant, coulombs/g-ion; $h$ is the peak height, cm; $S$ is the sensitivity of the instrument, A/cm; $l$ is the thickness of the mercury film, cm.
All peaks were recorded 4–10 times, and the calculation results presented in the table and in the figures are average values.
Table 1
Change in the characteristics of the anodic peak of cadmium
as a function of the concentration of S.A.S. in solution.
| Concentration of $\beta$-naphthol, mole fraction | $K_e$, $\dfrac{\mathrm{a}\cdot\mathrm{cm}}{\mathrm{g\text{-}ion}}$ | $q$, coulomb | $I\cdot 10^7$, a | $K_a$, $\dfrac{\mathrm{a}\cdot\mathrm{cm}}{\mathrm{g\text{-}atom}}$ |
|---|---|---|---|---|
| 0.00 | 3.26 | 4.80 | 185.20 | 2.82 |
| 0.02 | 2.36 | 3.32 | 141.31 | 3.40 |
| 0.05 | 1.29 | 2.56 | 100.10 | 3.74 |
| 0.10 | 0.636 | 1.60 | 57.94 | 4.32 |
| 0.20 | 0.304 | 0.76 | 29.78 | 4.70 |
| 0.30 | 0.060 | 0.24 | 3.70 | 4.93 |
| 0.40 | 0.031 | 0.16 | 2.73 | 5.17 |
As can be seen from Table 1, our assumptions concerning changes in the characteristics of the anodic peaks are confirmed experimentally: with increasing concentration of $\beta$-naphthol, the electrolysis constant, the area under the peak, proportional to the concentration of the amalgam, and the anodic dissolution current decrease. It should be noted that the change in the anodic-current constant presented here is affected by the electrolysis conditions, since $K_a$ was calculated for different concentrations of cadmium in the amalgam. To exclude this effect, we used the following procedure.
Cadmium peaks were recorded with increasing electrolysis time against backgrounds containing different concentrations of $\beta$-naphthol. The curves $q-\tau_1$, $h-\tau$ constructed from these data are shown in Fig. 1.
By cutting the $q-\tau$ curves with the straight line $q=\mathrm{const}$, we obtain the values of the times $\tau_1$, $\tau_2$, $\tau_3$, etc., over which amalgams of equal concentrations can be obtained at different contents of $\beta$-naphthol in the solution. Transferring the obtained values of $\tau$ to the curves, we find the corresponding values of the peak height, which allows us to construct the dependence of $K_a$ on the concentration of S.A.S. in solution (Fig. 2).
As in the first case, the anodic-current constant increases with increasing concentration of $\beta$-naphthol. The displacement of the curves
for different \(q=\mathrm{const}\) is apparently connected with the fact that adsorption of \(\beta\)-naphthol takes place over time.
Thus, the experimental results confirm our assumptions about the influence of adsorption of the surfactant both on the stage of delivery of cadmium ions to the electrode surface (a decrease in \(K_{\mathrm{e}}, h, q\)) and on the stage of ion removal (an increase in \(K_a\)).
Fig. 1. Change in the area under the peak and in the height of the cadmium peak with increasing electrolysis time at different concentrations of naphthol in the solution (indicated above the curves in fractions of solution saturation).
Fig. 2. Change in the anodic-current constant with increasing concentration of \(\beta\)-naphthol in the solution. The curve numbers correspond to the numbering of the sections in Fig. 1.
References
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