The study of Cr3+/Cr2+ redox system by polarographic method

Abstract

Aim. The relevance of the theme is the ability to determine kinetic and thermodynamic parameters of the redox system on the example of Сr³/ Cr²in order to explain the reaction mechanism.

In this paper, the electrochemical behavior of Cr⁺³/ Cr⁺²has been studied by polarography method. Polarogramshave been interpreted using the theory of irreversible polarographic waves and thermodynamically irreversible processes. Experiments have been carried out in a sealed cell with an outer casing for temperature control. Air oxygen removed from the feed solution for 15-20 minutes, purified nitrogen flowing from the cylinder. Characterization of the capillary in a 0.1N solution.H₂SO₄:  mass flowing mercury m = 2,39 mg / sec; droplet formation period τ = 1,25 sec.

Sulfate Cr (II) was prepared by dissolving an electrolytic chromium 150 cm³ of water and 46 grams of 98 % sulphate acid. Thus a saturated solution formed a blue color.

Sulfate salt solutions of two- and trivalent chromium in the medium of 0.1NH₂SO₄ give one wave. Anodic diffusion current sulfate Cr (II) was 4,67μa, and half-wave potential E ½ = -0,44 V. The cathode current diffuse sulfate Cr (III), respectively - 7,65μa and E ½ = - 1,01 V. Inconsistency half-wave potentials of anodic and cathodic processes indicates its irreversibility. Half-wave potentials are given with respect to a saturated calomel electrode.

Polarogramshave been investigated in the temperature range of 20-60⁰C every 10⁰C. Temperature coefficients have been calculated by the equation:

Β% = i_dt –i_d20/ i_d20Δt,

wherei_dtis diffusion current for a given concentration of the test solution at t⁰С; i_d20 is diffusion current for the same concentration at 20⁰C; Δt - temperature change relative to 20⁰C .

Irreversibility, obviously due to the change in the electron configuration of chromium electrode is accompanying the redox process.Using the theory of irreversible polarographic waves we have determined the kinetic characteristics of redox Cr⁺³/ Cr⁺². The specific reaction rate found by extrapolating the straight line constructed in the lgK = f (E) coordinates to the equilibrium potential Ep. Equilibrium potential found by the Eyring method as half-wave potential for the reversible waves.We have constructed the initial currents of irreversible waves, considering that the beginning of irreversible and reversible waves practically coincide.

Specific reaction rate (K) is associated with activation energy given by:

lgK= lg(ВТ/h∙ δ) – ΔF/2,3RT,

where: B is the Boltzmann constant, T is temperature Kelvin , h - Planck constant , δ - the distance between ions 2 ∙ 10-8 cm , R - universal gas constant , ΔF- changing activation energy. Substituting the values of the constants in the equation, we obtain an expression for calculating the activation energy: ΔF = 6,86∙10³ – 1,35∙10³lgК.

An important characteristic of irreversible processes is the change in entropy, as the derivative of the entropy increment in time (diS / dt) = ΔS. This quantity can be expressed in terms of current strength (J) and overvoltage (η): ΔS = (diS / dt) = 1 / T ∙ J η. In polarographic it indicates that the electrochemical process is complicated as compared with delivery of reactant to the electrode.

With increasing temperature, we saw insignificant changes in the kinetic characteristics of both anodic and cathodic reduction of chromium salts. Obviously, accelerate the process of discharging ions prevents complication.

Conclusion. This indicates a different state of ions of bivalent and trivalent chromium in solution and various ways of the electrode processes.