Deuterium electrolytic separation

Use of deuterium in the mechanistic studies of the HER began shortly after its discovery in 1931. It was used in two directions (a) as a tracer to measure the unidirectional component rate of the reaction, and (b) for examination of the kinetic isotope effect, namely, the electrolytic separation factor. 

In this method, each gas is produced in a separate compartment so they have high purity. In this process, deuterium oxide, D20, is electrolyzed more slowly so the water becomes enriched in the heavier isotope. The other electrolytic process that produces hydrogen is the electrolysis of a solution of sodium chloride. 

Deuterium. Naturally occurring hydrogen contains 0.0156 at.% D. Deuterium as D20 is separated from H20 by fractional distillation or electrolytic enrichment of normal water. 

It is only in the case of hydrogen H (weight i) and deuterium D (weight 2), discovered by Urey (1933), that the relative difference is so large that a small but observable difference in all physical and even in chemical properties occurs. Thus the separation of pure D2 is possible by repeated electrolysis of water, in which the separated hydrogen gas is less rich in the heavy isotope than corresponds to the ratio in the water 1 4000. The deuterium therefore accumulates in the residual liquid of technical installations for the electrolytic production of hydrogen (Norway). 

The emichment of tritium is usuaUy determined by the tritium separation (fractionation) factor during electrolysis, and by measuring the initial and final amounts of water. However, many workers have reported that the value of the separation factor of tritium depends on the electrode material, the type of electrolytic emiclunent ceU, the current density, the mode by which water is fed into the electrolytic cell, and the temperature of the electrolytic cell. In 1991, a rehable method was proposed for estimating tritium concentrations in water, based on a rehable correlation between the water electrolytic emiclunent of deuterium and tritium. The constancy of the ratio, k, during the electrolysis, k = a(fi — — 1), was... 

Hydrogen-Deuterium Separation.—When an aqueous solution is electrolyzed, the hydrogen gas evolved always contains relatively more of the lighter isotope than does the electrolyte the difference is expressed as the separation factor (s) defined by... 

As a result of experimental studies it appears that the electrolytic hydrogen-deuterium separation factors fall into two distinct classes " in the first 8 has a value of about 6, viz.,... 

Separation factors. Deuterium separation factors in the electrolytic plants described above, together with the types of cells used and operating conditions that may have had an effect on separation factor, are listed in Table 13.13. Separation factors of from 6 to 10 have been reported for the secondary plants, and from 3.8 to 7.0 for the primary plants. The lower values for the primary plants are attributed to their higher cell temperatures, their use of diaphragms, and the greater difficulty of keeping large equipment clean. 

In a detailed laboratory investigation of the effect of cell variables on the deuterium separation factor in electrolysis of water, Brun and co-workers [B13] have found that a depends on the cathode material, electrolyte composition, and cell temperature, generally as follows. The separation factor is higher for an alkaline electrolyte than for an add. With KOH, at 15°C, a pure iron cathode gave the highest value reported, 13.2. The separation factor for mild steel, the material used in most commercial electrolyzers, was 12.2. Values as low as S were reported for tin, zinc, and platinized steel. At 2S°C the separation factor with a steel cathode was 10.6, and at 75°C it had dropped to 7.1. 

Separation factors in electrolysis for other elements are much lower than for hydrogen. A few values that have been reported are listed in Table 13.15. These values are so low, and the cost of electric energy per unit electrolyzed is so high, that electrolysis is uneconomical for separating isotopes of any element other than hydrogen. Some concentration of takes place in an electrolytic deuterium plant. 

An electrolytic hydrogen plant fed with natural water containing 0.0149 percent deuterium is operated as a simple cascade, without recycle, to produce water containing 0.142 percent deuterium. The stage separation factor a has the value 7.0, and the heads separation factor 0 is independent of stage number. 

Solvents for nuclear magnetic resonance (NMR) spectroscopy, enriched in deuterium to an extent of >99%, are commercially available. The separation of deuterium from naturally occurring hydrogen is achieved electrolytically with the isotope in the form of D2O. When an aqueous solution of NaOH (natural isotopic abundances) is electrolysed (equation 3.36) using an Ni electrode, the separation factor defined in equation 3.37 is 6. The choice of electrode is critical to the optimization of this value. 

The electrolysis is continued until 90% of the liquid has been converted into O2 and H2. Most of the residual liquid is then neutralized with CO2, and the water distilled and added to the remaining electrolyte. This process is repeated to give <99.9% D2O. In the later stages of the separation, the gas evolved at the cathode is burned to yield partially enriched deuterium oxide that can be electrolysed further. Cheap electrical power is, of course, essential for the economic concentration of D2O by this method. 

Erdey-Gruz and Volmer and Butler/ to whom we owe a rate expression for charge transfer processes. Later, Frumkin " took into account the potential distribution at the electrode-electrolyte interface and its influence upon the process of discharge of ions. Experimentally observable quantities other than the Tafel constants have also been employed since then in elucidating the mechanism. For example, experimental observations of and attempts of theoretical interpretation of the electrolytic hydrogen-deuterium separation factor were presented by Topley and Eyring, Horiuti, " and others, " as briefly reviewed elsewhere. ... 

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