Sodium standard electrode potential

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

As may be seen from the diagram, silver in highly alkaline solution corrodes only within a narrow region of potential, provided complexants are absent. It is widely employed to handle aqueous solutions of sodium or potassium hydroxides at all concentrations it is also unaffected by fused alkalis, but is rapidly attacked by fused peroxides, which are powerful oxidising agents and result in the formation of the AgO ion Table 6.6 gives the standard electrode potentials of silver systems. 

In this equation, e stands for the electron, Na+ for the sodium ion, (aq) for the aqueous state, Na for elemental sodium, (s) for the solid state, and E° represents the standard electrode potential given in volts (v). The superscript ° on E indicates that the reaction is taking place under standard conditions (T = 298 K, Na+ concentration of 1.00 M). Values of E° may be readily obtained from tables in reference works such as A. J. Bard, R. Parsons, and... 

Sodium perxenate, first prepared in 1963, has a standard electrode potential estimated to be 3.0 V in acid solution. Without a catalyst and at room temperature it rapidly oxidizes Mn(II) to Mn(VII). In neutral solution it oxidizes Np(IV) and Pu(IV) hydroxides to Np(VI) and Pu(VI). Sodium perxenate decomposes in acid solution ... 

The standard electrode potential Eo is produced between the reference solution and the sample in the single use ion-selective electrodes via the paper bridge. Its magnitude (numerical value) and sign depend upon the overall ion concentration and the composition of the two solutions. It is primarily obtained from the concentration of the sodium ions and chloride ions. The value of o can be kept low if a reference solution is used the ion concentration of which is near the physiological range. 

Metals differ greatly in how easily they are oxidized. The most reactive metals are those with a very negative standard electrode potential. Of the commonly used metals, the alkali metals lithium, sodium, and potassium are highly reactive the next most reactive is magnesium, then zinc. 

Sodium-type amalgams with Na, K, Rb, Cs, Ca, Si, and Ba react completely with low solubility in mercury. They can be formed electrolytically and exhibit half-wave potentials positive to the M/M+ standard electrode potential. 

The standard potential of Na/Na+ at 25°C is -2.7142 V and the potential of the Na/Na amalgam cell is -0.75852 V when the mole fraction of sodium in the amalgam is unity. Hence, the standard electrode potential for the reaction... 

Flengas and coworkers [2,3] have measured the equilibrium potentials of chromium in a molten equimolecular mixture of sodium and potassium chlorides at various concentrations of its ions, based on the ratio to a silver reference electrode. They determined [2,3] the standard electrode potentials E cr(ii)/Cn E°ci(niycr and the redox potential E°cr(m)/cr(U)-... 

Often, other metals than sodium are employed as reducing agents in organic reduction reactions. The standard electrode potentials of the metals... 

Using standard electrode potentials, decide whether aqueous sodium h3q)ochloiite solution will oxidize Br to Br2 in basic solution under standard conditions. See Appendix I for data. 

The standard electrode potentials for active metals, such as calcium and sodium, are not measured experimentally since they undergo a direct reaction with water to release hydrogen. The values of their electrode potentials are calculated indirectly using Hess s law. 

However this approach is not invariably feasible. For example, if we wish to evaluate the standard electrode potential of the Na /Na couple it is clear that a sodium electrode would have such a short lifetime in contact with an aqueous solution as to preclude any chance of making appropriate measurements. Instead we have to rely on an indirect, two step approach to the problem. 

It is possible to discharge sodium electrolytically from an aqueous solution if a mercury cathode is used. On the basis of the standard electrode potentials [E (Na, Na) =-2.71V], the decomposition of water to form hydrogen at the cathode would be a far easier process even when allowance is made for the high hydrogen overpotential (q.v.) at mercury. However, the fact that sodium forms intermetallic compounds with mercury which are soluble in mercury and diffuse away so reduces the activity of the sodium at the cathode surface, and its tendency to re-ionise that the discharge of sodium becomes the preferred process. This method is not used commercially because of the high cost of extraction of sodium from its amalgam, and recourse is had to fused electrolytes. 

One of the difficulties in the use of molten electrolytes is the solubility of the metal in the electrolyte, and the formation of metallic clouds consisting of a colloidal dispersion of the metal and one of its compounds. The resulting losses increase with rise of temperature, and at 1073 K the efficiency of sodium production would be exceedingly low. The temperature of the melt is therefore reduced by using a mixture of sodium chloride and calcium chloride (or sodium carbonate) which is molten at about 873 K. The standard electrode potential of calcium, E (Ca, Ca) = -2.87 V, is slightly more negative than that of sodium only about 1% calcium is produced at the cathode, and most of this separates when the sodium solidifies. 

Lithium is the most electropositive of all metals, with a standard electrode potential of 3.045 V compared with 2.71 V for sodium and 0.76 for zinc. It thus can generate the greatest electrical power per unit weight or volume of any metal, but it is also extremely reactive and thus potentially dangerous. Special designs and applications... 

The extract was then diluted with 0.5 mL with 0.2 M sodium acetate buffer, pH 4.7 and analyzed by HPLC. Chromatographic conditions were the same as for the determination of benzidine in hair dye formulations. For the particular lot of diarylide yellow studied 46 Ug/kg of DCB was found. In an attempt to confirm the identity of the chromatographic peak, its response as well as the response for the authentic DCB standard was determined at several different electrode potentials. These data, shown in Figure 7, illustrate the ability of HPLC/EC to yield qualitative as well as quantitative information for unknown components. 

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu2+(aq), adjusting the pH to a weakly acidic level, allowing the Cu2+ to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure ... 

Although the entire discussion of electrochemistry thus far has been in terms of aqueous solutions, the same principles apply equaly well to nonaqueous solvents. As a result of differences in solvation energies, electrode potentials may vary considerably from those found in aqueous solution. In addition the oxidation and reduction potentials characteristic of the solvent vary with the chemical behavior of the solvent. as a result of these two effects, it is often possible to carry out reactions in a nonaqueous solvent that would be impossible in water. For example, both sodium and beryllium are too reactive to be electroplated from aqueous solution, but beryllium can be electroplated from liquid ammonia and sodium from solutions in pyridine. 0 Unfortunately, the thermodynamic data necessary to construct complete tables of standard potential values are lacking for most solvents other than water. Jolly 1 has compiled such a table for liquid ammonia. The hydrogen electrode is used as the reference point to establish the scale as in water ... 

Although the detailed mechanism of electron transport and transfer involving PVF electrodes may be complex, they show remarkable stability and rapidly reversible redox behaviour in non-aqueous solvents such as acetonitrile. This has led to the suggestion69 that they might function as standard electrodes for non-aqueous solvents. Such standards are required since the SCE is unsatisfactory in a number of respects. Particularly, the liquid junction potential between aqueous and non-aqueous solutions is unknown and irreprodudble also there is a danger that the test solution will become contaminated with water and with potassium and sodium ions. 

Standard [reduction] potentials for hundreds of electrodes have been determined (mostly in the period 1925-45, during which time they were referred to as oxidation potentials ) and are usually tabulated in order of increasing tendency to accept electrons. This ordering is also known as the electromotive series of the elements. As can be seen in the abbreviated version in Table 1, sodium is the most active of the metallic elements in the sense that its oxidation product Na+ shows the smallest tendency (as indicated by the highly negative voltage) to undergo reduction. 

The reversible potential of the sodium amalgam electrode, considering an amalgam concentration of 0.2 wt% sodium and the already mentioned sodium concentration, amounts to —1.78 V (the difference to the Na/Na+-standard potential —2.71 V is due to the fact that in the case of amalgam the discharged sodium ions must not be incorporated into a metallic structure). 

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