Cell, amalgam standard

Weston normal element (cell) — Electrochemical -> standard cell showing a particularly stable and reproducible cell voltage. In the international Weston normal element a cadmium amalgam (cadmium content in the solid phase approx. 15 wt %, in the liquid phase approx. 5wt%, total average 12 to 12.5 wt%, the electrode potential depends only on the temperature, not on the mass ratio of liquid and solid phases) and a mercury electrode (half-cell) are combined according to... 

Weston cadmium cell A standard cell that produces a constant e.m.f. of 1.0186 volts at 20°C. It consists of an H-shaped glass vessel containing a negative cadmium-mercury amalgam electrode in one leg and a positive mercury electrode in the other. The electrolyte - saturated cadmium sulfate solution - fills the horizontal bar of the vessel to connect the two electrodes. The e.m.f. of the cell varies very little with temperature, being given by the equation = 1.0186 - 0.000 037 (T - 293), where T is the thermodynamic temperature. 

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described. 

For a couple of years 0.25% Hg became the technical standard. It has to be pointed out that the mercury content of the metallic zinc has to be divided by a factor of roughly 10 to give the mercury content based on the total cell weight of an AA (LR-6) cell. These values are sometimes indicated on the cell labels. With decreasing amalgamation, other corrosion inhibitors had to take over the role of mercury. There are numerous papers and patents claiming corrosion-inhibiting activities of elements like A1, In, T1, Cd, Ga, Na, ... 

Gill and Fitzgerald [481] determined picomolar quantities of mercury in seawater using stannous chloride reduction and two-stage amalgamation with gas-phase detection. The gas flow system used two gold-coated bead columns (the collection and the analytical columns) to transfer mercury into the gas cell of an atomic absorption spectrometer. By careful control and estimation of the blank, a detection limit of 0.21 pM was achieved using 21 of seawater. The accuracy and precision of this method were checked by comparison with aqueous laboratory and National Bureau of Standards (NBS) reference materials spiked into acidified natural water samples at picomolar levels. Further studies showed that at least 88% of mercury in open ocean and coastal seawater consisted of labile species which could be reduced by stannous chloride under acidic conditions. 

All species are aqueous unless otherwise indicated. The reference state for amalgams is an infinitely dilute solution of the element in Hg. The temperature coefficient, dE°/dT, allows us to calculate the standard potential, E°(T), at temperature T E°(T) — Ec + (dE°/dT)AT. where A T is T — 298.15 K. Note the units mVIK for dE°ldT. Once you know E° for a net cell reaction at temperature T, you can find the equilibrium constant, K, for the reaction from the formula K — lOnFE°,RTln w, where n is the number of electrons in each half-reaction, F is the Faraday constant, and R is the gas constant. 

Amalgam Cell Starting with the chlorine electrode, the reversible standard potential is +1.37 V (all potentials are given versus NHE), due to the sodium concentration of 6 mol L-1 we have to note +1.33 V. The current-overpotential curve represents an unhindered process... 

Although equation (38.22) gives the ratio of the activities in any two fl.Tnn.lgaTn directly from a measurement of the e.m.f. of the appropriate cell, it is frequently desirable to express the actual activity of a metal in a given amalgam with reference to a particular standard state. Suppose the latter is chosen, in accordance with 37b, III B, so that the activity coefficient o /n is unity at infinite dilution the following procedure may then be used. Equation (38.22) may be written as... 

The E.M.F. of a cell consisting of cadmium amalgam (N2 = 1.781 X 10" ) as one electrode and a saturated amalgam as the other is — 0.05294 volt at 25 C. What are the activity and activity coefficient of the cadmium in the dilute amalgam with reference to that in the saturated amalgam as the standard state ... 

Clark cell. Standard cell for measuring electrical potential with mercury and zinc amalgam electrodes in zinc sulfate solution. 

Weston cell. An electrical cell used as a standard that consists of an amalgamated cadmium anode covered with crystals of cadmium sulfate dipping into a saturated solution of the salt, and a mercury cathode covered with solid mercury sulfate. 

The potentials of half-cells that are constructed using amalgams are important whenever one is interested in performing reactions in a media in which hydrogen evolution at the metal-solution interface might interfere with the reaction of interest. Consequently, half-cell potentials for alkali metal amalgams are important to synthetic chemists and electrochemists alike. The standard potential for a metal amalgam in contact with a solution of its monovalent cation ( °(M" /M(Hg))) can be related to that of the pure metal in contact with its monovalent cation (EJ ) as shown in Eq. (14)... 

Note, however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential First, the product of reduction may be stabilized by amalgam formation in metal ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and hnally, it can be shown that the potential Ey is the sum of two terms ... 

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... 

Since we know the values for the silver-silver halide electrodes versus the standard hydrogen electrode (see above), we can easily set up a series of standard potentials vs. H2 assuming (H+ H2) = 0.0 V. Recently Scrosati et made measurements on double amalgam cells in pure methanol. Their results for the alkali metal chlorides and bromides are in good agreement with others but the iodide systems appear to involve some error since a value of --0.2993 V is obtained for of cell (Vc). Table 2.7.2 lists the standard potentials of the alkali-metal electrodes. 

Clark cell A type of voltaic cell consisting of an anode made of zinc amalgam and a cathode of mercury both immersed in a saturated solution of zinc sulphate. The Clark cell was formerly used as a standard of e.m.f. thee.m.tat IS C is 1.4345 volts. It is named after the British scientist Hosiah Clark (d. 1898). 

Weston cell (cadmium cell) A type of primary voltaic cell, which is used as a standard it produces a constant e.m.f. of 1.0186 volts at 20°C. The cell is usually made in an H-shaped glass vessel with a mercury anode covert with a paste of cadmium sulphate and mercury(l) sulphate in one leg and a cadmium amalgam cathode covered with cadmium sulphate in the other leg. The electrolyte, which connects the two electrodes by means of the bar of the H, is a saturated solution of cadmium sulphate. In some cells sulphuric acid is added to prevent the hydrolysis of mercury sulphate. It is named after Edward Weston (1850-1936). 

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