Weston standard cell

Weston cell A widely used standard cell... 

Mercury is extensively used in various pieces of scientific apparatus, such as thermometers, barometers, high vacuum pumps, mercury lamps, standard cells (for example the Weston cell), and so on. The metal is used as the cathode in the Kellner-Solvay cell (p. 130). 

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. 

Mercurous Sulfate. Mercurous s Ai2LX.e[7783-36-0] Hg2S04, is a colodess-to-shghfly-yellowish compound, sensitive to light and slightly soluble ia water (0.05 g/100 g H2O). It is more soluble ia dilute acids. The compound is prepared by precipitation from acidified mercurous nitrate solution and dilute sulfuric acid. The precipitate is washed with dilute sulfuric acid until nitrate-free. Its most important use is as a component of Clark and Weston types of standard cells. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

The procedure is as follows. In switch position 1 and while repeatedly depressing tap key K, the variable resistor R, is adjusted once for each measurement to zero current through galvanometer G, so that the emf of the standard cell C8t (Weston 1.01832 V) becomes accurately compensated over the constant resistor Rj. Next, in switch position 2 the unknown emf of cell Cx is... 

A standard cell produces a precise voltage and, before the advent of reliable voltmeters, was needed to calibrate medical and laboratory equipment. It is generally agreed that the first standard cell was the Clark cell (see p. 299), but the most popular was the Weston saturated cadmium cell, patented in 1893. 

Edward Weston (1850-1936) was a giant in the history of electrical measuring instruments. In the held of measurement, he developed three important components the standard cell, the manganin resistor and the electrical indicating instrument. 

A brief discussion of standard cells, together with a short biography of Edward Weston, may be found at http //www.humboldt.edu/ scimus/HSC.54-70/Descriptions/ WesStdCell.htm. Alternatively, http //www.humboldt.edu/ scimus/Instruments/Elec-Duff/StdCellDuff.htm is also useful, although its symbolism does not always adhere to SI or IUPAC. 

The red form of HgO is of increasing importance in electronics in the production of miniature dry cells, and Hg2S04 is employed in both the Weston and Clark standard cells. Hg2Cl2 still finds use as an antiseptic and Hgl2 is used to treat skin diseases. Mercury fulminate, Hg(OCN)2, is an important detonator for explosives.142... 

The mercury-mercurous sulfate electrode. Several commercial suppliers offer the mercury-mercurous sulfate electrode with a saturated potassium sulfate electrolyte. The potential (E° + E ) of this electrode system is 0.658 V on the hydrogen scale at 22°C.34 The electrode constitutes one-half of the Weston standard cell,35 an international secondary voltage standard, and is outstanding in reproducibility,36 in spite of the slight tendency of mercurous sulfate to hydrolyze and its rather high solubility. 

One of the most satisfactory voltage standards is the Weston standard cell, shown in Figure 6.23. The cell is represented by the schematic... 

The Weston cell is the most suitable standard cell for measuring electromotive forces because of its excellent reproducibility it is shown in diagrammatical form in Fig. 9. 

Standard cell — An electrochemical cell composed of two - half-cells containing electrodes built according to standard (normal) conditions. Frequently the term is also used for electrochemical cells showing a well-defined, reproducible, and stable cell voltage suitable for calibration purposes (- Weston cell, - Clark cell). 

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... 

Standard Cell. The electrochemical cell used as a voltage standard is the Weston cell, shown in Fig. 13. The voltage of this cell changes only slightly with temperature and is given in absolute volts by... 

Instruments and Methods of Measurements. A Leeds and Northrup Type K-3 universal potentiometer, in conjunction with a General Electric Model 29 galvanometer, was used to measure electromotive force. The potentiometer was calibrated by means of a Weston Standard Cell which had been calibrated against a National Bureau of Standards (NBS) certified standard cell. Galvanic cells which were maintained at constant temperatures of 25°, 35°, and 45°C d= 0.01° by being immersed in a water bath at the desired temperature. The temperatures of the baths were set using a Fisher Scientific calibrated standard thermometer, with calibration traceable to the NBS. An adaptation of the cell sketched by Ives and Janz (II) was used. The modification of the cell was that described by Mclntrye and Amis (10). 

Use Pigments, fluorescent screens, electrolyte in Weston standard cell, electroplating. 

The pure metal is used in the cadmium and Weston standard cells, invaluable for the accurate determination of E.M.F s. It is sprayed on to steel to protect against corrosion sometimes it is plated on to steel prior to chromium plating. Alloys of cadmium with 2 per cent of Ni, or of 2 25 Ag plus o-2 Cu are used in -automobiles etc, as handles, and for other purposes. 

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. 

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