Weston cell

Woodcut line drawing of H-shaped cell in an enclosure with electrical terminals at the top.

Drawing from Edward Weston's US Patent 494827 depicting the standard cell.

The Weston cell is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893, it was adopted as the International Standard for EMF between 1911 and 1990.

Chemistry

The anode is an amalgam of cadmium with mercury with a cathode of pure mercury over which a paste of mercurous sulphate and mercury is placed. The electrolyte is a saturated solution of cadmium sulfate, and the depolarizer is a paste of mercurous sulfate.

As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made by platinum wires fused through the lower ends of the legs.

Anode reaction: Cd(s) → Cd²⁺(aq) + 2e⁻

Cathode reaction: (Hg⁺)₂SO₄²⁻(s) + 2e⁻ → 2Hg(l) + SO₄²⁻(aq)

Reference cells must be applied in such a way that no current is drawn from them.

Characteristics

The original design was a saturated cadmium cell producing a 7000101863800000000♠1.018638 V reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell.^[1]

One of the great advantages of the Weston normal cell is its small change of electromotive force with change of temperature. At any temperature $t$ between 7002273149999999999♠0 °C and 7002313150000000000♠40 °C, $E t /V = E 20 /V - 6995406000000000000♠ 0.000 0406 (t /°C - 20) - 6993950000000000000♠ 0.000 00095 (t /°C - 20) 2 + 6992100000000000000♠ 0.000 00001 (t /°C - 20) 3$ . This temperature formula was adopted by the London conference of 1908^[2]

The temperature coefficient can be reduced by shifting to an unsaturated design, the predominant type today. However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodic calibration against a saturated cell.

References

↑ Robert B. Northrop Introduction to instrumentation and measurements 2nd edCRC Press, 2005 ISBN 0-8493-3773-9 page 14
↑ "Electric units and standards". Circular of the National Bureau of Standards. Washington, D.C.: USA Government Printing Office. 1916 (58): 39. 25 September 1916. Retrieved 12 July 2016.

Literature

Practical Electricity by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 198–203
U.S. Patent 494,827, "Voltaic cell"
Standard Cells, Their Construction, Maintenance, and Characteristics by Walter J. Hamer, National Bureau of Standards Monograph 84, January 15, 1965.

External links

Galvanic cells

Types	Voltaic pile Battery Flow battery Trough battery Concentration cell Fuel cell Thermogalvanic cell

Primary cell (non-rechargable)	Alkaline Aluminium–air Bunsen Chromic acid Clark Daniell Dry Edison-Lalande Grove Leclanché Lithium Mercury Nickel oxyhydroxide Silicon–air Silver oxide Weston Zamboni Zinc–air Zinc–carbon

Secondary cell (rechargeable)	Automotive Lead–acid gel / VRLA Lithium–air Lithium–ion Lithium polymer Lithium iron phosphate Lithium titanate Lithium–sulfur Dual carbon battery Molten salt Nanopore Nanowire Nickel–cadmium Nickel–hydrogen Nickel–iron Nickel–lithium Nickel–metal hydride Nickel–zinc Polysulfide bromide Potassium-ion Rechargeable alkaline Sodium-ion Sodium–sulfur Vanadium redox Zinc–bromine Zinc–cerium

Cell parts	Anode Binder Catalyst Cathode Electrode Electrolyte Half-cell Ions Salt bridge Semipermeable membrane

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