Weston cell

TheWeston cellorWeston standard cellis awet-chemical cellthat produces a highly stablevoltagesuitable as a laboratory standard forcalibrationofvoltmeters.Invented byEdward Westonin 1893, it was adopted as the International Standard forEMFfrom 1911 until superseded by theJosephson voltage standardin 1990.

Chemistry

Theanodeis anamalgamofcadmiumwithmercurywith acathodeof pure mercury over which a paste of mercurous sulfate and mercury is placed. Theelectrolyteis asaturated solutionofcadmium sulfate,and thedepolarizeris a paste ofmercurous 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 byplatinumwires fused through the lower ends of the legs.

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

Cathode reaction: (Hg⁺)₂SO²⁻
₄(s) + 2e⁻→ 2Hg(l) +SO²⁻
₄(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 a1.018638Vreference and had the advantage of having a lowertemperature coefficientthan the previously usedClark 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 $t$ between0 °Cand40 °C,

E t /V = E 20 /V - 0.000 0406 (t /°C - 20) - 0.000 00095 (t /°C - 20) 2 + 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. NorthropIntroduction to instrumentation and measurements 2nd edCRC Press, 2005ISBN 0-8493-3773-9page 14
^"Electric units and standards".Circular of the National Bureau of Standards.1916(58). Washington, D.C.: USA Government Printing Office: 39. 25 September 1916.Retrieved12 July2016.

Literature

Practical Electricityby 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 Characteristicsby Walter J. Hamer, National Bureau of Standards Monograph 84, January 15, 1965.

External links

Standard Cells for E.M.F. Determinations Archived2011-08-07 at theWayback Machine
Special-purpose batteries

[1] Robert B. NorthropIntroduction to instrumentation and measurements 2nd edCRC Press, 2005ISBN 0-8493-3773-9page 14

[2] "Electric units and standards".Circular of the National Bureau of Standards.1916(58). Washington, D.C.: USA Government Printing Office: 39. 25 September 1916.Retrieved12 July2016.

[1]

[2]

v t e Electrochemical cells
Types	Galvanic cell Concentration cell Electric battery Flow battery Trough battery Fuel cell Thermogalvanic cell Voltaic pile
Primary cell (non-rechargeable)	Alkaline Aluminium–air Bunsen Chromic acid Clark Daniell Dry Edison–Lalande Grove Leclanché Lithium metal Lithium–air Mercury Metal–air electrochemical Nickel oxyhydroxide Silicon–air Silver oxide Weston Zamboni Zinc–air Zinc–carbon
Secondary cell (rechargeable)	Automotive Lead–acid gel–VRLA Lithium–air Lithium ion Dual carbon Lithium–iron–phosphate Lithium–polymer Lithium–sulfur Lithium–titanate Metal–air Molten salt Nanopore Nanowire Nickel–cadmium Nickel–hydrogen Nickel–iron Nickel–lithium Nickel–metal hydride Nickel–zinc Polysulfide–bromide Potassium ion Rechargeable alkaline Silver–cadmium Silver–zinc Sodium ion Sodium–sulfur Solid state Vanadium redox Zinc–bromine Zinc–cerium
Other cell	Atomic battery Fuel cell Solar cell
Cell parts	Anode Binder Catalyst Cathode Electrode Electrolyte Half-cell Ions Salt bridge Semipermeable membrane

Chemistry

Characteristics

See also

References

Literature

External links