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Consider the Galvanic cell shown in the adjacent image: it is constructed with a piece of zinc (Zn) submerged in a solution of zinc sulfate (ZnSO 4) and a piece of copper (Cu) submerged in a solution of copper(II) sulfate (CuSO 4). The overall reaction is:
The copper–copper(II) sulfate electrode is a reference electrode of the first kind, [1] based on the redox reaction with participation of the metal and its salt, copper(II) sulfate. It is used for measuring electrode potential and is the most commonly used reference electrode for testing cathodic protection corrosion control systems. [ 2 ]
Copper(II) sulfate is an inorganic compound with the chemical formula Cu SO 4.It forms hydrates CuSO 4 ·nH 2 O, where n can range from 1 to 7. The pentahydrate (n = 5), a bright blue crystal, is the most commonly encountered hydrate of copper(II) sulfate, [10] while its anhydrous form is white. [11]
Variations from these ideal conditions affect measured voltage via the Nernst equation. Electrode potentials of successive elementary half-reactions cannot be directly added. However, the corresponding Gibbs free energy changes (∆G°) must satisfy ∆G° = – z FE°,
3) Acid solution with an activity of H + = 1 mol/L, 4) Hydroseal for prevention of oxygen interference, 5) Reservoir via which the second half-element of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge, depending on the other electrode and solution. This ...
Daniell cells, 1836. The Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode.
In the above reaction, zinc metal displaces the copper(II) ion from the copper sulfate solution, thus liberating free copper metal. The reaction is spontaneous and releases 213 kJ per 65 g of zinc. The ionic equation for this reaction is: Zn + Cu 2+ → Zn 2+ + Cu. As two half-reactions, it is seen that the zinc is oxidized: Zn → Zn 2+ + 2 e −
Each solution has a corresponding metal strip in it, and a salt bridge or porous disk connecting the two solutions and allowing SO 2− 4 ions to flow freely between the copper and zinc solutions. To calculate the standard potential one looks up copper and zinc's half reactions and finds: Cu ++ + 2 e − ⇌ Cu : E o = +0.34 V