Daniell cell

The Daniell cell (also Daniell'sches element ) is a historical galvanic cell consisting of a zinc - and copper - half cell. It is named after John Frederic Daniell, who developed it in 1836.

Construction

A zinc rod (Z in the figure) in the interior of the cell is immersed in a solution of zinc sulfate. In the historic design, the zinc sulfate solution is filled into a container made ​​of porous earthenware ( P), which in turn is immersed in the outdoor area in a solution of copper sulfate. The two sulfate solutions have a concentration of 1 mol per liter. Add the copper sulphate solution bathes the second electrode of copper, C, designed as cylindrical curved copper plate. The entire cell is housed in an outer container J. Alternatively, the compound of the sulfate solution in separate vessels are connected via a special U-tube with a diaphragm as an ion conductor.

The diaphragm, also referred to as a salt conductor and realized in the original design in the form of a crucible made ​​of stoneware, serves to prevent the diffusion mixing of the different metal cations of the two solutions, and yet to allow a charge balance by anions by the porous material.

In the original design with earthenware as a diaphragm, the internal resistance of a cell is about 10 Ω, which element at a voltage of 1.1 V has a maximum discharge current of 100 mA in the range allows. Different gravity Daniell elements have been developed to avoid this high internal resistance, which omit the most significant difference, the porous diaphragm. Securing the vertically stacked sulfate solutions is achieved in a variety of approaches such as the constructive Meidinger element Callaud element or Lockwood element by different densities of the sulfate solution, and by gravity.

Function

Zinc has a lower standard potential than copper, i.e., zinc is less noble than copper, and the solution pressure of zinc is higher. Therefore go on zinc rod relatively large number of zinc ions in solution, while only relatively few copper ions replace the copper rod and leave their electrons in the metal. In the zinc electrode, the negative pole, and in this case, the anode, and more electrons are left as seen in the diagram, and therefore it is more negatively charged than the copper rod, which means the structure of an electric voltage.

The excess electrons in the oxidized zinc (Zn ) migrate through a conductor from the zinc to copper (Cu). This can be at standard conditions a voltage of 1.10 V measured. This is the electric source voltage from the redox potential of the copper (E0 (Cu) = 0.34 V ) and the zinc (E0 (Zn ) = -0.76 V) composed.

The dissolved copper ions absorb the electrons and are deposited as copper on the electrode, in this case the cathode decreases. Since going on the one hand, positive zinc ions in solution and is deposited on the other side of copper, a charge balancing must take place. About the salt bridge migrate sulfate anion [ SO4 ] 2 - to the anode chamber and zinc cations Zn 2 in the opposite direction. Thus, the circuit is closed.

The two subtasks of the redox reaction can thus be spatially separated. The electrons do not go directly from the system Zn/Zn2 on the system Cu/Cu2 over, but first migrate via a wire from the zinc to copper. Flowing a stream of electrons.

The two separate subsystems are called " half-cells ". The zinc electrode dissolves with time due to corrosion, while the copper electrode to ground is increased. There are two redox couples ( Zn/Zn2 and Cu/Cu2 ).

Reaction equation: