Product Code Database
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, which is immersed in unglazed earthenware container filled with sulfuric acid and a zinc electrode. He was searching for a way to eliminate the hydrogen bubble problem found in the voltaic pile, and his solution was to use a second electrolyte to consume the hydrogen produced by the first. Zinc sulfate may be substituted for the sulfuric acid. The Daniell cell was a great improvement over the existing technology used in the early days of battery development. A later variant of the Daniell cell called the gravity cell or crowfoot cell was invented in the 1860s by a Frenchman named Monsieur Callaud and became a popular choice for electrical telegraphy.
The Daniell cell is also the historical basis for the contemporary definition of the volt, which is the unit of electromotive force in the International System of Units. The definitions of electrical units that were proposed at the 1881 International Conference of Electricians were designed so that the electromotive force of the Daniell cell would be about 1.0 volts. With contemporary definitions, the standard potential of the Daniell cell at 25 °C (77°F) is actually 1.10 V.
At the cathode (positive electrode), copper is reduced as per the following reaction:
Note that positively charged copper ions move towards the positive electrode, driven by a reduction in chemical energy.
The total reaction is:
These processes result in the accumulation of solid copper at the cathode and the corrosion of the zinc electrode into the solution as zinc cations. In classroom demonstrations, a form of the Daniell cell known as two half cells is often used due to its simplicity. The two half cells each support one half of the reactions described above. A wire and light bulb may connect the two electrodes. Excess produced by the oxidation of zinc metal are “pushed” out of the anode, which is therefore the negative electrode, travel through the wire and are "pulled" into the copper cathode where they are consumed by the reduction of copper ions. This provides an electric current that illuminates the bulb. Since neither half reaction will occur independently of the other, the two half cells must be connected in a way that will allow to move freely between them. A porous barrier or ceramic disk may be used to separate the two solutions while allowing the flow of sulfate ions. When the half cells are placed in two entirely different and separate containers, a salt bridge is often used to connect the two cells. The salt bridge typically contains a high concentration of potassium nitrate (a salt that will not interfere chemically with the reaction in either half-cell). In the above wet-cell during discharge, nitrate anions in the salt bridge move into the zinc half-cell in order to balance the increase in ions. At the same time, potassium ions from the salt bridge move into the copper half-cell in order to replace the ions being precipitated onto the copper electrode.
If the cell is connected to a potential source (e.g. a battery charger) such that the potential difference of the source is slightly higher than the cell emf (1.1 V) then the current flow could be reversed and the reaction would become:
or,
Hence, the Daniell cell is reversible, if the current drawn from (or fed to) it is small. The Daniell cell can be used to ‘generate’ electricity, by consuming an electrode, or to store electricity.
Over time, copper buildup will block the pores in the earthenware barrier and cut short the battery's life. Nevertheless, the Daniell cell provides a longer and more reliable current than the Voltaic pile because the electrolyte deposited copper, which is a conductor, rather than hydrogen, which is an insulator, on the cathode. It is also safer and less corrosive. With an operating voltage of roughly 1.1 volts, it saw widespread use in telegraph networks until it was supplanted by the Leclanché cell in the late 1860s.
This variant, called a gravity cell, consists of a glass jar in which a copper cathode sat on the bottom and a zinc anode is suspended beneath the rim in the zinc sulfate layer. Copper sulfate crystals are scattered around the cathode and the jar then filled with distilled water. As the current is drawn, a layer of zinc sulfate solution forms at the top around the anode. This top layer is kept separate from the bottom copper sulfate layer by its lower density and by the polarity of the cell. A disadvantage of the gravity cell is that a current has to be continually drawn to keep the two solutions from mixing by diffusion, so it is unsuitable for intermittent use. In addition, it was vulnerable to loss of integrity if too much electric current is drawn, which also causes the layers to mix.
Sometimes called the crowfoot cell due to the distinctive shape of the electrodes, this arrangement is less costly for large multicell batteries and it quickly became the battery of choice for the American and British telegraph networks. Even after most telegraph lines started being powered by motor-generators, the gravity battery continued to be used in to power the local circuit at least into the 1950s. Tools of Telegraphy , Telegraph Lore; Last accessed Jul 30, 2010 In the telegraph industry, this battery was often assembled on site by the telegraph workers themselves, and when it ran down it could be renewed by replacing the consumed components.Gregory S. Raven, Recollections of a Narrow Gauge Lightning Slinger ; Last accessed on Jul 30, 2010. The zinc sulfate layer is colorless in contrast to the deep blue copper sulfate layer, which allows a technician to determine the battery life with a glance. On the other hand, this setup means the battery could only be used in a stationary appliance, otherwise the solutions would mix or spill.
|
|
