Chromic acid element

Two historical designs of the chromic acid element, on the left the Poggendorff design without diaphragm, on the right with a diaphragm

The chromic acid element is a historic galvanic cell , which consists of an anode made of zinc , an electrolyte made of dilute sulfuric acid and chromic acid and a cathode made of graphite . It is one of the wet batteries that are no longer in use today and delivers a cell voltage of 1.92  V to 2.2 V. The application area was from the mid-19th century to the 1920s in the area of ​​wired telegraphy as an electrical energy source.

The galvanic element is available in two designs: In a design with a mixture of dilute sulfuric acid and chromic acid, the latter is formed by adding potassium dichromate in the presence of sulfuric acid. This design, developed in 1842 by Johann Christian Poggendorff and also known as the Poggendorff element , has no diaphragm as a special feature . The two acids are present as a mixture in which the two electrodes, the zinc anode and the graphite cathode in the form of plates are immersed. The advantage of this arrangement is that by omitting the diaphragm, which at the time was usually made of porous earthenware, the internal resistance of the cell to contemporary galvanic elements such as the Daniell element could be reduced. The disadvantage was that the acid mixture attacked the zinc anode - the anode therefore had to be designed in such a way that when the element is not in use it can be drawn from the acid in order to avoid premature dissolution in the acid. The construction of the Poggendorff element was improved in the following years by Eugene Grenet , for which he received a patent in 1859.

The second design of the chromic acid element, shown on the outside right in the figure, consists of the diaphragm for separating the sulfuric and chromic acids. The graphite cathode is immersed in chromic acid, the zinc anode in sulfuric acid. This design, which is less common because of the higher internal resistance, had the advantage that in the area of ​​the zinc anode, adding a little mercury could reduce the decomposition process of the anode due to the amalgam formation .

Web links

• Wire Telegraphy. Retrieved April 14, 2015 . 

Individual evidence

1. ^ William Edward Ayrton: Practical Electricity . Cassell, London 1891, p. 222 ff . ( Online ).
2. ↑ Mark DiLuciano: Grenet Cell. Retrieved April 14, 2015 . 
3. ↑ Patent US25503 : Improved galvanic battery. Published September 20, 1859 , inventor: Eugene Grenet. ‌