Salt bridge
A salt bridge , also ion bridge , electrolyte bridge or current key called, is used as ion-conducting connection between electrolyte - solutions . It allows ions to flow freely between connected systems. In the case of a galvanic element consisting of two half-cells , the salt bridge prevents the build-up of charge in the half-cells, which would otherwise stop the flow of current prematurely. Salt bridges or their alternatives are generally used when several systems are to be connected to one another, but the free diffusion of the particles between the systems is to be minimized. The electrolyte in salt bridges is a concentrated salt solution. When selecting the salt, it must be ensured that the anion and cation have as similar conversion numbers as possible . This is especially the case with potassium chloride , but potassium nitrate and ammonium nitrate are also used. In addition, it must not react with the electrolytes to be connected.
The most important application of salt bridges is in analytics , more precisely in electroanalysis , e.g. B. the use of electrochemical sensors : For an accurate analysis result it is important that the connection to the reference electrode does not falsify potential measurements. Salt bridges also serve to connect the half-cells of galvanic elements. In both cases the following applies: If only one voltage is to be measured across the current bridge, the current bridge may have a relatively high resistance, i.e. H. the bridge can also be long and / or narrow. If a current should also flow, e.g. B. If you want to use a galvanic element as a powerful power source, you should pay attention to a small resistance. The bridge should then be shorter and of sufficient diameter.
U-shaped glass tubes (U-tubes), which are filled with the concentrated salt solution and whose legs are immersed in the electrolyte solutions to be connected, often serve as the salt bridge. Plastic or rubber hoses are also used. The solution can be thickened (for example with agar ), which prevents the saline solution from mixing with the contact fluids. The conductivity of this type of salt bridge increases as the concentration of the salt solution increases until a maximum is reached, from which the conductivity drops again. In addition, the conductivity increases as the diameter of the U-tube increases. Air bubbles in the salt bridge reduce the conductivity and should therefore be avoided.
If only small currents occur, e.g. B. for potential measurements with high-resistance electronic voltage measuring devices, filter paper strips can also be used, which are soaked with the concentrated salt solution and the ends of which are immersed in the electrolyte solutions to be connected. The conductivity of this type of salt bridge also depends on the properties of the filter paper.
Historical
Salt bridges were used to connect reference electrodes, without actually designating them. For example, according to his book published in 1893 , Wilhelm Ostwald established the connection between a calomel electrode and the half-cells under consideration by using a rubber tube filled with potassium chloride. This had a pinch cock and could be easily closed.
The discovery that potassium chloride solutions are more suitable in salt bridges than the solutions of other salts was made by the American Olin Freeman Tower (1872–1945) when he was a student at Ostwald in Leipzig. He published his results in 1896. In the second edition of Ostwald's book “Hand- und Helfsbuch zur Execution of physico-chemical measurements”, various electrolyte bridges are presented, whereby those made of glass are designated with the term “siphon tube”. Ostwald also mentions wicks made of cotton or asbestos . In 1905, Niels Bjerrum reported on his measurements on salt bridges with potassium chloride solution.
literature
- Daniel C. Harris: Textbook of Quantitative Analysis . 8th edition. Springer-Verlag, 2014, ISBN 978-3-642-37788-4 , Chapter 13: Fundamentals of electrochemistry , p. 318 ( limited preview in Google Book search).
Individual evidence
- ↑ Takashi Kakiuchi: Salt bridge in electroanalytical chemistry: past, present, and future . In: Journal of Solid State Electrochemistry . tape 15 , no. 7-8 . Springer, July 2011, ISSN 1432-8488 , p. 1661–1671 , doi : 10.1007 / s10008-011-1373-0 ( springer.com ).
- ↑ a b Wilhelm Ostwald: Manual and auxiliary book for the execution of physico-chemical measurements . 1st edition. Wilhelm Engelmann, Leipzig 1893, Chapter fifteen: Electrical measurements. Normal electrodes., P. 258 ( online in the Internet Archive [accessed on October 2, 2019]): "The second [...] tube carries a rubber hose about 10 cm in length, [...] everything is filled with the potassium chloride solution."
- ↑ Fritz Scholz, Takashi Kakiuchi: Salt Bridges and Diaphragms . In: Handbook of Reference Electrodes . Springer Berlin Heidelberg, Berlin, Heidelberg 2013, ISBN 978-3-642-36187-6 , pp. 49-76 , doi : 10.1007 / 978-3-642-36188-3_4 ( springer.com [accessed October 2, 2019]).
- ↑ Olin Freeman Tower: About potential differences at the contact surfaces of dilute solutions . In: Journal of Physical Chemistry . 20U, no. 1 . De Gruyter, May 1896, ISSN 2196-7156 , p. 198-206 , doi : 10.1515 / ZPCH-1896-2014 ( degruyter.com ).
- ↑ a b Wilhelm Ostwald, Robert Thomas Dietrich Luther : Hand and auxiliary book for the execution of physico-chemical measurements . 2nd Edition. Wilhelm Engelmann, Leipzig 1902, Chapter sixteenth: Electromotor force. Metal electrodes., P. 377–378 ( online at the University of Innsbruck [accessed on October 2, 2019]): "(defatted) cotton and asbestos wicks [...] siphon tubes [...] with agar or gelatine jelly containing electrolytes;"
- ↑ Niels Bjerrum: About the elimination of the diffusion potential between two dilute aqueous solutions by switching on a concentrated potassium chloride solution . In: Journal of Physical Chemistry . 53U, no. 1 , January 1, 1905, ISSN 2196-7156 , p. 428-440 , doi : 10.1515 / ZPCH-1905-5325 ( degruyter.com ).