Iodine coulometer

An iodine coulometer is - like all coulometers  - a historical device with the help of which the electrical charge (amount of electricity) flowing in a direct current circuit was measured. In combination with a time measurement, constant currents were also determined. It was also used to determine the Faraday constant and to determine the molar mass of iodine . Today it is only used for training purposes.

An iodine coulometer usually consists of two vessels connected by a salt bridge into which two platinum electrodes are immersed. On the side with the positive electrode (anode) there is a potassium iodide solution ; the iodide is oxidized to iodine I ₂ when current flows :

${\displaystyle \mathrm {2I^{-}\longrightarrow I_{2}+2e^{-}} }$

The amount of iodine formed is determined - as described under iodometry - by titration , either by titration with a set sodium thiosulphate solution :

${\displaystyle \mathrm {I_{2}+2\ S_{2}O_{3}^{2-}\longrightarrow 2\ I^{-}+\ S_{4}O_{6}^{2-}} }$,

or by titration with arsenic acid:

${\displaystyle \mathrm {I_{2}+AsO_{3}^{3-}+H_{2}O\longrightarrow AsO_{4}^{3-}+2\ I^{-}+2H^{+}} }$.

The iodine coulometer was proposed for measuring currents in 1895. The following disadvantages of the gas coulometers used at that time were mentioned : With these, the temperature and the air pressure must be measured for more precise measurements, and the gas volume must then be converted to standard conditions. In addition, their operation requires a minimum voltage that corresponds to the decomposition voltage of water. In comparison, the evaluation with the iodine coulometer is easier, and if iodine can be reduced at the cathode, the cell voltage is low.

In 1906 an atomic weight of iodine determined with the aid of an iodine coulometer and a silver coulometer was published.

The iodine coulometer has good reproducibility and is considered to be very accurate; therefore it was used several times to determine the Faraday constant , the results being published in 1912 and 1957.

Individual evidence

1. ↑ Heinz Wambach (Ed.): Materials manual course instruction chemistry . 2nd Edition. tape 4 Electrochemistry - Energetics . Aulis, 2012, ISBN 978-3-7614-1610-5 , pp. 46 .
2. ↑ EF Herroun FIC: IV. On the use of an iodine voltameter for the measurement of small currents . In: Philosophical Magazine, Series 5 . tape 40 , no. 242 , 1895, pp. 91-94 , doi : 10.1080 / 14786449508620711 .
3. ^ Gino Gallo: L'equivalente elettrochimico dell'iodio . In: Gazzetta Chimica Italiana . tape 36 , no. 2 , 1906, p. 116-128 .
4. ^ D. Albert Kreider: An Iodine Titration Voltameter . In: American Journal of Science, Series 4 . tape 20 , no. 115 , 1905, doi : 10.2475 / ajs.s4-20.115.1 .
5. ↑ Hans Peter Latscha, Gerald Walter Linti, Helmut Alfons Klein: Analytical Chemistry: Basic Chemistry III . 4th edition. Springer, Berlin Heidelberg 2003, ISBN 978-3-540-40291-6 , pp. 343 .
6. ^ Gustav Kortüm: Textbook of electrochemistry . 1st edition. Verlag Chemie, Weinheim 1952, p. 14 .
7. ^ Edward W. Washburn, Stuart J. Bates: The Iodine Coulometer and the value of the Faraday. In: Journal of the American Chemical Society . tape 34 , no. 10 , 1912, pp. 1341–1368 , doi : 10.1021 / ja02211a009 .
8. ↑ Stuart J. Bates: The Iodine Coulometer and the Value of the Faraday. A correction . In: Journal of the American Chemical Society . tape 34 , no. 11 , 1912, pp. 1515–1515 , doi : 10.1021 / ja02212a013 .
9. ^ DA Mac Innes, AR Pray: A redetermination of the value of the faraday with the iodine coulometer (Progress Report) . In: Il Nuovo Cimento Series 10 . tape 6 , no. 1 , 1957, pp. 232-241 , doi : 10.1007 / BF02724779 .
10. ^ DA MacInnes, Chia-chih Yang, Alfred R. Pray: A Redetermination of the Value of the Faraday with the Iodine Coulometer. A Precision Constant Current Apparatus . In: The Journal of Physical Chemistry . tape 61 , no. 5 , 1957, pp. 662-664 , doi : 10.1021 / j150551a036 .