Copper / copper sulfate electrode

Schematic of copper / copper sulphate electrode

Copper / copper sulfate electrode

A copper / copper sulfate electrode consists of metallic copper that is immersed in a copper sulfate solution and that has a contact option so that the electrode can be connected to an external circuit. Two types of copper / copper sulfate electrodes are important for applications:

• The copper / copper sulfate reference electrode is used - like all reference electrodes - to measure the electrical potential or the electrode potential . It is used in particular in corrosion protection and is commercially available from many manufacturers and suppliers. These copper / copper sulphate electrode is referred to with the abbreviation CSE (of English. C opper / copper s ulfate e lectrode). The potential of the CSE measured against the standard hydrogen electrode is 317 mV at 25 ° C.
• The electrodes in electrolytic copper refining . The solution used in this case only contains 3-4% copper sulfate and 10-16% sulfuric acid .

There used to be other applications for copper / copper sulfate electrodes, which today are primarily of historical or didactic importance:

• Use in concentration elements to demonstrate the Nernst equation
• in galvanic elements such as the Daniell element (+: copper / copper sulphate, -: zinc / zinc sulphate) and its further developments, e.g. B. the Gravity-Daniell-Element
• in the copper coulometer for measuring the charge in a direct current circuit.

Electrode potential

General case

If a copper wire is immersed in a solution, metal ions Cu ^{2+ go} into solution and an electrode potential is established. The so-called passage reaction

${\displaystyle \mathrm {Cu_{(s)}\rightleftharpoons Cu^{2+}+2e^{-}} }$

is the potential-determining reaction for the electrode. According to the Nernst equation, it follows for the electrode potential

${\displaystyle E=E_{\mathrm {Cu/Cu^{2+}} }^{o}+{{RT} \over {2F}}\ln a_{\mathrm {Cu^{2+}} }}$

The potential of the electrode therefore only depends on the activity or concentration of the copper ions. The copper / copper sulfate electrode is therefore regarded as a reference electrode of the first kind.

Saturated solution - potential of the Cu / CuSO ₄ reference electrode

If a saturated copper sulphate solution is used as the solution, the concentration of the copper ions is given by the solubility product at a certain temperature:

${\displaystyle K=a_{\mathrm {Cu^{2+}} }\cdot a_{\mathrm {SO_{4}^{-}} }=const.}$

With a constant concentration of copper ions, a constant electrode potential is thus achieved (reference electrode). For practical use as a reference electrode, a saturated copper sulphate solution is therefore usually used. The main advantage of this is that the functionality of the electrode can be checked visually: As long as there are blue copper sulfate crystals in the solution, it is saturated and the potential is constant (at a certain temperature).

The potentials of the Cu / CuSO ₄ reference electrode CSE measured against the normal hydrogen electrode are 316 mV at 15 ° C, 317 mV at 25 ° C and 319 mV at 35 ° C.

The temperature dependence of the potential E _CSE can approximately be represented by

${\displaystyle E_{\mathrm {CSE} }\mathrm {/mV} =313,4+0,17\cdot t\mathrm {/^{\circ }C} .}$

The potential of the CSE should be measured with the smallest possible measuring currents (10 µA for short-term measurements, 0.1 µA for continuous measurements), if possible with a measuring device with an impedance of 20 MOhm.

Maintenance and construction of the reference electrode and special versions

The electrode is filled with high-purity copper sulfate and distilled water .

For potential measurements on buildings or pipelines, copper / copper sulfate electrodes with large diaphragms (diameter approx. 10 cm) are generally used in order to keep the contact resistance as low as possible.

Versions of the electrode have also been made that can be used at elevated pressure up to a temperature of 150 ° C.

Applications of the reference electrode

• In the construction industry, copper / copper sulfate electrodes are used in the context of potential field measurements to determine the condition of reinforced concrete structures . Compared to other reference electrodes, the copper / copper sulfate electrodes are easy to manufacture and also very robust, which is why they are well suited for use on the construction site or on the object.
• Maintenance and intensive measurements on KKS-protected pipelines and tank systems
• Influence measurements on underground installations and structures to assess the risk of corrosion, e.g. B. by driving DC railways. The copper / copper sulfate electrode is the most common reference electrode for measuring the object / soil potential.

Historical

In 1939 the copper / copper sulfate reference electrode was proposed in connection with the corrosion of gas pipes for measuring potentials in the ground. The electrode was specially developed for investigations in the field and was then used more and more.

Individual evidence

1. ↑ Examples for manufacturers or suppliers of copper / copper sulphate reference electrodes: Example 1 ( Memento of the original from January 28, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Ex.2 , Ex.3 , Ex.4 
2. ↑ ^{a b c} Heather AG Stern, Donald Robert Sadoway, Jefferson William Tester: Copper sulfate reference electrode . In: Journal of Electroanalytical Chemistry . tape 659 , no. 2 , August 15, 2011, p. 143–150 , doi : 10.1016 / j.jelechem.2011.05.014 .
3. ↑ Klaus Walther: Use and maintenance of copper-copper sulfate electrodes . Part 2. In: Communications of the Association of Cathodic Corrosion Protection eV Volume 37 , September 2000, p. 1–4 ( online [PDF]).
4. ^ Klaus Walther: Influence of the impedance of measuring devices on the function of reference electrodes . Supplement to the specialist lecture given by Ing.Klaus Walther. In: Communications of the Association for Cathodic Corrosion Protection eV Volume 38 , December 2000, p. 3–4 ( online [PDF]).
5. ↑ Klaus Walther: Use and maintenance of copper-copper sulfate electrodes . Part 1. In: Communications of the Association of Cathodic Corrosion Protection eV Volume 36 , June 2000, p. 1–4 ( online [PDF]).
6. ↑ Walter von Baeckmann, Willi Fleig: Measurement technology for cathodic corrosion protection (=  contact & study . No. 93 ). 4th edition. expert verlag, Renningen-Malmsheim 2000, ISBN 3-8169-1879-4 , p. 32 ( limited preview in Google Book search).
7. ↑ Derek M. Hall, Justin R. Beck, Edward Brand, Margaret Ziomek-Moroz, Serguei N. Lvov: Copper-Copper Sulfate Reference Electrode for Operating in High Temperature and High Pressure Aqueous Environments . In: Electrochimica Acta . tape 221 , December 10, 2016, p. 96-106 , doi : 10.1016 / j.electacta.2016.10.143 .
8. ↑ Ulrich Bette, Wolfgang Vesper: Pocket book for cathodic corrosion protection . Vulkan-Verlag GmbH, 2005, ISBN 3-8027-2932-3 , p. 324 ( limited preview in Google Book search).
9. ^ Scott Preston Ewing: Soil Corrosion and Pipe Line Protection . American Gas Association, New York 1938.
10. ^ Scott Preston Ewing: The copper-copper sulfate half-cell for measuring potentials in earth . In: American Gas Association Proceedings . tape 21 , 1939, pp. 624-634 .
11. ↑ Patent US2338713 : Half-cell for making potential measurements in substances. Registered August 18, 1939 , published January 11, 1944 , inventor: Scott Preston Ewing. ‌

literature

• Wolfgang Bechmann, Joachim Schmidt: Entry into physical chemistry for minor subjects . 4th edition. Springer + Teubner, 2010, ISBN 978-3-8348-0991-9 , chapter 4.4.3, electrode types.