Silver coulometer
The Silbercoulometer is a historical device, with the aid of electric charges and constant currents in the DC circuit were determined. For more than half a century it was of great importance for precision measurements of charge and current strength, in particular it was also used to define these quantities and to determine the Faraday constant . Like all coulometers , it has hardly any practical significance today, but it is still used for training purposes.
The Silbercoulometer contains a silver salt solution through the current electrolysis is. The positive electrode consists of silver , which goes into solution through electrolysis. Silver is deposited on the negative electrode. By precisely determining the change in mass of one of the electrodes, the charge that has flowed through the device can be calculated. With a constant current strength, this can be calculated from the known electrolysis time.
Historical
The silver coulometer, formerly also known as the silver voltameter , was first mentioned in 1861 by Johann Christian Poggendorff . From 1881, the brothers Friedrich and Wilhelm Kohlrausch used various silver coulometers for precision measurements, including one in which both the cathode and the anode were made of silver. In 1881 at an international conference in Paris, the unit of amperage, the ampere , was defined on the one hand by the voltage of one volt and a resistance of one ohm, but on the other hand it was also stated that the practical determination of the ampere was made with the help of the silver coulometer. John William Strutt, 3rd Baron Rayleigh , studied the silver coulometer carefully. He used platinum vessels as cathodes and silver anodes, which were wrapped in filter paper, so that any falling metal flakes were caught during the dissolution of the silver and thus could not falsify the measurement of the crucible mass.
At international conferences in Chicago in 1893 and in London in 1908, the silver coulometer was established as the official primary standard for measuring current intensity and thus also for determining charge. The electrochemical equivalent of silver was established in 1893 as 1.118 mg / C. In Germany the ampere was defined in the Reichsgesetzblatt from 1898 on electrolysis.
In 1946 a new definition for the unit ampere was proposed, which is based on forces that are caused by the magnetic effect of the electric current. This new definition was formally established in 1948 at the ninth general conference on Weights and Measures . The ampere was thus no longer defined based on the chemical effect of the current, and the silver coulometer no longer had a special position in the system of measuring units. However, it was still used to determine the Faraday constant .
With the reform of the International System of Units in 2019 , the ampere and the mole were redefined by assigning fixed values to the elementary charge and the Avogadro constant . Since then, the Faraday constant has also had an exact value.
advantages
The molar mass of silver is a factor of about 1.7 higher than that of copper . In addition, double the amount of substance is deposited for a certain charge in silver. Hence the electrochemical equivalent of silver is 3.4 times that of copper. For this reason, too, a measurement with the silver coulometer is more precise than one with the copper coulometer , and it is more suitable for measuring smaller currents. Further reasons are that copper can be dissolved in acidic solutions by atmospheric oxygen, while such a disturbance is not possible with the more noble silver, and that copper oxidizes more easily in air. The copper coulometer can also deliver values that are too small if the copper is not reduced completely.
variants
The most important cathode materials were platinum, which can be easily annealed to constant weight, and silver. Silver nitrate was mostly used as the silver salt dissolved in the electrolyte , and sometimes also silver perchlorate . The two electrodes were either in the same vessel or in two beakers connected by a bridge filled with electrolyte.
The mass of silver deposited on the cathode was often used to determine the charge, but silver coulometers were also used, in which the amount of silver dissolved on the anode was weighed instead.
Charge calculation
According to Faraday's laws , the charge Q is proportional to the change in mass, and the following applies:
- Q : transferred electric charge
- Δ m : change in mass of the anode or cathode, determined with the most accurate balance possible
- z : number of electrons transferred per particle. For Ag / Ag + , z = 1.
- Q : Faraday's constant ( 196 485.3 As mol −1 )
- M : molar mass . For silver: M = 107.8682 g / mol.
- Ä e : electrochemical equivalent , Ä e = M / z F . For silver: Ä e = 1.118 mg / As.
Individual evidence
- ↑ EB Rosa, GW Vinal: The silver volta meter - Part I. First series of quantitative experiments . In: Bull. Bur. H . tape 9 , no. 2 , 1913.
- ^ F. Kohlrausch, W. Kohlrausch: The electrochemical equivalent of silver; at the same time an experimental test of geomagnetic intensity measurements . In: Wiedemann's annals of physics and chemistry. New episode . tape 27 , 1886, p. 1 .
- ↑ history of the ampere
- ^ A b A. Horsfield: The Faraday and Its Significance in Determining the Fundamental Constants . In: Precision Measurement and Fundamental Constants, Proceedings of the International Conference held at the National Bureau of Standards, Gaithersburg, MD, August 3-7, 1970 . 1971, p. 137 .
- ^ A b D. Norman Craig, James I. Hoffman, Catherine A. Law, Walter J. Hamer: Determination of the Value of the Faraday with a Silver-Perchloric Acid Coulometer . In: J. Res. Nato. Bur. Stand. (US) . 64A, no. 5 , 1960 ( Online @ NIST ).