Electrogravimetry

Electrogravimetry is a physical method for the quantitative analysis of a sample and represents a special application of electrolysis . The first application of this analysis technique goes back to Wolcott Gibbs .

Principles

Experimental setup

In electrogravimetry, like in gravimetry , the concentration of a substance is measured with the help of weight. To do this, the substance in question must first be precipitated from the solution. In electrogravimetry, electrons are used as precipitating agents instead of a chemical additive, as in gravimetry . The metal to be determined is deposited on the cathode . To determine the quantity, the gut with dist. Water washed and later dried cathode, each carefully weighed before and after the electrolysis. A platinum mesh is usually chosen as the cathode and a platinum spiral as the anode.

Chloride and especially nitrate ions can adversely affect the analysis result. Oxidations on the anode are easily possible. Electrolysis is therefore usually carried out in a sulfuric acid solution.

Knowledge of the oxygen overvoltage at the anode is very important for electrogravimetry (see electrolysis ). Depending on the nature of the anode metal (platinum or platinum-coated graphite) and current density, the overvoltage also differs.

At the anode, the water is broken down into oxygen and hydronium ions. If the solution was not strongly acidic (or basic) beforehand, the pH value can drop in the course of the electrolysis. This has an important influence on the decomposition voltage of water (in extreme cases this leads to a voltage increase of around 0.8 V). The change in the metal ion concentration during electrolysis also has a small influence on the selected voltage (up to 0.2 V).

At temperatures around 50–60 ° C, the conductivity (1 ° C increase in temperature increases the conductivity by about 1–2%), which means that higher currents and faster deposition can be achieved. Continuous stirring is also recommended so that the diffusion layer is reduced.

The use of depolarizers (reducing agents or oxidizing agents such as hydrazine or nitrate) is also recommended. Depolarizers can suppress the deposition of hydrogen or oxygen.

The voltage to be applied can be calculated from knowledge of the normal potential. Whether elements can be separated depends on the normal potential of the electrochemical series. The decomposition voltages should differ by at least 200 mV so that an element can be completely separated. Silver , copper , nickel , zinc , lead , tin , for example, can be separated from one another.

However, since silver and copper are often determined electrogravimetrically in the presence of nitrate ions, in this case, for example, ethanol (which is oxidized at the anode) must be added. Silver can sometimes only be obtained (unless cyanide ions are present) in the form of a granular precipitate that sometimes falls off the electrode. However, the gravimetric determination of silver and copper in particular are very popular, as no deposits of other metals can occur in this potential range.

When determining zinc, nickel and other metals, the presence of nitrate and chloride ions should be avoided (smoking, separation by means of anion exchangers). Lead is deposited almost exclusively as lead dioxide on the anode.

When depositing base metals, the platinum electrode should be copper-plated. Electro-gravimetric separations can be carried out at constant current strength or constant voltage. The current density should be between 5 and 50 mA / cm ² . Higher current densities lead to unclean layers with foreign inclusions. If the voltage is kept constant by means of a potentiostat, the current intensity drops to almost zero at the end of the material separation.

Example: Nickel (II) determination

The nickel (II) -containing sample solution with a ammonia / ammonium nitrate - buffer at about pH brought 10th This creates the hexaammine nickel (II) complex. When a decomposition voltage of approx. 3 V is set, nickel (II) is electrochemically reduced at the platinum cathode and solid, finely divided nickel is deposited. In return, oxygen is formed at the platinum anode. Since chloride is also oxidized at the anode (to chlorine ), only traces may be present. Otherwise you smoke beforehand with conc. Nitric acid .

• ${\displaystyle \mathrm {[Ni(NH_{3})_{6}]^{2+}+2\,e^{-}\rightarrow Ni\downarrow +6\,NH_{3}} }$

• ${\displaystyle \mathrm {4\,OH^{-}\rightarrow 2\,H_{2}O+O_{2}\uparrow +4\,e^{-}} }$

Faraday's law is used to relate the number of electrons used, the amount of charge Q, to the amount of substance precipitated .

${\displaystyle m={M\cdot Q \over z\cdot F}}$

in which:

• ${\displaystyle m}$ the mass of the precipitated substance
• ${\displaystyle M}$the molar mass of the precipitated substance
• ${\displaystyle Q}$the amount of charge ( amperage times time)
• ${\displaystyle z}$the number of electrons transferred per formula conversion
• ${\displaystyle F}$the Faraday constant = 96,485 As / mol

literature

• Georg Schwedt: Analytical chemistry. Wiley-VCH Verlagsgesellschaft, 2nd edition, Weinheim 2008, ISBN 978-3-527-31206-1 , pp. 170 ff.
• A. Schleicher: Electroanalytical rapid methods. Ferdinand Enke Verlag, Stuttgart 1947

Web links

Commons : Electrogravimetry  - collection of images, videos and audio files

• Experiments

Individual evidence

1. ↑ Georg Schwedt: Analytische Chemie, Wiley-VCH, 2nd edition 2008, p. 170 ff.