Electrogravimetry

Electrogravimetry is a physical method for the quantitative analysis of a sample, and provides a special application of the electrolysis represents the first application of this analytical technique is due to Wolcott Gibbs.

Principle

In Electrogravimetry the concentration of a substance, as in the gravimetry, measured by the weight. For this purpose, the target substance must first be precipitated from the solution. As the precipitant electrons are used in the Electrogravimetry instead of a chemical auxiliary, as in the gravimetry. To be determined, the metal is deposited on the cathode. For quantity determination is well with distilled water. Water washed and dried later cathode before and precisely weighed after electrolysis, respectively. One chooses as the cathode usually a platinum mesh, a platinum coil as the anode.

Chloride and particularly nitrate ions could affect the analytical result adversely. Oxidation at the anode are easily possible. Most electrolyses are therefore carried out in sulfuric acid solution.

Electrogravimetry is very important in the knowledge of the oxygen overvoltage at the anode (see electrolysis). Depending on the nature of the anode metal (platinum or platinized platinum, graphite) and current density and the overvoltage is different.

At the anode, the water is decomposed into oxygen and hydronium ions. If the solution was not previously strongly acidic ( or basic), the pH may fall in the course of electrolysis. This has an important influence on the decomposition voltage of water ( in the extreme case this leads to a voltage rise of about 0.8 V). The change in the metal ion concentration during the electrolysis has also a small influence on the selected voltage (up to 0.2 V).

At temperatures of about 50-60 ° C, the conductivity can (1 ° C increase in temperature increases the conductivity to about 1-2 %) is significantly improved, thereby to achieve higher currents and more rapid deposition. Also recommends continuous stirring, so that the diffusion layer is reduced.

Sometimes also recommends the use of depolarizers ( reducing or oxidizing agent such as hydrazine or nitrate). Depolarizers by hydrogen or oxygen precipitate can be suppressed.

The voltage to be applied can be calculated from knowledge of the normal potential. Whether elements can be separated, will depend on the normal potential of the electromotive 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, can be separated from each other for example.

Because silver and copper are, however, often elektrogravimetrisch determined in the presence of nitrate ions, for example ethanol, must in that case (which is oxidized at the anode ) are added. Silber (unless cyanide ions are present ) can sometimes win only in the form of a granular precipitate, which sometimes falls from the electrode. However, precisely the gravimetric determination of silver and copper are very popular, because in this potential range can not occur deposits of other metals.

In the determination of zinc, nickel and other metals, the presence of nitrate, chloride ions should be avoided ( fuming, separation by anion exchanger). Lead is used almost exclusively deposited at the anode as lead dioxide.

In the deposition of base metals, the platinum electrode should be coppered. Electrogravimetric separations can be carried out at constant current or at constant voltage. The current density should be between 5-50 mA/cm2. Higher current densities lead to messy layers with foreign inclusions. If you hold the tension by means of a potentiostat constant, falls at the end of the current material separation to almost zero.

For example, nickel (II) determination

Nickel (II) - containing sample solution is brought to about pH 10 with an ammonia / ammonium buffer. Here, the Hexamminnickel (II ) complex is formed. At the platinum cathode nickel ( II) is electrochemically reduced if you choose a decomposition voltage of about 3 V and solid finely divided nickel deposited. Forms at the platinum anode in return oxygen. Since chloride is also oxidized at the anode ( to chlorine ), traces may possibly be present. Otherwise you smoke previously acidified with conc. From nitric acid.

In order to set the number of electrons used, the charge amount Q in relation to the precipitated amount of substance, the Faraday's law is used.

Where:

• The mass of the precipitated material
• The molecular weight of the precipitated material
• The amount of charge ( current times time)
• The number of electrons transferred per formula sales
• Faraday's constant = 96485 As / mol