Informations about the analytical methods
Melt decomposition (docimasy / fire assaying)
The use of so-called docimasy to identify precious metals was developed over 2500 years ago in ancient mining and was derived from the refinement processes common at the time. It was primarily used to define the melting properties of ore. The term docimasy comes from the greek for testing.
One of the best known and also most commonly applied methods is the collection of precious metals in a melt rich in lead followed by a thermal separation of the lead, which is also known as cupellation. The remaining precious metal is separated and the elements they consist of are defined. The method being mentioned in bible texts is proof for its great age. “I have made you a tester of metals and my people the ore, that you may observe and test their ways.” (Jer. 6: 27-30).
Nowadays docimasy is performed using crucible and muffle furnaces. The use of purpose built crucibles, assay porringers, cupels and different kinds of flux agents make it possible to quantitatively reclaim and identify precious metals from a wide range of raw materials.
Wet chemical method
A typical wet chemical method in precious metal analytics is the separation (digestion) of alloys using acids. Due to the different chemical properties of metals within an alloy, it is possible to separate them from one another with the help of acids. A well-known example being nitric acid, also known by the name of aqua fortis. Nitric acid is able to dissolve most base metals such as copper, iron and zinc. Precious metals such as gold, platinum and iridium are not dissolved. Due to this characteristic for example the gold percentage in jewellery alloys can be seperated and gravimetrically determined. Thanks to their chemical properties dissolved precious metals such as silver can be percipitated using special chemicals in order to make it possible to weigh these also.
In addition to the above mentioned methods of seperation and weighing precious metals, there is a further substantial advantage to the use of wet chemical seperation of a sample. Modern analysers e.g. the ICP-OES (inductively coupled plasma with optical emission spectroscopy) allow an exact statement regarding precise concentration of all dissolved metals. Using certified standards, the device is calibrated and can then determine the exact concentration of metals within a solution This analyser makes a fast, precise and above all parallel analysis of many elements possible, which is why it plays an important role in our laboratory. Because meeting deadlines is amongst our highest priorities, we work with a number of high-end ICP-OES-analysers, to always be safeguarded in the case of a machine malfunction or necessary maintenance.
In titration a dissolved element is chemically altered by adding a reaction solution. This alteration is traced using an indicator. As the reaction solution is added it points out the end of the reaction. The end allows a conclusion as to the concentration of the sought-after elements. One of the most common titration methods in precious metal analytics is the titration acc. to Volhard. The titration acc. to Volhard is a percipitative titration method. This method of analysis is based upon the low solubility of some silver compounds. Making it possible to determine the silver concentration manually or automatically, using a Dosimat and a measuring probe. Titration is commonly applicable for analysing metals with a concentration above 10%. Hence titrations are well suited for determining the concentration of silver in alloys or lead in lead concentrate.