These titrations are based on complexation reactions.
Most often used reagent is EDTA - EthyleneDiamineTetraAcetic acid. There are also other similar chelating agents (EGTA, CDTA and so on) used. In some of other methods Ag+ is used as a titrant for determining cyanides and Hg2+ as a titrant in Cl- determination.
Changing property of the solution is usually the concentration of the complexed substance, although in some cases it can be much more convenient to express results in terms of titrant concentration. As its concentration changes by many orders of magnitude, and is almost always smaller than 1, we use negative logarithmic scale, similar to that used in pH definition.
In the case of determination of metals detection of the endpoint is mainly based on substances that change color when creating complexes with determined metals. One of these indicators is eriochrome black T, substance that in pH between 7 and 11 is blue when free, and black when forms a complex with metal, other examples are pyrocatechin violet and murexide. It is important that formation constant for these complexes is low enough, so that titrant reacts with complexed ions first.
Page was last modified on February 23 2009, 12:50:18.