This method describes how to determine the total hardness of water by means of complexometric titration.
Water intended for use as an ingredient in the production of beer (brewing liquor) or other foods
The ions causing hardness can be bound by disodium dihydrogen ethylenediaminetetraacetic acid (EDTA) under formation of complexes through chelation. A mixed indicator is used to form loosely bound complexes, which are red in color, together with the ions that constitute hardness on the basis of Eriochrome Black.
During complexometric titration, the following processes take place:
1. The ions causing hardness (metal ions) at first form a complex through chelation with the indicator:
Metal ion + indicator → metal-indicator complex (red)
2. The metal-indicator complex, however, is less stable than the EDTA complex.
Therefore, by adding EDTA, the indicator is eliminated from the complex:
Metal-indicator complex + EDTA → metal-EDTA complex + indicator (green)
In water samples that contain few to no magnesium ions, the color change takes place gradually. Therefore, a substitution titration is performed.
Conventional titrants usually contain a magnesium EDTA complex.
The principle is based on the fact that calcium ions form a more stable chelate with EDTA than magnesium ions do. Thus, the latter are displaced by Ca ions from the EDTA complex:
(Mg-EDTA)2- + Ca2+ → (Ca-EDTA)2- + Mg2+
If a Mg-EDTA complex is added along with the conventional titrant to the solution, the Mg ions present, due to the hardness of the water, in addition to the Mg ions that have been released through substitution with the Ca ions, are both in solution. The sum of Mg ions corresponds to the amount of Mg and Ca in the water and thus its total hardness. The Mg ion content is determined through titration using an EDTA standard solution.
In order to prevent substantial change in the pH during titration due to chelate formation of the released hydronium ions, ammonia and a mixture containing an indicator and a buffer are utilized.
Simple instructions have been created by various suppliers, of which the Titriplex method from Merck, Darmstadt, Germany (www.merck.de) is described below and will serve to represent the instructions for other methods as well (e.g., Riedel de Haën, Hannover, Germany (www.riedeldehaen.de)).