An important factor in the choice of chelates is the strength of the complex formed between the metal ion and the chelating agent. This determines whether the complex will be formed in the presence of competing anions. The stability or equilibrium constant (K), expressed as log K, has been determined for many metals and chelating agents. The higher the log K values, the more tightly the metal ion will be bound to the chelating agent and the more likely that the complex will be formed.
1 A.E. A.E. Martell, R.M. Smith, NIST Critically selected stability constants of metal complexes (NIST standard reference database 46, Version 7.0, 2003)
* As determined by AkzoNobel
** Z.A. begum et all, J Solution Chem (2012) 41: 1713-128
*** Z.A. Begum et all, Journal of Chemical & Engineering Data (2012), 57(10), 2723-2732Stability Constants: as determined at an ionic strength of 0.1M and at a temperature of 25°C, or if not available at 20°C.
The pH of the system and the oxidizing nature of the environment can affect the stability and effectiveness of the chelating system. For each metal complex there is an optimum pH and an active pH range in which the metal complex is stable.
Calculated for a hydroxide environment in demineralized water at 0.1 mol / l.
Lower pH limit: the conditional stability constant logK’ ≥ 3. Upper pH limit is based on the precipitation of the metal hydroxide.
At upper pH limit; fraction chelated ≥ 95%. For the Fe chelates deviations are possible in the calculation of the upper pH limit.
*= determined by measurement
The quantity of chelating agent needed depends on the concentration of metal ion to be chelated and the type of chelating agent used. Dissolvine chelating agents form a complex with a metal ion generally on an equimolecular basis (which means the higher the molecular weight of the chelating agent, the higher the quantity of chelating agent required to chelate the metal ion). See chelation equivalents in the product overview.
Chelating agent product range and applications