Raw material information

Complexing agents and their use in the chemical industry

22.02.2023

Donauchem

Business Development Manager Chemical Industry

Complexing agent/chelating agent/softener

Complexing agents are compounds that can form compounds (called complexes) with metal ions or atoms to mask undesirable chemical properties.

In terms of quantity, the removal of Ca²⁺ ions from various media, where precisely these Ca²⁺ ions have a disturbing effect, is of greatest importance - this is referred to as softening. Softening therefore means the removal of hardness-forming (Ca²⁺, Mg²⁺) ions from water or also from dirt.

Complexing agents or chelating agents also remove interfering ions from water or dirt, but in addition to Ca²⁺ und Mg²⁺ ions, complexing agents also bind (depending on their structure or composition) a whole range of other interfering metal ions such as Fe, Zn, etc.

Definition of water hardness:

1 °dH = 0.18 mmol/l alkaline earth. That is 7.1 mg Ca²⁺- or 4,3 mg Mg²⁺-ions per litre of water.

Hardness range	Water properties	°dH
1	Soft water	0 - 7
2	Medium-hard water	8 - 14
3	Hard water	15 - 21
4	Very hard water	> 21

Influence of metal ions during the cleaning or washing process.

Effect on surfactants/wetting agents (due to hard water):

Surfactants or wetting agents are impaired in their surface-active effect by the influence and presence of hardness-forming ions. Thus, soaps with calcium ions form poorly soluble lime soaps, which precipitate as residues:

Lime soaps have no surfactant properties and thus extremely reduce the washing performance.

Effect of metal ions in dirt:

Since metal ions strengthen the adhesion of dirt to the surface as well as the cohesion of dirt particles, they should be removed from the respective system if possible. Dirt can be removed much better if metal ions are removed from it.

Formation of gypsum and lime deposits:

With sulphate ions, Ca ions form poorly soluble gypsum:

At temperatures above 60°C, sparingly soluble calcium carbonate (lime) is formed from water-soluble calcium hydrogen carbonate:

The result in both cases is deposits that are difficult to remove, which damage appliances and greatly impair washing performance.

Decomposition of bleaching agents:

The bleaching effect of various bleaching agents is nullified by the presence of heavy metal ions, as these catalyse the decomposition of bleaching agents.

Mode of action of softeners or complexing agents

The use of complexing agents in washing liquors essentially achieves the following effects:

Prevention/disintegration of gypsum and lime deposits
Dispersion of dirt/dirt particles
Stabilisation of bleaching agents
Improvement of the swelling capacity of organic dirt

The following three mechanisms of action are distinguished:

Complexation
Precipitation
Ion exchange

Mode of action of various complexing substances and products

Na-triphosphates	Has a complexing effect. Rapid binding of Ca ions. Has a dispersing effect and thus supports the cleaning process. Non-toxic.	Na₅P₃O₁₀
Phosphonates	Comparable effect and properties as with Na-triphosphate.
Polycarboxylates	Water-soluble ion exchangers. Polycarboxylates also develop a very rapid binding, effect during softening. During softening/complexation, two Na ions are replaced by one Ca or Mg ion.
Gluconates	A softener without fertilising effect. Biodegradable. Made from renewable raw materials, as gluconates are produced from glucose.
EDTA (Ethylendiamintetraacetat)	Rapid complexing effect. Forms very stable Ca complexes and also complexes with other heavy metal ions. Disadvantage: EDTA is degraded only slowly.
Citric acid	Has a water-softening effect; at higher temperatures, however, the effect is somewhat worse. Biodegradable.
Soap	Hardness-forming ions are bound as lime soaps. Biodegradable.	----
Zeolith A	Component of most powder detergents. The effect is similar to that of polycarboxylates. The disadvantage is the relatively slow onset of action. According to current knowledge, the most environmentally friendly softener.	x Na2O · y Al2O3 · z SiO2.

Capacities of the various complexing agents

Naturally depends on the type of softener used, the temperature and the respective water hardness:

Binding capacities of softeners in mg Ca²⁺-ions per 1 g softener
Softener/ Temperature:	20 °C	90 °C
Phosphonate	281	270
NTA	204	144
Polycarboxylate	200	170
EDTA	156	110
Triphosphate	113	81
Zeolith	-	» 81 (bei 60 °C)
Citrate	139	21
Gluconate	90	-
Soap	» 65	-

Areas of application of complexing agents in the (chemical) industry

Surface finishing, electroplating, electrochemistry; electronics industry	Prevention or reduction of metal/metal salt precipitation in baths. Prevention of contamination by foreign metal ions in treatment baths. Improvement of the quality of the end products.
Textile and leather industry	Stabilisation of peroxide bleaching lyes. Removal of trace metals to protect dyes. Reduction of colour tone changes. Prevention of metal-containing stains. Prevention of limescale and scale formation.
Pulp and paper industry	Binding of non-complexed metal ions that catalyse the degradation of hydrogen peroxide and ozone. To increase shelf life and efficiency of bleaching agents. To improve the effectiveness of bleaching agents.
Chemical industry	In a wide variety of industries and manufacturing processes, metal ions have a negative impact on production and the quality of the end products. Complexing agents are used in: Polymerisation Synthesis of polyesters and polyethers Production of inorganic and organic peroxides Production of various inorganic products Cosmetics industry Oil and gas industry Production of dyestuffs Production of printing inks Etc.
Detergent / washing agents	Water softening. Dirt trap. Prevention of gypsum and limescale deposits. Reinforcement of the cleaning power.