Hydroxy-OH, in organic matter, is mainly alcohol, that is, alcohol hydroxyl. Its chemical properties are mainly reaction with sodium metal, esterification with acid, dehydration between hydroxyl groups to ether, oxidation to ketone, and finally acid reaction of aldehyde. There are also phenolic hydroxyl groups such as phenol. Due to the influence of benzene ring, phenolic hydroxyl group is more active than alcohol hydroxyl group, and phenolic hydroxyl group is generally easy to be oxidized.

Carboxyl-COOH, because it is an organic acid, the first thing to note is that it is acidic.

Typical hydroxyl complexing agents are hydroxyethylidene diphosphonate (HEDP), pyrophosphate, etc.

Structural formula: C2H8O7P2

Hydroxyethylidene diphosphonate (HEDP) is often used as an additive for copper plating: a complexing agent used in copper plating process.

The structure of HEDP is very similar to that of pyrophosphate, and the main difference is that the atoms connected between the two phosphate groups are different. Because the electronegativity of 0 is greater than that of C, and the polarity of P-O bond is much greater than that of P-C bond, pyrophosphate is easily hydrolyzed into orthophosphoric acid at high temperature and high pH value. On the contrary, HEDP is stable at high temperature and high pH value.

HEDP can be dissociated into five kinds of positive and negative ions, and can form a six-membered ring chelate with metal ions, especially calcium ions, forming a capsule-shaped macromolecular chelate with good scale inhibition effect. The chelation of HEDP is not based on chemical equivalence, but also has the general corrosion and scale inhibition and solubility limitation of general organic polyphosphoric acid. The complexing effect of HEDP on copper ions is as follows.

(1) Morphology of HEDP-Cu2+ complex

Cu2+ and HEDP(H5L) can form various kinds of complexes at different pH values, such as [Cu(HL)]2-, [Cu(H2L)] 1, [Cu(H3L)], [Cu(H2L)2]4-, [Cu2(HL)] and.

According to the recent research, without adding K2C03, when HEDP/Cu2+= 2 ~ 4 and pH = 9 ~ 1 1, the complex of HEDP/Cu2+= 2 is mainly formed in the plating solution. Its composition and structure may be: [Cu(H2L)2]4-, [Cu(HL)2]6-. In these two complexes, the two coordination positions of Cu2+ on the longitudinal axis may be occupied by 0H 1 or H20. In the anode region, the concentration of Cu2+ is usually higher than that of HEDP because of its large molecular volume and slow diffusion rate. Therefore, the possibility of metal ions dissolving in these two forms is relatively small. Therefore, complex ions in the form of [Cu(HL)]2-, [Cu(H2L)] 1, [Cu2(HL)] and [Cu2(H2L)]+ can be formed. Among these complex ions, the coordination number of Cu2+ cannot be saturated by HEDP, and the empty coordination position will be occupied by 0H- 1, which is likely to form a mixed complex of [Cu(OH)2(HL)]4- and [Cu(OH)2(H2L)]3. They can also form more complex polynuclear complexes through bridging of OH- or O2-. These long chain polynuclear complexes are usually dispersed in solution in the form of colloidal particles.

Complexing agent of carboxyl group with metal ion, such as EDTA.

citric acid

Tartaric acid is very common

Take EDTA for example.

Characteristics of EDTA forming complexes with metal ions

1.

The complex formed by EDTA and metal ions is very stable.

2.

The molar ratio of EDTA to metal ions is 1: 1.

3.

The complexes formed by EDTA and metal ions are mostly soluble in water.

4.

The color of the complex is mainly determined by the color of metal ions.

Generally speaking, the electrode potential of Cu2+-HEDP shows that the complexation of HEDP is still weak. It is not enough to inhibit the generation of substituted copper on the surface of steel parts and zinc die castings in HEDP copper plating bath, which also shows that the metal complex of hydroxyl group is not as stable as the metal ion of carboxyl complex.