Chemical properties of 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP)

1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP), also known as etidronic acid, is an organophosphonic acid featuring two phosphonic acid groups (-PO₃H₂) and a hydroxyl group (-OH) bound to the same central carbon atom (geminal bisphosphonate structure).

1. Physical & Identification Parameters

Parameter Value / Description

CAS Registry Number 2809-21-4

Molecular Formula C₂H₈O₇P₂

Molar Mass 206.03g/mol

IUPAC Name (1-hydroxy-1-phosphonoethyl)phosphonic acid

Appearance White crystalline powder (pure form) or clear colorless to pale yellow liquid (60% aqueous solution)

Solubility Highly soluble in water; soluble in alcohols; insoluble in non-polar organic solvents

2. Chemical Properties & Reactivity

Acid-Base dissociation (Multibasic Acid)

HEDP is a tetraprotic acid (H4L), possessing four ionizable hydroxyl protons across its two phosphonate groups. It dissociates stepwise in aqueous solution:

Ka1: ≈1.35

Ka2:≈2.87

pKa3: ≈7.03

pKa4: ≈11.3

Because of its multiple dissociation steps, HEDP acts as an effective buffer across neutral-to-alkaline pH ranges and forms various stable mono-, di-, tri-, and tetra-metal salts (e.g., HEDP.Na4).

Chelation & Complexation

The spatial arrangement of the two phosphonate groups alongside the central hydroxyl group allows HEDP to act as a multidentate ligand, forming h3 six-membered ring complexes with polyvalent metal cations (Fe³⁺, Fe²⁺, Cu²⁺, Ca²⁺, Mg²⁺, Zn²⁺).

Hexadentate Coordination: The central hydroxyl group (-OH) contributes electron density, enhancing the stability constants of the resulting metal-chelate ring relative to non-hydroxylated diphosphonates.

Iron Dissolution: Shows exceptionally high affinity for ferric ions (Fe³⁺), dissolving and masking rust oxides in water systems.

Thermal Stability & Hydrolysis Resistance

C–P Bond Stability: The covalent carbon-to-phosphorus bonds confer high thermal and chemical stability compared to inorganic polyphosphates (like sodium hexametaphosphate).

High-Temperature Resistance: Resists hydrolytic degradation at elevated temperatures (up to 250℃), preventing the premature formation of orthophosphate sludge in high-pressure boilers.

Chlorine & Oxidative Stability

Unlike nitrogen-containing organophosphonates (such as ATMP or EDTMP), HEDP contains no central nitrogen atom.

This carbon-backbone structure provides superior resistance to active chlorine, bromine, and chlorine dioxide oxidative attack, preventing breakdown in halogen-treated cooling water loops.

Threshold Scale Inhibition & Crystal Modification

Sub-stoichiometric Effect: Inhibits scale precipitation (especially CaCO₃ and calcium sulfate) at micro-molar doses (1 to 10 ppm).

Lattice Distortion: Adsorbs onto active crystal growth sites on forming nuclei, disrupting crystal lattices and keeping mineral ions dispersed.