Hexamethylenediaminetetra(methylenephosphonic acid), commonly abbreviated as HMDTMPA or HDTMPA, is an organophosphonic acid with strong chelating and scale-inhibiting properties. Its chemical behavior is defined by its four phosphonic acid groups, high polarity, and strong affinity for metal cations.
Key Chemical Properties
Acidic Nature
HMDTMPA is a strong acid. Its predicted pKa value is approximately 0.70, and a 1% aqueous solution typically exhibits a pH of ≤ 2.0 . The molecule contains eight acidic protons (four phosphonic acid groups, each with two ionizable hydrogens), though they dissociate at different pH levels .
Chelation and Complexation
The core chemical function of HMDTMPA is its ability to chelate metal ions. As pH increases, the molecule becomes more ionized and binds divalent and trivalent cations—such as calcium (Ca²⁺), magnesium (Mg²⁺), and iron (Fe³⁺)—more strongly than monovalent ions like potassium. It forms stable complexes with these metals, which is the basis for its scale inhibition performance .
Solubility
In its pure acid form, HMDTMPA is white crystalline powder that is only slightly soluble in water, with a solubility of less than 5% at room temperature . This limited solubility is a notable distinction from its potassium salt form, which is a liquid with much higher water solubility .
Adsorption Behavior
Despite having a very low octanol-water partition coefficient (log Kow ≈ -4.7), indicating high hydrophilicity and low bioaccumulation potential, HMDTMPA and its salts adsorb strongly to inorganic surfaces, soils, and sediments. The presence of calcium in solution significantly increases this adsorption tendency .
Stability
The compound is hygroscopic, meaning it absorbs moisture from the air, and should be stored in a dry environment . Its chemical structure is stable under normal conditions, with an estimated boiling point of 546.04°C and a density of 1.4353 g/cm³ .
Summary of Chemical Identity
| Items | Index |
|---|---|
| Appearance | White crystal powder |
| Active component (as HDTPMA–), % | 97.0 min |
| chloride (Cl–), % | 0.5 max |
| pH (1% water solution) | 2.0 max |
| Fe, mg/L | 35 max |