Hydroxyethylidene diphosphonic acid (HEDP) and its sodium salt exhibit good stability in a variety of environments, but their stability and effectiveness can be affected by factors such as pH, temperature, and coexisting ions. The following is an overview of the stability of HEDP and its sodium salt in different environments:
Hydroxyethylidene diphosphonic acid (HEDP) and its sodium salt exhibit good stability in a variety of environments, but their stability and effectiveness can be affected by factors such as pH, temperature, and coexisting ions. The following is an overview of the stability of HEDP and its sodium salt in different environments:
1. pH value
Neutral to weakly alkaline environment: HEDP and its sodium salt exhibit high stability within the pH range of 6–9. Within this pH range, HEDP can effectively chelate metal ions such as calcium and magnesium, preventing scale formation.
Acidic environment: At lower pH values (e.g., pH 3–5), HEDP maintains some stability, but its corrosion inhibition performance may decrease. Under acidic conditions, HEDP tends to exist in a protonated form, which may affect its chelating ability.
Strongly alkaline environment: At high pH values (e.g., pH > 10), the stability of HEDP decreases because phosphate ions may undergo hydrolysis to form orthophosphate. In such cases, the tetrasodium salt (HEDP·4Na) is more advantageous than the disodium salt (HEDP·2Na) because it exhibits better solubility and stability in high pH environments.
2. Temperature
HEDP and its sodium salt exhibit good stability within the range of ambient to moderate temperatures (approximately 20°C to 80°C). However, under high-temperature conditions (above 100°C), HEDP may undergo decomposition, producing orthophosphates and other byproducts, thereby losing its chelating and corrosion-inhibiting properties.
3. Coexisting Ions
Hardness Ions: HEDP has strong chelating ability toward hardness ions such as calcium and magnesium, so it performs well in water containing these ions.
Iron Ions: The presence of iron ions may form complexes with HEDP, affecting its stability. Special attention is required when treating water with high iron content.
Chloride ions: Under certain conditions, chloride ions may compete with HEDP for metal ions, affecting its chelating efficacy. However, this effect is typically minor in general industrial applications.
4. Oxidizing agents
HEDP exhibits some resistance to common oxidizing agents (such as chlorine and hypochlorite salts). However, degradation may occur in the presence of high concentrations of oxidizing agents. Therefore, when using HEDP, direct contact or mixing with strong oxidizing agents should be avoided.
5. Light exposure
HEDP and its sodium salts exhibit good stability under light exposure, but prolonged exposure to intense ultraviolet light may cause some degree of degradation.
6. Microorganisms
HEDP itself does not have bactericidal properties, but under appropriate pH and temperature conditions, it can be used in conjunction with other biocides to enhance the overall stability of the system.
Application Scenarios
Circulating Cooling Water Systems: HEDP and its sodium salts are widely used in circulating cooling water systems, particularly under neutral to weakly alkaline conditions, where they effectively control scaling and corrosion.
Boiler water treatment: In boiler water treatment, HEDP is used to prevent scale formation caused by hardness ions such as calcium and magnesium, especially in neutral to weakly alkaline environments.
Reverse osmosis membrane pretreatment: As a pretreatment agent, HEDP prevents scale formation on membrane surfaces, improving the efficiency of reverse osmosis systems.
Textile and dyeing industry: HEDP also has widespread applications in softening hard water and improving dyeing effects.