Increase of pH and of the Calcium concentration heavily affects  the calcium tolerance of phosphonates, increasing the possibility of precipitation.

This effect is called CALCIUM- SENSITIVITY and is explained by a precipitation of sparingly  soluble Ca -INHIBITOR salt.

Threshold effect: stabilisation of supersaturated solutions (which would normally be expected to precipitate) by means of substoichiometric concentrations of highly selective polyelectrolytic compounds.

The inhibition mechanism is explained by the absorption of the inhibitor molecules on to the surface of the growing nucleus during crystallisation.

The final effect is a  modification of the structure of the nucleus causing delay or even complete inhibition of the crystal growth.

The inhibitor was added to 250 ml beaker containing hard water having a pH 8,3 and:

Test conditions:

500 ppm CaCO3 – 0,05M Borate Buffer Initial T @ 60°C and final T @ 100°C

  1. Prepare stock solutions of inorganic salts and antiscalant at suitable dilution in deionized
  2. Mix appropriate volumes of the previous stock
  3. Bring to the desired pH with HCl or NaOH and let at the desired temperature for 24
  4. Filter the turbid solution with appropriate paper
  5. Measure by complexometric titration the remaining hardness in filtrate

The following figure shows comparison between Standard phosphonates and HYDRODIS :

Antiscalant inhibitor comparison with standard phosphonates

The structure of Hydrodis RO improve the so called “Calcium Sensitivity” of Phosphonates, avoiding the problem of possible precipitation of the product in extremely hard waters.

In these conditions the product, staying in solutions, can perfectly perform as threshold agent and so no loss of activity is expected even in extreme hardness conditions.

Performance data: Crystal modification


It’s the property to distort crystal shapes.

As crystals begin to form at the submicroscopic level, negative groups located on the antiscalant molecule influence the kinetic of growth, alter the edges of the crystals that become rugged, as a result of the adsorption-desorption equilibrium.

HYDRODIS RO terminates crystal growth by attaching to the positively charged calcium ions located at the corners of the crystals.

Dispersion evaluation

Dispersancy is the ability to adsorb on crystals or colloidal particles present in solutions and impart them a high anionic charge, adding a steric barrier which tends to keep the crystals separated. The high anionic charge also separates particles from fixed anionic charges present on the membrane surface.


HYDRODIS RO have strong dispersion power and therefore act as deflocculants.

For this reason it makes solids more flowing, it brings with it a more homogeneous distribution of colloidal suspended particles.

In this way these negatively charged compounds impede flocculation, effect that remain for long periods of time, due to the high resistance to hydrolysis of these products.