Silicate(2-), hexafluoro-, disodium, reaction products with lithium magnesium sodium silicate

Hydrolysis

Description of key information

This substance does not hydrolyse in the pH range of 4 to 8.4

Additional information

This substance is an inorganic layered silicate structure with a unit cell of the following composition: Na_0.7[Mg_5.3Li_0.7Si₈O₂₀(OH)_0.0F_4.0].  

The material is non-biodegradable, retaining its clay structure in the environment. At normal water course pH’s this material is stable and insoluble. It does not dissolve in water but disperses to form a sol so the usual methods to determine water solubility do not work because it appears by observation that total dissolution has taken place whereas in reality the structure has remained unchanged but simply dispersed to be invisible to the naked eye. To demonstrate whether any solubility to the component ions takes place, a dialysis method was devised to determine solubilised ions present. Based on the structure above, lithium, sodium and magnesium, and in a later study fluoride, were determined to be key identifiers of whether the structure under defined conditions dissociates in water.

The natural pH of this substance in water at a concentration of 0.5% by weight is 8.4. At that pH, no free lithium or magnesium ions are found in solution and only low level concentration of sodium ions which are present from dissociation of free sodium sulphate left over from the manufacturing process as a low level impurity.

To demonstrate hydrolysis at lower pH, the dialysis test was carried out by adjusting the dispersion to pH1, pH4 and pH7 and measuring Li, Mg, Na and F ions that pass through the membrane into demineralised water on the other side of the membrane. At pH4, and pH7 no Li and Mg ions were measured thus concluding that hydrolysis of the actual structure does not take place at the range of pH covered in the prescribed OECD method (4-9).

The hydrolysis of the fluoride content is somewhat more complicated. The fluoride is incorporated into the lattice during processing using sodium hexafluorosilicate as the source. The total fluoride in the material as sold is ~ 11.0% by weight as measured by XRF analysis.

Of that amount, ~ 9.5% is present in the layer structure, present as bound + free fluoride.

(The 1.5% residual F is associated with insoluble neighborite (NaMgF₃) formed during processing and is variable, measured as sediment (sediment values are around 3%). This material is insoluble and the fluoride content will not be measured by ISE)

The free fluoride is likely to be simply F^-ions since it can be shown that the following reaction occurs for the hexafluorosilicate at pH values above 3.2, and processing liquors are alkaline.

SiF62- + 4 H2O ↔ Si(OH)4 (aq) + 6 F- + 4H+
SiF62- + 2 H2O ↔ SiO2 + 6 F- + 2H+

The substance decompose at pH>3.2. Hydrolysis reaction of hexafluorosilicate was studied by 19F NMR in situ (REACH dossier for disodium hexafluorosilicate CAS 16893-85-9). As the pH of the solution increased, hydrolysis continues until at pH 3.2 no hexafluorosilicate remains in solution.

For the purpose of investigating the status of fluoride within the material, Laponite type 1 was dispersed in water at 0.1%w/w and the pH of the resulting dispersions adjusted to pH 1, 4 and 7.

At a pH of 1, the structure disintegrates into component parts, the fluoride ions being measured by ion selective electrode at a buffer adjusted solution pH of ~ 6 confirms that the level of available fluoride is ~9.5%.

At pH values of 4, 7 and natural pH of 8.4, the following values were obtained,(an average of ~2.5%) which implies that 7% w/w of the available fluoride is bound in the structure whilst ~ 2.5% w/w is loosely ionically bound and free to be measured at the higher pH values which have no significant effect on the Laponite structure.

	%w/w available F in LP
pH1	9.44
pH4	2.70
pH7	2.42
pH8.4	2.38

 

The conclusion that can be drawn from the results as seen from the measurement of Li, Mg, Na and F is that there is no actual hydrolysis of the layer structure until pH values lower than 4 are reached, and the Na and F in solution at pH values between 4 – 8.4 are the result of processing impurities that have not been completely removed from the material by washing.