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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
February-May 2010
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Tests carried out by competent laboratory personnel using fit for purpose equipment, calibrated to national standards where possible. The test that has been conducted to obtain data for the hydrolysis of this substance have been designed in-house rather than by execution of a recognised hydrolysis method. The reason being the nature of this material once dispersed in water does not allow usual methodology to be applicable. The substance disperses to a nanoparticulate colloidal suspension rather than actually dissolving.
Reason / purpose for cross-reference:
reference to other study
Qualifier:
no guideline followed
Principles of method if other than guideline:
A 0.1% w/w dispersion ofthe test material in demineralised water is dialysed against demineralised water at a range of pH values. The degree of solubility is determined by measuring the concentrations of the displaced ions, (e.g. Na+, Li+, Mg2+) by atomic absorption spectroscopy.
Silicic acid, lithium magnesium sodium salt has a layer structure which, in dispersion in water, is in the form of disc-shaped crystals.
The substance is insoluble at its natural pH as described in 4.8. In order to demonstrate the hydrolysis in water a dialysis method was devised to determine solubilised ions present whe the substance is dispersed in water adjusted to pH 1, 4, 7 and 9. Lithium, sodium and magnesium were determined to be key identifiers of whether the structure under defined conditions dissociates in water based on the structure shown
The total concentration of each element that passes through the dialysis membrane is measured and compared versus the original stock dispersion to demonstrate if any hydrolysis to the individual elements has taken place
GLP compliance:
no
Remarks:
Compliance to ISO 9001:2008
Radiolabelling:
no
Preliminary study:
Results indicate that this substance does not dissociate in the pH range 4 - 9.8 but at pH < 4 dissociation starts to take place and significant dissociation is observed at pH 1.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Details on hydrolysis and appearance of transformation product(s):
- Formation and decline of each transformation product during test: No lithium or magnesium ions were analysed under the conditions of the test at pH = to or above 4. Sodium ions were observed but were present because of dissociation of the sodium sulphate impurity rather than from the dissociation of the substance structure itself.
Remarks on result:
hydrolytically stable based on preliminary test
Remarks:
the substance is hydrolytically stable and no further study was performed

 

Lithium, average mg/L

Description

pH 1

pH 4

pH 7

pH 9.8

Water surrounding dialysis tubing

1.22

0.00

0.00

0

Laponite Dispersion from Tubing

1.17

2.13

2.27

2.73

Original Stock dispersion analysis

3.34

3.01

2.80

3.05

Difference (%)

71.46

70.66

81.11

89.69

 

Sodium, average mg/L

Description

pH 1

pH 4

pH 7

pH 9.8

Water surrounding dialysis tubing

10.87

9.05

10.44

9.69

Laponite Dispersion from Tubing

12.74

11.23

14.06

12.88

Original Stock dispersion analysis

19.32

20.24

24.81

20.18

Difference (%)

122.24

100.20

98.75

111.80

 

Magnesium, average mg/L

Description

pH 1

pH 4

pH 7

pH 9.8

Water surrounding dialysis tubing

66.21

1.49

0.00

0.00

Laponite Dispersion from Tubing

67.96

152.45

162.20

128.09

Original Stock dispersion analysis

143.27

134.88

150.37

121.98

Difference (%)

93.64

114.13

107.87

105.01

Conclusions:
No hydrolyis has taken place between pH 4 and 9.8
Executive summary:

This substance is an inorganic layered silicate structure with a unit cell of the following composition Na0.7+ [(Si8Mg5.5Li0.3) O20(OH)4]0.7- 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 clear 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, adialysis method was devised to determine solubilised ions present. Based on the structureabove, lithium, sodium and magnesium 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 2% by weight is 9.8. At that pH, no free lithium or magnesium ionsare 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 and Na 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 does not take place at the range of pH covered in the prescribed OECD method (4-9)

Description of key information

This substance does not hydrolyse in the pH range of 4 to 9

Key value for chemical safety assessment

Additional information

This substance is an inorganic layered silicate structure with a unit cell of the following composition Na0.7+ [(Si8Mg5.5Li0.3) O20(OH)4]0.7- 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 clear 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, adialysis method was devised to determine solubilised ions present. Based on the structureabove, lithium, sodium and magnesium 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 2% by weight is 9.8. At that pH, no free lithium or magnesium ionsare 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 and Na 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 does not take place at the range of pH covered in the prescribed OECD method (4-9)