Silicic acid, aluminum sodium salt

Dissociation constant

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Sep. 10, 2019 - Feb. 20, 2020; experimental phase: Sep. 18-19, 2019

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Version / remarks:

1981-05-12

Deviations:

yes

Remarks:

The test item solution was membrane filtrated. This can be considered as uncritical, as the test item contains insoluble parts which must be removed before the measurement and without this treatment a measurement is not possible.

GLP compliance:

yes (incl. QA statement)

Specific details on test material used for the study:

Name: Silicic acid, magnesium aluminium sodium salt
Batch no.: 201810LC007
Appearance: white fluffy powder
Composition: Sodium; Magnesium; Aluminium; Silica
CAS No.: 12040-43-6
EC-No.: 234-919-5
Purity: >95%
Homogeneity: homogeneous
Production date: October 2018
Expiry date: Nov. 2020
Specific surface area: 57.9 m²/g

Dissociating properties:

no

Remarks:

the majority of SMAS is present in non-dissociated form

No.:

#1

pKa:

ca. 9.161 - ca. 9.249

Temp.:

20 °C

No.:

#2

pKa:

ca. 7.405 - ca. 7.519

Temp.:

20 °C

No.:

#3

pKa:

ca. 6.143 - ca. 6.485

Temp.:

20 °C

Above values are the means of three independent determinations ± their standard deviations

Six Main Tests were performed. The first Main Test and the second Main Test were not taken into account for evaluation.

The measurement of the third Main Test, fifth Main test and sixth Main Test were used for evaluation and calculation (see chapter 7.2). Main Test four was not used as the deviation at EQP 3 was too high.

The pKb1 was calculated as 4.795 ± 0.044

The pKb2 was calculated as 6.538 ± 0.057

The pKb3 was calculated as 7.686 ± 0.171

These values are also the means of three independent determinations ± their standard deviations

Conclusions:

Results of this study are stated as follow:

The pKa1 was calculated as 9.205 ± 0.044
The pKa2 was calculated as 7.462 ± 0.057
The pKa3 was calculated as 6.314 ± 0.171
and
The pKb1 was calculated as 4.795 ± 0.044
The pKb2 was calculated as 6.538 ± 0.057
The pKb3 was calculated as 7.686 ± 0.171
These values are the means of three independent determinations ± their standard deviations

Executive summary:

This study was performed in order to determine the dissociation constant of Silicic acid, magnesium aluminium sodium salt.

Reference 2

Endpoint:

dissociation constant

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study based on accepted scientific principles, conducted as part of a comprehensive testing programme, basic data given, in accordance with published data

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline available

Principles of method if other than guideline:

Experiments carried out with a well-defined amorphous silica, Monospher 250 (Merck Darmstadt), included dependence on pH and NaCl concentration at 40 °C. pK determination was based on calculations combined with experimental studies.

GLP compliance:

no

Dissociating properties:

yes

No.:

#1

pKa:

6

Temp.:

40 °C

Remarks on result:

other: for silanol groups: mean value of 4 (+-0.08) at 0.1 mol NaCl/L

No.:

#2

pKa:

6.42

Temp.:

40 °C

Remarks on result:

other: for silanol groups: mean value of 4 (+-0.10) at 0.01 mol NaCl/L

No.:

#3

pKa:

6.65

Temp.:

40 °C

Remarks on result:

other: for silanol groups: mean value of 4 (+-0.11) at 0.001 mol NaCl/L

According to the authors (publication, p. 4391 - 4393, Tab. 2), the data are largely consistent with others given in the literature:

pKa = 6.8 (titrated in 0.1 mol NaClO₄/L, 25 °C) [Schindler P, Kamber HR: Helv. Chim. Acta 51 (1968), 1781]

(Note: NaClO₄ is correct.)

pKa = 6.13 - 6.24 (0.1 mol NaCl/L, 25 °C) [Sonnefeld J: J. Colloid Interface Sci. 183 (1996), 597]

Note: The pKa for the dissolved ortho (mono-) silicic acid is higher: pKa = 9.8

(based on data given by Iler RK;: The Chemistry of Silica. John Wiley & Sons, N.Y., 1979)

Reference 3

Endpoint:

dissociation constant

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Description of key information

There is no information on the substance Silicic acid, aluminum sodium salt (CAS 1344-00-9, NAS) available.

pKa values of the silanol groups of the structure analogue, well-defined amorphous silica (read-across substance) are reported at different concentration of NaCl:

pKa = 6 (40 °C, 0.1 mol NaCl/L)

pKa = 6.42 (40 °C, 0.01 mol NaCl/L)

pKa = 6.65 (40 °C, 0.001 mol NaCl/L)

 

Results for the other read-across substance Silicic acid Aluminium Magnesium Sodium Salt are as follow:

The pKa1 was calculated as 9.205 ± 0.044
The pKa2 was calculated as 7.462 ± 0.057
The pKa3 was calculated as 6.314 ± 0.171
and
The pKb1 was calculated as 4.795 ± 0.044
The pKb2 was calculated as 6.538 ± 0.057
The pKb3 was calculated as 7.686 ± 0.171
These values are the means of three independent determinations ± their standard deviations