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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Physical & Chemical properties

Dissociation constant

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Administrative data

Link to relevant study record(s)

Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
test procedure in accordance with national standard methods with acceptable restrictions
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)

Description of key information

Experiments have been carried out with a well-defined amorphous silica (Read-Across Substance). The pKa value at 40°C for silanol groups was 6 to 6.65.

Key value for chemical safety assessment

Additional information