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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2013
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
Temp.:
19.5 °C
Remarks on result:
other: Dissoziationskonstante bei 0,05 mol/l ist 0,06
No.:
#2
Temp.:
19.5 °C
Remarks on result:
other: Dissoziationskonstante bei 0,03 mol/l ist 0,06
No.:
#3
Temp.:
19.5 °C
Remarks on result:
other: Dissoziationskonstante bei 0,01 mol/l ist 0,07
No.:
#4
Temp.:
19.5 °C
Remarks on result:
other: Dissoziationskonstante bei 0,005 mol/l ist 0,08
No.:
#5
Temp.:
19.5 °C
Remarks on result:
other: Dissoziationskonstante bei 0,003 mol/l ist 0,09

-      Umrechnung Leitfähigkeiten in Äquivalentleitfähigkeiten

Konzentration mol/L

0,003

0,005

0,010

0,030

0,050

MgAA - Leitfähigkeit mS

0,580

0,908

1,585

4,165

6,400

NaAA - Leitfähigkeiten mS

0,300

0,455

0,878

2,390

3,900

ϫ MgAA mS/m2 mol

0,037

0,035

0,031

0,027

0,025

ϫ NaAA mS/m2 mol

0,039

0,035

0,034

0,031

0,030

 

-      Auswertung Äquivalentleitfähigkeit gegen Wurzel der Konzentration
(für starke Elektrolyte ergibt dies eine Gerade, für schwache Elektrolyte nicht)
Ermittlung der Grenzleitfähigkeit für c = 10-20mol/L

 

(c mol/L)1/2

ϫ MgAA

ϫ NaAA

0,055

0,037

0,039

0,071

0,035

0,035

0,100

0,031

0,034

0,173

0,027

0,031

0,224

0,025

0,030

 

  Graph siehe Anhang

-      Berechnung der Dissoziationskonstanten

 

Für MgAA: αi=ϫiMgAA/ϫ0MgAA oder analog für NaAA:αi=ϫiNaAA/ϫ0 NaAA

 Tabelle: Dissoziationskonstanten in Abhängigkeit von der Konzentration  

Konzentration mol/L

α NaAA

α MgAA

0,05

0,10

0,06

0,03

0,10

0,06

0,01

0,11

0,07

0,005

0,12

0,08

0,003

0,13

0,09

Magnesiumdiacrylat zeigt in verdünnten wässrigen Lösungen das typische Verhalten eines schwachen Elektrolyten. Der Dissoziationsgrad nimmt mit steigender Verdünnung zu.
Bei 0,05 mol/L sind ca. 6 % vollständig dissoziiert.
Bei 0,003 mol/L sind ca. 9% vollständig dissoziiert.

Conclusions:
Magnesium diacrylate shows the typical behavior of a weak electrolyte in dilute aqueous solutions. The degree of dissociation increases with increasing dilution. The dissociation constant at 0,05 mol/L is 0.06 and at 0,003 mol/L 0.09.

Description of key information

Magnesium diacrylate shows the typical behavior of a weak electrolyte in dilute aqueous solutions. The degree of dissociation increases with increasing dilution. The dissociation constant at 0,05 mol/L is 0.06 and at 0,003 mol/L 0.09.

Key value for chemical safety assessment

Additional information

For Acrylic acid, the pKa value of 4.25 was determined in a peer-reviewed risk assessment (European Union Risk Assessment Report acrylic acid, European Chemicals Bureau, European Commision Institute for Health and Consumer Protection European Chemicals Bureau, 2002).

For Magnesium acrylate: It is expected that at neutral pH, which is relevant for the environment, Magnesium acrylate is not completely dissociated into magnesium cation and the acrylate anions. As typical for a salt of an organic acid, the dissociation is not complete at neutral pH in water. According to Arrhenius theory of electrolyte dissociation, the molecules of an electrolyte in solution are constantly splitting up into ions and the ions are constantly reuniting to form unionized molecules. Therefore, a dynamic equilibrium exists between ions and unionized molecules of the electrolyte in solution. This is also true for the weak base magnesium diacrylate which is in equilibrium with its ions magnesium cation, magnesium monoacrylate cation and acrylate anion. According the law of Ostwald the degree of ionization increases on dilution. It is expected that at the used test concentrations in toxicity testing magnesium acrylate is only partly dissociated.

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