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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH and ANALOGUE APPROACH JUSTIFICATION
Based on reliable textbook data (HoWi, 2017) chromic(VI) oxide is soluble in water and forms immediately chromic acid, which is only known in solution:
CrO3 + H2O ---> H2CrO4

Based on this fact the dissociation properties of chromic acid have to be considered, as this is the species existing in water, not chromium(VI) oxide.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Please refer to the respective test item in the respective endpoint study record for details!


Reason / purpose for cross-reference:
read-across source
Dissociating properties:
yes
No.:
#1
pKa:
-0.61
Remarks on result:
other: Temperature not indicated, room temperature assumed
No.:
#2
pKa:
6.488
Remarks on result:
other: Temperature not indicated, room temperature assumed

When chromium(VI) oxide is dissolved in water it forms immediately chromic acid:
CrO3+ H2O ---> H2CrO4

Chromic acid is only known in solution.

The chromic acid dissociates in water and is in the first stage strongly dissociated and in the second stage only weakly:

1) H2CrO4<==> H++ HCrO4-with pKa= -0.61

2) HCrO4-<==> H++ CrO42-with pKa= 6.488

Conclusions:
When chromium(VI) oxide is dissolved in water it forms immediately chromic acid: CrO3+ H2O ---> H2CrO4

Chromic acid is only known in solution.

The chromic acid dissociates in water and is in the first stage strongly dissociated and in the second stage only weakly:
1) H2CrO4<==> H++ HCrO4-with pKa= -0.61
2) HCrO4-<==> H++ CrO42-with pKa= 6.488
Executive summary:

Based on reliable textbook data (HoWi, 2017) chromic(VI) oxide is soluble in water and forms immediately chromic acid:


CrO3+ H2O ---> H2CrO4

Chromic acid is only known in solution.

The chromic acid dissociates in water and is in the first stage strongly dissociated and in the second stage only weakly:

1) H2CrO4<==> H++ HCrO4-with pKa= -0.61

2) HCrO4-<==> H++ CrO42-with pKa= 6.488

Description of key information

Based on reliable textbook data (HoWi, 2017) chromic(VI) oxide is soluble in water and forms immediately chromic acid:


CrO3+ H2O ---> H2CrO4

Chromic acid is only known in solution.

The chromic acid dissociates in water and is in the first stage strongly dissociated and in the second stage only weakly:

1) H2CrO4<==> H++ HCrO4-with pKa= -0.61

2) HCrO4-<==> H++ CrO42-with pKa= 6.488

Key value for chemical safety assessment

pKa at 20°C:
-0.61

Additional information

Temperature is not stated in the key-value source, but room temperature is assumed. The test substance dissociates in a second step weakly, but for risk assessment the first dissociation step is of relevance based on a worst-case approach, as this first step is the reason for the acidic properties of the substance.