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Diss Factsheets

Physical & Chemical properties

Dissociation constant

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Endpoint:
dissociation constant
Type of information:
other: Handbook data
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Data from handbook or collection of data
Guideline:
other: Handbook data
Dissociating properties:
yes
No.:
#1
pKa:
10.5
Temp.:
25 °C
Remarks on result:
other: hypoiodous acid (HIO)
No.:
#2
pKa:
0.78
Temp.:
25 °C
Remarks on result:
other: iodic acid (HIO3)
Conclusions:
The dissociation constants pKa values established are 10.5 for hypoiodous acid (HIO), 0.78 for iodic acid (HIO3).
Executive summary:

As iodine in contact with water is rapidly hydrolysed in a two stage equilibrium and different iodine species are formed, the determination of the dissociation constant of iodine is scientifically not feasible. The half-life of iodine in water is less than one second (pre-equilibrium, formation of HIO and iodide). Thus no data on the dissociation constant of iodine is presented.

However, the main disproportionation products of iodine are hypoiodite (IO-) and iodide (I-). Hypoiodite is only an intermediate (half-life of few minutes to hours depending on environment) and disproportionates in iodine and iodate (IO3-). In principle, for these transformation products the measurement of the dissociation constants is feasible. The established pKa values for the corresponding acids are 10.5 for hypoiodous acid (HIO), 0.78 for iodic acid (HIO3) (CRC Handbook of Chemistry and Physics) and -8.56 for hydriodic acid (HI) (Lange's Handbook of Chemistry). All values are determined at 25 °C.

 

Endpoint:
dissociation constant
Type of information:
other: Handbook data
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Data from handbook or collection of data
Guideline:
other: handbook data
Dissociating properties:
yes
No.:
#3
pKa:
-8.56
Temp.:
25 °C
Remarks on result:
other: hydrogen iodide (HI)
Conclusions:
The established pKa value is -8.56 for hydriodic acid (HI) at 25 °C.
 
Executive summary:

As iodine in contact with water is rapidly hydrolysed in a two stage equilibrium and different iodine species are formed, the determination of the dissociation constant of iodine is scientifically not feasible. The half-life of iodine in water is less than one second (pre-equilibrium, formation of HIO and iodide). Thus no data on the dissociation constant of iodine is presented.

However, the main disproportionation products of iodine are hypoiodite (IO-) and iodide (I-). Hypoiodite is only an intermediate (half-life of few minutes to hours depending on environment) and disproportionates in iodine and iodate (IO3-). In principle, for these transformation products the measurement of the dissociation constants is feasible. The established pKa values for the corresponding acids are 10.5 for hypoiodous acid (HIO), 0.78 for iodic acid (HIO3) (CRC Handbook of Chemistry and Physics) and -8.56 for hydriodic acid (HI) (Lange's Handbook of Chemistry). All values are determined at 25 °C.

 

Endpoint:
dissociation constant
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is hydrolytically unstable (half-life less than 12 hours)

Description of key information

Test data: the study does not need to be conducted because the substance is hydrolytically unstable (halflife less than 12 hours).

Handbook data: The established pKa values for the corresponding acids are 10.5 for hypoiodous acid (HIO), 0.78 for iodic acid (HIO3) (CRC Handbook of Chemistry and Physics) and -8.56 for hydriodic acid (HI) (Lange's Handbook of Chemistry). All values are determined at 25 °C.

Key value for chemical safety assessment

Additional information

As iodine in contact with water is rapidly hydrolysed in a two stage equilibrium and different iodine species are formed, the determination of the dissociation constant of iodine is scientifically not feasible. The half-life of iodine in water is less than one second (pre-equilibrium, formation of HIO and iodide). Thus no data on the dissociation constant of iodine is presented.

However, the main disproportionation products of iodine are hypoiodite (IO-) and iodide (I-). Hypoiodite is only an intermediate (half-life of few minutes to hours depending on environment) and disproportionates in iodine and iodate (IO3-). In principle, for these transformation products the measurement of the dissociation constants is feasible. The established pKa values for the corresponding acids are 10.5 for hypoiodous acid (HIO), 0.78 for iodic acid (HIO3) (CRC Handbook of Chemistry and Physics) and -8.56 for hydriodic acid (HI) (Lange's Handbook of Chemistry). All values are determined at 25 °C.