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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:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Data from reviews Read across from a similar substance which has the same main component and with a different counter ion that doesn't influence the characteristics related to the specific end-point
Reason / purpose for cross-reference:
reference to same study
Principles of method if other than guideline:
Colourimetry method. The speed at which the equilibrium (I <--> II) was attained was measured at various pH values, and the degree of ionization, at each pH value, was calculated only after equilibrium had been established. Ionization constants were calculated from the degrees of ionization of the bases at equilibrium at various pH values, by means of the law of mass action.
Dissociating properties:
yes
pKa:
ca. 6.9
Temp.:
25 °C

Effect of pH on degree of ionization; (α) Malachite Green 27 x 10-6M at 25°C

pH Extinction coefficient (E) (red filter) Degree of ionization (α) logα/ (1 α) pK 
4.00 0.113 1.00 -
6.09 0.097 0.86 0.80 6.89
6.60 0.075 0.66 0.30 6.90
6.78 0.060 0.53 0.06 6.84
6.97 0.052 0.46  - 0.07 6.90
7.10 0.044 0.39 - 0.20 6.90
7.42 0.026 0.23 - 0.52 6.90
10.1 0.000 0.00 - -

T1/2 = time required for degree of ionization to fall half-way from 100% to its equilibrium value, at a pH value equal to the pK.

MG (4:4'-bisdimethylaminotriphenyl-carbinol) 2.7 x 106M pK: 6.90 T1/2 hours: 2.1 

Rate of disappearance of the coloured ion of Malachite Green

Concentration: 1.3 x 10-5M at pH 7.90 (20°C)

Time, mins Extinction coefficient (red filter)  k x 103 
2 0.424 -
12 0.386 -
22 0.362 6.4
32 0.341 6.0
42 0.320 6.4
52 0.300 6.4
0.100 -
Conclusions:
The basic triphenylmethane dyes can exist in the form of coloured cations (as in the acid salts) or as undissociated bases, the carbinols. The percentage of the dye in the cationic state is therefore dependent on the hydrogenion concentration of the system, and the greater the hydrogen-ion concentration, the greater the dissociation into the cationic form. Experiments with such dyes are complicated by two of their inherent properties. First, the equilibrium between the cation and the carbinol often requires an appreciable time to become established; and secondly, the carbinol form is relatively insoluble. At certain hydrogen-ion concentrations the carbinol form began to precipitate. In these circumstances a true equilibrium could not be established.
pK = 6.9
Executive summary:

Data was reported in two review, in which are reported different aspects of the original study (Goldrace and Phillips, 1949).

Data is referred to Malachite Green (MG); read across between chloride, oxalate and acetate salts form of MG have the same main component and have a different counter ion that doesn't influence the characteristics related to the specific end-point.

Resuts:

The ionisation constant (pK) of MG is 6.90. It is 100% ionised at pH 4.0, 50% at pH 6.9, 25% at 7.4 and 0% at pH 10.1

Conclusion:

The basic triphenylmethane dyes can exist in the form of coloured cations (as in the acid salts) or as undissociated bases, the carbinols. The percentage of the dye in the cationic state is therefore dependent on the hydrogenion concentration of the system, and the greater the hydrogen-ion concentration, the greater the dissociation into the cationic form. Experiments with such dyes are complicated by two of their inherent properties. First, the equilibrium between the cation and the carbinol often requires an appreciable time to become established; and secondly, the carbinol form is relatively insoluble. At certain hydrogen-ion concentrations the carbinol form began to precipitate. In these circumstances a true equilibrium could not be established. Malachite Green it is 100% ionised at pH 4.0, 50% at pH 6.9, 25% at 7.4 and 0% at pH 10.1

Description of key information

Key value for chemical safety assessment

Additional information

The dissociation constant (pKa) of Malachite Green has been considered and derived from the solvolytic reaction indicated by Goldacre in 1949. MG is 100% ionised at pH 4.0, 50% at pH 6.9, 25% at 7.4 and 0% at pH 10.1