Pirimiphos-methyl

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

GLP compliance:

yes

Specific details on test material used for the study:

.

Dissociating properties:

yes

pKa:

4.3

Temp.:

20 °C

Table 1: pH measurements

x cm3 HCI	50.0	9.0	8.0	7.0	6.0	5.0	3.0	-
x cm3 KOH	-	-	-	-	-	-	-	10.0
pH	1.58	3.90	4.16	4.37	4.55	4.72	5.07	9.02

Table 2: Dissociation Constant Calculations

1	2	3	4	5 (pKa)
Rep.	pH	Absorbance, 235.8 nm	log((A - Am)/(Ai -A))	pH+COL4
	1.54	0.5243	-	-
	3.84	0.4558	+0.3752	4.22
	4.11	0.4240	+ 0.1150	4.22
1	4.32	0.4036	-0.0391	4.28
	4.50	0.3839	- 0.1902	4.31
	4.67	0.3664	- 0.3345	4.34
	5.04	0.3290	- 0.7380	4.30
	9.25	0.2933	-	-
	1.59 3.87	0.5242 0.4590	- + 0.4108	- 4.28
	4.13	0.4285	+ 0.1571	4.29
2	4.35	0.4056	-0.0153	4.33
	4.54	0.3803	- 0.2077	4.33
	4.70	0.3593	- 0.3834	4.32
	5.05	0.3279	-0.7271	4.32
	9.22	0.2911	-	-
			Mean	4.30

Where: A = Absoroance.

Am = Absoroance of molecular form.

Ai = Absoroance of fully ionized form. pH = Measured on day of determination.

Conclusions:

The dissociation constant of pure test substance at 20 °C is 4.30.

Executive summary:

The study was performed to evaluate the dissociation constant of the test substance in water according to OECD TG 112 and in compliance with GLP. The dissociation constant at 20 °C was found to be 4.30 i.e. the substance can be characterised as a weak base.

Description of key information

pKa = 4.3 at 20 °C, OECD TG 112, Husband 1997

Key value for chemical safety assessment

pKa at 20°C:

4.3