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Multiple pKas

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SandraR
SandraR's picture
Multiple pKas

Hello,

how is the issue with acids with multiple pKas treated?

Should I use the value for the first or the second pKa (2.0 and 6.59 in this case)? 

Best, Sandra

USEtox Team
USEtox Team's picture
pKa in USEtox

Dear user,

For ionizing organic chemicals, USEtox version 2.0 and higher requires as input the first acid pKa (i.e. if pKa(acid)>1) and base pKa (i.e. if pKa(base)<14).

SanTra
SanTra's picture
pKa and SPARC

Dear USEtox-Team,

 

Could you please tell me how to deal with results presented by SPARC concerning a dissociating substance?

I cannot find any information on this topic – neither on archemcalc.com, nor in van Zelm et al. 2003, Hilal et al. 2003a, Hilal et al. 2003b or Hilal et al. 2007.

Unfortunately, I do not understand the advice you gave to SandraR in this forum on 23 June 2016.

The USEtox Documentation (Fantke et al. 2017, p. 58) says that “chemical insight and expert judgement was used to decide (…) whether the substance is an acid, a base, or a zwitter ion.”

As I am studying lifestock sciences (currently doing my master thesis), I am definitely no expert in chemistry… So I wonder whether it is possible for me to use SPARC, and if yes, how.

 

The substance of interest is Chlorhexidine gluconate, CAS RN: 18472-51-0. According to EPI Suite, it is an aliphatic amine. It is not included in the database of USEtox.

The table presented below show the results given by SPARC. (At least, I assume that these are the results.) Are these data the pKa? If yes, which one am I supposed to use as an input for the User Interface Wizard of USEtox 2.12?

pH

Species 1

Species 2

Species 3

Species 4

Species 5

8,8

0

0

0

0

0

9

0,001

0

0

0

0

9,2

0,001

0

0

0

0

9,4

0,002

0

0

0

0

9,6

0,004

0

0

0

0

9,8

0,006

0

0

0

0

10

0,008

0

0

0

0

10,2

0,011

0

0

0

0

10,4

0,014

0

0

0

0

10,6

0,016

0

0

0

0

10,8

0,018

0

0

0

0

11

0,019

0

0

0

0

11,2

0,02

0

0

0

0

11,4

0,02

0

0

0

0

11,6

0,02

0

0

0

0

11,8

0,02

0

0

0

0

12

0,02

0,001

0

0

0

12,2

0,019

0,001

0

0

0

12,4

0,018

0,001

0

0

0

12,6

0,016

0,002

0

0

0

12,8

0,014

0,003

0

0

0

13

0,012

0,003

0,001

0

0

13,2

0,009

0,004

0,001

0

0

13,4

0,007

0,005

0,002

0

0

13,6

0,004

0,005

0,003

0,001

0

13,8

0,003

0,005

0,005

0,003

0

14

0,001

0,004

0,006

0,006

0,002

 

Thank you!

Kind regards,

Sandra Travnitzky

 

USEtox Team
USEtox Team's picture
SPARC for pKa

From SPARC, we have used the different reported pKa values for acid and base parts of each molecule, and then selected the pKa of the strongest acid and strongest base. We used the batch output from SPARC. Acids will give one or more pKa and all will be for the acid parts of the molecule. Bases will give one or more pKa and all will be for the base part of the molecule. Amphoters will give one or more pKa and they will be for both acid and base parts of the molecule.

Example for acid: CAS 99-50-3 will give 3 pKa, acid 1: pKa=4.06, acid 2: pKa=8.30, acid 3: pKa=13.39. From those, we select as pKa.loss=4.06 the strongest acid as input for USEox, while pKa.gain=0 by default.

Example for base: CAS 67-43-6 will give 3 pKa, base 1: pKa=0.66, base 2: pKa=4.68, base 3: pKa=8.14. From those, we select as pKa.gain=8.14 as input for USEox the strongest base, while pKa.loss=14 by default.

Example for amphoter: CAS 38641-94-0 will give 3 pKa, acid 1: pKa=2.20, acid 2: pKa=6.35, base 1: pKa=11.23. From those, we select as pKa.loss=2.20 the stongest acid and as pKa.gain=11.23 the strongest base as input for USEox.