[inazir]

Dear All,

I need to calculate the pH of a salt solution. The weak acid is adipic acid, strong base is HMD (Hexamethylendiamine), the salt solution is nylon 6,6 and the stoichiometry is 1:1.

HMD (aq) is delivered in isotainers at 90% (wt) concentration and is diluted down to 40% concentration at 60 degC before being charged to the reactor.

The adipic acid (powder) is added to the reactor along with the 40% HMD (aq) solution. The salt solution in the vessel needs to be maintained at 55% concentration at a pH of between 6.9 to 7.5 at a temperature of 90 degC. THe reaction is exothermic. I know that at a pH of 6.9, it is close to the equivalence point and a very small change can result in a large swing on the titration curve. I would look at calculating the pH away from this point, say pH of 7.3.

The pH of the salt solution is adjusted by changing the adipic acid feed rate to the vessel. I understand that temperature has an effect on the pH of a solution but I don't know how to account for this. From literature, I know the adipic acid Ka1 and Ka2 values, given at 25 degC. For HMD the Ka1 (not sure about using ka value for base?) and Kb2 values are given 0 degC. I would have used ka1 value but not i'm not sure if or how to account for ka2 value. I've tried to calculate the pH of the salt solution but I'm not getting the correct answer. I'm not sure if my methodology is correct.

It may be that I need to use the activation coefficients to calculate the pH, this is more detailed and i'm not entirely sure on the way forward.

I would appreciate any help you can offer or provide a stepwise approach to resolve the problem.

Regards,
Imran

 

[Borek]

At these concentrations calculations will be quite inaccurate, no matter what. I must admit your post is rather convoluted and I have problems following the exact question.

The best approach - or the one I would take - would be to use either BATE or Buffer Maker (follow chembuddy link from my signature). They are both capable of calculating pH of such mixtures - assuming the ionic strength is not too high and concentrations are low enough that standard approach to equilibrium calculations works.