# Calculating the Ionic strength of sodium acetylsalicylate

With the addition of aqueous NaOH to an aspirin solutions in water the reaction will yield products of sodium acetylsalicylate and water. I would like to measure the amount of sodium apsirin formed that is disolved in water right after the NaOH reaction.

Attempt for the solution: Given that sodium acetylsalicylate dissociates in water to form sodium ions and salicylate ions, I can consider finding the ionic strength i.e. ion concentration. However in order to calculate the ionic strength I need the molality which require the amount of solutes disolved, which is basically what I am trying to find in the first place.

Is there a recorded value for the Ionic strength of of sodium acetylsalicylate, which would intern allow me to calculate the mass of the solute or I'm going the wrong way about this?

Is there any other way I can use to algebraic calculate the mass of the sodium acetylsalicylate dissolved?

Any help will be greatly appreciated!

• why not to calculate it from amount of added NaOH ? – Poutnik Nov 10 '19 at 15:04
• Then it would be the amount of solute added not dissolved – EPIC Tube HD Nov 11 '19 at 6:36
• As it is added to aspirin solution and as sodium aspirin as much more soluble..... it is still dissolved. – Poutnik Nov 11 '19 at 6:38
• ahhh due to sodium aspirin being more soluble in water, we can assume the amount of sodium aspirin formed due to the REACTION with NaOH...as the amount of sodium aspirin dissolved. Is that right? – EPIC Tube HD Nov 11 '19 at 6:46
• It is a simple acid base reaction. – Poutnik Nov 11 '19 at 6:49

Aspirin has a solubility of ~3 g/L. In addition from a patent by Galat :

Sodium aspirin is almost 1000 times as soluble in water as aspirin itself.

All of the $$\ce{NaOH}$$ can be assumed to react with acetylsalicylic acid (aspirin) to form the sodium salt. This means the amount of solubilized sodium aspirin is roughly equal to the amount of added $$\ce{NaOH}$$ (up to the equivalence point).

When computing the ionic strength you can therefore assume that the concentration of solubilized sodium aspirin is roughly equal to the amount of added $$\ce{NaOH}$$.

### References

1. Galat, A. Stable Sodium Acetylsalicylate and Method for Its Manufacture. US3985792A, October 12, 1976. (PDF)
• But we can assume that the amount of sodium aspirin formed by its reaction with NaOH, is the amount dissolved – EPIC Tube HD Nov 11 '19 at 6:43
• @EPICTubeHD I thought you wanted to compute the ionic strength? In that case if the concentration of added NaOH is significantly greater than the concentration of dissociated acetylsalicylic acid then you can assume most of the ions are from Na aspirin. – Buck Thorn Nov 11 '19 at 9:29