Rhiannon, I need a chemist! (and I've bored anyone else)
I just was looking over the exact wording on the requirements for my essay, and I see I need to discuss, "What are differences between salicylic acid and acetylsalicylic acid in terms of structure, function, pH, and effect on the stomach?"
My question is about pH.
I figured I'd just look up pH in my Merck index and compare the two, hoping they are listed at the same concentrations and conditions. Anyway, I've got salicylic acid at a pH 2.4 for a saturated solution and no pH info for aspirin. I decided to scrounge MSDSs, and while many have that 2.4 value (but include no concentration information), all I'm finding for aspirin is stuff like "no information found." I'm not sure MSDSs are that reliable as sources since I think someone at the company just pulls the numbers off another MSDS or a reference book.
I didn't see it in my CRC handbook, and I am stumped. I see discussions of it, but nothing worth citing. I know it's not particularly soluble in water, and since we take it in solid form, it really doesn't have a pH as it sits on my desk. I've looked in Lange's and can't find pH of those materials, but I found the pKa (specifically pK1) of acetylsalicylic acid (listed as 2-acetoxybenzoic acid) in water at 25 °C is 3.48. I've found salicylic acid's (listed as 2-hydroxybenzoic acid) pK1 of 2.98 and a pK2 of 12.38. Fortunately, these are also in water at 25 °C. I don't know the connection between pK1, pK2, pKa and pH. I'm guessing the pK1 and pK2 go into a formula somewhere to get the pKa. It's been about 20 years since I've done this sort of thing, and I don't think this essay was meant to require problem solving of this nature.
By the way, I got my CRC used. It's probably older than you--the 1974-1975 edition. I just looked in it for pKa values to try to see what pK1 and pK2 mean, and I see only salicylic acid listed in a pK table but at slightly different temperatures and with slightly different values than Langes used. The big difference is, the data is on two lines with a reference to step number. Now I'm thinking there's a two-step dissociation for salicylic acid, and pK1 is for the first step and pK2 is for the second step. This stinks and I'm sleepy.
Do you have any ideas where I could get pH data, or can you direct me to what I do with the values from my table to get to pH? If you don't know of a simple way to do this, I think I'll have to post the question to my professor about where to find the pH data he wants. I don't know how helpful he'll be; he often talks around questions instead of answering them.
Maybe I'm taking the question too literally; I don't know, but to me to compare pH values, you need first to have them.
Thanks for any help you can provide. Thanks for just reading this if you made it this far. It's pretty garbled.
4 Comments:
It's going to be hard to describe this to you in a blog post, so I'll reference to wikipedia (so wrong, I know) so you know what I'm talking about.
The relationship between pH and pKa is listed here
http://en.wikipedia.org/wiki/Acid_dissociation_constant
There are two protons that can be lost in salycilic acid, so the pka1 is the dissociation of the first one and the pka2 is the dissociation of the second. I would say the first would be in the carboxylic acid and the second the phenol, but you should probably check. I'm sure you'd be able to find a reference somewhere that would say.
You do need to know the concentration for the pH, so unless it's given to you in the question, you can't really figure it out (my vague memory is telling me).
I would think that what the question is getting at is the difference that the acetyl group would make on pH, so perhaps instead of looking for an actual pH, you could compare the pKa's of the 2 and thus which has the more acidic group. That's what I would do.
Good luck with your assignment!!
Thank you for the information.
I just assumed I'd assume a solution of say 1 M of each or otherwise assume they had the same concentration (although I don't even know enough about their solubilities to determine if I can; I figured I'd just ignore that).
If I just compare pKas, what do I do when I have two pKas for one molecule? Do I add them? I already expect the salicylic acid is more acidic at a given concentration of both.
Thanks again.
Okay. I've had sleep and coffee and have worked on my paper again today. I have this issue under control. I even think I know what to do with both pKas. I'm going with the "the lower the pKa, the lower the pH" as my method for the comparison of the pHs.
I dug out my textbook from high school and pretended to have a 1.0 M solution of the acetylsalicylic acid. A quadratic equation and a bit later, I was able to get to the pH of the solution so if the professor calls me on this, I can run the numbers (at least I think I can). Also I believe I figured out what to do with the two pKas for salicylic acid. Mathematically, I don't know if I could combine them before going to the [H+], but if I add the two [H+] together (I assume the one from the smaller pKa will overwhelm the one from the larger pKs so I could assume it doesn't contribute to [H+]), I'd get the total [H+] concentration to use to convert to pH. I've only worked that through theoretically, though, and I assume my professor will buy the comparison of pKas.
Thanks so much for your help and motivation to work this out. It's taking me back to the 80s. I think I'm going to see if I can't find some Bon Jovi on YouTube.
Chemistry now for me consists of noting that when my baking includes baking soda instead of or along with baking powder, it also calls for yogurt, buttermilk, sour cream or some other acid.
That sounds pretty good. The lower pKa does indeed overrule the higher one, so you go with that. It's the first proton to dissociate and then the other does.
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