[nortorius]

We prepared aspirin as a lab on friday, and I have one question I'm unsure of. ASA is made from salicylic acid and acetic anhydride.

Q. The acidic charatcer of ASA may be used to quantify the amount of pure ASA in a sample. Suggest a procedure that may be preformed in a school labratory to test the purity of the ASA sample you prepared based on the knowledge of acids and bases. Shows materials, how the results may be interpreted, safety precautions, ect.

Note: Oh, and this is how we made the ASA...

1. 4g of salicylic acid in a flask
2. Added 8mL of acetic anhydride (stirred till solids dissolved)
3. Added 4 drops sulfuric acid
4. Placed contents within flask into a hot water bath for 15 min
5. Added to ice water bath and stirred for 10 min
6. Put contents in a filter paper, and let it dry overnight.

All I've been able to find that COULD work for purity testing purposes is the following:

"Test the product for relative purity as follows. Place a few crystals of salicylic acid in one test tube; in a second, place a little crushed commercial aspirin; in the third test tube, add a few crystals of your prepared acetylsalicylic acid. Now add a few drops of iron(III) chloride to each tube and shake to mix the contents.

The SA should turn dark purple; the prepared sample should be dark purple (due to SA contamination), and the commercial aspirin sample will probably be a pale orange color (indicating no SA contamination).

Many phenols (R-OH) produce colored coordination compounds with iron(III) ions. These complex anions are composed of 6 molecules of the phenol combined with one iron(III) atom. Since salicylic acid has a phenolic –OH group, it produces the positive (purple) test with iron(III) chloride.When SA reacts completely to produce ASA, the phenolic group is replaced with an acetate (acetyl) group, so the iron test would be negative. Of course, students will not get 100% yield, so some SA remains to make their sample purple in the iron test."

[Yggdrasil]

Have you thought about trying some type of acid/base titration?  Titrations are fairly standard ways to quantify the amount of acid/base you have.

[nortorius]

A titration was the first thing to cross my mind, but I don't understand how it can show the purity of the ASA obtained?

Does ASA always have a set pH level it should be at if it's pure? How would titrating the ASA with a base (ie. sodium hydroxide) show the purity of ASA obtained from the inital lab? I'm guessing it has something to do with the neutralization point.

EDIT: I read up on it, brings back memories. Would using standardized NaOH as a titrant, along with ASA dissolved in 50mL of water as a sample (bromothymol blue indicator) work? Using C1V1 = C2V2 would show the concentration (purity) of the salicylic acid if I'm not mistaken.

[Yggdrasil]

The edit idea sounds like a good one to me.  The basic principle is to titrate a known quantity of your product in order to determine the number of equivalents of acid.  Then compare the number of equivalents determined by titration to the number of equivalents that should be in your solution assuming your product is 100% pure.

[pantone159]

But both ASA and salicylic acid have one COOH acid group.  Wouldn't you then end up with the same number of equivalents of base used to titrate?  The pH at the titration point would be different, but I figure you'd use the same amount for either.

[Yggdrasil]

The molecular weights are different, so if you use say 5.00 g of pure ASA your titration will give about 0.0278 equivalents of acid.  But, if you have 4.00g of ASA and 1.00g of salicylic acid (80% purity) you would find 0.0295 equivalents of acid in 5.00g of your product.

[pantone159]

Ahh, that makes sense.   

[blusapphire00]

How do you calculate % purity?

I know the equation is (actual moles of acid)/(theoretical moles of aspirin in sample)

but which values are you suppose to use?


Thanks!

[Yggdrasil]

I don't think you could get a reliable quantitative number for % purity from a titration.

[ARGOS++]

Dear Nortorius;

Why not doing the analysis with Spectroscopy and you will have additional “Identity” too.
Both Spectra you can see on “NIST” under their formulas.
But even better would be to use reference samples under identical measuring conditions.

That’s how it is often done for production (But adjust pH!).
In special cases you can use also in this method the Iron(III)-Ion too.

Good Luck!
                   ARGOS⁺⁺

P.S.:    ----> "NIST  Webbook”
.