Creating and Titrating ASA: What You Need to Know

[decamij]

For a lab, i am to titrate a sample of ASA, which i am to create myself. I have two questions:

Are 5g of salycilic acid, 10mL of acetic anhydride, and some sulfuric acid enough to create some ASA (obviously using a hot and cold water bath as well)?

What should i use to titrate the ASA, and how do i do it?

I think i'll use NaOH. Also, what is he thing that i titrate with called, do i need any other materials (I have NaOH, ASA, a beaker/flask/test tube, and a pH meter)? Thanks a bunch

 

[chem_tr]

I think acetylation step should be in an aprotic solvent like chloroform or diethyl ether. If I were you, I wouldn't use sulfuric acid, as it will cleave the anhydride. Any reactant with acidic proton will give acetic acid under these circumstances.

When you are ready with your ASA sample, you can do the titration with dilute NaOH or better yet, tetrabutylammonium hydroxide. Unless you'll do the analysis with a pH meter, you'll need an indicator like phenolphthalein, with this one, you are to stop titration when you see the pale pink color. The instrument we use in titrating is called buret, with a stopcock and grades (marks for certain milliliters). I recommend that you do the titration triplicate to obtain a more reliable result, by averaging the titers.

Your pH meter will need calibration before starting your titrations. Please don't skip this step, since your results will be affected greatly with proper calibration. Secondly, I think that your pH values will be within pH=4 and pH=10, so use pH=4 and pH=10 buffer solutions to calibrate your pH-meter. Lastly, ensure that your reference electrode has 6M KCl solution to provide a constant ionic strength. Leave your working electrode in the clean working solvent before and after titrations (in your case, water).

 

[decamij]

For the first question, would i only use a few drops of chloroform like i would have used only a few drops of sulfuric acid? (considering my ratios)

For the NaOH, what would be the best concentration, and which one would probably be most available?

Oh, and one more thing. After i react the acetic anhydride with the salycylic acid and create ASA, how do i separate the ASA from the other contents in the flask (****USING EASILY ACCESIBLE MATERIALS, that i can access at school*****)

 

[chem_tr]

Sulfuric acid was a catalyst in your reaction, but chloroform may be more than a few drops, normally, enough to dissolve all the reactants.

You'll purchase solid NaOH and prepare solutions by taking proper amounts of it. This is the cheapest and nearly the easiest method.

I recommend that you use a great excess of anhydride, as it is naturally liquid; the excess can easily be distilled off and therefore, removed with this simple measure. You'll need a boiling flask, a liebig condenser, two pieces of ptfe tubing (to carry water into the condenser), a tap (as water source; or a bucket of water and an aquarium motor to drive it, the second option needs cooling the water with ice at least one time a day). Distilling first takes off chloroform of course, finally leaving ASA in the flask.

 

[decamij]

This may be the best way to separate the two, but what would be the quickest/easiest way?

 

[chem_tr]

Okay. Use acetic anhydride as solvent; don't use any other. Reflux on an oil bath; for this you'll need a reflux condenser. After say, 16 hours, you'll only have asa and acetic anhydride in your flask, along with some acetic acid produced. You'll filter it in an efficient fume hood, and wash the precipitate with petroleum ether and finally dietyl ether. The quickest way is this, but a reflux condenser is nearly essential in my opinion. Other members may propose alternative ways of course.

 

[decamij]

Back to the actual titration. I am using ASA, which is solid, a phenophthalien indicator, and NaOH to titrate the ASA. And THAT's ALL.
1) do i need the ASA to be in solution?
2) after i find out the volume of NaOH added to the ASA to reach an endpoint, how can i find the pH level of the ASA?

 

[chem_tr]

Yes, you need it to be dissolved in a proper solvent (I recommend water plus some known amount of acid to facilitate dissolving).

If you are doing the titration with a pH-meter, it is very easy; just record the equilibrium pH values after each addition of base. A jump around pH=5 to pH=9 will indicate that no asa is present, all of it is converted into acetylsalicylate, and that's why the pH rises so sharply because of the added excess NaOH. Finish the titration until no change of pH around 10-11, and plot a graph to find the middle neutralization point; it is the middle point of the sharp increase.

You can very easily find the pH level by simply measuring the aqueous solution of asa itself I am sure this is not what you mean.

 

[decamij]

I'm supposed to tirtate it the ASA with a strong base to determine the amount of acid present in my product. Then, I'm supposed to compare it with Bayer Aspirin. But when i find out how much acid is present, does this mean finding the mass of acetylsalicylic acid (in the water solution), or findind how acidic it is?

 

[chem_tr]

It depends on what you find... If you find the hydrogen ion concentration, then you know the acidity of your sample, even acidic dissociation constants. Titration may be useful for both things you mentioned in your post; finding the unknown amount of acid in a sample provided no additional acid is present, or find the acidity constant of a known amount of acid.