Determing Concentration and Type of Acid?

Question

Hello,

I have an assignment to write a lab practicum and experiment for a college course. Here is the assignment:

Goal: To determine BOTH the concentration and the identity of an unknown acid. You and your lab partner will be given an unknown acid. It will be one of the following: hydrochloric acid, hydroiodic acid, sulfuric acid, or phosphoric acid.

I need to write up a protocol explicitly detailing how you will determine the concentration of the unknown acid. And then a protocol to determine the type.

We are only given a certain amount of acid to work with. If we run out, there are no refills.

so, what procedure would I use to figure this out? titration? What are teh steps, etc. I have to be VERY detailed. Thanks for any help!

Answer 1

Been ages since I have done anything like this. Some ideas;

1. do a titration. Look for more than one end point. Sulphuric acid and Phosphoric acid will both have two.You need to check this but with Phosphoric and NaOH you will get an end point at approx pH of four and 9 (you won't see the third). If available use a pH meter.
If you get one end point only then you have narrowed it down to monoprotic acid (HCl or HI). two end points and you have the diprotic acid H2SO4 or the triprotic H3PO4.

Also work out the theoretical end point as these may give you a further clue to the nature of the acid

2. If it is monoprotic then look for a reaction that will produce Iodine. This will then be able to be tested via starch indicator.

3. If the result in 1 is not a monoprotic acid and is not conclusive you need to look at a method of distinguishing between H2SO4 and H3PO4. I can't think of one at the moment but if I do I will post later.

This should put you on the right track.

PS: the titration should give you the concentration

Addition:
To differentiate between SO4(2-) and PO4(3-) look at adding AgNO3. A precipitate should form. If it is Yellow it is the PO4(3-) if it is White it is SO4(2-). NOTE: this will also differentiate between Cl- and I- with the precipitate being white and yellow respectively

Answer 2

Lots of luck, brother. First you would want to determine which acid it is. There are qualitative analysis procedures to identify these, but you will have to track them down. Once you know the acid, the pH should be useful in figuring out the concentrations.

Answer 3

Try this WS
http://geowww.geo.tcu.edu/richards/Volumetric%20Analysis%20Feb%2016.pdf
As the first answer states though, you need to determine the acid. Can you use a pH meter? If not, find a qualitative test that can help.

GL