Expert: Leo Carr - 3/27/2006

Question
Hello,

I have a sample that is either barium chloride, magnesium sulfate, sodium carbonate, sodium sulfate or sodium acetate and i need to find out what it is. I am  a TA and have this lab comming up and i am alittle bad at chemistry.  Thanks for any help,

Andrew

Answer
Andrew,
 One way to approach this problem is too break it down into analysis for anions and/or cations. You can run some preliminary tests for the anions.  You can test them all against concentrated sulfuric acid.  If you get a white, milky precipitate chances are it's formed BaSO4 (so you know you have Ba present). Also, for Ba you can add about 8drops of 6M acetic acid, 1 drop of potassium chromate and mix. A yellow ppt. indicates Ba.  If you add concentrated sulfuric acid to you original sample and a gas forms(bubbles) you probably have carbonate present.  If you add conc. sulfuric to your original sample and do not get a ppt., you can probably conclude that it is sodium sulfate or the acetate anion. (since there is no Ba present to form BaSO4, which is highly insoluble).  The addition of a silver nitrate soln. to your sample will ppt out AgCl. (possibly indicating BaCl2) Silver will probably ppt out the acetate ion as well, forming AgC2H3O2. This is a sparingly soluble salt.

You can add some sodium hydroxide to your sample (6M) and if a ppt. forms you may have magnesium; since MgOH is an insolube ppt. Be aware that Ba will also form the insoluble hydroxide since its a group II element as well.  See above for Ba tests.

Remember, a good test for cations are flame tests. Ba, Mg,test well.  Na is in everything, so this would be harder to see.  Also, use pH paper or a meter.  The carbonate should be higher than 7. The sodium acetate should be 7 or lower due to the acetate ion.

USE A SPOT PLATE OR A WELL PLATE AND ENSURE YOU USE THE FUME HOOD FOR ALL TESTS!
Good luck