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Weird sulphate test

(9701 Jun02 p3 no2)
Hello folks,
Here is a funny question about qualitative analysis:

The solution contains the sulphate anion and here is the test together with the mark scheme answers:

======================================================
Test:
Place 1 cm depth of
FA 3 in a test-tube
and add an equal
depth of dilute nitric
acid.

Add aqueous
barium nitrate.

Observation: permanent white ppt.

Deduction: sulphate ion present.
========================================================

Now, check this from my textbook:

Test for sulphate ions:
make a solution of the salt. Add a solution of barium nitrate or chloride. If a precipiate forms add dilute nitric acid. If a white precipitate forms which does not re-dissolved in acid then sulphate is present

So, my question is, why does the question ask to put acid first? I find CIE papers to be very silly.

The mark scheme said that candidates didn't realize the significance of adding the acid as they left sulphite in the deduction. But the acid is added after isn't it? I am so confused.

Now, if a sulphite were really present instead of the sulphate, and the acid was added first, then what would have I seen when I added the Ba2+(aq)? Would the white ppt appear then disappear? (sulphites ppt do dissolve in excess acid). See what I mean by adding the acid after?

I am so confused.

Thanks for all your help friends.

Love you all.

Reply 1

EierVonSatan

Well if the sulphite only is present adding the barium nitrate after the acid means that no percipitate would be formed. Its just a variant of letting it percipitate out then finding that it dissolves upon addition of acid as suggested in your text. So the observation for the sulphite would be no percipitate forms (or maybe forms and re-dissolves quickly?), as opposed to a permanent percipitate being formed with sulphate.

Reply 2

dannyboy383

oooooooooooooooooo

I think I guess what you are saying; if we added acid first, and if a sulphite were present, then a reaction will take place (sulphur dioxide liberated with dilute acids). So, when Ba2+ is added, no ppt forms.

But not so for sulphate as sulphates don't react with acids to give SO2 gas.

hmmm...

Reply 3

charco

Study Forum Helper

This is the same for interference by carbonates, which would also give a white ppt with barium chloride (or nitrate) solution.

If you are using barium chloride to carry out the test they you also add HCl. As EVS says the order is not important.

Ba²⁺ + SO₄^2- --> BaSO₄ (white ppt)

add H⁺ ions and no change

Ba²⁺ + SO₃^2- --> BaSO₃ (white ppt)

add H⁺ ions and ppt redissolves

Ba²⁺ + CO₃^2- --> BaCO₃ (white ppt)

add H⁺ ions and ppt redissolves.

------------------------------------------
When you are testing for the presence of chloride, bromide or iodide ions by adding Ag⁺ in the form of silver nitrate you also add some nitric acid to prevent interference by carbonate ions.

In this case you have to be sure that sulphate ions are not present as they interfere with the test. For this reason the sulphate test is always the first carried out.

The usual order for carrying out identifying tests is:

1. Test for sulphate (at the same time identify the presence of sulphite or carbonate)
2. Test for chloride, bromide, iodide.
3. Test for nitrate

Reply 4

dannyboy383

mmkay.

I think I have an idea what you dudes are talking about.

What confuses me slightly is the statement by charco:

In this case you have to be sure that sulphate ions are not present as they interfere with the test.

How does this interference occurr? I have a feeling the answer is right in front of me.

____
So in this test, when I add the acid first, and then the Ba2+, the test is no different from the test given in my text?

Reply 5

charco

Study Forum Helper

Sulphate ions only interfere with the AgNO3 test for chlorides, bromides and iodides as sulphate ions also give a white ppt with Ag(+) ions.

Therefore the first test that has to be done on an unknown is the Ba(2+) test to identify sulphate, sulphite or carbonate.

Reply 6

EierVonSatan

dannyboy383

What confuses me slightly is the statement by charco:

In this case you have to be sure that sulphate ions are not present as they interfere with the test.

How does this interference occurr? I have a feeling the answer is right in front of me.

SIlver sulfate can be formed (which is sparingly soluble in water)

So in this test, when I add the acid first, and then the Ba2+, the test is no different from the test given in my text?

pretty much :wink:

Edit: beaten by charco

Reply 7

dannyboy383

Thanks friends,
Sorry for not replying sooner. Health is weath - and these days I am not that wealthy. LOL.

Hey charco - thanks a million and one for this tip:

"If you are using barium chloride to carry out the test they you also add HCl"

I was confused about the Barium nitrate and choride and the nitric hydrochloric acids.

I did not know about silver sulphate ppt.

And the silver carbonate. Wikipedia tells me that silver carbonate has a very low solubility.

Question:
If I take aqueous sodium carbonate and add a dash of silver nitrate, would I get a delicious precipitate of silver carbonate?
See, the reason I am confused is that the qualitative analysis notes I get for exams use silver nitrate only to test for the halides.

I could probably use the carbonate test to act as a confirmatory test.

Again, sorry for not replying sooner. You guys whisper a little prayer for me.

Reply 8

charco

Study Forum Helper

Question:
If I take aqueous sodium carbonate and add a dash of silver nitrate, would I get a delicious precipitate of silver carbonate?
See, the reason I am confused is that the qualitative analysis notes I get for exams use silver nitrate only to test for the halides.

If you add carbonate ions in any form to silver ions in solution you will get a ppt of silver carbonate... BUT you would also get a precipitate with most metal ions and carbonate ions. (only Na⁺, K⁺ and NH₄⁺ do NOT give a ppt)

The idea of qualitative analysis is to EXCLUDE ions one at a time gradually getting down to the only possible answer. That's why the tests are carried out in a logical order. Silver ions make precipitates with many other anions apart from halide ions. Therefore to test for the halide ions you must first eliminate the possibility of there being other ions present.

The first test using Ba²⁺(aq) is carried out specifically because there is no interference from other ions. It allows you to eliminate sulphate, sulphite and carbonate from the equation.

The second test can then be carried out safe in the knowledge that sulphate, sulphite and carbonate (which would interfere) are NOT present. The second test uses Ag⁺(aq) ions to test for Cl^-(aq), Br^-(aq), and I^-(aq).

If the second test is negative then you can carry out a nitrate test and if THAT test is negative you can suspect phosphate, or an organic anion such as ethanoate.

It is often helpful to draw out a flow scheme.