Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Swiften on 02/02/2008 11:03:29

Title: Rationales behind Steps in Testing for Ions ?
Post by: Swiften on 02/02/2008 11:03:29

Hi all, I'm testing for ions but am clueless to what is/are the rationale(s) behind the steps that were carried out. Can someone enlighten me on the following questions:

===============================================
Tests conducted
===============================================
A. To test for Chloride:
1) Acidify with dilute Nitric Acid
2) Add Silver Nitrate solution.

B. To test for Sulphate:
1) Acidify with dilute Hydrochloric Acid
2) Add a few drops of Barium Chloride solution.

C. To test for Nitrate:
1) Acidfy with dilute Sulphuric Acid
2) Add a few crystals of Ferrous Sulphate
3) Shake to dissolve
4) Add concentrated Sulphuric Acid slowly, allowing it to settle at bottom of test tube.

===============================================

Q1: Why are different acids being used for the 3 tests?

Q2: How do we know if the solution is completely acidified?

Q3: In Test C, brown ring is observed if Nitrate ions are present. What is the brown ring due to?

Q4: Is it possible to write chemical equation for the above questions? If yes, how do i write them?

Q5: In the preliminary test, I added excess Sodium Carbonate to an unknown sample. I then added distilled water, placed it over a bunsen flame, heated it to slow boiling with intermitten stirring for 5 minutes. Why do I have to add excess Sodium Carbonate in this preliminary step?

Title: Rationales behind Steps in Testing for Ions ?
Post by: lightarrow on 02/02/2008 13:47:36

Quote from: Swiften on 02/02/2008 11:03:29

Hi all, I'm testing for ions but am clueless to what is/are the rationale(s) behind the steps that were carried out. Can someone enlighten me on the following questions:

===============================================
Tests conducted
===============================================
A. To test for Chloride:
1) Acidify with dilute Nitric Acid
2) Add Silver Nitrate solution.

B. To test for Sulphate:
1) Acidify with dilute Hydrochloric Acid
2) Add a few drops of Barium Chloride solution.

C. To test for Nitrate:
1) Acidfy with dilute Sulphuric Acid
2) Add a few crystals of Ferrous Sulphate
3) Shake to dissolve
4) Add concentrated Sulphuric Acid slowly, allowing it to settle at bottom of test tube.

===============================================

Q1: Why are different acids being used for the 3 tests?

Q2: How do we know if the solution is completely acidified?

Q3: In Test C, brown ring is observed if Nitrate ions are present. What is the brown ring due to?

Q4: Is it possible to write chemical equation for the above questions? If yes, how do i write them?

Q5: In the preliminary test, I added excess Sodium Carbonate to an unknown sample. I then added distilled water, placed it over a bunsen flame, heated it to slow boiling with intermitten stirring for 5 minutes. Why do I have to add excess Sodium Carbonate in this preliminary step?

Q1:
You certainly cannot use HCl to test for Cl^-; you can't use H₂SO₄ because Ag₂SO₄ is not very water soluble (so you could see a precipitate even in the absence of Cl^-). The only choice among common acids is HNO₃. For the sulphate test, you can't use H₂SO₄; however I think you could use HNO₃ as well, instead of HCl. For the nitrate test you cannot use HNO₃; you don't also have to use HCl because Cl^- is oxidated as well by the nitrates in strongly acidic solutions.

Q2:
Usually you use diluted solutions of the sample, so adding 2 drops of acid in a test tube are enough to acidify it, but if you have doubts you can always use litmus paper, I don't think that's forbidden!

Q3.
NO₃^- oxide Fe²⁺ in acidic solution to form Fe³⁺ and NO. If I remember correctly (I'll check on my book later), NO forms a brown complex with Fe²⁺ in presence of concentrated H₂SO₄, so you will notice that brown ring at the interface between the solution and the conc. acid.

Q4:
A. Ag⁺ + Cl^- --> AgCl (white precipitate)
B. Ba²⁺ + SO₄^2- --> BaSO₄ (white, sericeous precipitate). 
C. NO₃^-+ 3Fe²⁺ + 4H⁺ --> 3Fe³⁺ + NO + 2H₂O
FeSO₄ + NO --> FeSO₄NO (brown).

Q5:
To transform possible insoluble chlorides, sulphates, nitrates, into the corresponding soluble sodium salts.

Title: Rationales behind Steps in Testing for Ions ?
Post by: Swiften on 03/02/2008 07:52:32

Wow! Thanks lightarrow!  [:D]