Question

Qualitative Analysis Lab

Introduction
design and carry out a chemical analysis scheme to separate and identify a series of cations and anions. By this point in your chemistry career you will have encountered a fair amount of chemistry and are familiar with most of the fundamental techniques that we use in general chemistry. This lab will require two class sessions to complete. You will have the chance to apply knowledge you have gained from previous labs and learn about the properties of cations and anions.

The cations and anions we will be analyzing are:
Ba+2 Cl-
Cu+2 SO4-2
CO3-2 Ag+
Pb+2

Procedure
When you leave college and embark upon a career in whatever your field is, you will be expected to apply your knowledge to your work. You will often be venturing into new territory and there will not be someone who can tell you exactly what to do. This applies no matter what field you might pursue. If you find that you are stuck, one way to find answers is to do research and look into what has been done before. This is one aspect of the “real world” we are attempting to duplicate with this series of labs.
The procedures for these next labs will not be spelled out for you. You will be expected to use your knowledge of solubility rules (see “solubility rules” in your text), group chemistry (group 2A-8A elements), and additional sources (I strongly recommend http://www.public.asu.edu/~jpbirk/ or another (simpler) site is http://www.ausetute.com.au/solrules.html ). Qualitative analysis labs are plentiful, and if you get stuck planning a procedure a quick search on the internet or library should reveal the relevant information.

We will provide you with the following chemicals to perform your tests with:
6 M NH3 6 M HCl
6 M HNO3 6 M NaOH
3 M H2SO4 0.2 M AgNO3
Saturated Ba(OH)2 0.2 M BaNO3

If you have a procedure that would require additional chemicals email your instructor in advance and we will try to provide them for you.
In the lab you will be provided with two known solutions. One solution will contain all three of the anions (Cl-, SO4-2, CO3-) and the other known solution will contain all of the cations (Cu+2, Ag+,Pb+2, Ba+2). These are for you to perform the separations on. Once you have completed your known solutions you may proceed to your unknown solutions.

Write down the procedure that you follow step by step and why you are doing it.

Thank you

Answer 1

We will have two separate solutions, one containing all the three anions (Cl^-, SO₄^2- and CO₃^2-). The second solution contains all the cations (Pb²⁺, Cu²⁺, Ag⁺ and Ba²⁺). Work them out separately.

Separation of anions (Cl^-, SO₄^2- and CO₃^2-)

Detection of anions is somewhat misleading since there isn’t a systematic procedure like metal ions. We need to perform separate tests to identify the ions.

1) Take a small portion of the solution of the solution and add 2 mL of 6 M nitric acid (note we cannot add HCl or H₂SO₄ since the salt solution contains the two anions). Heat if required. A brisk effervescence occurs due to the evolution of carbon dioxide, CO₂ gas. CO₃^2- decomposes in acid giving CO₂ and H₂O.

CO₃^2- (aq) + 2 HNO₃ (aq) -------> 2 NO₃^- (aq) + CO₂ (g) + H₂O (l)

2) To another small portion of the solution taken in a test tube, add 2 mL of 6 M HNO₃ solution and then add 0.2 M silver nitrate, AgNO₃ dropwise. The appearance of a white precipitate confirms the presence of Cl^-. Continue the addition until no further precipitation occurs. The precipitate is due to silver chloride, AgCl as shown below.

Cl^- (aq) + AgNO₃ (aq) -------> AgCl (s) + NO₃^- (aq)

3) To a third portion, add 2 mL of 6 M HNO₃ and then add 0.2 M Ba(NO₃)₂. The appearance of a white precipitate of barium sulfate, BaSO₄ indicates the presence of SO₄^2-.

Ba(NO₃)₂ (aq) + SO₄^2- (aq) -------> BaSO₄ (s) + 2 NO₃^- (aq)

Separation of cations (Pb²⁺, Cu²⁺, Ag²⁺ and Ba²⁺)

1) Dissolve the solution is 6 M HCl. A white precipitate appears. Filter the precipitate and retain the filtrate. Call this filtrate F1.

2) Wash the precipitate with water and boil with water and centrifuge. Retain the residue and the filtrate. Call this filtrate F2

3) Divide the filtrate F2 into two parts:

a) Add 3 M H₂SO₄ to one part; a white precipitate appears indicating the presence of Pb²⁺.

b) Cool the second portion. A white precipitate appears, indicating the presence of Pb²⁺.

4) Add 6 M NH₃ to the residue. The residue dissolves, indicating the presence of Ag⁺. Acidify the solution with 6 M HNO₃. A white precipitate of AgCl appears indicating the presence of Ag⁺.

5) Pass H₂S gas through the filtrate F1. A precipitate appears; separate the residue and the filtrate. Call this filtrate F3.

6) The precipitate contains Cu²⁺. Confirm the presence of Cu²⁺ by taking a fresh solution of the sample and adding 6 M NaOH solution. The appearance of a blue solution indicates the presence of Cu²⁺.

7) Acidify the filtrate F3 with 6 M NH₃ and add (NH₄)₂CO₃. The appearance of a white precipitate indicates the presence of Ba²⁺.

8) Alternatively take a fresh solution of the sample and add 3 M H₂SO₄. The appearance of a white precipitate indicates the presence of Ba²⁺.