EXPERIMENT 21

[qual1.lab]

OBJECTIVES:

To learn to analyze a solution for Ag⁺¹, Pb⁺² and Hg₂⁺²

DISCUSSION:

Forensic chemists are those who use chemistry to solve a crime. Often they have only a small amount of evidence to work with. Methods have been developed to use only a few drops of solution to find out what metals are present (or absent). These methods involve forming precipitates and separating them from the rest of the solution. The following experiment illustrates this method. The ions of the "Silver Group" are Ag⁺¹, Pb⁺² and Hg₂⁺² [ mercury(I) ]. Each of these ions will react with Cl ^-1 to form a precipitate. If no precipitate forms when HCl is added, we know that all of these ions must be absent. Study the Procedure for the Analysis. Find the formulas for all of the precipitates obtained, and for the ions remaining in the solution. Complete the Flow Chart by filling in these formulas.

List the colors for all precipitates. This will serve as a guide in outlining the separation steps and final identification tests.

GENERAL PROCEDURE:

Part A:

Obtain a KNOWN solution containing Ag⁺, Pb⁺⁺, and Hg₂⁺⁺, and analyze this according to the Procedure below. Follow your results on the Flow Chart. What you SHOULD see is in the Analysis Summary. The four columns are, respectively:

- Sample (the solution or precipitate involved at that step)

- Reagents (what is added to the solution or ppt)

- Observations (what you see happen)

- Conclusions (what inference or deduction you make)

The Summary provides a record of your actual analysis. You will make a similar summary for your UNKNOWN solution.

Part B:

Obtain an UNKNOWN and follow the same procedure as for the KNOWN. The UNKNOWN may not contain all 3 of the ions. Therefore, some of the tests will be NEGATIVE (ie: no precipitate will be obtained when the test for that ion is performed). This means that ion is ABSENT. Complete the ANALYSIS SUMMARY for this Unknown. Report which ions you found to be present.

Note: if you wish, Parts A and B may be run at the same time. You must be careful not to mix up the test tubes!

Flow Chart - The Silver Group

Solution of

@ = centrifuge metal ions

Step 1 add HCl

@

P1 S1 Cations

of groups 2-5

Step 2 add H₂O, heat

@

P2 S2

Step 4 add NH₄OH Step 3 add K₂CrO₄

@

P5

P3 S3

Color:

Color:

Step 5 add KI

P4

Color:

Key:

Reagents are in italics with arrows.

Precipitates (ppts) or residues are in double-line boxes.

Solutions of ions are in single-line boxes.

@ means to centrifuge and decant, getting P (ppt or residue) and S (solution or decantate).

EXPERIMENTAL PROCEDURE:

1. Precipitation of the Group.

Put 10 drops of the solution to be tested in a small test tube (tt). Add 5-10 drops of 6 M HCl and mix. The white precipitate consists of AgCl, Hg₂l₂and/or PbCl₂. (NO precipitation indicates ABSENCE of Ag⁺, Hg₂⁺⁺, and of much Pb⁺⁺.) Centrifuge.

Decant the solution into another small test tube. This solution is S1. The residue that remains in the test tube is P1.

DISCARD S1. [If we were analyzing for Groups 2-5, we would use S1 for this purpose.]

Take P1. Add 1-3 mL of distilled water from your wash bottle to wash the ppt. Shake, centrifuge and discard the liquid.

2. Separation of Lead Ion from AgCl and Hg₂Cl₂.

Heat 25-50 mL of water to boiling in a small beaker. Add 15 drops of H₂O to P1 and shake, then place the tt in the beaker of hot water for 2-3 minutes. Centrifuge and decant, obtaining P2 and S2. P2 contains AgCl and Hg₂Cl₂. (AgCl and Hg₂Cl₂ do not dissolve in hot water.) The PbCl₂ dissolves in hot water and S2 contains Pb⁺² ions.

3. Precipitation of Lead Ion.

Add several drops of K₂CrO₄ solution to the S2 decantate. A yellow precipitate of PbCrO₄ proves Pb⁺⁺ is present. Discard this test tube when done.

4. Test for Mercury(I) Ion and Separation of Silver Ion.

To residue P2, add 1 mL of water. Heat for 1 minute. Shake, centrifuge and decant. Discard the decantate. To P2, add 12 drops of concentrated NH₄OH and mix. Centrifuge and decant. Label the decantate S3. A black residue (P3) in the test tube indicates that Hg₂⁺⁺ is present. This residue is a mixture of Hg and HgNH₂Cl₂. Discard.

5. Test for Silver Ion.

To S3, add 4-5 drops of KI solution. A light-colored ppt of AgI proves that Ag⁺ is present. Discard.

Exp 21 - ANALYSIS OF AN UNKNOWN SOLUTION FOR Ag⁺¹, Pb⁺², Hg₂⁺²

Name ____________________

Analyze the Unknown and report your actual observations for each step of the procedure. At the bottom, list the ions found.

Unknown No.____ Appearance of sample: __________________________

Ions found:___________________________