Google Adsense

Thursday, November 8, 2012

The Systematic Analysis of Cation Group 2 - The Hydrogen Sulfide Group - Hg++, Cd++, Cu++, Pb++, Sn++, Sn(IV), Bi+++, Sb(III), Sb(V), As(III), As(V)

Cation Group 2 – The Hydrogen Sulfide Group
Hg++, Cd++, Cu++, Pb++, Sn++, Sn(IV),
Bi+++, Sb(III), Sb(V), As(III), As(V)

The eight elements of this group form sulfides that are precipitated by hydrogen sulfide or thioacetamide from solution that are that are 0.1 to 0.3 M in hydrochloric acid. Of the chloride only that of lead is sparingly soluble in dilute hydrochloric acid and precipitates with Cation Group 1 to an appreciable extent. Silver and mercury(I), had they not already been removed as chlorides, would precipitate as sulfides with Group 2.

Thioacetamide is a white crystalline solid. It is soluble in water and serves as a source of sulfide ions in the systemic analysis of Group 2.

TA (Thioacetamide) solution in pH 0.5

 Outline 1

Testing sample

 Warm the solution containing TA at pH 0.5 in a bath of vigorously boiling water for 5 minutes. 

  After added TA solution into testing sample, warmed in water bath

Centrifuge and withdraw solution to another test tube.

 Check the acidity with short range indicator paper. Add 2 drops TA and heat for several minutes in the boiling water. Centrifuge if more precipitation appears. Continue heating with TA as long as precipitation is obtained. Then dilute several drops of the clean solution with twice its volume of water, add 1 drop TA, and heat again. If a precipitation appears, treat the whole solution in the same way; divide it between several test tube if necessary.

  Combine precipitation if several portions have been obtained. Wash the precipitated sulfides twice with about 20 drops hot water containing 1 drop 6M NH4Cl. Add the first washing to solution2-1 in a general analysis for all groups.

Solution 2-1

Add 10 drops 0.5M KOH and heat briefly in the water bath. Centrifuge and transfer the solution to a centrifuge tube. Repeat the extraction with a second portion of KOH solution. Combine solutions.
Precipitation 2-2
Wash twice with hot water to which either 1 drop 0.2M NH4NO3 or crystal of the solid salt has been added.
Analysis according to Outline 2

Solution 2-2
 Recentrifuge to remove the last traces of precipitation. Transfer the clear, yellow solution to a test tube.
Analysis according to Outline 3

 Outline 2

Treat precipitation with 5 drops water and an equal volume 6M nitric acid. Warm it in water bath until reaction occurs. Centrifuge and wash precipitation with water. Add wash to solution3-1.

Solution 3-1
 Transfer to a casserole and add 2 drops 18M sulfuric acid. Evaporate over flame and directly under a ventilating duct or in the hood until white fume of SO3 are copiously evolved. Cool for a while. Use many drops of water to wash to casserole to collect the solution. Transfer the solution quickly to a centrifuge tube. Centrifuge it.

Precipitation 3-1
Add a few drops aqua regia and warm. Transfer solution to a crucible. Evaporate over a microflame to a small volume but not to dryness. Dilute with about 1 mL water and transfer to a centrifuge tube. Centrifuge if not clear and discard residue. Add SnCl2 solution.
White precipitation turning gray:
Hg + Hg2Cl2: Presence of Hg(II)

Solution 3-2
Add 15M NH3 until is alkaline.
Centrifuge it.
 Blue solution shows copper(II) ions

Precipitation 3-2
Warm it with a few drops NH4OAc solution. Centrifuge and discard residue. To solution add 1 drop 6M HOAc and a few drops K2CrO4
Yellow precipitation: Pb CrO4 presence of Pb.

Precipitation 3-3
 Add sodium stannite solution.
Jet black precipitation(Bi0):
Presence Bi
 Bismuth formed in the solution

Solution 3-3
 Add some Na2S2O4. Warm it for 1~2 minutes. Centrifuge quickly and remove solution. Repeat the treatment again. Add some drops TA and warm it.
Yellow precipitation(CdS): Presence of Cd

Outline 3

Add 1 drop TA and some 2M HCl. Centrifuge the solution, Draw off the solution as possible.

 Precipitation 4-1

Add 5~10 dros 12M HCl and warm it in hot water bath. After reaction occurred, centrifuge the solution.

 Precipitation 4-2
 Add 2 drops 15M NH3 and 1 drop 3% H2O2. Warm it and centrifuge. Discard residue. To solution add several drops of magnesia mixture. Let it react 5 minutes. Centrifuge and discard solution. Wash once with water.

 Precipitation 4-3

 Add 1 drop 6M HOAc and several drops AgNO3 solution.

Reddish brown precipitation:
Presence of As

Solution 4-2
 Evaporate the solution to half the original volume. Dilute with about 1 mL water and divide between two test tubes.

Sb Test 

 Add some oxalic acid and a few drops TA.

Deep orange: Sb2S3 ; presence of Sb.

Sn Test
 Add some drops of HCl and a peace of Al. Warm the solution in hot water bath for some minutes.

Centrifuge the solutiton.

 Precipitate 4-4
 Black precipitation: Presence Sb0

Solution 4-4

 Add 1~2 drops HgCl2 solution.

White precipitation (Hg2Cl2):  Presence of Sn

Experimental Record

No comments:

Post a Comment