How could you detect the presence of the so42 ion

Project:

In a vacated company laboratory, you find an unlabeled bottle of aqueous solution. It is known that the cation(s) is(are) from the following: Ag+, Hg22+, Pb2+, Cu2+, Bi3+, As3+, Co2+, Ni2+, Fe3+, and Al3+; and the anions from SO42-, CO32-, PO43-, Cl-, and NO3-. Design an experiment to identify what ion(s) is(are) present in this unlabeled solution.

· Design a detailed procedure to separate the ions into groups

· Identify the ions in the unknown solution using the information in the Background Section.

· Develop a flow chart to clearly show the steps, and add individual tests to the flow chart to show the identification. List the techniques and equipment you would need to use.

Background:

The first step in qualitative analysis is to separate the ions into groups, because many ions may give similar results when the same reagents are added, and hence may interfere with each other. If present, Ag+, Hg22+ and Pb2+ will all form white precipitate when HCl is added, thus separate from the rest of the ions in the solution. After Ag+, Hg22+, Pb2+ are removed from the solution, Cu2+, Bi3+, As3+, if present, will all form precipitate with H2S in acidic conditions , thus separate from the rest of the solution. Co2+, Ni2+, Fe3+, and Al3+, if present, will precipitate when NaOH is added, thus separate from the rest of the solution.

After separation of the ions, individual tests are to be performed to identify the ions. For example, a white precipitate forms when HCl is added to the unknown solution, indicating one, two, or all three cations from Group I (Ag+, Hg22+, Pb2+) may exist. After adding excess HCl to ensure that all of the Group I cations have precipitated out of the solution, the precipitate is collected (using which technique?), washed (using which technique?) and dried. Always keep the rest of the clear solution because you need to analyze it for other ions later! The precipitate collected is then analyzed for existence of Ag+, Hg22+ or Pb2+:

PbCl2 is soluble in hot water while AgCl and Hg2Cl2 are not. Therefore adding water the precipitate and heating the mixture will dissolve PbCl2 and separate it from AgCl and Hg2Cl2 (using which techniques?). AgCl will redissolve if NH3 is added,

AgCl(s) + 2 NH3 (aq) ⇌ Ag(NH3)2+(aq) + Cl-(aq)

but will form a white precipitate again if the solution is acidified with HNO3,

Ag(NH3)2+(aq) + Cl-(aq) + 2H+(aq) ⇌AgCl(s) (white) + 2 NH4+(aq)

This series confirms the existence of Ag+.

Hg22+ is identified by the following reaction,

Hg2Cl2(s) + 2 NH3(aq) ⇌ Hg(s) (black) + HgNH2Cl(s) (white) + NH4+(aq) + Cl-(aq)

Pb2+ is confirmed by the K2CrO4 test:

Pb2+(aq) + CrO42-(aq) ⇌ PbCrO4(s) (yellow)

After Group I cations are removed from the unknown solution, Group II cations are precipitated by adding 6M HCl and thioacetamide (CH3CSNH2) which hydrolyzes to give H2S:

CH3CSNH2(aq) + 2H2O(l) + H+(aq) ⇌ CH3COOH(aq) + NH4+(aq) + H2S(aq)

H2S will then precipitate Cu2+, Bi3+, As3+ from the solution, if they are present. Copper and bismuth sulfides are insoluble in basic thioacetamide while arsenic sulfide dissolves in the form of thio complex ion, AsS33-, thus separating arsenic from copper and bismuth.

Arsenic is tested as follows,

As2S3(s) + H2S (aq) + NaOH (aq) → AsS33-(aq)

AsS33-(aq) + HCl(aq) → As2S3(s)

As2S3(s) + H2O2 (basic) → AsO43-(aq)

AsO43-(aq) + Mg(OH)2 (magnesia mixture) → MgNH4AsO4 (s) (white)

CuS and Bi2S3 dissolve in hot HNO3 forming Cu2+ and Bi3+ ions; addition of NH3 will form a white precipitate, Bi(OH)3, with Bi3+, and a deep blue color complex solution, Cu(NH3)42+, with Cu2+.

Co2+, Ni2+, Fe3+, and Al3+ all form insoluble hydroxides when NaOH is added, but Al(OH)3 will re-dissolve if excess NaOH is added because of its amphotericity,

Al(OH)3(s) + OH-(aq) ⇌ Al(OH)4-(aq)

Therefore, Co(OH)2(s, lavender), Ni(OH)2(s, green), Fe(OH)3(s, reddish-brown) are separated from Al(OH)4-(aq). They will dissolve again if the pH is adjusted to acidic. Since the tests for these three ions, Co2+, Ni2+, Fe3+, do not interfere significantly with each other, the solution is simply divided into three portions for individual tests:

Ni2+(aq) + 6NH3(aq) ⇌ Ni(NH3)62+(aq)

Ni(NH3)62+(aq) + 2HDMG ⇌ Ni(DMG)2(s) (red) + 2NH4+(aq) + 4NH3(aq)

where HDMG is demethylglyoxime, (CH3)2C2(NOH)2.

Co2+ is tested by first adding NaF to complex with Fe3+ (FeF63-) to prevent interference from Fe3+, followed by addition of saturated NH4SCN in ethanol solution:

Co2+ + 4SCN- ⇌ Co(SCN)42-(blue)

Fe3+ is tested by adding KSCN:

Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq) (red)

Aluminum is tested by the following steps:

1. Al(OH)4-(aq) + 16M HNO3 → Al3+(aq)

2. Al3+(aq) + 15M NH3 → Al(OH)3(s)

3. Al(OH)3(s) + 3M CH3COOH → Al3+(aq)

4. Al3+(aq) + aluminum reagent + 6M NH3 → red precipitate (Al(OH)3-Al)

The anions are analyzed in a similar fashion. The BaCl2 group where addition of BaCl2 forms white precipitates with SO42-, CO32-, PO43-,

SO42-(aq) + Ba2+(aq) ⇌ BaSO4(s)

CO32-(aq) + Ba2+(aq) ⇌ BaCO3(s)

2PO43-(aq) + 3Ba2+(aq) ⇌ Ba3(PO4)2(s)

The difference is BaSO4(s) will not dissolve in HCl , while both BaCO3(s) and Ba3(PO4)2(s) do dissolve in HCl; BaCO3(s) also effervesce with release of CO2.

BaCO3(s) + 2H+(aq) ⇌ Ba2+(aq) + CO2(g) + H2O(l)

The AgNO3 group , where addition of AgNO3 precipitates CO32-, PO43-, and Cl-. Ag2SO4 is slightly soluble.

CO32-(aq) + 2Ag+(aq) ⇌ Ag2CO3(s) (white)

PO43-(aq) + 3Ag+(aq) ⇌ Ag3PO4(s) (yellow)

Cl-(aq) + Ag+(aq) ⇌ AgCl(s) (white)

Addition of HNO3 causes Ag2CO3 and Ag3PO4 to dissolve, with Ag2CO3 also causing effervescence, while AgCl remains as a white precipitate.

The soluble group, NO3-, is tested by the brown ring test. The addition of H2SO4 and FeSO4 causes a brown ring of Fe(H2O)5NO2+ to appear at the interface of the two layers.

NO3-(aq) + 4H+(aq) +4Fe2+(aq) + 3H2O → 3Fe3+(aq) + Fe(H2O)5NO2+(aq)

Identification of the Silver group ions: Ag+, Hg22+, and Pb2+

These ions are removed from the solution by adding excess 6M HCl to the solution and stirring with a stirring rod. After centrifugation, the precipitate will stick to the wall and bottom of the test tube and the supernatant solution will be clear. Test for complete precipitation by adding 2 more drops of 6M HCl to the clear supernatant liquid. If the solution turns cloudy, add two more drops of HCl, stir and centrifuge. Repeat until the supernatant liquid is clear when HCl is added.

Decant the supernatant solution and wash the precipitate with some cold water, centrifuge and decant the wash solution. The silver group ions, if any, are now in the precipitate.

Add some distilled water to the precipitate and place the test tube in a hot water bath, stir and heat for a few minutes. Quickly centrifuge and decant the solution in a clean test tube labeled “Pb2+”. Repeat the above step two more times and combine the solutions after centrifuging in the labeled test tube. The remaining precipitate, if any, will be analyzed for Ag+ and Hg22+.