Lab, CSI: Chemistry Solubility Investigation (Qualitative Analysis) · AP Chemistry

A forensic chemist receives an unidentified victim sample and must report exactly which metal ions it contains. In this two-day qualitative-analysis lab you separate and confirm seven cations, NH₄⁺, Ag⁺, Pb²⁺, Fe³⁺, Al³⁺, Ni²⁺, Co²⁺using selective precipitation: each separation is a solubility equilibrium you push one way or the other, and each confirmation is a colour or precipitate driven out of equilibrium by Le Châtelier's principle.

CED 7.11–7.13 (Solubility Equilibria · Ksp · Common-Ion & pH effects) + 7.9–7.10 (Le Châtelier) · Unit 7. ~2 × 70-min blocks (TWO-DAY lab) · FORMATIVE-category practical: observation tables + Day-2 case report, scored on the shared AP Chem lab rubric. Pegged to the AP recommended lab "Separation and Qualitative Analysis of Cations and Anions."

Background CED 7.11–7.13

Qualitative analysis answers one question: which ions are here? You cannot test all seven at once (they interfere) so you separate them into groups by adding a reagent that precipitates some ions and leaves the rest dissolved, then drain off (centrifuge) and work each fraction. Every move is a solubility equilibrium.

Ksp and selective precipitation. A salt dissolves to an equilibrium: for example AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq), with Ksp = [Ag⁺][Cl⁻]. A precipitate forms only when the reaction quotient Q exceeds Ksp. Because the seven chlorides have hugely different Ksp values, adding Cl⁻ drives Q > Ksp for only AgCl and PbCl₂ (both barely soluble) while NH₄Cl, FeCl₃, AlCl₃, NiCl₂, and CoCl₂ stay fully dissolved (very large Ksp). That single reagent cleanly pulls out two ions, selective precipitation.

Le Châtelier runs every step. Add a common ion (Cl⁻) to force precipitation; raise temperature to redissolve endothermic PbCl₂; add excess OH⁻ to dissolve amphoteric Al(OH)₃ as aluminate; add excess SCN⁻ or solid reagent to pull a confirmation complex into existence. Each confirmation colour is an equilibrium you have deliberately shoved off-balance.

The seven cations, in the order you separate them: NH₄⁺ (tested in place, it cannot be precipitated); Ag⁺ and Pb²⁺ (insoluble chlorides); Fe³⁺ and Al³⁺ (insoluble hydroxides, very small Ksp); Ni²⁺ and Co²⁺ (stay dissolved as ammine complexes while the hydroxides drop out). You will confirm each with its own signature reaction.

Why a buffer, not just base? When you precipitate Fe(OH)₃ and Al(OH)₃ you add NH₃ with NH₄Cl. The NH₃/NH₄⁺ buffer fixes [OH⁻] high enough that Q > Ksp for the extremely insoluble Fe(OH)₃ (Ksp ≈ 10⁻³⁹) and Al(OH)₃ (Ksp ≈ 10⁻³⁴), yet low enough (and with enough NH₃ to form [Ni(NH₃)₆]²⁺ / [Co(NH₃)₆]²⁺) that Ni²⁺ and Co²⁺ stay in solution. Controlling [OH⁻] is itself a selective-precipitation tool (CED 7.13).

Safety read before you start

6 M NaOH and 6 M HCl, corrosive. Both cause severe skin and eye burns. Add acids and bases dropwise, keep them off your skin, and rinse any contact immediately for 15 minutes. Neutralize spills with the bench kit and tell Dr. Mulinta.

Ammonia (6 M NH₄OH), pungent, irritating vapour. Dispense and use it under the fume hood (Room 3035) or with the bench fan on. Never put your face directly over an open tube; waft to smell only if instructed.

Acetone (flammable; solid KSCN) toxic. Keep acetone well away from any flame or hot plate; the cobalt step is done at the bench, not at the boiling-water bath. Thiocyanate releases toxic vapour if mixed with strong acid in quantity, use only the small amounts specified, and never combine KSCN with concentrated acid.

Heavy-metal waste, NOT down the sink. Everything containing lead, silver, chromate, or nickel goes in the labelled heavy-metal waste container: all chloride precipitates, the chromate tube, the nickel and cobalt tubes, and any rinse from them. The yellow chromate solution is a special hazard. When in doubt, it goes in the heavy-metal container.

PPE, every student, the whole period: splash goggles, lab coat or apron, nitrile gloves whenever you handle a reagent. Tie back hair. No food or drink. Wash hands before you leave.

Materials & Equipment Mrs. Kathy pre-sets all reagents

This lab uses many small-volume reagents. Mrs. Kathy (lab tech) pre-sets every solution at the reagent bench in labelled dropper bottles: you carry your test tubes to the reagents, not the reagents to your bench.

Per student / pair: rack of several small test tubes (label them S1, S2, S3, S4, P1–P4 as you go) · 2 watch glasses · glass stirring rod · medicine droppers · 10 mL graduated cylinder · small beaker · wash bottle of distilled water · grease pencil or labels.

Shared at stations: centrifuge (balanced, run in opposed pairs) · ice-water bath · boiling-water bath on a hot plate · red & blue litmus paper · heavy-metal waste container · spill-neutralizing kit.

Reagents Mrs. Kathy pre-sets (labelled dropper bottles): the All-Ion-Solution (Day 1, contains all seven cations) · the sealed numbered "victim" unknowns (Day 2) · 6 M NaOH · 6 M HCl · 1 M K₂CrO₄ · 6 M NH₄OH (aqueous ammonia) · 6 M NH₄Cl · 0.1 M KSCN · aluminon test reagent · dimethylglyoxime (DMG) solution · crystalline (solid) KSCN · acetone.

Note for Mrs. Kathy: keep the All-Ion-Solution and Day-2 unknowns acidified (trace HNO₃) so nothing pre-precipitates on the shelf; set the solid KSCN and acetone at a bench station away from the boiling-water bath; have a fresh heavy-metal waste container per bench and the chromate flagged. Full reagent recipes are in the teacher key.

Procedure: Day 1: Training on the Known All-Ion-Solution · all 7 present

Day 1 you run the full scheme on the All-Ion-Solution, which contains all seven cations. Because every ion is present, every confirmation should turn positive, this is how you learn what a true positive looks like before you face an unknown. Start Part I, then begin Part II while it develops.

Part I: Ammonium (NH₄⁺) · test in place

Place 20 drops of All-Ion-Solution on a clean watch glass.
Dampen a strip of red litmus with distilled water and stick it to the underside of a second watch glass.
Add 10 drops 6 M NaOH to the solution and immediately invert the litmus watch glass over it like a lid. OH⁻ drives NH₄⁺(aq) + OH⁻(aq) ⇌ NH₃(g) + H₂O(l) forward; the escaping basic NH₃ gas turns red litmus blue.
Wait at least 5 minutes (run Part II meanwhile). Red litmus turning blue = NH₄⁺ present. Record the litmus colour change.

Part II: Silver (Ag⁺) & Lead (Pb²⁺)

Get the boiling-water bath going. Put 20 drops All-Ion-Solution in a small test tube. Add 5 drops 6 M HCl → white precipitate forms: Ag⁺ + Cl⁻ ⇌ AgCl(s) and Pb²⁺ + 2 Cl⁻ ⇌ PbCl₂(s).
Cool in the ice-water bath ~1 min (cold lowers solubility → drives precipitation), stir, then centrifuge ~1 min. Transfer the supernatant to a tube labelled S1: it holds the other five cations for Parts III & IV. Do not discard S1.
Wash the precipitate P1: add 1 drop 6 M HCl + 1 mL distilled water, stir, centrifuge, and add this wash to S1. (The HCl supplies common-ion Cl⁻ so the wash does not redissolve your precipitate.) Set S1 aside.
Add 2 mL distilled water to P1 and heat in the boiling bath ~2 min, stirring. PbCl₂ dissolution is endothermic (hot water dissolves it) while AgCl stays solid. Centrifuge while hot. A white/cream precipitate that persists = AgCl = silver present.
Quickly transfer the hot supernatant to a tube S2. Add 2 drops 1 M K₂CrO₄: a bright yellow precipitate = Pb²⁺ + CrO₄²⁻ ⇌ PbCrO₄(s) = lead present.

Part III: Iron (Fe³⁺) & Aluminum (Al³⁺) · from S1

S1 is acidic. Add 6 M NH₄OH dropwise, stirring, until basic to litmus; then add 10 drops excess NH₄OH and 10 drops 6 M NH₄Cl. A gelatinous precipitate P2 forms: Fe(OH)₃ + Al(OH)₃.
Centrifuge and separate P2 from the supernatant S3: S3 holds Ni²⁺ and Co²⁺ for Part IV. Save S3.
To P2 add 6 M HCl dropwise just until it dissolves; then add 6 M NaOH dropwise until basic, plus 2 mL excess NaOH. Amphoteric Al(OH)₃ redissolves as aluminate [Al(OH)₄]⁻; the rust-coloured precipitate P3 = Fe(OH)₃.
Centrifuge and separate P3 from supernatant S4 (S4 holds the aluminum).
Confirm iron: dissolve P3 in 6 M HCl dropwise; add 3 drops 0.1 M KSCN. A deep blood-red solution = Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺ = iron present.
Confirm aluminum: to S4 add 6 M HCl dropwise until acidic; add 5 drops 6 M NH₄Cl; then 6 M NH₄OH dropwise until basic → white/grey flocculent Al(OH)₃ (P4). Centrifuge, discard the solution. Dissolve P4 in 6 M HCl; add 10 drops aluminon reagent; add 6 M NH₄OH dropwise until basic; warm in the water bath. A reddish-maroon "red lake" precipitate = aluminum present.

Part IV: Nickel (Ni²⁺) & Cobalt (Co²⁺) · from S3

Put 10 drops S3 into each of two test tubes.
Tube 1, nickel: add 5 drops DMG (dimethylglyoxime) and warm gently. A strawberry-red precipitate = Ni(DMG)₂ (NiC₈H₁₄N₄O₄) = nickel(II) present.
Tube 2, cobalt: add 2 drops 6 M HCl to acidify and stir; add excess solid KSCN and stir to dissolve; add acetone equal in volume to the solution. A blue colour = Co²⁺ + 4 SCN⁻ ⇌ [Co(SCN)₄]²⁻ = cobalt(II) present. (Done at the bench, acetone is flammable.)

Separation Flow one sample → seven answers

SAMPLE (All-Ion-Solution / Day-2 unknown) | |-- Part I [test in place] + 6 M NaOH --> NH3 gas turns RED litmus BLUE ........ NH4+ | +-- Part II + 6 M HCl, ice, centrifuge | | | +-- PRECIPITATE P1 (AgCl + PbCl2) | | + hot water, centrifuge hot | | |-- precipitate PERSISTS (white/cream AgCl) ...................... Ag+ | | +-- hot solution S2 + K2CrO4 --> YELLOW PbCrO4 .................... Pb2+ | | | +-- SUPERNATANT S1 (Fe3+, Al3+, Ni2+, Co2+) | + NH4OH / NH4Cl buffer, centrifuge | | | +-- PRECIPITATE P2 (Fe(OH)3 + Al(OH)3) | | dissolve in HCl, + excess NaOH, centrifuge | | |-- RUST precipitate P3 (Fe(OH)3): + HCl + KSCN --> RED FeSCN2+ .. Fe3+ | | +-- solution S4 (aluminate): reform Al(OH)3, + aluminon --> RED LAKE Al3+ | | | +-- SUPERNATANT S3 (Ni2+, Co2+) [split into two tubes] | |-- + DMG, warm --> RED precipitate ......................... Ni2+ | +-- + HCl + solid KSCN + acetone --> BLUE complex ........... Co2+

Procedure: Day 2: Working a Case numbered unknown · any subset of the 7

You now receive a sealed, numbered victim sample. It may contain all, some, or none of the seven cations, you do not know in advance. Run the identical scheme from Day 1 (Parts I–IV) on your unknown. A confirmation that produces no precipitate and no colour change is a valid negative: that ion is absent. Record carefully, especially at each confirmation step, and watch that you carry S1 and S3 forward without losing them.

Process notes: if no white precipitate forms with HCl in Part II, both Ag⁺ and Pb²⁺ are absent, but still run the hot-water and chromate steps to be sure. Keep every fraction until you have read its confirmation. A negative is only meaningful if the earlier separations were done correctly, so do not skip steps even when you expect "nothing."

Ion	Confirming test	Your observation	Present / Absent
NH₄⁺	6 M NaOH; NH₃ gas on red litmus
Ag⁺	HCl precipitate survives hot water
Pb²⁺	hot solution + 1 M K₂CrO₄
Fe³⁺	rust Fe(OH)₃; then HCl + 0.1 M KSCN
Al³⁺	reform Al(OH)₃; aluminon, warm
Ni²⁺	S3 + DMG, warm
Co²⁺	S3 + HCl + solid KSCN + acetone

Present?	Ion	The observation that confirms it
☐	NH₄⁺
☐	Ag⁺
☐	Pb²⁺
☐	Fe³⁺
☐	Al³⁺
☐	Ni²⁺
☐	Co²⁺

Analysis Questions show all work · balanced equations

Answer on your own paper / in your typed report. Every equilibrium argument needs a balanced equation and a Q-vs-Ksp or Le Châtelier sentence, not just "it dissolves."

Why HCl is selective. Of the seven cations, adding 6 M HCl precipitates only Ag⁺ and Pb²⁺. Explain in terms of Ksp and Q why the other five chlorides stay dissolved. Write the two precipitation equilibria.
Selective precipitation, quantitative. A test portion is 0.10 M in Ag⁺ and 0.10 M in Pb²⁺. Ksp(AgCl) = 1.8×10⁻¹⁰, Ksp(PbCl₂) = 1.7×10⁻⁵. (a) Find the [Cl⁻] at which each just begins to precipitate. (b) Which precipitates first? (c) Find [Ag⁺] still in solution at the moment PbCl₂ begins, is the separation essentially complete?
Hot water splits PbCl₂ from AgCl. Dissolving PbCl₂ is endothermic and its Ksp is ~10⁵ times larger than AgCl's. Use Le Châtelier (temperature) and the Ksp comparison to explain why hot water dissolves PbCl₂ but leaves AgCl solid.
Le Châtelier in the NH₄⁺ test. For NH₄⁺ + OH⁻ ⇌ NH₃(g) + H₂O, identify two things the procedure does that drive the equilibrium to the right. Why must you invert the litmus cover quickly and read within a few minutes?
Amphoterism. Write the equation for Al(OH)₃ dissolving in excess NaOH. Why does this single step separate Al³⁺ from Fe³⁺, when both first precipitated together as hydroxides?
The buffer's job. Fe(OH)₃ and Al(OH)₃ are precipitated using NH₃ plus NH₄Cl rather than NaOH alone. Explain how the NH₃/NH₄⁺ buffer and ammine-complex formation keep Ni²⁺ and Co²⁺ in solution while the two hydroxides drop out.
Confirmation equilibria. Write the balanced net-ionic / complex-ion equilibrium for each confirmation: PbCrO₄, FeSCN²⁺, Ni(DMG)₂, and [Co(SCN)₄]²⁻. For the cobalt test, explain how adding excess solid KSCN and then acetone uses Le Châtelier to produce the blue colour.
Common-ion wash. The P1 precipitate is washed with dilute HCl, not pure water. Use the common-ion effect to explain why pure water would cost you some of your AgCl/PbCl₂.
Reasoning about a negative (Day 2). Your unknown gives no red colour with KSCN at the iron step. Can you conclude Fe³⁺ is absent? State what must have been true at the earlier separation steps for that conclusion to be valid.
Your verdict (SP 6). For your Day-2 sample, write an evidence-based claim listing exactly which ions are present. For each one, cite the specific observation and the equation that confirms it. For any ion you marked absent, state the negative observation that rules it out.
Error analysis, a FALSE POSITIVE. Identify one procedural error that would make you wrongly report a cation as present when it is truly absent (e.g. carryover from an incompletely washed precipitate or decanted supernatant, or contaminated glassware between samples). Explain through the relevant Ksp / Le Châtelier equilibrium how that carryover produces a confirming colour or precipitate the sample should not give.
Error analysis, a FALSE NEGATIVE. Identify one procedural error that would make you miss a cation that is truly present (e.g. too little precipitant to drive Q > Ksp, not centrifuging or heating long enough, or losing precipitate when you decant). Explain through the same equilibrium ideas why the ion fails to give its confirmation even though it is in the sample.

Victim / Sample ID number	__________
Block / Date analyzed	__________

Lab · CSI: Chemistry Solubility Investigation