[crushedcorn]

I've been tasked with creating a quantitative analysis flow chart to separate the ions Ag⁺, Fe³⁺, Al³⁺, and Cu²⁺ from a compound using the results from preliminary tests with the probes NaOH, NH₃, H₂SO₄, and HCl that I obtained in the lab. I am working on the flow chart starting with one of my basic probes and I keep getting stuck. This is what I'm working with so far:

Ag⁺, Fe³⁺, Al³⁺, Cu²⁺

solution   precipitate

NaOH

Al(OH)₄-   Ag⁺, Fe³⁺, Cu²⁺

NH₃

Cu(NH₃)₃-, Ag(NH₃)₂-    Fe(OH)₃

H₂SO₄

Cu(SO₄)    Ag₂(SO₄)

Key:

• Next to the down arrow is the probe I would add to reach the next step of separation.
• All ions/compounds next to a left arrow remain in solution, all ions/compounds next to a right arrow form precipitates

Could I get some feedback on the following flow chart? Am I correct in assuming that if I add a probe to a precipitate that the precipitate will dissolve based on the probes I'm adding? It seems like it would be better to separate the ions in solution instead of from a precipitate but I'm not finding a way to do that without getting stuck with Al³⁺ and Cu²⁺ unable to be separate with the given probes. Are there any glaring errors or does it seem like I'm on the right track? Thank you!

[Arkcon]

You skipped one of your probe reagents.  Could that be the source of your problem? Try it and see.

[crushedcorn]

I don't have to use all of the probes. I just have to start with a basic one and use whatever combination of probes needed to separate the ions.

Could this be a better method?

Ag⁺, Fe³⁺, Al³⁺, Cu²⁺

solution   precipitate

NH₃

Ag⁺, Cu²⁺    Fe(OH)₃, Al(OH)₃

H₂SO₄            NaOH

Cu(SO₄)   Ag₂SO₄          Al(OH)₄-     Fe(OH)₃

[AWK]

Mass solubility of Ag₂SO₄ is at least 1000 times greater then that of AgCl.

[crushedcorn]

Are you saying that Ag₂SO₄(s) wouldn't be likely to form if I combined my Ag⁺ with H₂SO₄? I could use HCl as the probe instead of H₂SO₄ in that step so that AgCl(s) would form as my precipitate and CuCl₂ would remain in solution.

[Borek]

Are you saying that Ag₂SO₄(s) wouldn't be likely to form if I combined my Ag⁺ with H₂SO₄?

That's what he is saying. Not only is Ag₂SO₄ relatively well soluble (compared with other salts in the context), fact that sulfuric acid below pH 2 (more or less 0.007 M - quite diluted) is mostly protonated to the HSO₄^- form doesn't help.

[crushedcorn]

Thanks for the help, everyone. I completed my flow charts, performed my experiments on my unknowns, and am now writing equations for the reactions seen between my metal ions and NaOH, NH₃, HCl, and H₂SO₄. There is one equation that I'm having trouble with, so I'm turning to all of your wonderful brains again for some direction.

The reaction I'm struggling to express is between Ag+ and HCl. With the guidelines I'm given, the reactants would be Ag⁺ + H₃O⁺ + Cl^-. I believe that AgCl_(s) is formed, but that would mean that H₃O⁺ is untouched. I would remove it from my equation but was told that all acids should be expressed as H₃O⁺ and an anion and it seems strange that it wouldn't take part in this reaction.

I'd appreciate any nudges in the right direction. Thanks!

[Burner]

The reaction I'm struggling to express is between Ag+ and HCl. With the guidelines I'm given, the reactants would be Ag⁺ + H₃O⁺ + Cl^-. I believe that AgCl_(s) is formed, but that would mean that H₃O⁺ is untouched. I would remove it from my equation but was told that all acids should be expressed as H₃O⁺ and an anion and it seems strange that it wouldn't take part in this reaction.

I'd appreciate any nudges in the right direction. Thanks!

When writing ionic equations, we cancel out any spectator ions(ions that don't take part in the reaction) from both sides. Indeed, in ionic equations we need to write HCl in its ionised form initially, but the H₃O⁺ does not take part in the reaction and appears in the product side also, and finally cancelled out.

[Borek]

The reaction I'm struggling to express is between Ag+ and HCl. With the guidelines I'm given, the reactants would be Ag⁺ + H₃O⁺ + Cl^-. I believe that AgCl_(s) is formed, but that would mean that H₃O⁺ is untouched. I would remove it from my equation but was told that all acids should be expressed as H₃O⁺ and an anion and it seems strange that it wouldn't take part in this reaction.

This is probably more confusing than it should be, as the counteranion of Ag⁺ is not given. There is no such thing as isolated Ag⁺, so in the full equation there would be another anion - and it will be a spectator, just like H⁺ (or H₃O⁺ if you prefer it this way).

[AWK]

In the student qualitative analysis of cations practically only nitrates are used (exclusions - salts of Sn, Sb and As). For anions - potassium salts, and in a few cases - ammonium salts (in my lab only oxalate). The most difficult case is a solid mixture of up to 12 cations + anions (~40 possible).

[crushedcorn]

Ok, that makes sense. Ag⁺ was the only cation I had that reacted with HCl. It was part of AgNO₃. So if I were to write the ionic equation, it would be (unless indicated, all ions are aqueous):

Ag⁺ + NO_3- + H₃O⁺ + Cl^- = AgCl_(s) + H₂O_(l) + HNO₃

Right?

[AWK]

Right.

[Borek]

No, HNO₃ is still dissociated after the precipitation.

[AWK]

Correct to pain.
Frankly saying, if student wrote down shortcut
Ag⁺ + Cl^- = AgCl or AgCl(s)
reaction is fully accepted

[crushedcorn]

Ok. Thanks, you guys!