precipitation prac

[Courtney]

Hi all,

I'm having some trouble getting a precipitation prac to work and was wondering whether anyone had any ideas (my background is more human biology than chemistry).

They are doing a prac where they react Silver nitrate, lead nitrate and magnesium nitrate with potassium nitrate, chloride, iodide, sulphate and carbonate. I get all the right reactions occurring with everything but the silver nitrate with the potassium sulphate (should form a precipitate).

I'm told that prior my being here it worked with 0.1M solutions of each. I've tried making up fresh solutions of each and using a more conc (0.5M and 1M) solution of potassium sulphate but I get nothing. I'm hesitant to try increasing the strength of the silver nitrate as 1: we've never had it made up to anything stronger so I'm sure it should work and 2: it's so expensive.

I'm sure I'm probably just missing something really simple but at the moment I'm stumped so if anyone has any ideas I would be very appreciative.

[Courtney]

Just spoke to a friend doing his PHD in chemistry. Even though I was told they always used 0.1M it is utterly impossible that this ever worked. It will only work using 1M silver nitrate and potassium sulphate.

[Lyn]

I have just tried it with my chemicals and got no precipitate when adding 0.1M silver nitrate to 0.1M potassium sulfate. Sounds like you may have to make up a minute quantity of 1M for a teacher demonstration. I'ts always possible that it worked previously because the lab. tech used a 1M solution trying to please the teachers and didn't say anything. (Hang the expense!!!!)
Lyn.

P.S. Background more commonsense than anything and asking others when I don't understand. Still learning after 20 odd years and appreciating this website immensely.

[Courtney]

Thanks, I think the teacher demonstration idea is good. Now they will do the prac by doing all the various reactions and when that particular one doesn't work the teacher will tell the kids why and show them the demonstration with the stronger solutions.

You may well be right about the old lab tech, when I came into this job there were several bottles just labelled things like A or B with no explanation!

[Robb]

Hi All,

Hey Courtney,

Your friend is right, the Silver Nitrate needs to considerably higher in concentration as the Sulfate Radical is more stubborn to bond with the Silver Ions, this is why primarily Sulfate compounds are much more stable than Nitrates and Iodates, Iodides,Perchlorates, Dichromates etc. (Eg Silver Nitrate it light sensitive).

Right on the Money,

Cheers,

Robb.....

[RosalieM]

I think we usually use a 5% solution of Silver Nitrate.

[rosaliemb]

Am i stupid or something, but why must you get a precipitate of silver sulphate? An experiment is to record what happened.
Anyway text book Quest 4 says
"Compounds containing the SO₄ ion are soluble except for Ba, Pb and Ca.
Some compounds are slightly soluble. These include Ca(OH)₂, PbCl₂, PbBr₂,CaSO₄ and Ag₂SO₄"
Simply, because that's how I work, does this not mean Silver sulphate will not be expected to precipitate unless in high concentrations.
Rosalie