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Hi,

We are doing the HSC ion testing prac and we cannot get CaSO4 to ppt. We have tried a range of concentrations of cation and anion (0.1-1M).

The flame test works fine and the BaSO4 precipitates just fine...

Is there a trick to precipitating calcium sulfate?

Calcium sulfate is more soluble than BaSO4, could that be the problem?
Lada

hi
go to hsc site, you can find what do you need
http://hsc.csu.edu.au/chemistry/core/mo ... .html#net1
scroll down to flow chart all you need is there.
I do have exam paper with this prac. if you need it I can send it to you

What Iron II would people use in Precipitating test?

Thanks Jazz,

We are doing exactly what this site said and we are still not getting the results we are meant to be getting......

Hi all,
Had a lok over the Cation and Anion Testing prac, and according to my notes, I have Ba++ forming a ppte with Sodium Sulfate, but Ca++ doesn't. And Ca++ forms ppte with Sodium Hydroxide, but Ba++ doesn't. I did fiddle with concentrations until I got the "right" result!
For the IronII, we used Iron II Sulfate, but it must be fresh daily.
Hope this helps.
Ellice.

Hi All,

The pKa values for CaSO4 is very high, this means we need another chemical that has a pKa value higher. This is not the case for most available chemicals. What I am trying to say is that each recipitation constant needs to be higher than that of the other at the point of which a precipitate forms. Not just that one is higher than the other. (it's constant must support precipitation).

I hope this does not confuse you....

Cheers,

Robb.....

Thanks Ellice, Iron II Surface, thats just what we have been useing.

try putting a Iron nail or two in reagent bottle of Iron ll Sulfate this keeps it as Iron ll
If left without nail, it reverts to Iron lll

Hi Ocker,
I will try your suggestion and add a nail to one of my bottles. When I make up Fe2+ the brown Fe3+ colouration appears within minutes (probably cheap chemical!). I add just enough 1M H2SO4 to reduce it back and it remains in the 2+ state. Because it doesn't need a lot of acid there isn't a problem with it reacting with carbonates to form CO2 when performing precipitation reactions.
Cheers
Fran

Acidifing turns it green, but can affect reactions.

Forgot to mention! my reagent bottle looks Brown as xxxx but when you tip it sideways liquid inside is green

Let me see if I have this all right. By putting a iron nail (New) of course in a bottles of solution of Iron II Sulfate, it will keep if much fresher, and stop it going to Iron III Sulfate, is that correct??

I only make up Iron II Sulfate when it is ordered, have a label on the bottle make up fresh.

You Got it!

I have been sublimely ignorant.. here I was thinking that the teachers actually KNEW everything they had to teach .. and here it is given to them step by step... oh the magic is now tarnished!

Thanks for the information below Ocker:

"Acidifing turns it green, but can affect reactions."

Using acidified FeSO4 we get all the right results with the solutions we use for precipitation reactions here, but I'd like to know what reactions can be affected because - surprise, surprise - teachers sometimes change their requirements.

Cheers
Fran

Before I learnt about the nail trick, I use to acidify it also
Then teachers complained because they didn't get the desired reaction and quized me to find out if I had made fresh!
When I told them about acidifying, they blamed me for the bad results.
So after some research, I found out about the nail trick, and had no troubles since