Confusion in gravimetric analysis

[Martoman]

We want to determine the sulfate S04 (2-) content of a certain fertilizer. It is weighed, filtered and diluted with water to 200ml. Now we want to precipitate the sulfate as barium sulphate BaS04.

HOWEVER, to do this we add HCL to the diluted fertilizer sample. Then heat it to boiling point. After this, BaCl is titrated into it. The barium ions then supposedly bond to the sulfate ions to form the precipitate. I don't understand why the HCL is needed here. Does it form the basis of a catalyst or is it just a spectator in itself?

[longy1991]

LOL, mhs man? this part of the prac killed me as well till i found out. The fertiliser solution will actually contain various ions like phosphate, carbonates etc. that are soluble but will precipitate out of solution when Barium ions are added. The HCl reacts with these ions so they will not precipitate out when BaCl is added. I'll give you the actual reactions if you want them, but they are not really all that relevant. And don't worry, on the actual sac we did not need to know this piece of info.

[Martoman]

Ahhhh nice. I did know that there were others there but I get it now. Thank you so much. What is mhs by the way?

[Martoman]

cl ions are spectators here.

So the overall ionic equation is Ba2+ + S042- => baS04

[vexx]

Ohh wait.. they are spectators because the solution is aqueous yeah?

i hope i didn't bump in and answer the question to something you didn't want but..
the spectator ion (Cl) above does not play any part in the formation of the precipitate so it is not included in the ionic equation...

[longy1991]

As BaCl is an ionic compound, it will ionise in solution. Technically speaking, water can "break up" the barium ions from the chlorine ions, but how this is done is beyond vce. Then there will be Sulfate ions and Barium ion floating around in the solution, among other things. Now all these ions are running around the solution bumping into each other, but as most are soluble, the ionic compounds that are formed when the ions bump together will simply ionise again. However, when sulfate ions and barium ions bump into each other they will form a solid insoluble precipitate, and will not ionise.

I hope i haven't misunderstood what you asked dude..

[superflya]

hey do any of u have the info for this prac (disscussion questions, procedure etc) I think we may be doing this soon.

[superflya]

I'm doing it this weekend - I'll post my prac by Sunday if it's ready you can only have it if you look through and pick up any mistakes though!

lol sweet