Why dissolve potassium sulphate in hydrochloric acid before adding barium chloride in a gravimetric analysis?

[SkaletteRose]

In labs today we analyzed sulphate ion content in a sample of K2SO4 by first dissolving in HCl and then reacting the solution with Barium chloride. Potassium sulphate dissolves in water, and the reaction for the displacement is K2SO4 + BaCl2----> BaSO4 + KCl. So why use HCl in the first place? Is there an intermediary step in the reaction I'm missing, or is it simply to dissolve the potassium sulphate? I have to write this up, and as it's not part of the reaction (as far as I know) I can't work it out. Assistance appreciated!

[gingejbee]

I think this may be a bit specialized for people on here at the moment - I can't think why hydrochloric acid would be needed as a solvent myself, and I did Molecular Sciences at University! I'd like to know the answer myself.

[squarebear]

Mollykins will probably know.

[gingejbee]

Oh , thinking about it a bit more - doesn't barium sulphate only form a good precipitate in acidic solutions?

[jomifl]

Is it to increase the ionisation of the potassium sulphate and speed up the reaction?

[gingejbee]

Yes, think that's it - either dissolve it in the acid first or acidify after mixing the two solutions . Otherwise, you won't get a good colloidal precipitate that will filter off properly!

[SkaletteRose]

Thanks for your help, is there another answers site which is more specialised? The acidity is a good idea, do you know in more detail why acidity would cause the precipitate to form larger crystals?

[gingejbee]

Sorry, but I surprised myself at remembering this much - A level Chemistry was some time ago! Don't know why it does but Ba2SO4 is a fine precipitate until acidified.

[Teddio]

The presence of hydrogen ions (from the hydrochloric acid) will get rid of any carbonate or sulphite ions which may be present. These will produce a precipitate with bariun chloride.

[Teddio]

Sorry a typo, meant barium chloride!

[theprof]

OK time for a definitive answer from a specialist. I hope you'll accept it as I'm a mere university Science Dean at a Russell Group university right now and hold Biochemistry Chairs in the UK and USA. It's quite a few years now since I was a Chemistry Reader at a university down the road but I'll give it my best shot. Hope it's good enough!

The K2SO4 solution is acidified with HCL for two reasons:

The first is that BaSO4 tends to crystallise in very tiny particles that have a tendency to occlude the formation of impurities. The acidification of the solution allows larger crystals of BaSO4 to form unhindered.

The second reason is that you have to remember that barium ions will precipitate other anions such as carbonates present in the K2SO4 as impurities. Acidifying the solution will largely prevent the precipitation of any carbonates, sulphites (as Teddio mentions) etc present.

[gingejbee]

Prof: thanks for that (and it wasn't my question!). Don't think I did so badly, considering.....

[theprof]

gingejbee, you were on the right track and I'm sure you'd have got there in the end with some prompting. As a result, I won't be sending you to the back of the class today!