[jubba]

I was performing a experiment on working out the sulfate content of fertiliser by dissolving it and precipitating the sulfate as barium sulfate and then filtering. My results were lower than the expected results. I used a sintered glass funnel and vaccum and heated the barium sulfate containing beaker before filtration but it cooled down whilst filtering. Barium sulfate passed through the filter.
 
Why does heating help reduce the barium sulfate passing through the filter and what are other reasons for the results being less and how do you think I can improve them. I've already tried using a higher grade sintered glass funnel but it didn't help that much.

Thanks
Jubba

[Borek]

Why does heating help reduce the barium sulfate passing through the filter and what are other reasons for the results being less and how do you think I can improve them. I've already tried using a higher grade sintered glass funnel but it didn't help that much.

Not sure how this process is called in English - precipitate aging? Small crystals get dissolved, large ones grow.

Obvious advices that you are probably aware off: try to cool solution down before filtering. Use excess barium chloride. Don't let your solution to be too acidic, as SO₄^2- gets protonated and BaSO₄ solubility grows.

Other reasons for low results: coprecipitation (when part of the barium in precipitate is replaced by lighter cations), reduction of sulfate during roasting (carbon from cellulose filter paper can reduce sulfate). Not sure if you used filter paper or not, and whether you dried or roasted your precipitate.

http://www.csudh.edu/oliver/che230/labmanual/gravsulf.htm

[jubba]

you

Obvious advices that you are probably aware off: try to cool solution down before filtering.

what your saying theoretically makes sense but the filtering works better at higher temperatures. Once it is cooled basically all the precipitate passes through. I don't understand why this happens.

excess barium chloride was used, so wouldn't coprecipitation increase the weight in this case?

And we didn't use filter paper, just a sintered glass funnel.

[Alberto_Kravina]

My holy textbook says that if it is possible you should leave the crystals grow overnight and filtrate the day after....

[Borek]

Have you aged your precipitate? It should stay at higher temperature (80/90 deg C) for at least an hour before filtering.

I was taught to filter BaSO₄ on paper and then to roast solid, which can't be done on sintered glass  

Is there iron in your sample? If so, you may try to reduce it to Fe²⁺ before precipitation, to lower coprecipitation problems. Adding some EDTA may help to mask coprecipitating cations as well.

Hmm, Alberto posted about aging in the meantime... that happens when you must answer some calls while editing post

[jubba]

thanks guys i shall try that

but does coprecipitating would increase the mass, right ? just checking

[Alberto_Kravina]

but does coprecipitating would increase the mass, right ? just checking

Sure

Other interferences that could increase the precipitated mass could be occlusion and adsorption.

[Borek]

thanks guys i shall try that

but does coprecipitating would increase the mass, right ? just checking

Sorry, nomenclature problem. I was not really meaning coprecipitation. Coprecipitation increases mass of solid, but if some other cation replaces Ba²⁺ precipitate can be lighter. This second effect can be present here and I was all the time referring to it.