Precipitation of barium sulfate?

[ASidd]

Precipitation of barium sulfate??

We had to determine the content of sulfate found in lawn food for a class prac.

Anyway what we did was to dissolve the fertilizer in HCl and then add barium cholride to it in order to precipitate sulfate as barium sulfate. We later determined its concentration using gravimetric analysis.

The results of my group was substantially different to the actual amount of sulfate present in the fertilizer. Our result was about 2% less than the actual value.

Now I am trying to find some possible sources of error in the prac. I know that we spilled a couple of drops of the mixture but it was not that much. I was wondering if somehow impurities can impact upon the mass being less than it's supposed to be? I know impurities usually cause mass to be larger but I read somewhere that occulsions can causesul mass to be smaller than the actual value?

Is this true?

 

[ASidd]

Also can someone tell me what is the meaning of washing the precipitate. Like with such a fine solid like Barium sulfate how would you wash it?

 

[Borek]

Was all fertilizer dissolved, or was there some solid residue?

Have you aged the precipitate before filtering?

Most impurities will mean your final result would be too high, not too low.

 

[ASidd]

No all of the fertilizer had definitely dissolved

We did let the precipitate sit for about 48 hours but what happened was that some of it got stuck to the bottom so we had to wash it with about 2 bottles of DE-ionized water in order to make it unstuck. So I am wondering if some of the precipitate peptized upon adding the huge amounts of de-ionised water?
Although when we filtered the solution the filtrate didn't look cloudy but I am also thinking if some of the precipitate went through without the filtrate becoming cloudy?

 

[Borek]

If you wash with copious amounts of water yo always risk dissolving some of the precipitate. It is better to wash with solution containing ion used for precipitation.

 

[ASidd]

But barium sulfate is insoluble in water?

 

[Borek]

There is no such thing as "insoluble". There are varying degrees of solubility.

Google for "solubility product".