Mystery Precipitate in Prac to determine sulphate content in fertiliser sample (1 Viewer)

[annh]

Hey guys, I've just done my assessment today which was the gravimetric analysis to determine the sulphate content in the fertiliser sample. It was supposed to be straightforward: 1g of fertiliser dissolved in distilled water with 2MHCl and filtered to remove solids. Then 0.1Mbarium nitrate solution is added to form the white barium sulfate precipitate and that would have been filtered with ashless filter paper. The problem with this was, a brown precipitate had formed and we have no idea what it is. The solution had already been filtered and was clear. But it seems that upon heating, this mystery precipitate had formed. We suspect it's a fine substance that upon heating had coagulated but we have no idea what it is. What could it be?

HELP IS VERY MUCH APPRECIATED. =D The experiment is already botched, we have the wrong precipitate. I'm just not sure how to explain it.

 

[golgo13]

Umm just a question when u filtered the precipitate from solution the whole thing was white? (no contaminants nothing)
But when u heated it (i'm assuming this was done on the hot plate), there was brown precipitate.

From what i can see there is 2 possibilities. Either:
a)More likely it is contaminated
b)Iron precipitate form the colour description, but unlikely because u made the solution acidic, but it's always possible

Did you run the HCl into excess and were you allowed to do it more than once

 

[annh]

Umm just a question when u filtered the precipitate from solution the whole thing was white? (no contaminants nothing)
But when u heated it (i'm assuming this was done on the hot plate), there was brown precipitate.

From what i can see there is 2 possibilities. Either:
a)More likely it is contaminated
b)Iron precipitate form the colour description, but unlikely because u made the solution acidic, but it's always possible

Did you run the HCl into excess and were you allowed to do it more than once

um, I wouldn't say white, but it was a dark yellow solution. No visible impurities.
It was heated with a bunsen burner. and the HCl was meant to break down some existing bonds as later Ba(NO3)2 would be added in excess to produce barium sulfate precipitate

I did suspect contamination, but we another group which used the same fertiliser produced the same results. we also had another set up but with a larger mass of fertiliser and the brown precipitate was more obvious and occured before Ba(NO3)2 was added but after heating.

I suspect its some form of insoluble brown precipitate with a size as small or smaller than barium sulfate which upon heating had coagulated.

 

[golgo13]

The thing is the method looks fine to me, but now that you say it was a dark yellow solution i'm thinking there's a complex being formed. The thing when i remember doing it, the HCl is heated and put in, the barium nitrate is then put in. The solution is allowed to settle (in ice) allowing the precipitate to clump together. When you look at it should be a fairly clear solution with like whiteish dust at the bottom of the beaker. Filter that and solution is kind of milky in a way.

Still thinking it's not acidic enough maybe
And you have precipitate phosphate instead of sulphate,

As a side note i'm not sure how bunsen burner drying affects it since i did it via incubator so i'd assume that maybe of played some role

 

[annh]

The thing is the method looks fine to me, but now that you say it was a dark yellow solution i'm thinking there's a complex being formed. The thing when i remember doing it, the HCl is heated and put in, the barium nitrate is then put in. The solution is allowed to settle (in ice) allowing the precipitate to clump together. When you look at it should be a fairly clear solution with like whiteish dust at the bottom of the beaker. Filter that and solution is kind of milky in a way.

Still thinking it's not acidic enough maybe
And you have precipitate phosphate instead of sulphate,

As a side note i'm not sure how bunsen burner drying affects it since i did it via incubator so i'd assume that maybe of played some role

It was a clear solution and then brown precipitate just randomly formed. If we were in the 1700s, it'd be spontaneous generation, lol.
oh, and just to clarify, barium sulfate will only form precipitate in an acidic solution right? so, if it was made basic, nothing would happen?

 

[golgo13]

If it was basic from memory barium sulphate would form which is what you wanted to avoid, but brown precipitate is quite weird which i agree, but your meant to make the solution as a acidic as possible, cause i reviewed the method u outlines above and compared it to when i did it. Basically i added a second part of HCL after the initial filtration, which is why i have come up with the reasoning it might not be acidic enough and it has formed the unkown brown precipitate
Hope that logic makes sense

 

[barbernator]

It was a clear solution and then brown precipitate just randomly formed. If we were in the 1700s, it'd be spontaneous generation, lol.
oh, and just to clarify, barium sulfate will only form precipitate in an acidic solution right? so, if it was made basic, nothing would happen?

this isn't correct. The reason you add acid is to fully dissolve the fertiliser into solution. Barium sulfate precipitate will still form if the solution is slightly basic.

 

[annh]

this isn't correct. The reason you add acid is to fully dissolve the fertiliser into solution. Barium sulfate precipitate will still form if the solution is slightly basic.

Initially I had assumed that it was simply that, to aid in dissolving the fertiliser. However, I read on and there were little hints from my teacher that pH will affect precipitation. I guess HCl performs two roles in this experiment.

oh, and I never thought of adding HCl twice in the experiment. I guess it does make sense to do so.

 

[golgo13]

Most of this is in the textbook but not many people pay attention to it LOL, but yeh, i guess for actually looking for improvements and writing about it, improvements would be too add HCl in excess after initial filtration (preferably warmed) and use an incubator instead of a bunsen to dry (reduces contamination).

Hope this has helped in some way

 

[annh]

Most of this is in the textbook but not many people pay attention to it LOL, but yeh, i guess for actually looking for improvements and writing about it, improvements would be too add HCl in excess after initial filtration (preferably warmed) and use an incubator instead of a bunsen to dry (reduces contamination).

Hope this has helped in some way

Thanks
I've had so much to write about errors, limitations, improvements and further research that my discussion section of the prac is 1400 words long!