Gravimetric SAC tomorrow!(URGENT)

[Bozo]

Its a gravimetric analysis finding the proportion of sulfate in a brand of commercial fertilizer.
BaCl2 is added after filterting etc, to form the precipitate. You guys get the gist of it.

I need to know;

Discuss the discrepancies between your % and manufacturers %.

Possible answers for if my percentage is


HIGHER and






LOWER



Thanks in Advance!

[Water]

Higher:

- Not all of Water has not been dried up in the precipitate
- Did not remove the insolubles prior to Prac
- Weighing of sample (Our School, weighers are dodgy, so we have to remember, the weighing machine that we use, or else diff mass)


Lower
- Not enough precipitating agent
- Loss of sample during the process of the pract
- Heating of Reaction, when cooling, did not let it cool properly, which would allow for more precipitate (solubility is higher at higher temperatures )

[meli001]

Hi Bozo (LOL!)

I did the same SAC just two weeks ago, it's quite likely that you'll get a % lower than the manufacturer's claim but anyways, just in case ^^

Higher:
•The precipitate was not washed thoroughly (still traces of Cl- ions) leading there to be a higher % than the expected value. If I can remember correctly, you’re expected to test for chloride ions with AgNO3.
•The precipitate was not dried thoroughly and therefore there would be a higher % than the expected value.
•Residue not all filtered

Lower:
•Maximum amount of sulphate ions not obtained during diluting process
•Loss of precipitate whilst washing it
•Barium chloride was not added in excess (this is a really unlikely one seeing as the prac will give you a reasonable amount to ensure that it is in excess)
•Not all the precipitate was obtained


It's also quite important to note that the manufacturer's claim is mean % of the sulfate.

Good Luck!

[pi]

I also did this prac, everyone got a lower percentage.

If you are using a Buchner vacuum funnel to filter, then there will be the error that very small particles of BaSo4 will pass through the filter paper, hence decreasing measured mass and hence, calculated percentage.

When transferring solutions from one beaker to a measuring cylinder (for example), there will always be loss of sulfate ions, even after many washes, hence decreasing mass.

A systematic error in the weighing device could move your percentage either way (but will still be lower due to other more significant errors)

[Bozo]

If you are using a Buchner vacuum funnel to filter, then there will be the error that very small particles of BaSo4 will pass through the filter paper, hence decreasing measured mass and hence, calculated percentage.

Just on that, i didn't use filter paper in that step i actually used a crucible, would that matter.

And also would another reason BOTH ways be that the fertilizer is a non-homogeneous mixture meaning one sample of fertilizer can vary in SO4 ions to another sample.

[pi]

And also would another reason BOTH ways be that the fertilizer is a non-homogeneous mixture meaning one sample of fertilizer can vary in SO4 ions to another sample.

I should have mentioned that (as I wrote it in my prac report), my bad, forgot. But that is definitely a valid error.

Just on that, i didn't use filter paper in that step i actually used a crucible, would that matter.

I'm not sure on this one...

[Bozo]

Last question i'm not to sure about, its something like this....

Fertilizer contains N P and K. This is in the form of (NH4)2SO4, Ca(H2PO4)2 and K2SO4. Could  other elements in this fertilizer be analysed by gravimetric analysis? If so, what solution would be added to carry out a precipitation reaction?

[pi]

You could add Na2SO4 to precipitate the Ca 2+ ions?

[Bozo]

I don't get it though, what am i supposed to do

[pi]

Isolate Ca 2+ ions (in a similar way to the sulfate ions) and then precipitate it with sulfate ions from Na2SO4 solution (acts like BaCl2, with sulfate ions)

Although an ion other than sulfate would be better to use, but I just don't know my solubilities well enough...

EDIT: I'm not sure if you need to precipitate the N, P and K ions though... I don't know the solubilities for H2PO4

[Bozo]

BUT in the chem text book page 22 it says that only P can preciptate and be used for gravimetric analysis.

[Bozo]

Wait can anyone clarify this,

So....
Ca(H2P04)2 can be analysed by gravimetric analysis because it forms a precipitate when combined with another reactant(BaCl2)

[Blair1994]

And also would another reason BOTH ways be that the fertilizer is a non-homogeneous mixture meaning one sample of fertilizer can vary in SO4 ions to another sample.

how would you be able to minimise this?