Fertilizer Prac (1 Viewer)

[brollyman_27]

Hey all
I need a bit of help in writing up a first hand invetigation to determine the the sulfate content of a lawn fertilizer then after i have perfomed this i need to measure and calculate the % of sulfate in the fertilizer.
I know this is very similar to a previous post but that post did not help me very much, any help at all would be greatly appreciated.
Thanks

 

[Jumbo Cactuar]

I'd do a Br2+ titration, but this is all off the top of my head so I'm not too sure about any problems you come into.

DISSOLVE IT HOWEVER POSSIBLE (if issued a solution skip to TITRATION)

Accurately weigh approx. 2.0g of fertiliser and transfer to a beaker with 50mL of water and about 2mL 1M HCl.

Heat slightly (not boil) and agitate with stiring rod for 10 mins.

Filter solution if full of crud and wash filter paper with additional 10 mL water.

Transfer solution to 250mL volumetric flask and make up to the mark.

TITRATION

In conical flasks combine 25mL of fertiler solution combined with 2.0 mL potassium chromate (K2CrO4). Titrate with a standard of known BaCl2, barium acetate, BaBr2, BaI2 or Ba(NO3)2 (all white solids and very soluble). A white precipitate should form, then when a yellow precipitate forms, that is the end-point.

Be sure to not make too much barium chromate (yellow precipitate) as it is carcinogenic.

I am going to try this in my lab today, as it is all off the top of my head really. Expect a serious edit tomorrow

 

[bEAbEA]

if you have conquering chem, look at page 212

 

[brollyman_27]

Thanks alot for your help this is very useful but i only have 1 gram of fertilizer to use,
but once agin thanks for your help, i cant wait to hear your reply tommorow

 

[Templar]

You could do a direct precipitation, filter and measure the mass of the solid and use that to calculate sulfate content, but it does require more than your average filter paper.

 

[Jumbo Cactuar]

You could do a direct precipitation, filter and measure the mass of the solid and use that to calculate sulfate content, but it does require more than your average filter paper.

I was filtering proteins today .. OMG it took 40 mins to filter

Will post tomorrow with thoughts on this prac.

 

[Jumbo Cactuar]

OK, that didn't work because chromate reacts first and the change is subtle...

Any way this is an adaptation of another experiment so it should work (though it is a bit long winded)!

btw filtration and dessication is boring but plausible (where's the pretty colours I ask?!!!?).
_______________________________

REAGENTS
Fertiliser Unknown
10% BaCl₂ solution (roughly 0.5M) <--doesn't matter what concentration
Solid Na₂CO₃
0.1M HCl

DISSOLVE IT

Accurately weigh all of fertiliser (on watchglass) and transfer to a 250mL beaker with 40mL of water (use a washbottle to get it all off) and NO HCl. (sulphate in fertilizer is meant to be soluble or else what is the point).

If it doesn't want to dissolve stir it with a glass stirring rod and heat slightly (though don't waste time unless you have to).

Filter solution if neccessary.

Transfer solution to 100mL volumetric flask and make up to the mark.

STANDARDISATION

You are going to need standardised Ba²⁺ as well as standardised HCl. Chances are you will have BaCl₂ solution which isn't standardised, but if you have solid or already standardised you are very lucky.

You'll need to make a standard of Na₂CO₃ by weighing solid (2.0g) and making it up in a volumetric flask (250mL).

Make roughly 300mL 0.05M BaCl₂ in a beaker.

To standardise HCl take a large beakerfull, fill a burette with it. Pipette 20 mL Na₂CO₃ solution into conical flask and add two drops bromocresol green indicator (solution should be strongish blue). Titrate until green is observed.
moles HCl = 0.5 * moles Na₂CO₃

To standardise BaCl₂ pipette 5mL into conical flask and pipette 20 mL Na₂CO₃ solution into conical flask and add two drops bromocresol green indicator (solution should be mily blue). Titrate until milky green is observed.
moles HCl = 0.5 * ( moles Na₂CO₃ - moles BaCl₂)
moles BaCl₂ = moles Na₂CO₃ - 2 * moles HCl

TITRATION

In conical flasks pipette 20mL of fertiler solution and pipette 5mL aliquots of BaCl₂ until no more precipitate forms (Then directly pipette an equivelent amount to others). pipette 20 mL Na₂CO₃ solution into conical flasks and add two drops bromocresol green indicator (solution should be mily blue). Titrate until milky green is observed.
moles BaCl₂ = moles Na₂CO₃ - 2 * moles HCl - moles SO₄^2-
moles SO₄^2- = moles Na₂CO₃ - 2 * moles HCl - moles BaCl₂



ISSUES

Could be ammonium sulphate fertiliser. This means the pH becomes an issue. However at pH 5 (our end point) nearly all ammonia will be ammonium again so there won't be significant error.

Sodium carbonate draws water out of the atmosphere. Normally analytical carbonate is roasted (250C) for several before use, then allowed to cool in a dessicator. Once dissolved there isn't a problem.

If you want to do this prac I'd seriously talk to my teacher about practicality.

As you can see this method is LONG. Check up on how conquering chemistry does it, I don't have that text book. I'd be greatful to see how they do it.

 

[Jumbo Cactuar]

Any suggestions from the floor welcome --- I am far from a perfect chemist

 

[Templar]

I don't think any school will attempt this on their own.

I was filtering proteins today .. OMG it took 40 mins to filter

To filter out barium sulfate precipitate using a silicon matrix filter will take a few hours at least...when my prac group did the it we had to cheat at the end by pour out some of the solution without filtering. Not best lab procedures, but the group got the best results for that prac (if not the only result in that session).

 

[Jumbo Cactuar]

It is an investment of time as much as anything. I reckon it'd take about 3 hours (if Na2CO3 is ready to go).

Barium can be replaced by strontium or radium at a pinch, but not calcium, beryllium or magnesium.

I can't think of another way that is within your means to do that doesn't involve filtration.

Personally I'd be straight to a mass spectrometer

On the subject of BaCO₃, it is a big source Ba. The mineral it baked/roasted in really hot temperatures to make BaO. BaO crystals are used in some dessicators since it super hydrophilic (forms Ba(OH)₂.8H₂O).