gravimetric determination of sulphate in fertiliser

[dinosaur93]

Could anyone help me go through the theory and significance behind each step of the procedure that contributes towards the aim in determining the amount of sufate that is present in a fertiliser?

Ba²⁺ + SO₄^2- ----> BaSO₄

1. Add 50 mL of de-ionise water unto a 100mL beaker and stir to dissolve as much of the ground fertilier as much as possible. FILTER the mixture into a 600mL beaker washing the residue using de-ionised water.

2.  Add 3 mL of 0.5 M of HCL and add more water so that the volume is about 200mL.

3. Boil the solution and add 15mL (or more) of BaCl drop-by-drop to the hot solution.

What would happen to the solution if BaCl is less that 15mL? Would it have any effect on the result's obtain, How?


4. Cool off the solution allowing the precipitate to settle. Add several drops of BaCl to ensure that no sulfate ions are present.

What would happen if the procedure was not tested with more BaCl solution?


5. Collect the precipitate through vacuum filtration. Wash any precipitate that remains in the beaker into the filter funnel and collect the last few drops of filtrate to test if chloride ions is present through dropping silver nitrate solution to the filtrate.

What would happen if the filtrate is not tsted with AgNO₃ when the solution is washed?
What is the main purpose of cleansing the beaker with de-ionised water so that no precipitate will be left behind? Will this have any effect on the results obtained, Why?


6. Place the filter paper in an oven heated to 100-110 C and record the mass when fully dried.

[Aurelian]

What would happen to the solution if BaCl is less that 15mL? Would it have any effect on the result's obtain, How?

You run the risk of not adding an excess of Ba²⁺ ions and, as a result, of failing to precipitate all the SO₄^2- ions. This will mean that your collected ultimate mass will be less than it should, resulting in you determining a lower amount of sulphate being in the fertilizer than is actually the case.

What would happen if the procedure was not tested with more BaCl solution?

This is done to ensure that an excess of Ba²⁺ ions has indeed been added (if more precipitate forms, clearly this is not the case). This is done for the same reasons as described in the answer to your previous question.

What would happen if the filtrate is not tsted with AgNO₃ when the solution is washed?
What is the main purpose of cleansing the beaker with de-ionised water so that no precipitate will be left behind? Will this have any effect on the results obtained, Why?

My presumption for why the filtrate is tested with AgNO₃ is to ensure that the chloride ions have not precipitated with anything else in the fertilizer, which would interfere with your ultimate result.

The main purpose of cleansing the beaker with deionized water is to collect all the precipitate, so that you can be sure it accounts for all the sulphate in the fertilizer. You don't want to leave any behind, otherwise you aren't getting the full story. Suppose a total of 2.00g of precipitate is formed, but 0.20g remains in the beaker, then you'll only end up weighing 1.80g of BaSO₄. This will result in a lower final calculated amount of sulphate in the fertilizer. I hope this point is fairly obvious.

Deionized water is used so as to not degrade or react with the newly-formed precipitate.

[nina_rox]

I have a few question on this, I'm doing the same prac.
Why are the elements nitrogen and sulfur included in lawn food? Apparently there is some
biology reason behind it?
and What distinguishes gravimetric analysis from other forms of analysis?

[oliverk94]

I have a few question on this, I'm doing the same prac.
Why are the elements nitrogen and sulfur included in lawn food? Apparently there is some
biology reason behind it?
and What distinguishes gravimetric analysis from other forms of analysis?

Gravimetric analysis involves finding the amount of a particular substance by forming a precipitate (solid).. while volumetric analysis is used to measure the unknown concentration of a solution by reacting it with a known concentration of a standard solution.

Correct me if I'm wrong guys

[nina_rox]

Thanks Oliver, I think that's right but there are other forms of analysis as well right?

[oliverk94]

You'll only need to use volumetric and gravimetric analysis for the SAC as far as I'm aware.
I'm doing this SAC soon as well.

[nina_rox]

You'll only need to use volumetric and gravimetric analysis for the SAC as far as I'm aware.
I'm doing this SAC soon as well.

Yeah thats true but I'm looking for something that is more broad since the question is 'from other forms of analysis' and not "from volumetric analysis.' Something that gravimetric has that other analysis' don't...? Use of precipitation maybe? :/

[|ll|lll|]

What would happen if the filtrate is not tsted with AgNO₃ when the solution is washed?
What is the main purpose of cleansing the beaker with de-ionised water so that no precipitate will be left behind? Will this have any effect on the results obtained, Why?

My presumption for why the filtrate is tested with AgNO₃ is to ensure that the chloride ions have not precipitated with anything else in the fertilizer, which would interfere with your ultimate result.

The main purpose of cleansing the beaker with deionized water is to collect all the precipitate, so that you can be sure it accounts for all the sulphate in the fertilizer. You don't want to leave any behind, otherwise you aren't getting the full story. Suppose a total of 2.00g of precipitate is formed, but 0.20g remains in the beaker, then you'll only end up weighing 1.80g of BaSO₄. This will result in a lower final calculated amount of sulphate in the fertilizer. I hope this point is fairly obvious.

Deionized water is used so as to not degrade or react with the newly-formed precipitate.

If required to add more solutions to the precipitate (so as to ensure spectator ions would not intefere), wouldn't the experiment be prone to more inaccuracies and errors?
Why isn't it precipitated with barium nitrate to begin with?

[thebiologybandit]

Why was the HCl added?

[dinosaur93]

anyone has practise papers regarding gravimetric analysis?

[jaydee]

HCl is added to react with the fertiliser so that when you add BaCl2 it won't form any other precipitates and interfere with the experiment?

[nina_rox]

5. Wash any precipitate that remains in the beaker into the filter funnel and collect the last few drops of filtrate to test if chloride ions is present through dropping silver nitrate solution to the filtrate.

I don't understand this step... What is the filtrate and where is the silver nitrate dropped into? On top of the precipitate or in the filter flask? Also I understand that silver ions with chloride ions will precipitate but we do this test until silver nitrate test comes out clear. So where has all the chloride ions gone?

Any help is much appreciated.

[nina_rox]

Also why does boiling the sample and reducing its volume affect calculations? Apparently there is no effect?