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August 12, 2020, 04:56:09 pm

Author Topic: Adding HCl in gravimetric analysis  (Read 11633 times)  Share 

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Christiano

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Adding HCl in gravimetric analysis
« on: March 10, 2011, 10:00:18 pm »
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Why is HCl added and boiled to the filtrate when finding the % of sulfate in lawn fertiliser?
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pi

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Re: Adding HCl in gravimetric analysis
« Reply #1 on: March 10, 2011, 10:01:35 pm »
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Why is HCl added and boiled to the filtrate when finding the % of sulfate in lawn fertiliser?

It helps with the 'clumping' of the solid parts, making it easier to filter

Aurelian

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Re: Adding HCl in gravimetric analysis
« Reply #2 on: March 12, 2011, 11:02:22 am »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
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Re: Adding HCl in gravimetric analysis
« Reply #3 on: March 12, 2011, 11:55:59 am »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
yep - acidifying the solution means that other things (e.g. carbonates (e.g. BaCO3)) will not precipitate and therefore won't affect the mass.
and also yep - heating it helps the precipitate go from colloidal (dispersed throughout solution) to crystalline (clumped together) to make filtration more effective and accurate.
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luffy

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Re: Adding HCl in gravimetric analysis
« Reply #4 on: March 12, 2011, 01:08:54 pm »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
yep - acidifying the solution means that other things (e.g. carbonates (e.g. BaCO3)) will not precipitate and therefore won't affect the mass.
and also yep - heating it helps the precipitate go from colloidal (dispersed throughout solution) to crystalline (clumped together) to make filtration more effective and accurate.


Sorry, I don't know if this is a very basic question; but, why does heating it help the precipitate become crystalline?

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Re: Adding HCl in gravimetric analysis
« Reply #5 on: March 12, 2011, 02:30:14 pm »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
yep - acidifying the solution means that other things (e.g. carbonates (e.g. BaCO3)) will not precipitate and therefore won't affect the mass.
and also yep - heating it helps the precipitate go from colloidal (dispersed throughout solution) to crystalline (clumped together) to make filtration more effective and accurate.


Sorry, I don't know if this is a very basic question; but, why does heating it help the precipitate become crystalline?

This is by no means a VCE-level concept :P

Suspended colloid is a metastable phase. Crystalline solid is the lowest energy phase. The transition between colloid and crystal has a potential barrier, that is it requires energy to go from colloid to crystal, because the intermediate between the two phases is not stable. As such, heating it speeds up the process of colloid -> crystal by providing energy. You should note that in ordinary cases this transition is spontaneous, i.e. you can do nothing and it will just happen over a long time. By heating you are making this process much faster. [In some cases though you have to heat it for this to happen due to equilibrium effects and such, and in some cases even heating won't change anything. Colloidal science is a big study in chemistry]
« Last Edit: March 12, 2011, 02:33:17 pm by Mao »
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luffy

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Re: Adding HCl in gravimetric analysis
« Reply #6 on: March 12, 2011, 02:46:13 pm »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
yep - acidifying the solution means that other things (e.g. carbonates (e.g. BaCO3)) will not precipitate and therefore won't affect the mass.
and also yep - heating it helps the precipitate go from colloidal (dispersed throughout solution) to crystalline (clumped together) to make filtration more effective and accurate.


Sorry, I don't know if this is a very basic question; but, why does heating it help the precipitate become crystalline?

This is by no means a VCE-level concept :P

Suspended colloid is a metastable phase. Crystalline solid is the lowest energy phase. The transition between colloid and crystal has a potential barrier, that is it requires energy to go from colloid to crystal, because the intermediate between the two phases is not stable. As such, heating it speeds up the process of colloid -> crystal by providing energy. You should note that in ordinary cases this transition is spontaneous, i.e. you can do nothing and it will just happen over a long time. By heating you are making this process much faster. [In some cases though you have to heat it for this to happen due to equilibrium effects and such, and in some cases even heating won't change anything. Colloidal science is a big study in chemistry]

Wow - That actually sounds interesting Haha.

I was just confused because I thought it had something to do with the solubility curve (i.e. heating increases solubility), which would make it less crystalline. I don't know what I was thinking. :P

Mao

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Re: Adding HCl in gravimetric analysis
« Reply #7 on: March 12, 2011, 02:49:56 pm »
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Acidifying the solution also helps to prevent other unwanted things precipitating (or, at least, so we were told when we did this in class).
yep - acidifying the solution means that other things (e.g. carbonates (e.g. BaCO3)) will not precipitate and therefore won't affect the mass.
and also yep - heating it helps the precipitate go from colloidal (dispersed throughout solution) to crystalline (clumped together) to make filtration more effective and accurate.


Sorry, I don't know if this is a very basic question; but, why does heating it help the precipitate become crystalline?

This is by no means a VCE-level concept :P

Suspended colloid is a metastable phase. Crystalline solid is the lowest energy phase. The transition between colloid and crystal has a potential barrier, that is it requires energy to go from colloid to crystal, because the intermediate between the two phases is not stable. As such, heating it speeds up the process of colloid -> crystal by providing energy. You should note that in ordinary cases this transition is spontaneous, i.e. you can do nothing and it will just happen over a long time. By heating you are making this process much faster. [In some cases though you have to heat it for this to happen due to equilibrium effects and such, and in some cases even heating won't change anything. Colloidal science is a big study in chemistry]

Wow - That actually sounds interesting Haha.

I was just confused because I thought it had something to do with the solubility curve (i.e. heating increases solubility), which would make it less crystalline. I don't know what I was thinking. :P

Actually, you are correct. What you have to realise is colloidal is suspended solids (like small floaties you can't get rid of). To force them to become part of a crystal, you need them to dissolve first. Thus why we heat it. When we eventually cool it down the crystal will naturally form.

Colloidal --(heat)--> dissolved --(cool down eventually)--> crystalline

The actual process is a bit more involved, but on a microscopic scale that is what actually happens.
« Last Edit: March 12, 2011, 02:51:54 pm by Mao »
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