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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Blair1994 on April 04, 2011, 07:27:49 am

Title: How would you minimise these gravimetric analysis errors?
Post by: Blair1994 on April 04, 2011, 07:27:49 am

- The precipitate not being completely dry
- Not all of the insoluble parts of the sample being analysed are removed (not sure if you can minimise..)
- Incomplete precipitation reaction

If you guys cant think of any for these ones, could you please suggest an error which I would be able to explain how to minimise it, as i need to list both. Thanks :)

Title: Re: How would you minimise these gravimetric analysis errors?
Post by: TortMequila on April 04, 2011, 09:47:14 am

Quote from: Blair1994 on April 04, 2011, 07:27:49 am

- The precipitate not being completely dry
- Not all of the insoluble parts of the sample being analysed are removed (not sure if you can minimise..)
- Incomplete precipitation reaction

If you guys cant think of any for these ones, could you please suggest an error which I would be able to explain how to minimise it, as i need to list both. Thanks :)

These errors can be minimised via the following methods:

- Putting the precipitate in volcanic heat
- The insoluble parts can be removed via filtration
- The precipitation reaction can be accounted for via the usage of excess reactants

Title: Re: How would you minimise these gravimetric analysis errors?
Post by: azn_dj on April 04, 2011, 03:04:56 pm

The precipitate not being completely dry:
- heat it for longer to evaporate water. Or in a dessicator.
- If it is absorbing moisture from the atmosphere,
a) Gravimetric is the wrong technique to use, OR
b) You should get the hell out of Queensland.

Not all of the insoluble parts of the sample being analysed are removed
- Be careful when you filter it. Maybe two lots of filter paper of bigger filter paper would help. This is the sort've error that sounds like the paper has been pierced or something.

Incomplete precipitation reaction
- Use an excess of one reactant
- Solution may be saturated (as in, you don't have enough water). Dilute it.
- Reaction may take time. Be patient.
Worst case is that the reaction does not actually go to completion. If this is the case, you would have to filter it and add excess of one reactant that is soluble, so you can increase the amount that will react.

Title: Re: How would you minimise these gravimetric analysis errors?
Post by: schnappy on April 04, 2011, 03:46:13 pm

Using higher heat can cause the sample to decompose... that's definitely not good.

For precipitation, as above, something should be in excess. Whatever you aren't trying to find the content of should be reactant not in excess.

Title: Re: How would you minimise these gravimetric analysis errors?
Post by: VCEMan94 on April 04, 2011, 05:18:57 pm

what about these errors:
-precipitate not being pure e.g. other ions (present in the precipitate increasing the weight of the precipitate)
- in an experiment where BaSO4 is used and some of it isnt dissolved in the wash due to the 'low solubility' of BaSO4 - resulting in a lower weight, decreasing the percentage by mass of the precipitate.
-impurities in the sample