What is the most efficient way to seperate and obtain salt from a
sand/salt mixture. I need to be accurate to within a 100th of a gram.

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Request for Question Clarification by mvguy-ga on 09 May 2004 19:19 PDT

0.01 gram for how big of a quantity to start out with?

This is a common problem that is encountered in experimental chemistry
and the standard method of solving it is this:

1) Heat an amount of pure distilled water in a clean flask to boiling
point.  If the water is not pure the final result will be inaccurate.

2) Leave this water boiling for a minute or two.  Using a hot solvent
means that you will require less to dissolve the salt.

3) Place the salt/sand mixture onto a filter paper on top of a Buchner
flask.  You should know the weight of the dry Buchner Flask.

4) Attach a vacuum to the bottom of the Buchner flask to provide a
pressure to the filter paper.

5) Gradually pipette hot water onto the salt/sand filter paper.  How
much water you use at this stage will help determine the accuracy of
the result.

6) After the salt/sand mixture has been fully saturated carefully
remove the vacuum from the flask.  The solution in this flask contains
the majority of the salt.

7) Repeat steps 4,5 and 6 using a new Buchner flask and copious
amounts of water to ensure all the salt has been dissolved.

8) With the two flasks of salt water carefully transfer these to an
oven and bake them slowly rising the temperature to  200C.  Preferably
the oven should be attached to a vacuum to remove the water vapor as
it evaporates.  How long depends on the amount of water/solvent used. 
An oven is used as boiling the water may result in some loss of salt
when bubbles are formed.

9) Periodically stir the salt mixture as the water evaporates to
ensure all of the water is being evaporated.

10) After the water has evaporated crush the remaining salt in the
Buchner flask to a powder (if possible) and bake for an additional few
minutes to remove any residual water.

11) The amount of salt in your salt/sand mixture is the difference in
mass from the inital mass of the Buchner Flasks and when you weigh the
cooled Buchner flasks (remember to cool the flasks as the hot ones
will weigh more!).  Also remember that the mass obtained will also
have a small contribution from dissolved solids that were present in
the sand (unless the sand was sterile), this value will probably be
insignificant when compared to the mass of salt (especially if the
salt/sand mixture was obtained from the beach or sea).

Apparatus needed:
2x Buchner Flasks (masses known)
2x Filter papers
Drying oven (preferably vacuum capable)
Dropping pippette
Spatula (to mix and crush the salt as it evaporates in the oven)

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Request for Answer Clarification by jakeberv-ga on 10 May 2004 12:06 PDT

I will most likely be using less than 50g to start out with. If I do
not have a vacuum available, is there some other method that will
produce similarly accurate results in a relatively short period of
time?

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Request for Answer Clarification by jakeberv-ga on 10 May 2004 12:09 PDT

Also, I may not have an oven available either. Could this be done with a hot plate?

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Clarification of Answer by palitoy-ga on 10 May 2004 13:09 PDT

If you do not have a vacuum pump available you will need to proceed
under gravity, in other words just let the hot water drip through the
filter paper (do not fold the filter paper so many times as you might
normally to speed things up but beware that saturating the filter
paper may cause it to rip).  The vacuum pump sucks the solvent through
allowing the procedure to be quicker and therefore allows you to use
less solvent.  By proceeding under gravity the solvent will also cool
whilst the filtration takes place which means you should use more
washes of the salt/sand mixture in the second phase to ensure all the
salt is extracted successfully.

If you use a hot plate you will need to take more care upon the
evaporation.  Try not to boil the liquid to vigorously as this may
result in splashes and the loss of some of the salt.

Is the mixture wet or dry?

Answer: Add water and heat this mixture so that all the salt is
dissolved. Filter this heated mixture.
Sand will remain on filter paper/filtering agent. wash this sand with
some more quantity of water in batches. Dry the sand in sunlight or IR
lamp.
Salt is in hot water. Boil this to evaporate all the water. you will get salt only.

Good answer Ravi :)