I figured that making Salt from seawater was a simple process of boiling down a saucepan of seawater, then adding more seawater and boiling down, then
drying out the precipitate. As usual a "But" appeared in my methodology:
Studying the information on chemical composition and saline composition of common seawater (with variances of course) I read that the order of
precipitation went thus:
--CaCo3 first precipitate making up the first 50 to 90%
--Gypsom (CaSo4) second making up 20 to 50%
--Then comes the NaCl at around 1 to 20%
--and lastly what you do not want are the sulphates and potassium and magnesium precipitated.
So a little confused on this whole idea of boiling the water in the first place. I read that in the preparation of sea salt the seawater is passed
through several separate stages of precipitation to rid the product of evrything except the NaCl stage of course, . If this is the case the boiling
or as in the old days drying in the sun would mean that all these ingredients were in the salt and were fit for human consumption.
[Edited on 5-5-2012 by CHRIS25]bbartlog - 5-5-2012 at 06:59
Well, if you can tell us where you read the above (completely bogus) information, that would be interesting. I could possibly believe that CaCO3 would
precipitate first... but it would be a tiny fraction of the salt(s) contained in seawater, not '50 to 90%'.
See http://en.wikipedia.org/wiki/Seawater . I know wikipedia isn't a legitimate reference per se, but the information there is certainly better than
what you give above. NaCl and MgCl are going to be the main constituents of sea salt. watson.fawkes - 5-5-2012 at 07:31
The percentages above are those of water volume remaining when
precipitation happens. Procedurally, that translates into a procedure something along the lines of this: At 50% volume loss, filter out mostly CaCO3.
At 20% volume loss, filter out mostly CaSO4. At 1% volume loss, filter and take out predominantly NaCl. Extract something else from the remaining 1%
or discard it. This is a batch process, not a continuous process where you can just keep topping up the vat.
The one complication is that these percentages are percentages of solution (or perhaps just solvent), not of the combination of precipitated crystals
and solution. As such, measurement down at the 1% level may
be not trivial, though I doubt it's very difficult. Realistically, this means multiple filtrations in the NaCl stage.
More generally, this kind of procedure is called fractional crystallization. In general, it's not just about removing solvent, but also manipulation
of temperature of the solution.bbartlog - 5-5-2012 at 08:01
Ah, now it makes more sense. Thanks for the clarification.CHRIS25 - 5-5-2012 at 12:18
Off to read about fractional distillation.....smaerd - 5-5-2012 at 13:19
Fractional Distillation will not help you obtain salt from sea-water any better than a simple distillation or boiling would.
Ah I see you meant to write fractional crystallization, or perhaps confused the two terms momentarily.
Is the goal of doing this experiment to obtain a specific chemical or for fun and learning purposes? If you want to play with fractional crystallization it might be best to try something more controlled where the
majority of the variables are more known.ScienceSquirrel - 5-5-2012 at 15:04
You want to replicate the sea water ponds used to make salt by natural evaporation.
Take a large shallow container and fill it with fresh sea water.
It is best to have some way to stir it so add an air pump, and stone to stir it around.
Allow natural evaporation to take place, eventually fleur de sel will deposit.
Scoop off and enjoy.
it will be anywhere between white and light grey depending on your conditions and it can be very salty to sightly bitter depending on if it contains
just sodium chloride or a little magnesium chloride, etc.CHRIS25 - 6-5-2012 at 00:20
Yes Smaerd fractional crystallization - crossed cranial neuro wires and a blocked up synaptic junction are to blame. Oh it's for fun and learning and
putting on my potato wedges, and if there is any left over to throw with onto my copper art. I know I can buy it, but that really is so boring and
totally un-phonecian like.
Science squirrel, I get your point.
weiming1998 - 6-5-2012 at 01:00
To purify the sea salt obtained, maybe you can add NaOH to the crude salt, add water, filter, then neutralize the solution with HCl and evaporate.
That would get rid of the MgCl, MgSO4, as well as other non alkali metal salts, so the purified salt would taste considerably less bitter because of
the Mg2+ removal. You could then boil the salt water down until crystals start to form, cool the solution and then filter out the crystals of NaCl.Pyridinium - 6-5-2012 at 10:33
Magnesium is such an important nutrient, I'd leave it in there. Fleur de sel sounds good to me.
If the concentration of sulfates in your seawater-evaporate is too high, your GI tract will tell you.
Speaking of sea salt, I noticed the Wikipedian line goes something like this:
If we really believe that's a valid equivalence, we could always do a series of precipitations and fractional crystallizations, then add the flow
agents, etc. Of course even the Wikipedians can't possibly believe that (NaCl) == (NaCl + XYZ), because NaI is added to provide trace iodine.
Sarah Myhill, M.D. has this to say:
"The benefits of micronutrients in modern medicine have been hugely dumbed down."
If you want to play with fractional crystallization it might be best to try something more controlled where the
majority of the variables are more known.