BlackPaw
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A simple questions about making NaOH with CaO.
I wanted to make a pure solid form of NaOH in another way other than the common process of electrolysis of NaCl in water for the obvious reason -
because of the poisonous gas.
Another method which i want to try is by:
I have CaO and NaHCO3.
1. I heat at about 150-200 Celsius the Sodium Hydrogen Carbonate until it realises all the hydrogen atoms and i get:
NaHCO3 --heat-----> Na2CO3(s) + H2O(g) + CO2(g)
2. I put the Na2CO3 in a beaker together with the CaO and i mix them briefly and then add water and I get the following important reaction beside the
return reaction which give uneccessary substances.
CaO(s) + H2O(l) + Na2CO3(aq) -----> CaCO3(aq) + 2NaOH(aq)
After i boil down the water I will be left with this substances:
-Ca(OH)2(s)
-Na2CO3(s)
-CaCO3(s)
-NaOH(s)
Is all above correct and if not what is my mistake?
Is there any way to make the reaction of Ca(OH)2 and Na2CO3 a bit more effective to yeild more NaOH?
And do you know how can i separate or filtrate the NaOH from all other substances that will be present and mixed together in the beaker?
Thanks in advance for all the answers comments and advices.
Edit: and please don't make any comments - saying that it's cheaper to buy or that it's readily available.
[Edited on 9-1-2011 by BlackPaw]
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not_important
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Per The principles of chemistry: illustrated by simple experiments
By Julius Adolph Stöckhardt , the process using K2CO3 & CaO (hydrated to Ca(OH)2) dissolves 1/2 oz of K2CO3 in 6 oz of water, then slowly
adds the Ca(OH)2, followed by boiling "for some time".
This is the general method used traditionally, the alkali carbonate is in hot solution forfor contacting it with the lime, and the mixture is boiled
and stirred for an extended period.
Separation consists of allowing the solids to settle out, but keeping the solution hot, and filtering while hot. Evaporate the filtrate to concentrate
the NaOH solution; to drive off all the water takes elevate temperatures and needs to be done in containers not readily attacked by strong alkalies.
In the laboratory silver dishes were traditionally used, industrially iron, stainless steel, or nickle or nickle alloys were more common.
The relative solubilities of the compounds drives the reactions, in particular compare the k<sub>sp</sub> of Ca(OH)2 and CaCO3.
You can use NaHCO3 in place of Na2CO3, avoiding the calcining, provided you boil the solution long enough, as calcium bicarbonate is unstable and
losses H2O and CO2 which escapes with the steam from boiling.
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unionised
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You won't lose CO2 from that alkaline solution, it will react with the Ca(OH)2
You could use the bicarbonate, but it would use twice as much Ca(OH)2
On the other hand, if you boil the solution of sodium bicarbonate first to remove CO2 and convert the stuff to carbonate then add the lime you should
be OK.
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BlackPaw
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OK, but how do I purify the mixture so I get only the NaOH from it?
Because there will be lot of other substances : Ca(OH)2, Na2CO3, CaCO3 etc.
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not_important
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I think you meant that either of the hydroxides would capture the CO2. However if the CO2 ends up as carbonate it will form CaCO3, and if it ends up
as HCO3(-) it forms some calcium bicarbonate which decomposes to CaCO3 releasing CO2 - some of which will escape along with the steam. The amount of
Ca(OH)2 needs is higher than with Na2CO3 but less than 1 mole of Ca(OH)2 per mole of NaOH formed.
Note that if you boil a sokution of Na2CO3 it will lose some CO2 and become increasingly alkaline.
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| Quote: | OK, but how do I purify the mixture so I get only the NaOH from it?
Because there will be lot of other substances : Ca(OH)2, Na2CO3, CaCO3 etc. |
CaCO3 has a low solubility, almost all of it will be precipitated.
If you start with anhydrous Na2CO3 ( AKA soda ash) it is easy to calculated the equal amounts of it and Ca(OH)2, although a slight excess of lime is
desirable as some of the hydroxide may have combined with CO2 in the air.
Settling and filtering of the dilute NaOH solution removes almost all the CaCO3. Concentration of the dilute NaOH (away from air because of the air's
CO2 content) will cause some of the remaining carbonates and Ca(OH)2 to drop out of solution. For really pure NaOH you heat the solution until all
the water is driven off leaving molten NaOH, which is poured onto chill iron sheets or otherwise solidified in some thin or small pellet form. The is
dissolved in anhydrous EtOH or MeOH, the solution filtered to remove impurities, and then evaporated to recover solid NaOH.
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ScienceSquirrel
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It is practical to make dilute sodium hydroxide this way.
Mixing a hot solution of sodium carbonate with the calculated amount of lime, boiling and then vacuum filtering will yield a dilute solution of sodium
hydroxide.
The calcium carbonate formed is very fine and voluminous so vacuum filtration is best.
Concentration becomes increasingly hazardous as the sodium hydroxide solution can spit as it gets stronger and stronger. It is also very hot and
corrosive.
Eventually you will end up with molten sodium hydroxide which is very hazardous indeed, pouring onto a metal sheet and then breaking it up with a
spatula will produce flakes of the almost pure material
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Random
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What will be concentration of NaOH solution after reaction without concentrating by boiling? Can we use excess Na2CO3?
I mean, if I add to that weak NaOH solution more Ca(OH)2 and Na2CO3, CaCO3 falls out and more soluble than Na2CO3, NaOH forms.
Can I dissolve then in that NaOH solution even more Na2CO3 until again saturated and again react it with Ca(OH)2 until I could get 110g/100mL
saturated solution of NaOH?
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not_important
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As the concentration of the reactants or products is increased, the reaction slows down and the product is less pure. So unless all you want is to
make soap, you need to run fairly dilute.
You want a slight excess of Ca(OH)2 as it is easier to remove than Na2CO3.
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ScienceSquirrel
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As you increase the concentration of sodium hydroxide the calcium hydroxide becomes less soluble due to the common ion effect.
Thus the reaction was run fairly dilute with excess calcium hydroxide. Concentration would precipitate the excess calcium hydroxide and any carbonate
formed during to reaction with carbon dioxide in the air.
A patent describing the process is here;
http://www.freepatentsonline.com/5993772.html
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Renaissance Man
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Is it actually need of NaOH that is driving you to make it or just for the sake of the accomplishment itself?
If you have a decent torch you could go directly to sodium oxide at 1000 C and then add water which would eliminate isolation steps. Propane gets to
2000 C and methyacetylene-propadiene (MAPP gas, its a bit more expensive than propane) gets to nearly 3000, you can get both at any hardware store
without in hand held tanks. However, if you want to keep any volume of material over 1000 c your probably going to have to use MAPP, oxy-propane, or
get use a pretty high output handhold propane torch. You could build a fire too, which might be the easiest option.
Also, chlorine isn't too terribly bad to work with to be honest, as long as you do it outside it is very low risk at the amounts produced by
electrolysis. Separating the NaCl might be really easy, I cant seem to find out if NaOH is soluble in acetone, but if it is you could boil the water
off of the hydrolysis solution and dissolve the powder in water to get only the NaOH. The only problem with this method would be finding a suitable
electrode material. Oh and don't get the anode and cathode too close or some hypochlorites the like will form.
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