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Author: Subject: NaOCl how konz. could it be?
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[*] posted on 19-5-2007 at 04:59
NaOCl how konz. could it be?


Theoretically a NaOCl solution produced with NaOH and Ca(OCl)2 could have a very strong concentration depending on the NaOH concentration:

NaOH Ca(OCl)2
36% 51%
50% 65%
70% 81%

But that´s just the theory and NaOCl is a quite nasty and an very strong oxidiser when it´s very concentrated, also it´s very sensitive to temperature and it decomposes very quickly.

So my question is how strong a solution of NaOCl produced of Ca(OCl)2 could really be and what are the hazards to handle with such a concentrated solution?
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[*] posted on 19-5-2007 at 05:29


According to my merc-index, NaOCl can be freeze dried in a vaccum, over con. H2SO4 till anhydrous (very explosive !!). The sol. of the pentahydrate at 0C is 29.3gr/100ml H2O. After that I think it becomes unstable. (not sure)
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[*] posted on 19-5-2007 at 05:38


A patent for making a slurry of NaOCl . 5H2O, which is pretty concentrated

http://www.freepatentsonline.com/5194238.html

Alternatively by evaporation under vacuum you can concentrate (cold) stock NaOCl to the point of forming crystals of the pentahydrate, which are not terribly stable.
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[*] posted on 19-5-2007 at 06:47


Quote:
(very explosive !!).

That´s the reason why I´m asking bevore attempting;)

Thanks for the patent, as far as now I know that it should be possible to produce a ca. 30% solution and then the NaOCl would be converted into solid NaOCL x 5H2O.
So I think stirring is really important in my process, attempting with small amounts would be the best way I think.
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[*] posted on 19-5-2007 at 08:23


Dont use NaOH! Na2CO3 is both so much cheaper and the precipitate is way easier to filter. How did you come up with NaOH in the first place?
Also, if you are going to use a glass filter, the NaOH will attack it, another reason for not using it. Or do you have a sintered ceramic filter? If not, you must use Na2CO3.

As chance would have it, yesterday I have been doing the metathesis of Na2CO3 with Ca(OCl)2 (70%). I have found out that it is not possible to prepare a stronger NaOCl solution than 15% without major losses due to the large amount of precipitate.
Also, solutions stronger than 15% NaOCl decompose very rapidly.
Even the 15% solution can only be stored in the freezer without too much decomposition.
There is a reason why no stronger NaOCl solutions than 12% are sold by industry, since they would lose a lot of their active chlorine within a few days.

In order to calculate the stochiometry, it is important to take into account the amount of other calcium salts (like CaCl2) that are present in the HTH (high-test-hypochlorite, abbreviation for the product you bought from ebay).
You have to treat the 70% Calcium hypochlorite as if it was 100% hypochlorite when calculating the amount of Na2CO3 to use!
And in the filtrate, you will of course have only as much NaOCl as is produced by the 70% Ca(OCl)2 content of the HTH.

The amount of water to use in order to make 15% NaOCl is so small that you will have to use a supersaturated solution of Na2CO3 ( I used 37,8g Na2CO3 in 80ml water- boil until dissolution, then let cool. If it starts crystallizing, heat again until dissolution, but dont use it hot!).
The HTH is finely powdered first (very important!) and then dissolved in the precisely calculated and weighed amount of water/ice mix under stirring (taking into account the amount of water you already used for dissolving the Na2CO3).
Then the Na2CO3 solution of room temperature is added.

Now there is a curious and unexpected phenomenon: after some stirring, the mixture slowly turns into viscous goop. After about ten minutes of magnetic stirring, the sirbar will decouple from the magnetic field due to the viscosity. Now it must be stirred by hand using a glass rod or plastic spoon (not metal!). It continues to become more viscous, until the beaker can be turned upside down with no product coming out! But keep on stirring, as after this point, it will slowly become less viscous again, and after ca. 10 min of kneading, it will have returned to the consistence of yoghurt. Now magnetic stiring can be started again, and the mixture will be flowing like water after ca. 20 minutes of strong stirring. It now looks like milk, due to the CaCO3 precipitate.
The temperature must stay below 15°C in all those operations.

Now you can filter it with suction through a sintered glass filter of porosity 3.
DO NOT WASH THE FILTER CAKE!
Measure the amount of filtrate you get, and divide it by the amount you should be getting. Multiply by 100, and you have your yield in % (mine was 90%). The solution is 15% by weight in NaOCl.
Either use immediately or put in the freezer at -18°C (partially solidifies due to some Na2CO3 crystallizing out).

For your convenience, I give the weight of the substances to use:

102,2g HTH (70% Ca(OCl)2) contains 0,5 mol calcium hypochlorite and therefore 1 mol hypochlorite ions.

75,7g Na2CO3 (0,714 mol) is used for the metathesis.

In total, 422,2ml of water (preferrably partially as ice with the HTH) have to be used for dissolving both HTH and Na2CO3.

This theoretically gives about the same volume of 15% NaOCl solution than the amount of water used. It theoretically contains 1 mol NaOCl.
Measure the amount of NaOCl solution actually obtained as filtrate, divide by 422,2ml and you have the molar amount of NaOCl actually obtained.


This research is part of my project to employ the cheap and very well stored HTH calcium hypochlorite as the oxidiser for hydrazine production instead of the badly stored NaOCl. If I can obtain worthwhile yields, expect a complete writeup.




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[*] posted on 19-5-2007 at 09:18


Garage Chemist, excellent post! I was just about to post a similar one on the current permangante thread, but yours says it all.

The best figures I have for the solubility of hypochlorites is:

NaOCl, 29g/100g at 0 C. or 22% wt per wt solution.
KOCl about the same.
Ca(OCl)2 21 g/100g aq at 0C, 23g/100g at 40 C.

I do not have a source for these figures.

Producing NaOCl by the method outlined usually resuts in a 15% solution, according to titration.

I have tried to produce the hydrate by cooling below 0 C, but have never been successful yet. It is unstable, apparently, and melts at 10 C but is not explosive.

It is not possible to concetrate NaOCl by evaporation unless a very low pressure is used at a low temp. Then actual anhydrous hypochlorite can then be made by drying over a dessicant. It is exolosive (CRC handbook). Regards,

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[*] posted on 19-5-2007 at 16:54


My 15% NaOCl solution deposited a lot of crystals upon cooling to -18°C, I believe they are sodium carbonate decahydrate (from the excess of Na2CO3). But they may just be NaOCl*5H2O, I dont know.
Upon removing the jar from the freezer, the crystals redissolved long before the solution reached 0°C, so isolation would have to be carried out under strong cooling.

Hypochlorous acid, as used in the patent, is made from Cl2O (the anhydride of HClO) which in turn is made by passing chlorine over moist mercuric oxide. The HgO binds the HCl which is produced by disproportionation of chlorine as basic mercuric chloride, and Cl2O is obtained as a yellow extremely explosive gas. I dont think this is a method for obtaning any useful amount of HClO.




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[*] posted on 20-5-2007 at 04:52


Thanks a lot for this great informations.

I´ve a 36%solution of NaOH in a glass jar and it doesn´t attack it at all, but former I tried to concentrate this solution by heating and my reagence glass became quickly a hole. But you´re right, Na2CO3 is much cheaper than sodium hydroxide.

15°C, sounds like a method for winters.

When you just want to produce N2H4, why don´t you try to use solid HTH and a mixture consisting of high concentrated NaOH and NH3, should be no problem to heat this mixture because the N2H4 could escape.
But that´s just an idea.
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