mycotheologist
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Hypochlorites - What does available chlorine mean?
My neighbour gave me a bottle of sodium hypochlorite solution which says >5% hypochlorite but >10% chlorine availability. It also says 16% w/v
chlorine availability. I have no idea how to use this information to do the stoichiometry for the haloform reaction. How can one hypochlorite product
have greater chlorine availability than another, if they both contain the same amount of hypochlorite?
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AJKOER
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Some chemistry may be helpful. First, how is NaOCl made? Answer, treating NaOH aqueous with Chlorine:
Reactions:
Cl2 + H2O <---> HOCl + HCl
NaOH + HOCl <--> NaOCl + H2O
NaOH + HCl --> NaCl + H2O
So an equal amount of NaOCl and salt are created. However, the Bleach solution can be cooled and precipitate out some of the relatively less soluble
NaCl. But why? Answer: To increase stability of the Bleach as upon acidification:
2 NaOCl + H2SO4 --> Na2SO4 + 2 HOCl
2 NaCl + H2SO4 --> Na2SO4 + 2 HCl
HCl + HOCl <--> Cl2 + H2O
So, a chloride must be present for a hypochlorite in the presence of a strong acid to form chlorine (or, more precisely, the lower the pH the higher
the concentration of free Chlorine in the bleach). The amount capable of being so produced is the definition of 'available chlorine'. Decreasing the
chloride content, may increase the stability of the bleach solution as in the presence of even a weak acid (like CO2 which in water forms Carbonic
acid) the very weak acid HOCl can form:
2 NaOCl + H2CO3 --> Na2CO3 + 2 HOCl
and with light, the unstable Hypochlorous acid can decompose:
HOCl --uv--> HCl + [O]
and per above:
HCl + HOCl <---> Cl2 + H2O
or, in the presence of a chloride, the Hypochlorous acid is more likely to decompose:
NaCl + HOCl <--> Cl2 + NaOH
where both reactions move to the right with lower pH (more acidic conditions). So the bleaching/disinfectant power of the bleach (from HOCl and the
hydrolysis of NaOCl) can be loss as the Chlorine is formed and escapes.
So higher available chlorine with the same % NaOCl by weight is good if you are making Cl2 now, but not so good if you are looking to store most of
the solution for use later. If your intended reactions requires only NaOCl (or HOCl), the amount of NaCl present (which relates to available free
chlorine generation), should not be a driving factor and perhaps a negative as the bleach may even be less potent.
For calculation purposes, 100 ml of Bleach weights about 111 grams. If 5% by weight is NaOCl, then the expect amount of Sodium hypochlorite is 5.55
grams, or actually less as the solution loses strength with age.
[Edited on 12-5-2012 by AJKOER]
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mycotheologist
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Highly informative reply, thanks. I haven't fully grasped all this yet though. So because NaCl + HOCl <--> Cl2 + NaOH, the greater the amount of
NaCl present, the less stable the solution is to spontaneous, equilibrium induced Cl2 formation which would cause the solution to lose its chlorine
content over time. Have I got the right idea there? So in swimming pool supply shops, I notice they stock NaHSO4 as a pH lowering agent. I'm guessing
then that, being a weak acid, it reacts with the NaClO very slowly to produce HOCl in order to maintain safe levels of HOCl (and thus Cl2). That
reaction would be:
NaHSO4 + NaClO -> Na2SO4 + HClO
H2SO4 on the other hand would produce massive quantities of HClO and probably kill everyone in the swimming pool. To produce Cl2, I usually add dilute
HCl so the reaction would be:
HCl + NaClO <--> HClO + NaCl
Now what I need to know is where all this comes in in the haloform reaction. Is it actually HOCl or Cl2 that reacts with the acetone? If so, then
wouldn't adding NaCl make the reaction more effective by increasing the availability of Cl2?
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Nicodem
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Mycotheologist, have you considered searching for the definition of "available chlorine" before opening a new thread for an old topic? Also, do you
seriously believe this topic has never been disused on this forum? Please read the forum guidelines for posting.
A stupid joke. It didn't make me laugh.
| Quote: | To produce Cl2, I usually add dilute HCl so the reaction would be:
HCl + NaClO <--> HClO + NaCl |
HCl is oxidized by HClO, so HClO is not a formal product in the above reaction (though it is always present due to the aqueous disproportionation of
chlorine).
| Quote: | | Now what I need to know is where all this comes in in the haloform reaction. Is it actually HOCl or Cl2 that reacts with the acetone? If so, then
wouldn't adding NaCl make the reaction more effective by increasing the availability of Cl2? |
This has nothing else to do with the haloform reaction, except in that you can use the "available chlorine" value of bleach for calculating its
nominal concentration (the actual concentration will always be lower due to disproportionation of hypohalites). Both, Cl2 and HClO, can react with the
acetone enolate, but Cl2 is obviously not going to be on the favorable side of any equilibrium at the basic conditions required for the formation of
the enolate.
Adding NaCl would likely decrease the rate of the reaction due to the salting out of acetone and the formation of a biphasic system. Besides, as far
as I know, the rate determining step in the haloform reaction is the enolization of acetone, so the exact nature of the electrophilic chlorine species
is pretty much irrelevant.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Nicodem
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Thread Moved
12-5-2012 at 08:21 |
woelen
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Available chlorine is a number, which tells how much chlorine can be obtained if the compund is treated with excess HCl.
If you have 5% available chlorine, then 100 grams of the compound (or mix) makes 5 grams of Cl2 when sufficient HCl is added. It is as simple as this.
Below follow some examples to make the concept more clear.
- Swimming pool chlorine calcium hypochlorite has formula Ca(ClO)2.2H2O and reacts as follows:
Ca(ClO)2.2H2O + 4 HCl ---> CaCl2 + 2 Cl2 + 4 H2O
If 100 grams of Ca(ClO)2.2H2O is taken, then you get 79.2 grams of Cl2. Hence, the available chlorine content of Ca(ClO)2.2H2O is 79.2%. Commercial
swimming pool chlorine has appr. 70% available chlorine, due to impurities, such as CaCl2, CaCO3 and Ca(OH)2.
Available chlorine can even be larger than 100%. This is the case for solid pure lithium hypochlorite.
LiOCl + 2HCl ---> LiCl + Cl2 + H2O
If 100 grams of LiOCl are taken and sufficient HCl, then 121.4 grams of Cl2 is produced and hence its available chlorine content is 121.4%
Most important of this concept of 'available chlorine' is that the chlorine need not come from the compound itself, as is shown by the example of
LiOCl. In fact, the concept of 'available chlorine' can be used for any oxidizer, such as KMnO4, which is capable of making Cl2 from HCl. In practice,
however, the concept of 'available chlorine' is used for products, which contain chlorine-based oxidizers only, such as hypochlorites, TCCA and
Na-DCCA.
[Edited on 12-5-12 by woelen]
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