Traveller
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sodium hypochlorite
Kind of a dumb question but....
Sodium hypochlorite 5% solution, as I'm told, is basically 5% NaClO in water with NaOH added to raise the ph over 12 to stabilize the NaClO.
I know that NaClO added to a large volume of drinking water will give me hypochlorous acid and NaOH. My question is, why does the NaClO in solution
not all turn to HClO way before I think about adding it to a large volume of water? Is it because of the high ph? By adding it to water with a ph of,
say, 7, does the subsequent lowering of the ph allow the NaClO to become HClO?
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AJKOER
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OK, see if this helps. Adding a weak acid (or a very dilute mineral acid) is known to release the HOCl. Higher pH most likely hypochlorite ion,
neutral to slightly acidic HOCl, even more acidic Cl2O increases along with the HOCl, lower pHs more free Cl2 and less HOCl.
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sargent1015
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Actually, not quite the case. You should think of NaOH as Na+ and OH- in solution. When you add lots of water, you are just
diluting everything. you will still have ClO- and Na+ and OH- floating around.
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Traveller
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Thanks for the responses. I think I see what you are pointing out, AKJOER. As I suspected (I think), NaClO is quite stable in its water solution at
+12 ph. Lowering it will release HOCl until it is slightly acidic, more acidic Cl2O along with HOCl, even lower ph's (ph 1 ?) gives free Cl2 and less
HOCl.
sargent1015...if the NaOH exists in solution as Na+ and OH-, does its addition to solution still raise the ph of that solution?
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Traveller
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There is a very important reason for asking these questions about NaClO, HOCl and NaOH. It has to do with a discussion that has been going on for
several years on a refining forum about dissolving gold with Cl2.
Over a hundred years ago, Plattner's Process was invented for recovering gold from ore. The subsequent invention of cyanide leaching pretty much
eliminated it from the mining scene. It is beginning to make a comeback in the world of electronic gold scrap recovery and the world of small time
mining due to the increasing difficulty of obtaining cyanide as well as aversion to working with such a dangerous compound.
One of the original variations of the invention involved manufacturing Cl2 gas onsite and piping it under pressure to a reaction vessel. This vessel
was capable of holding pressure; how much I do not know. In this vessel was placed finely ground ore and water and then it was sealed. Cl2 gas was
piped in under pressure through a petcock and, at a certain pressure, the petcock was closed. The vessel was then turned on rollers for several hours,
the liquid drained off, a precipitant added and the gold recovered.
From what I can deduce the Cl2, under pressure, becomes dissolved in water and becomes HCl and HOCl. It was said this leach was quite selective for
gold but sometimes would attack base metals, as well.
[Edited on 29-10-2012 by Traveller]
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sargent1015
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Quote: Originally posted by Traveller  |
sargent1015...if the NaOH exists in solution as Na+ and OH-, does its addition to solution still raise the ph of that solution?
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I think you mean lowers the pH, and yes it will a little bit after gallons of water. There is an equation somewhere that summarizes this point...
Actually, it's just the equation for pH=14-[OH-]
Eventually, you will get back to around 7, but OH- is such a strong base in water that the solution will still be basic. Give me an hour
and I will find a good example.
Also, not something I want to try (adding acid to sodium hypochlorite, since it releases a fair amount of Cl2 gas). Just sayin
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sargent1015
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so assuming a pH of 12, we find the concentration of OH- to be 1.0x10-2 Molar. Lets assume we have a liter of this solution. So
we have .01 moles of OH-. If we dilute the solution to 2 liters, we get .01mol/2L= 5x10-3 Molar.
If we find the pH of this ==> pH = 14-log([OH-])= 11.70.
So the amount of dilution will be so immense, that any HClO formed will be so dilute it is not worth your time. Go buy it 
Did one more calculation, in 2000L, the pH would be 8.70 
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Poppy
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The big tip here is, HOCl is strong acid, but no infinitely strong. Lowering the pH will raze its buffering effects, so you won't find out HOCl in the
ionized state. But its still so small I'd put a guess its not pratical for any particular niche uses.
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Vargouille
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HOCl is not classified as a strong acid. It's pKa value, the value determining dissolution, and thus, strength, is 7.53. Compare this to vinegar,
which has a pKa of 4.76, and to hydrochoric acid, with a pKa of -7. This is why addition of a hypochlorite to water causes an increase in pH, as does
an acetate or a carbonate, because the conjugate acid is weak. To answer the OP, the addition of hydroxide shifts the dissolution equilibrium to the
right, by consuming hydronium produced by the dissoution.
HOCl + H2O <=> OCl- + H3O+
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Traveller
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The following passage I c/p'ed is from a mining journal from the 1890's. It may not seem relevant to the topic of the thread but please read it
anyways and I will elaborate later on today.
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The most scientific and perfect mode of gold extraction (when the conditions are favourable) is lixiviation by means of chlorine, potassium cyanide,
or other aurous solvent, for by this means as much as 98 per cent of the gold contained in suitable ores can be converted into its mineral salt, and
being dissolved in water, re-deposited in metallic form for smelting; but lode stuff containing much lime would not be suitable for chlorination, or
the presence of a considerable proportion of such a metal as copper, particularly in metallic form, would be fatal to success, while cyanide of
potassium will also attack metals other than gold, and hence discount the effect of this solvent.
The earlier practical applications of chlorine to gold extraction were known as Mears' and Plattner's processes, and consisted in placing the material
to be operated on in vats with water, and introducing chlorine gas at the bottom, the mixture being allowed to stand for a number of hours, the
minimum about twelve, the maximum forty-eight. The chlorinated water was then drawn off containing the gold in solution which was deposited as a brown
powder by the addition of sulphate of iron.
Great improvements on this slow and imperfect method have been made of late years, among the earlier of which was that of Messrs. Newbery and Vautin.
They placed the pulp with water in a gaslight revolving cylinder, into which the chlorine was introduced, and atmospheric air to a pressure of 60 lb.
to the square inch was pumped in. The cylinder with its contents was revolved for two hours, then the charge was withdrawn and drained nearly dry by
suction, the resultant liquid being slowly filtered through broken charcoal on which the chloride crystals were deposited, in appearance much like the
bromo-chlorides of silver ore seen on some of the black manganic oxides of the Barrier silver mines. The charcoal, with its adhering chlorides, was
conveyed to the smelting-house and the gold smelted into bars of extremely pure metal. Messrs. Newbery and Vautin claimed for their process decreased
time for the operation with increased efficiency.
At Mount Morgan, when I visited that celebrated mine, they were using what might be termed a composite adaptation process. Their chlorination works,
the largest in the world, were putting through 1500 tons per week. The ore as it came from the mine was fed automatically into Krom roller mills, and
after being crushed and sifted to regulation gauge was delivered into trucks and conveyed to the roasting furnaces, and thence to cooling floors, from
which it was conveyed to the chlorinating shed. Here were long rows of revolving barrels, on the Newbery-Vautin principle, but with this marked
difference, that the pressure in the barrel was obtained from an excess of the gas itself, generated from a charge of chloride of lime and sulphuric
acid. On leaving the barrels the pulp ran into settling vats, somewhat on the Plattner plan, and the clear liquid having been drained off was passed
through a charcoal filter, as adopted by Newbery and Vautin. The manager, Mr. Wesley Hall, stated that he estimated cost per ton was not more than
30s., and he expected shortly to reduce that when he began making his own sulphuric acid. As he was obtaining over 4 oz. to the ton the process was
paying very well, but it will be seen that the price would be prohibitive for poor ores unless they could be concentrated before calcination.
The Pollok process is a newer, and stated to be a cheaper mode of lixiviation by chlorine. It is the invention of Mr. J. H. Pollok, of Glasgow
University, and a strong Company was formed to work it. With him the gas is produced by the admixture of bisulphate of sodium (instead of sulphuric
acid, which is a very costly chemical to transport) and chloride of lime. Water is then pumped into a strong receptacle containing the material for
treatment and powerful hydraulic pressure is applied. The effect is stated to be the rapid change of the metal into its salt, which is dissolved in
the water and afterwards treated with sulphate of iron, and so made to resume its metallic form.
It appears, however, to me that there is no essential difference in the pressure brought to bear for the quickening of the process. In each case it is
an air cushion, induced in the one process by the pumping in of air to a cylinder partly filled with water, and in the other by pumping in water to a
cylinder partly filled with air
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