Questions You Should Know about electrolytic sodium hypochlorite generator

Author: Subject: ph of NaOCl made through electrolysis of NaCl brine ph of NaOCl made through electrolysis of NaCl brine


Hello
I'm looking at purchasing a small sodium hypochlorite generator. It makes a 1.0% solution of NaOCl by means of electrolysis of a NaCl brine.
The question I have is, what would be the ph of the NaOCl solution, if the ph of the brine was between 7 and 8. I read somewhere that the ph of seawater was 8.
If the electrolysis process:

NaCl + H2O + e = NaOCl + H2

did not alter the original ph of the brine, would this, technically, not be a hypochlorous acid generator, as the shift from OCl to HOCl begins if the ph of NaOCl bleach falls under 11.86?

NaClO is relatively stable at a pH above ~12.
At lower pH the readily decomposed oxyacid forms . . .



I don't think you two quite understand the question. I know that commercially sold bleach has a ph of over 12. To achieve this, sodium hydroxide is added to it to increase its ph.

My question is, what is the ph of sodium hypochlorite made from the electrolysis of salt brine BEFORE the sodium hydroxide is added to raise its ph over 12?

Hmm, I would think they add the NaOH first (to achieve a pH 10 or higher) before electrolysis.

NaCl subjected to electrolysis, without any adjustment, tends towards a pH of 9 or 10, depending on temperature, electrodes and current density I suppose, but a lot of chlorate is formed, particularly at high temperatures.

Tim




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The electrolysis of salt produces Cl2 at the anode and H2 (and also NaOH) at the cathode. The NaOH and Cl2 reacts in situ to form NaClO. Because some chlorine gas will inevitably escape from the solution instead of reacting, there will be a small amount of NaOH in the solution. This will raise the pH.

NaClO dissociates to HOCl in neutral solution. That is true, but NaOH is also formed. Since HOCl does not contribute to the pH that much, it will only take somewhere near 0.01M of NaOH+HOCl before the pH goes above 12. Then the OCl- ion is generated.

So, technically HOCl is generated at the beginning of electrolysis, but only a very small amount will be generated before it starts generating NaClO instead.

Quote: Originally posted by weiming  The electrolysis of salt produces Cl2 at the anode and H2 (and also NaOH) at the cathode. The NaOH and Cl2 reacts in situ to form NaClO. Because some chlorine gas will inevitably escape from the solution instead of reacting, there will be a small amount of NaOH in the solution. This will raise the pH.

NaClO dissociates to HOCl in neutral solution. That is true, but NaOH is also formed. Since HOCl does not contribute to the pH that much, it will only take somewhere near 0.01M of NaOH+HOCl before the pH goes above 12. Then the OCl- ion is generated.

So, technically HOCl is generated at the beginning of electrolysis, but only a very small amount will be generated before it starts generating NaClO instead.

Weiming

Thank you for your reply. I had not thought about this but it seems to make quite a lot of sense.

So, it seems, in a solution of salt water at a ph of 7-8, electrolysis will make NaClO but, mandated by ph, a percentage of the OCl will instantaneously shift to become HOCl.

NaClO + H2O = HOCl + NaOH

Brilliant, an NaClO generator that makes its own preservative. I operate a small water system for the town I live in and we disinfect the water at each pumphouse as it is pumped out of the ground. In slow times (like winter) our pumps can be off for up to 24 hours before restarting. As the NaClO generator we are looking at has a small ten litre tank on it, as a reservoir, we were concerned that it would all be oxidised if it sat for 24 hours and it was mostly HOCl. We just assumed it was designed to run non-stop and, that way, the chance to decompose would not be there.

Weiming

Coincidentally, there is another reason I am interested in NaClO and HOCl. I am pursuing an effective way of recovering ultra fine gold from a bench of clay.

There is a method that was used right up until the late 's that involved chlorine, both in hypochlorite and gas form. From what I can gather, Cl2 gas was pressurised into vessels of ore and water at 60 psi. I believe hypochlorous acid was made this way. It also seems that the higher the percentage of HOCl present, the faster gold went into solution as a chloride.

Cl2 + H2O = HOCl + HCl

Because it made HCl, I would think the ph would be lowered slightly here. Interestingly, the OCl/HOCl shift mandated by ph will have the ratio of HOCl to OCl at 100%/0% at a ph of 5.

The other recovery method I read about involved the manufacture of what was called a "chlorine solution" from the electrolysis of salt water. Read the following article from the 's and tell me what you think. If what you say is true, the solution would have been far too basic to allow any HOCl to be present. It is possible they were adjusting the ph with HCl and the author was unaware of this. Also, do you think it possible they were making sodium chlorate (NaClO3)? I know that if the temperature of the electrolysis chamber gets over 50° C. it will stop producing NaClO and produce NaClO3. Would it be just as or more effective at putting gold into solution? Anyways, here is the article:

--------------------------------------------------------------------


Electrolytic Precipitation of Gold


In order to perform the electrolytic precipitation of gold, from the filtration vessel the gold chloride solution was conducted into the outer or battery jar of an electrolytic cell. The electrode of the outer cell was connected with the negative pole of a dynamo and the electrode on the inner cell was connected with positive pole or terminal of the dynamo. The gold solution was sent into the jar near the bottom and slowly circulated upwards, and at the same time a current of electricity was passes through the cell. When the reaction was produced, the gold chloride was decomposed and felt like a shower of fine spangles to the bottom, while the liberated chlorine passed into the inner cell where it was absorbed by the water circulating and formed a chlorine solution.

The receiver was charged with chlorine solution generated by the electrolysis of salt. The unit comprised a battery, a conductor from the electrode in the large battery jar to the negative pole of the electrical generator, the conductor from the electrode in the porous cell to the positive pole of the generator, a reservoir containing a saturated solution of sodium chloride, which passed to the battery through a pipe and a reservoir containing water connected by a pipe with a porous cell.

In order to treat the ore more effectually with the chlorine solution it was advantageous to expel the air from the chlorinator. For this purpose the chlorinator was provided with a valve, so that the air contained in the chlorinator passed out as the chlorine solution passed in. the valve was closed immediately the air was expelled. The chlorinator after disconnected was slowly revolved by means of a pulley and strap from an engine, or in other suitable way until the gold was dissolved as a gold chloride. The time required for treatment in the chlorinator varied from one to two hours according to the characteristics of the gold ore treated. The ore and solution were discharged into a suitable filtration vessel placed beneath the chlorinator. The vessel was a shallow vat constructed of oak or other material, the lower part was made cone-shaped and of the same capacity of the chlorinator.

The vat was closed by a cover bolted down. In the center of the cover there was hopper-shaped inlet for receiving the ore and solution from the chlorinator. A perforated diaphragm covered with asbestos cloth, over which was advantageously placed a layer of other suitable filtration media such as ground asbestos, which was fixed from one to two inches below the top of the vat. The gold chloride was washed out of the ore by a stream of water from a tank. The water entered the vat through a pipe at the lowest part and percolated upwards through the ore until the gold content in the solution was extremely low. It was important to take samples of the solution to determine the presence of gold. A sliding door in the bottom of the vat was opened and the residue from the ore as discharged by means of a large outlet opened into a truck placed underneath.

The gold chloride and water descended through a pipe into a receiver and was conducted from the receiver into the outer or battery jar of the electrolytic cell. The gold solution flowed into and entered the jar at the bottom and slowly circulated upwards and at the same time a current of electricity was passé through the cell to reduce and precipitate gold from the solution into a perfectly pure state, upon the bottom of the jar, from where it was removed. The chlorine was liberated at the same time at the electrode in the inner or porous cell and in contact with the water circulating and formed a chlorine solution, which was sent to receiver vessel.

Quote: Originally posted by Traveller  Weiming

Coincidentally, there is another reason I am interested in NaClO and HOCl. I am pursuing an effective way of recovering ultra fine gold from a bench of clay.

There is a method that was used right up until the late 's that involved chlorine, both in hypochlorite and gas form. From what I can gather, Cl2 gas was pressurised into vessels of ore and water at 60 psi. I believe hypochlorous acid was made this way. It also seems that the higher the percentage of HOCl present, the faster gold went into solution as a chloride.

Cl2 + H2O = HOCl + HCl

Because it made HCl, I would think the ph would be lowered slightly here. Interestingly, the OCl/HOCl shift mandated by ph will have the ratio of HOCl to OCl at 100%/0% at a ph of 5.

The other recovery method I read about involved the manufacture of what was called a "chlorine solution" from the electrolysis of salt water. Read the following article from the 's and tell me what you think. If what you say is true, the solution would have been far too basic to allow any HOCl to be present. It is possible they were adjusting the ph with HCl and the author was unaware of this. Also, do you think it possible they were making sodium chlorate (NaClO3)? I know that if the temperature of the electrolysis chamber gets over 50° C. it will stop producing NaClO and produce NaClO3. Would it be just as or more effective at putting gold into solution? Anyways, here is the article:

--------------------------------------------------------------------


Electrolytic Precipitation of Gold


In order to perform the electrolytic precipitation of gold, from the filtration vessel the gold chloride solution was conducted into the outer or battery jar of an electrolytic cell. The electrode of the outer cell was connected with the negative pole of a dynamo and the electrode on the inner cell was connected with positive pole or terminal of the dynamo. The gold solution was sent into the jar near the bottom and slowly circulated upwards, and at the same time a current of electricity was passes through the cell. When the reaction was produced, the gold chloride was decomposed and felt like a shower of fine spangles to the bottom, while the liberated chlorine passed into the inner cell where it was absorbed by the water circulating and formed a chlorine solution.

The receiver was charged with chlorine solution generated by the electrolysis of salt. The unit comprised a battery, a conductor from the electrode in the large battery jar to the negative pole of the electrical generator, the conductor from the electrode in the porous cell to the positive pole of the generator, a reservoir containing a saturated solution of sodium chloride, which passed to the battery through a pipe and a reservoir containing water connected by a pipe with a porous cell.

In order to treat the ore more effectually with the chlorine solution it was advantageous to expel the air from the chlorinator. For this purpose the chlorinator was provided with a valve, so that the air contained in the chlorinator passed out as the chlorine solution passed in. the valve was closed immediately the air was expelled. The chlorinator after disconnected was slowly revolved by means of a pulley and strap from an engine, or in other suitable way until the gold was dissolved as a gold chloride. The time required for treatment in the chlorinator varied from one to two hours according to the characteristics of the gold ore treated. The ore and solution were discharged into a suitable filtration vessel placed beneath the chlorinator. The vessel was a shallow vat constructed of oak or other material, the lower part was made cone-shaped and of the same capacity of the chlorinator.

The vat was closed by a cover bolted down. In the center of the cover there was hopper-shaped inlet for receiving the ore and solution from the chlorinator. A perforated diaphragm covered with asbestos cloth, over which was advantageously placed a layer of other suitable filtration media such as ground asbestos, which was fixed from one to two inches below the top of the vat. The gold chloride was washed out of the ore by a stream of water from a tank. The water entered the vat through a pipe at the lowest part and percolated upwards through the ore until the gold content in the solution was extremely low. It was important to take samples of the solution to determine the presence of gold. A sliding door in the bottom of the vat was opened and the residue from the ore as discharged by means of a large outlet opened into a truck placed underneath.

The gold chloride and water descended through a pipe into a receiver and was conducted from the receiver into the outer or battery jar of the electrolytic cell. The gold solution flowed into and entered the jar at the bottom and slowly circulated upwards and at the same time a current of electricity was passé through the cell to reduce and precipitate gold from the solution into a perfectly pure state, upon the bottom of the jar, from where it was removed. The chlorine was liberated at the same time at the electrode in the inner or porous cell and in contact with the water circulating and formed a chlorine solution, which was sent to receiver vessel.

I have read this. Maybe they used a membrane in their cell to separate the cathode and anode. This means that the anode will produce chlorine, the cathode sodium hydroxide and those two won't react. They were also talking about a reservoir of water, so I think the "chlorine solution" is a solution of chlorine in neutral water generated by electrolysis of brine, which will dissolve gold.

I have thought the same thing myself. Many mines in this period had their own chlorine works where the gas was made from sulphuric acid and chloride of lime. It is just possible that the inner and outer jar of the cell the author spoke of was the membrane required to keep Cl2 and NaOH from reacting. It may have been simpler to produce electricity onsite than to haul in H2SO4.

It may also have been they were making NaClO and bringing the ph to neutral, by adding the appropriate amount of HCl, and thus still ending up with HOCl at a neutral ph.

I've found these 100+ year old articles can be both articulate in detail and maddeningly vague at the same time. Unfortunately, they are about the only source of information, as cyanide leaching came on the scene about in a big way and chlorine leaching was abandoned almost overnight.

What do you think of the gold oxidizing potential of chloric acid (HClO3) made from sodium chlorate as opposed to hypochlorous acid?

Quote: Originally posted by Traveller  I have thought the same thing myself. Many mines in this period had their own chlorine works where the gas was made from sulphuric acid and chloride of lime. It is just possible that the inner and outer jar of the cell the author spoke of was the membrane required to keep Cl2 and NaOH from reacting. It may have been simpler to produce electricity onsite than to haul in H2SO4.

It may also have been they were making NaClO and bringing the ph to neutral, by adding the appropriate amount of HCl, and thus still ending up with HOCl at a neutral ph.

I've found these 100+ year old articles can be both articulate in detail and maddeningly vague at the same time. Unfortunately, they are about the only source of information, as cyanide leaching came on the scene about in a big way and chlorine leaching was abandoned almost overnight.

What do you think of the gold oxidizing potential of chloric acid (HClO3) made from sodium chlorate as opposed to hypochlorous acid?

It could be possible that they added HCl to the newly electrolysed brine solution. This produces Cl2 instead of HClO in situ as HCl reacts with HOCl and OCl- to form Cl2 and water. One of the main problems of that is the chlorine could start to rapidly bubble out of solution uncontrollably, poisoning everyone there, but a slow, controlled drip of HCl should do the trick.

With acidification by HCl, any formed NaClO3 will first be acidified to HClO3, but then be reduced to ClO2, a yellowish explosive gas. In the presence of HCl, a variety of oxy-acids of chlorine are generated by the hydrolysis/reaction of ClO2 with HCl. I think this will also dissolve gold to some extent, but probably not much better than just Cl2.

I thought this too, at first, and this is also the basis of a leach promoted worldwide as the Igoli Process. Large amounts of HCl are added to a NaOCl solution and great amounts of Cl2 gas are given off. Its promoters sincerely believe it is the Cl2 gas coming in contact with gold that puts the gold into solution. However, with Cl2 having no oxidising potential in the gaseous state, I do not believe this to be the case. I believe it to be HOCl doing the work, just as it is the active ingredient when disinfecting.

If a very minute amount of HCl was added to NaClO, would we not see the following reaction, given the amount of NaOH in our solution?

NaOH + HCl = NaCl + H20

If this were the course it followed, it would seem possible to gently lower the ph of the NaClO solution to neutral, thus converting most of the OCl ions to HOCl. It should also be noted that Cl2 gas will not come out of solution until the ph drops below 5.


[Edited on 27-1- by Traveller]

Quote: Originally posted by Traveller  I thought this too, at first, and this is also the basis of a leach promoted worldwide as the Igoli Process. Large amounts of HCl are added to a NaOCl solution and great amounts of Cl2 gas are given off. Its promoters sincerely believe it is the Cl2 gas coming in contact with gold that puts the gold into solution. However, with Cl2 having no oxidising potential in the gaseous state, I do not believe this to be the case. I believe it to be HOCl doing the work, just as it is the active ingredient when disinfecting.

If a very minute amount of HCl was added to NaClO, would we not see the following reaction, given the amount of NaOH in our solution?

NaOH + HCl = NaCl + H20

If this were the course it followed, it would seem possible to gently lower the ph of the NaClO solution to neutral, thus converting most of the OCl ions to HOCl. It should also be noted that Cl2 gas will not come out of solution until the ph drops below 5.


[Edited on 27-1- by Traveller]

Both HOCl and Cl2 are active ingredients in gold dissolution. Especially in a dilute solution of HCl, some Cl2 dissolve in water without dissociating to HOCl and HCl. The Cl2 reacts directly with the gold, forming AuCl3. This is why hydrochloric acid with dissolved chlorine in it can attack noble metals.

A very small amount of HCl into a solution of electrolysed brine might just react with some NaOH in solution, but if the pH is lowered so that even some HOCl forms (higher than 5), the mixture start to evolve Cl2. Chlorine only has a high solubility in very basic mixtures, and will bubble out when the formed HOCl start to react with the HCl.

Swimming pool water requires the addition of HCl regularly to maintain a close to neutral ph. If the active disinfectant in swimming pools is HOCl, with hypochlorite as a bank or reservoir of chlorine, wouldn't the addition of HCl, even at neutral ph, cause the release of chlorine gas from a swimming pool?
The entire purpose of adding HCl, to my understanding, is to keep the ph of the swimming pool from becoming basic; in other words, neutralizing NaOH. I would think the release of Cl2 gas to be the minor portion of the reaction.



[Edited on 4-2- by Traveller]