Sciencemadness Discussion Board

sodium hypochlorite bleach

Traveller - 15-9-2013 at 09:58

I know that NaClO bleach can be made in two ways; passing chlorine gas through sodium hydroxide or by the electrolysis of sodium chloride brine. I also know that the pH of household bleach is 12+. (I tested Clorox with a pH tester and got a figure of 12.2).

While it is easy to see how the pH of NaClO bleach would be controlled in the Cl2 and NaOH process by the percentage of NaOH in solution, what would be the pH of sodium hypochlorite made, through electrolysis, from a brine solution with a pH of 7? Would NaOH have to be added, as a preservative, to bring the pH over 12? Would the electrolysis process be making hypochlorous acid, as well as sodium hypochlorite?

[Edited on 15-9-2013 by Traveller]

plante1999 - 15-9-2013 at 10:06

in el;ectrolytic operation some chlorine is lost and pH increase. sodium hypochlorite is really unstable in near neutral and acid solution. If one wanted pure hypochlorite, making tert butyl hypochlorite with sodium hydroxide would be the way to go. then it could be hydrolysed to pure sodium hypochlorite.

Traveller - 15-9-2013 at 10:39

Quote: Originally posted by plante1999  
in el;ectrolytic operation some chlorine is lost and pH increase. sodium hypochlorite is really unstable in near neutral and acid solution. If one wanted pure hypochlorite, making tert butyl hypochlorite with sodium hydroxide would be the way to go. then it could be hydrolysed to pure sodium hypochlorite.


If nothing is done except the electrolysis of brine, and a little chlorine is lost and the starting pH of the brine is 7, what would be the resulting pH of the hypochlorous acid/hypochlorite solution?

[Edited on 15-9-2013 by Traveller]

bbartlog - 15-9-2013 at 17:29

There is no net production of hypochlorous acid by electrolysis of brine. Hydrogen is evolved at the cathode and the overall net reaction for the main path is

H<sub>2</sub>O + NaCl -> H<sub>2</sub> + NaOCl

Of course this doesn't go in to all the individual reactions that cause this to happen; sometimes Na+ is reduced at the cathode and *then* reacts with water to form hydrogen and NaOH, and indeed there is production of HCl and HOCl at the anode, but it can't exceed the amount of NaOH produced at the cathode and thus the solution will always be composed of NaOCl + NaOH rather than any free HOCl (except that there is a very small amount, pH dependent, because it exists in equilibrium...). Not least because some chlorine does escape at the anode, and to the degree this happens the solution will have an excess of NaOH.

Anyway, to answer the original question, the pH will continue to rise as electrolysis continues.

[Edited on 16-9-2013 by bbartlog]

Traveller - 15-9-2013 at 18:04

If the pH of sodium hypochlorite bleach was lowered to 5.5, all of the chlorine would exist as hypochlorous acid.

woelen - 15-9-2013 at 23:24

Electrolysis of brine is not a good way to make hypochlorite. You will have many side reactions and the main product will be chlorate. Just have a look at all the posts about chlorate cells. These simply use a solution of NaCl or KCl to make chlorate. Hypochlorite only is an intermediate and (depending on temperature and pH) is quickly (or not so quickly) converted to chlorate and chloride.

The real way to go is bubble Cl2 through a solution of NaOH until no more Cl2 is absorbed anymore, and at the same time keeping the liquid ice cold (otherwise you again get a lot of chlorate as byproduct). After the reaction then you can add a little NaOH to raise the pH and stabilize the hypochlorite somewhat.

Traveller - 17-9-2013 at 08:30

I am really not trying to make my own bleach. I am constructing an electrolytic cell that will, hopefully, dissolve gold from ore and deposit it at a cathode inside of a porous silica cup.

If the cell makes chlorate instead of hypochlorite, it makes no difference, as chloric acid (HClO3) is a superior oxidizer to hypochlorous acid (HClO).

All I would like to know is, if I begin with a brine solution with a pH of 7, what will the pH of the solution be if I transform it, through electrolysis, to a mixture of hypochlorite/chlorate. I understand, of course, that the ratio of hypochlorite to hypochlorous acid, as well as the ratio of chlorate to chloric acid, is mandated by the pH of the chlorine solution.

plante1999 - 17-9-2013 at 08:35

The PH will raise to about 10-12.

AJKOER - 25-9-2013 at 05:08

In my opinion, the pH may well be dependent also on the placement and surface area of the electrodes and stirring. My experience with a so called Bleach battery (Al, Cu and aqueous HOCl with a touch of NaCl) is that moving the Copper electrode from the base of the cell to the top forms visibly more greenish Copper oxychloride and Chlorine gas as opposed to the clear gelatinous Aluminum Hydroxide and Aluminum oxychloride together with less Cl2 gas when positioned at the base of the cell.

Also, is the Chlorine allowed to escape? For, improved hypochlorite and chlorate formation, continued solution contact (like periodic shaking) should increase yield (as it increases HOCl formation via the hydrolysis of the Chlorine).

The cell temperature will also be significant factor in the efficiency of the chlorate formation (60 to 80 C). Source, see page 2 at http://utahpyro.org/compositions/PreparingChlorates.pdf which is a generally good reference on the production of chlorates with electrolysis.

The allowed pH range (3.5 to 6.5 for HOCl, or 5.5 to 6.5 for highest joint HOCl and ClO- concentrations) is also significant as Chlorate formation is favored in near neutral to mildly acidic conditions as this allows free chlorine present in the solution to exist primarily as HOCl (fostering chlorate formation) as opposed to Cl2 gas (in lower pH ranges). High pH can also form chlorate, albeit, not as efficiently. Source: see prior link and also the chart on 'Available Chlorine Present as HOCl' versus pH on the last page of this link: http://aquaox.wordpress.com/category/hypochlorous-acid-sodiu...

Also, per my knowledge of the photolysis reaction creating chlorate from Cl2 and HOCl suggests that the action of light on Chlorine gas itself with solution contact of HOCl will increase chlorate formation and the decomposition of the hypochlorite. One source on the photolysis reaction chain, please see "Photolysis of aqueous free chlorine species (HOCl and OCl) with 254 nm ultraviolet light" by Yangang Feng,..

Also, I have read in a 2000 paper (see http://pubs.acs.org/doi/abs/10.1021/ic991486r ) and another source ("Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous Acid" at http://www.researchgate.net/publication/23141635_Effect_of_c... , to quote:

"Moreover, they found that acetate ion accelerates the formation of · ClO2 enormously. It was interpreted by a steady-state formation and further reactions of acetyl hypochlorite. "

So, in the presence of organics (like Acetic acid and Sodium acetate) one can promote the creation of chlorate in a non-electrolysis setting. But, as the electrolysis of aqueous KCl is, to some extent, a more complex chlorination route, the chemistry (caution: additional toxic products could be formed) may still work to catalyzed the reaction with less energy.

[EDIT] An important point that occurred to me is that the chief way to move from hypochlorite to chlorate, other than pH considerations, is by having highly ionic, hot and concentrated solutions. So, if one really wants ClO- and not ClO3-, employ (and maintain) a cold dilute electrolyte with reduce presence of non-essential salts (for example, do not add all the KCl at once, but slowly over time, making the solution less ionic and concentrated).


[Edited on 26-9-2013 by AJKOER]

papaya - 25-9-2013 at 05:34

Quote: Originally posted by woelen  
Electrolysis of brine is not a good way to make hypochlorite. You will have many side reactions and the main product will be chlorate. Just have a look at all the posts about chlorate cells. These simply use a solution of NaCl or KCl to make chlorate. Hypochlorite only is an intermediate and (depending on temperature and pH) is quickly (or not so quickly) converted to chlorate and chloride.

The real way to go is bubble Cl2 through a solution of NaOH until no more Cl2 is absorbed anymore, and at the same time keeping the liquid ice cold (otherwise you again get a lot of chlorate as byproduct). After the reaction then you can add a little NaOH to raise the pH and stabilize the hypochlorite somewhat.


But isn't electrolysis one of the industrial ways of hypochlorite production? A good discussion will be the difference of conditions while producing hypochlorite vs chlorate and possibility of preparing your own high percent(>5% at least) bleach at home.

woelen - 25-9-2013 at 06:02

Electrolysis is used to make Cl2 and NaOH, but these chemicals are prepared separately in a cell with anode space and cathode space separated by means of some membrane.

The produced chlorine and solution of NaOH are mixed, while keeping the solution cold to assure that (nearly) no chlorate is produced. It might be that the chlorine and solution of NaOH are not produced on the same location as where they are used for making bleach.

papaya - 25-9-2013 at 09:09

Woelen while what you're saying makes sense and surely will work, I found this on wiki: "Large amounts of sodium hypochlorite are also produced electrochemically via an un-separated chloralkali process" https://en.wikipedia.org/wiki/Hypochlorite
Doesn't un-separated process mean there's no diaphragm used?
Also here: "Today, an improved version of this method, known as the Hooker process (named after Hooker Chemicals, now Occidental Petroleum), is the only large scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold and dilute sodium hydroxide solution. It is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate." https://en.wikipedia.org/wiki/Sodium_hypochlorite
Of course wiki is not something too reliable, but from these two I concluded that CL2 and NaOH form and react in situ if electrodes are held very close, low temperatures are needed, but not the membrane, or?

[Edited on 25-9-2013 by papaya]

elementcollector1 - 25-9-2013 at 09:27

Quote: Originally posted by papaya  
Woelen while what you're saying makes sense and surely will work, I found this on wiki: "Large amounts of sodium hypochlorite are also produced electrochemically via an un-separated chloralkali process" https://en.wikipedia.org/wiki/Hypochlorite
Doesn't un-separated process mean there's no diaphragm used?
Also here: "Today, an improved version of this method, known as the Hooker process (named after Hooker Chemicals, now Occidental Petroleum), is the only large scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold and dilute sodium hydroxide solution. It is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate." https://en.wikipedia.org/wiki/Sodium_hypochlorite
Of course wiki is not something too reliable, but from these two I concluded that CL2 and NaOH form and react in situ if electrodes are held very close, low temperatures are needed, but not the membrane, or?

[Edited on 25-9-2013 by papaya]


In an electrolytic brine cell, to produce NaOCl instead of NaClO3 one must keep the electrodes far apart and the temperatures cold. No diaphragm is used. I would imagine that the farther the electrodes are apart, the less vigorous the reaction is, and the less heat produced. Of course, this is armchair speculation based on hearsay. Take it with a grain of NaCl.

woelen - 25-9-2013 at 10:00

I sometimes learn things as well ;) In my perception, making NaClO by means of direct electrolysis is a bad process, but apparently it can be done in a satisfactory way without separation of anode and cathode electrolyte.

I indeed think that having the electrodes close to each other is advantageous, because in that way, the chlorine formed at the anode directly mixes with the hydroxide, before it can escape as gas from the liquid. Cooling of the liquid is very important, otherwise you get chlorate instead and I think that the nature of the electrodes also is important. At some anode materials, there may be oxidation of hypochlorite and at some cathode materials there may be reduction of hypochlorite. So, a cell for making hypochlorite must be constructed carefully, using the right types of electrodes.

papaya - 25-9-2013 at 12:25

Hmmm, what about the voltage ? Is there a potential at which an oxidation to hypochlorite occurs but not further? Do you know any numbers?
Also stirring may eliminate the need of electrodes putting too close.

Traveller - 25-9-2013 at 20:17

If the brine solution in the cell is kept close to neutral pH, the loss of Cl2 to the atmosphere is minimal to nonexistent. This is because Cl2 easily dissolves in water to great volume, giving us HClO and HCl. However, as HClO is a weak acid (minimizing the production of HCl), and there is no Cl2 to combine with the NaOH at the cathode, the pH of the brine will slowly climb to between 8 and 9. As this will produce, at this pH, an unstable mixture of hypochlorite/hypochlorous acid, once the electrolysis is turned off, the hypochlorus acid will decompose to HCl and convert hypochlorite to replace itself; eventually consuming almost all of the hypochlorite in the process. The conversion of HClO to HCl lowers the pH and makes a higher percentage of the hypochlorite into hypochlorous acid, accelerating the decomposition. Once a pH of 5 is reached, Cl2 gas will leave the solution.

The only way to stabilize hypochlorite/hypochlorous acid, made through electrolysis of NaCl/H2O, is to add enough NaOH to the solution to raise the pH over 12. This effectively converts all of the hypochlorous acid to the stable hypochlorite, stabilizing and preserving the solution.

[Edited on 26-9-2013 by Traveller]

JasonHerbalExt - 28-9-2013 at 19:52

2NaCl + 2H2O = 2NaOH + H2↑ + Cl2↑
CL2 + H2O <=> HCLO + HCl
--------------------------------------------------
2NaCl + H2O = NaClO + H2 + NaCl

I guess adding OH- to products is to stabalize the ClO-, prevent it from changing back to Cl2