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Natural Science Forum / Chemistry / Electrochemistry / July 2008Electrolysis question: MgSO4, H20
Hi All, I built a water electrolysis chamber. I am using MgSO4 in H20 as the electrolyte. I built it with 2 plexiglass boses, each with one side open, that bolt together on the open side, with a small chamber between the 2 boxes, allowing me to try different materials for the semi-permeable membrane in the middle section. The chamber is working nicely. I am getting H2 from the negative side. It produces about 1 liter of H2 every 30 minutes. I have a few questions on the whole process: On the half of the chamber with the negative terminal, the water is turning a dark bluish color. Is this the Magnesium ions moving to that side of the chamber? On the half of the chamber with the positive terminal, the water is turning a orange color. Is this the SO4 ions moving to that side of the chamber? Would the reaction for the electrolyte be: MgSO4 + H2O --> MgOH + HSO4 As I understand it, the ions will move to the electrode of opposite polarity and combine with either the H+ proton or the OH- ion to neutralize them. i.e. the Mg+ will combine with OH- at the cathode side, and SO4- will combine with the H+ on the anode side. Is this correct? Of course the standard 1/2 reactions would happen as well at each electrode, to produce H2 gas at the cathode and O2 gas at the anode from electrolysis of the H20. For now I am just using a thick material as a membrane that isn't doing much to prevent the O2 and H2 gases from combining in the electrolyte. But it is preventing the ions from easily re-combining. I am going to try Agar, which is commonly used as a salt bridge, for the membrane material. I understand that the H+ protons and electrons can migrate through it, but not, Ions or gases. Is this correct? Once I use the membrane, I don't understand how the Mg and SO4 ions will be allowed to move across the chamber, as they do. Do I need to separate them first? and put the Mg ions in one half and SO4 ions in the other half? Any suggestions, or references to what really is going on in an electrolysis cells would be appreciated. Thanks Roger To add to my post: Why wouldn't the H+ ions migrate over to the negative electrode, and the OH- ions migrate over to the + electrode? From what I read they stay near the electrodes they are created at and combine with electrolyte. Likely the can't move in the H20 as it is not conductive enough. On first look, electrolysis sounds quite simple. But the more I read on it the more complex, and interesting it seems. For example, I understand by using KOH as the electrolyte, in a alkaline cell, the OH- ions would flow (being taken from the KOH, not the ones involved in the electrolysis of water). So the membrane needs to allow the OH- ions to flow through it, and not the gases, or KOH. I take it that there would also be acidic cells, where the H+ ions flow, has anyone heard of this? What would be the simplest, most practical acid to use for it? Thanks again, Roger >Hi All, > [quoted text clipped - 8 lines] >The chamber is working nicely. I am getting H2 from the negative >side. It produces about 1 liter of H2 every 30 minutes. at STP. >I have a few questions on the whole process: > >On the half of the chamber with the negative terminal, the water is >turning a dark bluish color. Is this the Magnesium ions moving to >that side of the chamber? No. most likely its corrosion from your copper contacts making a concentrated acid solution of copper sulfate. >On the half of the chamber with the positive terminal, the water is >turning a orange color. Is this the SO4 ions moving to that side of >the chamber? i'm not sure what the orange color is. do you have any iron in that solution? >Would the reaction for the electrolyte be: > >MgSO4 + H2O --> MgOH + HSO4 no. the MgSO4 is the electolyte. it makes the water more conductive. pure water is a lousy conductor >As I understand it, the ions will move to the electrode of opposite >polarity and combine with either the H+ proton or the OH- ion to >neutralize them. >i.e. the Mg+ will combine with OH- at the cathode side, and SO4- will >combine with the H+ on the anode side. Is this correct? not really. if you add a pinch of MgSO4 or a cup, its probably going to give you the same peformance. that's a good experiment. plot the weight of MgSO4 added to your standard volume versus the volume of H2 generated at STP. repeat making up the solution by adding known weights of MgSO4. record teh room temp and barometric pressure to compensate for STP. what does the slope of the line tell you? >Of course the standard 1/2 reactions would happen as well at each >electrode, to produce H2 gas at the cathode and O2 gas at the anode >from electrolysis of the H20. those half reactions are what you should focus on. >For now I am just using a thick material as a membrane that isn't >doing much to prevent the O2 and H2 gases from combining in the [quoted text clipped - 3 lines] >the membrane material. I understand that the H+ protons and electrons >can migrate through it, but not, Ions or gases. Is this correct? yes >Once I use the membrane, I don't understand how the Mg and SO4 ions >will be allowed to move across the chamber, as they do. they don't. > Do I need to >separate them first? >and put the Mg ions in one half and SO4 ions in >the other half? you can't. >Any suggestions, or references to what really is going on in an >electrolysis cells would be appreciated. > >Thanks > >Roger look up the "Nernst Equation." Check out amperometry. remember. one mole of electrons will produce 1/2 mole of H2.... > >The chamber is working nicely. I am getting H2 from the negative > >side. It produces about 1 liter of H2 every 30 minutes. > > at STP. Yes, at STP. > >On the half of the chamber with the negative terminal, the water is > >turning a dark bluish color. Is this the Magnesium ions moving to > >that side of the chamber? > > No. most likely its corrosion from your copper contacts making a > concentrated acid solution of copper sulfate. there is no copper in the cells, only stainless steel > >Would the reaction for the electrolyte be: > > >MgSO4 + H2O --> MgOH + HSO4 > > no. the MgSO4 is the electolyte. it makes the water more conductive. > pure water is a lousy conductor understood, but how does the electrolyte help the H+ and OH- ions travel through the water? I'd like to know the specifics, so I know what is actually moving through the membrane, > that's a good experiment. plot the weight of MgSO4 added to your > standard volume versus the volume of H2 generated at STP. repeat > making up the solution by adding known weights of MgSO4. record teh > room temp and barometric pressure to compensate for STP. > > what does the slope of the line tell you? (liter of H2)/(g of Mg) ? roughly the conductance of the electrolyte?? > >I am going to try Agar, which is commonly used as a salt bridge, for > >the membrane material. I understand that the H+ protons and electrons > >can migrate through it, but not, Ions or gases. Is this correct? > > yes electrons? they can't free flow through a fluid, so they must attach to an ion, then pass through the bridge, then attach to another ion??? Looking forward to your responses. Thanks Roger >> >The chamber is working nicely. I am getting H2 from the negative >> >side. It produces about 1 liter of H2 every 30 minutes. [quoted text clipped - 11 lines] > >there is no copper in the cells, only stainless steel is there any chance that your stainless plates are soldered/connected with copper? as salmon egg says, nickel and chromium can produce some colordul species. actually, the orange color could be oxidized Cr species (CrO4-2) at the cathode. >> >Would the reaction for the electrolyte be: >> [quoted text clipped - 7 lines] >I'd like to know the specifics, so I know what is actually moving >through the membrane, the protons (H+) are very mobile and make it across the bridge. locally (at the cathode and anode), you can drive the pH high or low. i had the pleasure of toying with coulometric titrations for an analytical chem lab. >> that's a good experiment. plot the weight of MgSO4 added to your >> standard volume versus the volume of H2 generated at STP. repeat [quoted text clipped - 5 lines] >(liter of H2)/(g of Mg) ? roughly the conductance of the >electrolyte?? drylabbing it (an executable offense in college) ( : ) ) the line will be flat (zero slope) you can add all the epsom salt you like, and the H2 generation will still be related to teh number of electrons you put through the solution, less losses. >> >I am going to try Agar, which is commonly used as a salt bridge, for >> >the membrane material. I understand that the H+ protons and electrons [quoted text clipped - 10 lines] > >Roger i'm reminded of the analogy of a straw filled with peas. you put one in one end and another pops out the other almost instantaneously. see: http://en.wikipedia.org/wiki/Electrochemistry specifically Electrolysis of water Main article: Electrolysis of water Water can be converted to its component elemental gasses, H2 and O2 through the application of an external voltage. Water doesn't decompose into hydrogen and oxygen spontaneously as the Gibbs free energy for the process at standard conditions is about 474.4 kJ. The decomposition of water into hydrogen and oxygen can be performed in an electrolytic cell. In it, a pair of inert electrodes usually made of platinum immersed in water act as anode and cathode in the electrolytic process. The electrolysis starts with the application of an external voltage between the electrodes. This process will not occur except at extremely high voltages without an electrolyte such as sodium chloride or sulfuric acid (most used 0.1 M). Bubbles from the gases will be seen near both electrodes. The following half reactions describe the process mentioned above: \mbox{Anode (oxidation): }2H_{2}O(l) \rightarrow O_{2}(g) + 4H^{+}(aq) + 4e^{-}\, \mbox{Cathode (reduction): }2H_{2}O(g) + 2e^{-} \rightarrow H_{2}(g) + 2OH^{-}(aq)\, \mbox{Overall reaction: }2H_{2}O(l) \rightarrow 2H_{2}(g) + O_{2}(g)\, (my comment)( this text didn't copy clearly. click the link for much more readable text) Although strong acids may be used in the apparatus, the reaction will not net consume the acid. While this reaction will work at any conductive electrode at a sufficiently large potential, platinum catalyzes both hydrogen and oxygen formation, allowing for relatively mild voltages (~2V depending on the pH). (long story short, the respective gas gets evolved and the 'leftover ions' neutralize each other through the bridge.) Wikipedia actually provides a wealth of supporting explanation. > conductive electrode at a sufficiently large potential, platinum > catalyzes both hydrogen and oxygen formation, allowing for relatively [quoted text clipped - 4 lines] > > Wikipedia actually provides a wealth of supporting explanation. Good points, I took at look at that article in Wiki. And it had an interesting statement: "The electrolysis starts with the application of an external voltage between the electrodes. This process will not occur except at extremely high voltages without an electrolyte such as sodium chloride or sulfuric acid (most used 0.1 M)." Which is commonly known as the reason either a salt, acid or base is added in the electrolyte. However, what is the "extremely high voltage" needed without the presence of an salt, acid or base for the electrolysis reaction to continue? That may be an interesting way to do it? Yes, with the higher voltage there is a chance of a spark within the cell, which could be a big problem, but maybe there is a way around it, depending on what voltage level is needed. Thanks Roger In article <7419f447-78f2-4b22-8abf-07056192befc@m36g2000hse.googlegroups.com>, > I built a water electrolysis chamber. I am using MgSO4 in H20 as the > electrolyte. I do not remember you telling us what the electrodes are made from. If you used copper somewhere, it might be dissolving. A negative electrode will attract copper ions which could provide the blue color. Magnesium and sulfate ions are colorless. Bill > I do not remember you telling us what the electrodes are made from. If > you used copper somewhere, it might be dissolving. A negative electrode > will attract copper ions which could provide the blue color. Magnesium > and sulfate ions are colorless. > > Bill The electrodes are stainless steel sheets with a grid of small holes all over them (for now only 304 grade stainless, I will switch to 316 grade when they need to be replaced). I know nickel plated iron is better, but the price was right for this set of electrodes - free. The conductors within the chamber are all stainless steel bolts, washers and nuts, not sure the grade of them. That is all that is conductive or metal in the cells, there is no copper used in it. Thanks Roger In article <f109bfa3-e0ad-483c-8354-5b49375d42ad@v39g2000pro.googlegroups.com>, > The electrodes are stainless steel sheets with a grid of small holes > all over them (for now only 304 grade stainless, I will switch to 316 [quoted text clipped - 6 lines] > That is all that is conductive or metal in the cells, there is no > copper used in it. Certainly nickel and chromium can provide strongly colored ions. Bill > In article > <7419f447-78f2-4b22-8abf-07056192befc@m36g2000hse.googlegroups.com>, [quoted text clipped - 6 lines] > will attract copper ions which could provide the blue color. Magnesium > and sulfate ions are colorless. Bullshit. You don't get a blue colour because Cu ions are attracted to the cathode, but you might because there are Cu ions in solution. I am wondering though how they got into the cathode compartment and not the other, where they might be generated, if there is copper in contact with the solution. You will not dissolve Cu from a cathode, so this blue colour mystifies me. There are things Roger has not told us, that he may not be aware of himself. A cautionary remark, which might sound a bit arrogant: if you, Roger, have to ask these very basic questions on electrolysis, then you should not be generating hydrogen (and oxygen). People have come to grief doing this (explosions).  SignatureDieter Britz (britz<at>chem.au.dk) he may not be aware of himself. > A cautionary remark, which might sound a bit arrogant: if you, Roger, > have to ask these very basic questions on electrolysis, then you > should not be generating hydrogen (and oxygen). People have come to > grief doing this (explosions). Point taken, and I fully understand that H2 is and explosive gas, and am taking all precautions with my set ups. A perhaps sounding arrogant, response is: I don't need to know the exact chemistry and physics in my cell to safely produce H2. There are standards on how to do it. I want to learn more about it to improve my setup. And as per the responses, I don't think these are simple questions. The more I read about electrolysis, the more I realize there is to know about it. For example there is nothing in the responses in this group that indicate that there are various types of cells, for example where either the OH- ions migrate (in an alkaline cell) or the H+ ions migrate (in a high temperature cell) and what the membrane material needs to be for each type of cell, and that there could be different anolyte solutions than catholytes solution, etc. Nobody here mentioned that asbestos is a standard membrane used in the industry for some alkaline cells. I think that the knowledge base here is University and educational based, rather than industrial based. Which, being an electrical engineer myself, I can appreciate, however the industry knowledge base is the type of practical input I need. I just want to learn all I can about electrolysis, and specifically the practical aspects of materials used in real industrial productions. Thanks Roger > I just want to learn all I can about electrolysis, and specifically > the practical aspects of materials used in real industrial > productions. Specifically right now I need to improve the choice of membrane material, which my line of questioning was geared to solve. If the colors of the anolye and catholyte are due to iron in the solution, that can be solved later, but is not a significant problem to me now. I can go to 316 metal later, and nickel coated metal for the electrodes, which are standard. The reason for asking is to see what exactly is happening in the cell in terms of ion migration. The other approach is to see what is done in the industry, and go with it for now. There must be long term proven setups and configurations for this. And methods of stacking cells to increase output while still keeping them safe and efficient. Regards, Roger Frist of all I need to know what kind of electrodes are you using, is platinum or carbon or copper. >Frist of all I need to know what kind of electrodes are you using, >is platinum or carbon or copper. I have a sistem that work very well without membrane or any thigh else. The electrodes are of stainless steel. The electrolite is NaOH. The electrodes are in the same bottle and one of them has separated with a PVC tube. > I have a sistem that work very well without membrane or any thigh else. > The electrodes are of stainless steel. The electrolite is NaOH. > The electrodes are in the same bottle and one of them has separated > with a PVC tube. Hi Jose, Could you describe your setup further? Is there a PVC tube within another bottle, so the PVC is separating the electrodes? I don't quite understand it. Thanks Roger OK Rogers If I got your email I to send you a picture because is to long for explain. A photo is worth a 1000 words But inside of the PVC is a steel electrode and out side is the other. At the top of the PVC tube I open a hole for H2 exit, and a tygon tube. The PVC tube is almost at the botom of the bottle. The O2 go free to the environment, I don't need it. José >> I have a sistem that work very well without membrane or any thigh else. >> The electrodes are of stainless steel. The electrolite is NaOH. [quoted text clipped - 10 lines] > >Roger Roger I put the photo in my group. http://groups.google.co.ve/group/electroquimicos?hl=es Any coment is welcome José >> I have a sistem that work very well without membrane or any thigh else. >> The electrodes are of stainless steel. The electrolite is NaOH. [quoted text clipped - 10 lines] > >Roger On Jul 28, 2:01 pm, "jose_gregorio_contreras via NatScience.com" <u44857@uwe> wrote: > Roger I put the photo in my group. > > http://groups.google.co.ve/group/electroquimicos?hl=es > > Any coment is welcome Hi Jose, I see your picture there, thanks for posting it. I can see how you have the PVC tube in it. But is the bottom of the PVC tube closed off? or is it open to the electrolyte? If the bottom of the PVC tube is closed, this means that the ions are traveling right through the PVC tube, correct? If so that gives me great options for another material to use as a divider. If the bottom of the tube is open, then the ions are taking that path. Thanks Roger The botton is open. The H2 has low solubility in the water and not go down. Unless the H2 exit is closed. José >On Jul 28, 2:01 pm, "jose_gregorio_contreras via NatScience.com" ><u44857@uwe> wrote: [quoted text clipped - 20 lines] > >Roger On Jul 28, 5:13 pm, "jose_gregorio_contreras via NatScience.com" <u44857@uwe> wrote: > The botton is open. The H2 has low solubility in the water and not go down. > Unless the H2 exit is closed. > > José Hi Jose, I understand your setup now, thanks. Roughly what voltage are you using on the electrodes? I am concerned that the effective distance between the electrodes is increased, so the voltage would have to be increased to compensate for this. Because the conductive path is the distance down to the bottom of the tube and back up inside it (rather than just the distance directly between electrodes). It would still work OK, but with the increased voltage, the efficiency of the cell would drop. Thanks Roger On Jul 24, 5:51 pm, "jose_gregorio_contreras via NatScience.com" <u44857@uwe> wrote: > Frist of all I need to know what kind of electrodes are you using, > is platinum or carbon or copper. The electrodes are stainless steel, 304 grade for now. Later I 'll switch to 316 grade on the cathode and nickel plated on the anode, which is common. I am not to concerned about them degrading for now. I mainly need to determine a good material for the membrane so the O2 and H2 stay apart. Thanks Roger > he may not be aware of himself. >> [quoted text clipped - 27 lines] > the practical aspects of materials used in real industrial > productions. There are books on industrial electrochemistry. As for university types, they often do know their stuff, even on practical matters. I find it hard to believe that people still use asbestos as dividers, except maybe in old cells. The bit about extremely high voltages in pure water, this is due to ohmic drop, because pure water is a very poor conductor. Add some salt, and this drops, and so does the cell voltage needed. You should read a text on electrochemistry, such as that by Bard and Faulkner, which will explain a lot of all this. SignatureDieter Britz (britz<at>chem.au.dk) > > he may not be aware of himself. > [quoted text clipped - 40 lines] > -- > Dieter Britz (britz<at>chem.au.dk) Hi Dieter, I understand that H20 is a poor conductor, hence the need for an electrolyte. But if the electrolyte could be eliminated with usage of a high voltage, that could be advantageous. I believe the electric field would pull the ions across, rather than conduction by the Ohmic process. The book by Bard and Faulkner looks more theoretical than practical. Do you have any suggestions for industrial based books on this topic? Thanks Roger >> > he may not be aware of himself. >> [quoted text clipped - 52 lines] > > Do you have any suggestions for industrial based books on this topic? Try Derek Pletcher. I think the title was in fact "Industrial Electrochemistry". Incidentally, a good membrane would be fritted glass or ceramics. You can get these with various porosities. The more porous, the smaller the electric resistance across, but also the greater the chance of mixing of the cathode and anode chambers' contents. Glass frit of about no. 3 seems a good compromise to me. SignatureDieter Britz (britz<at>chem.au.dk) > Try Derek Pletcher. I think the title was in fact "Industrial > Electrochemistry". [quoted text clipped - 5 lines] > -- > Dieter Britz (britz<at>chem.au.dk) Hi Dieter, Great tips on this subject. The fritted glass looks like a very practical way to go, and the book looks perfect for what I am after. Thanks again Roger > Try Derek Pletcher. I think the title was in fact "Industrial > Electrochemistry". [quoted text clipped - 5 lines] > -- > Dieter Britz (britz<at>chem.au.dk) Hi Dieter, I picked up the book you suggested. It has a lot of good practical and theoretical detail on electrolyzers. It does mention that a common divider material still seems to be asbestos, in a membrane type cell. Another common one is the flouropolymer type membrane. It doesn't mention any other materials. I am still trying to find someplace to get fretted glass. I am still hopeful that there is a more common/low cost material to use for the divider. Roger |
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