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Photo Forum / Film Photography / Darkroom / November 2004Chelating agent for copper (II) in acidic media
Hello, I'm looking for a chelating agent, which binds to copper (II) in acidic media with pH somewhere in the range 1 ... 2. I know of a few chelating agents for copper (II), but they all tend to be most effective at the basic side of the pH range. Does anyone of you know of a chelating agent, suitable for such low pH? As an added constraint I want to mention, that this compound must be available for the public at reasonable price. Hence, it should not be very toxic, unstable, air sensitive, etc. I intend to use it as part of a photographic toner solution. Thanks, Wilco > Hello, > [quoted text clipped - 12 lines] > > Wilco Wilco, The best chelating agent for copper is EDTA (ethylenediaminetetraaacteic acid whose di-sodium salt is usually used. The best pH for copper chelation is around 2.0. Once the complex is formed at this pH it is stable in highly alkaline and acidic media. EDTA is stable, perhaps easily available, non-toxic and stable with respect to air-oxidation. farooq_w@hotmail.com wrote:> > Wilco, > The best chelating agent for copper is EDTA [quoted text clipped - 3 lines] > EDTA is stable, perhaps easily available, non-toxic and stable with > respect to air-oxidation. Farooq, I actually have some EDTA-Na2 and EDTA-Na4. Both of these perfectly chelate with copper (II), but as soon as a few drops of 1 M H2SO4 are added, then the chelate is destroyed. I tried this, by mixing a solution of CuSO4 and K3Fe(CN)6. Without EDTA this mixture gives a precipitate. With EDTA (either -Na4 or -Na2) the solution remains clear. However, when the solution is somewhat acidified, then a precipitate is formed again. One can also see that the complex is destroyed without the ferricyanide. The complex has a deep blue color. On acidification with even a small amount of H2SO4, the deep blue color disappears and the color of plain aqueous Cu(2+) appears again. As you write, the complex should be stable. Did I miss something? Should the complex be prepared in a special way? Is the blue complex I have not the same as what you are referring to? What I want is that Cu(2+), [VO2](+) and [Fe(CN)6](3-) all are in solution at the same time. The pervanadyl only can exist at low pH. I can have the pervanadyl and ferricyanide in solution at the some time, but adding the copper is a problem. When I manage to have all these three in solution, I probably would have created a bright orange toner, which is quite special in photography. If you are interested in this aspect of chemistry, the idea behind metal-ferricyanide toning is explained in http://www.woelen.nl/photo/toner.pdf which I wrote approximately half a year ago. The second part of this document may be most interesting for you. Wilco > farooq_w@hotmail.com wrote:> > > Wilco, [quoted text clipped - 37 lines] > which I wrote approximately half a year ago. The second part of this > document may be most interesting for you. You need a hard base that sticks around at low pH without reducing the copper. Cu(II) is substitution labile. Nitrate isn't much of a grabber. What about hydroxamic acids (react esters with hydroxylamine to displace the alcohol)? Dimethyl tartrate is available, and the extra hydroxyls confer water solubility.  SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz.pdf > farooq_w@hotmail.com wrote:> > > Wilco, [quoted text clipped - 23 lines] > Should the complex be prepared in a special way? Is the blue complex I > have not the same as what you are referring to? Copper does form a precipitate with [Fe(CN)6](3-). Doesn't the ferricyanide ion decompose in highly acidic medium to HCN? Na4EDTA is very alkaline, but using disodium salt is better. Dissolving H4EDTA is frustating. I don't think here is a specific requirement for the preparation of its EDTA complex. Admittedly, I never went below the pH of 2(which is failry acidic) for the preparation of copper-EDTA , but once the complex is prepared you can increase the pH to 12 without any signs of precipitation, even the blue color is not destroyed. Below the recommended pH of 2, the complex doesn't form at all because EDTA, then exists totally in protoned form, the six binding sites are "blocked" by H. Another thing that might be relevant to your problem is copper-bromo complex, add few drops of ammonium bromide in a copper solution acidified with HCl (ie at very low pH~1), the solution becomes violet...a qualitative test for copper. There are many organic complexing reagents for copper but most of them are used in gravimetric analysis, ie the complexes are highly insoluble. Copper-tartarate requires alkaline medium and perhaps citrate too. > What I want is that Cu(2+), [VO2](+) and [Fe(CN)6](3-) all are in > solution at the same time. The pervanadyl only can exist at low pH. I > can have the pervanadyl and ferricyanide in solution at the some time, > but adding the copper is a problem. I don't know whether Cu-EDTA, pervanadyl ion and ferricyanide can coexist together. If by chance a better chelating agent is found it must have very low affinity for vanadium and iron. Is pervanadyl ion a true peroxy compound- I guess it should be [VO2](3+), assuming V(+5). > When I manage to have all these > three in solution, I probably would have created a bright orange toner, [quoted text clipped - 3 lines] > > http://www.woelen.nl/photo/toner.pdf Very well written. > which I wrote approximately half a year ago. The second part of this > document may be most interesting for you. > > Wilco I have the impression that postings sometimes are not displayed. My reply on your posting does not display at google groups-beta. Are there other people who have similar problems? My reply however, does display on groupsrv, but over there, two other replies are missing. > I have the impression that postings sometimes are not displayed. My > reply on your posting does not display at google groups-beta. Are there > other people who have similar problems? Yes. It happens sometimes. > My reply however, does display > on groupsrv, but over there, two other replies are missing. I would recommend trying L-cysteine hydrochloride hydrate (or L-cysteine; it is the same thing). It is reasonably cheap and nontoxic - edible. The only disadvantage is that while the stuff is stable as a solid, the aqueous solutions have to be stored in fridge to slow down the air oxidation. Oxidized cysteine solution can develop precipitate. If you add some antioxidant (ascorbic acid!), the solution will keep longer. [Also, try to sell your cysteine/ascorbic photo solution to a nutrition supplement stores as well - the body builders will quaff it down by a pint] > farooq_w@hotmail.com wrote:> > > Wilco, [quoted text clipped - 39 lines] > > Wilco > I would recommend trying L-cysteine hydrochloride hydrate (or > L-cysteine; it is the same thing). It is reasonably cheap and nontoxic [quoted text clipped - 3 lines] > If you add some antioxidant (ascorbic acid!), the solution will keep > longer. This sounds good. Storage is no problem, because the solutions are not stored. They are prepared, just before use and then they are discarded. I only wonder if L-cysteine withstands the oxidizing power of copper (II) or [VO2](+). The latter is a fairly strong oxidizer with a redox potential (for V(+5) to V(+4) at pH=0) equal to 1.0 V. Using ascorbic acid as antioxidant does not work, this reduces copper (II) to copper(I) at once and the pervanadyl is reduced to blue vanadyl immediately. > [Also, try to sell your cysteine/ascorbic photo solution to a > nutrition supplement stores as well - the body builders will quaff it > down by a pint] I also have a bottle containing some nice blue VOSO4.xH2O, the ideal stuff for bodybuilders... I, however, prefer to keep my health :-) Sorry, this would not work then. I did not realize that you wanted to have vanadate around. Cysteine is a pretty good reducing agent, it would get eaten by it. > > I would recommend trying L-cysteine hydrochloride hydrate (or > > L-cysteine; it is the same thing). It is reasonably cheap and [quoted text clipped - 20 lines] > I also have a bottle containing some nice blue VOSO4.xH2O, the ideal > stuff for bodybuilders... I, however, prefer to keep my health :-) |
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