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Showing papers on "Ferric published in 1981"


Journal ArticleDOI
TL;DR: In this article, the reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide.

405 citations


Journal ArticleDOI
TL;DR: No correlation was observed between the virulence of different strains or colony types of gonococci and the ability to scavenge iron in vitro from transferrin or other chelators and theAbility of Neisseria species to use iron compounds and to compete with iron-binding proteins was examined with deferrated defined medium and the iron chelator deferoxamine.
Abstract: The ability of Neisseria species to use iron compounds and to compete with iron-binding proteins was examined with deferrated defined medium and the iron chelator deferoxamine. All Neisseria species were able to assimilate a variety of ferric and ferrous iron salts. They were not able to efficiently solubilize an inorganic iron salt such as ferric nitrate, but were able to use iron chelated by citrate, oxalacetate, pyrophosphate, or nitrilotriacetate. Each of the 95 Neisseria isolates examined was able to use hemin as a sole source of iron, and most, but not all, of the isolates were able to obtain iron from hemoglobin. Heated human serum stimulated growth of all gonococci, meningococci, and some commensal Neisseria species in iron-deficient medium. All gonococci and meningococci were able to scavenge iron from 25% saturated transferrin, whereas most commensal organisms were inhibited by this iron-binding protein. The ability to compete with transferrin was specific, since partially saturated conalbumin was bacteriostatic for all Neisseria species. Although the pathogenic Neisseria species were able to compete more efficiently with transferrin for iron than were the nonpathogenic Neisseria species, no correlation was observed between the virulence of different strains or colony types of gonococci and the ability to scavenge iron in vitro from transferrin or other chelators.

268 citations


Journal ArticleDOI
TL;DR: A reaction cycle involving humic substances and iron as a catalyst acts as an oxygen sink in humic-colored waters was investigated in this paper, showing that rates of oxygen consumption are linear functions of iron and humic color concentration and a nonlinear function of light energy and pH.
Abstract: A reaction cycle involving humic substances and iron as a catalyst acts as an oxygen sink in humic-colored waters. The cycle consists of photoreduction of Fe(II1) to Fe(I1) by humic matter and subsequent oxidation of Fe(I1) back to Fe(II1) by dissolved oxygen. Laboratory experiments to parameterize the reaction cycle indicate that rates of oxygen consumption are linear functions of iron and humic color concentration and a nonlinear function of light energy and pH. Esterification of carboxylic groups in dissolved humic material decreased oxygen consumption rates by 50%, suggesting that oxidation of humic matter results from a ligand-to-metal charge transfer through iron chelated by these functional groups. In situ consumption rates of 0.12 mg of 0 2 L-l h-l caused by this reaction sequence were measured in the surface water of Lake Mize, a highly colored lake in Florida. Such rates could account for the frequently observed low oxygen concentrations (ca. 3-4 mg L-I) in the surface waters of colored lakes.

252 citations


Journal ArticleDOI
TL;DR: The products of surface oxidation of pyrite and pyrhotite have been determined from analysis of linear potential sweep voltammograms as discussed by the authors, showing that pyrrhotite oxidizes to both sulphur and sulphate.

207 citations


Journal ArticleDOI
TL;DR: The literature on the ferric ion leaching of chalcopyrite has been surveyed to identify those leaching parameters which are well established and to outline areas requiring additional study as mentioned in this paper.
Abstract: The literature on the ferric ion leaching of chalcopyrite has been surveyed to identify those leaching parameters which are well established and to outline areas requiring additional study New experimental work was undertaken to resolve points still in dispute It seems well established that chalcopyrite dissolution in either ferric chloride or ferric sulfate media is independent of stirring speeds above those necessary to suspend the particles and of acid concentrations above those required to keep iron in solution The rates are faster in the chloride system and the activation energy in that medium is about 42 kJ/mol; the activation energy is about 75 kJ/mol in ferric sulfate solutions It has been confirmed that the rate is directly proportional to the surface area of the chalcopyrite in both chloride and sulfate media Sulfate concentrations, especially FeSO4 concentrations, decrease the leaching rate substantially; furthermore, CuSO4 does not promote leaching in the sulfate system Chloride additions to sulfate solutions accelerate slightly the dissolution rates at elevated temperatures It has been confirmed that leaching in the ferric sulfate system is nearly independent of the concentration of Fe3+, ka[Fe3+]012 In ferric chloride solutions, the ferric concentration dependence is greater and appears to be independent of temperature over the interval 45 to 100 °C

205 citations


Journal ArticleDOI
TL;DR: In 5 volunteers inhalation of 100% oxygen decreased the mean observed spin-lattice relaxation time of blood within the left ventricular cavity, and although the change was small, it may have important uses in monitoring metabolic processes.

178 citations


Journal ArticleDOI
TL;DR: The ferric oxide formed by oxidation of Fe(II) in Esthwaite Water, U.K., during the lake's seasonal thermal stratification and deep-water anoxia consists of amorphous particles which are approximately spherical or ellipsoidal, with diameters in the range 0.05-0.5 μm as mentioned in this paper.

152 citations



Journal ArticleDOI
TL;DR: It is proposed that the inhibitory effect of NO3- on microbial reduction of Fe(III) is due to a secondary reaction, which involves the chemical oxidation ofFe(II) by NO2-.
Abstract: A Pseudomonas sp. isolated from crude oil reduced ferric ions (Fe(III)) to ferrous ions (Fe(II)). In the presence of nitrate (NO3−) after prolonged incubation, the amount of Fe(II) was lower than i...

103 citations


Journal ArticleDOI
TL;DR: In this paper, the photochemical nature of the reduction of ferric to ferrous iron in the presence of oxidizable organic ligands is investigated, and a field test for ferric-organic complexes in soils, using the same vials, is proposed.
Abstract: A 1 M solution of ammonium acetate containing a, a?-dipyridyl indicator is recommended for use in field tests for exchangeable and water-soluble ferrous iron in soils. In using the test, a soil sample may be added to a vial containing the solution, or the solution can be sprayed on to a freshly exposed soil face. A field test for ferric-organic complexes in soils, using the same vials, is proposed. This is based on the photochemical nature of the reduction of ferric to ferrous iron in the presence of oxidizable organic ligands. Two subsamples of soil are added to separate vials, and light is excluded from one. After 5-15 min, a positive test for ferrous iron in the vial exposed to light, and a negative test in the other, indicates the presence of ferric-organic complexes.

100 citations



Journal ArticleDOI
TL;DR: In this paper, the influence of dissolved sulfide ion and the ferri/ferrocyanide redox system on the photoelectrochemical behavior of iron oxide has been investigated.
Abstract: The influence of dissolved sulfide ion and the ferri/ferrocyanide redox system on the photoelectrochemical behavior of iron oxide has been investigated. Particular attention was given to transient behavior and to dark current phenomena. An interpretation of the results is given in terms of a back‐reaction of photogenerated species with conduction band electrons via surface states.

Journal ArticleDOI
TL;DR: In this article, it was shown that a layer of sulfur forms on corroding chalcopyrite, but this is not the species which slows transport of ions and transfer of electrons.
Abstract: Electrochemical aspects of semiconductors are used to interpret well established observations on the kinetics of leaching of chalcopyrite. The oxidation of this n-type semiconductor is dominated by a surface film which is thermally unstable and breaks down in CS2, acetone or acidified water, or under dry nitrogen, over comparable time periods. The film is thought to be a semiconductor metal-deficient polysulfide which slows transport of Cu+ and Fe2+ products, slows electron transfer to oxidants such as Fe3+ and Cu2+, and dramatically slows supply of holes and thus electron transfer from reduced species such as Fe2+ on corroding chalcopyrite. Thus the Fe3+/Fe2+ couple (especially as sulfate) is much less reversible on corroding chalcopyrite than on pyrite or platinum. The couples Cu2+/Cu+,I3-/I- and Fe(CN)63-/Fe(CN)64- are more reversible than Fe3+ /Fe2+ but all couples are much less reversible on chalcopyrite than on pyrite. A layer of sulfur forms on corroding chalcopyrite, but this is not the species which slows transport of ions and transfer of electrons. A mixture of Fe3+/Cu2+ chlorides is one of the more effective oxidants for CuFeS2 because of relatively fast electron transfer from corroding chalcopyrite to Cu2+ and oxidation of Cu+ by Fe3+. Catalysis by iodine and by inclusion of Ag2S or FeS2 in natural chalcopyrite is explained by the electrochemical model.

Journal ArticleDOI
TL;DR: Cysteine significantly increases the absorption from nonheme iron present in vegetable foods, hemosiderin, and of a ferric salt, when this amino acid is administered during the ingestion of foods.

Journal ArticleDOI
TL;DR: Chocolate milks fortified with nine iron compounds commonly used for food fortification and with ferripolyphosphate and with a protein complex were evaluated for changes in color and flavor.

Journal ArticleDOI
TL;DR: In this article, the polarization characteristics of mild steel were studied under microaerobic conditions in four media, including a synthetic medium and the produced water from an oil field, and the results showed that both anodic and cathodic depolarization of the steel could occur, although transiently.
Abstract: The polarization characteristics of mild steel were studied under microaerobic conditions in four media, including a synthetic medium and the produced water from an oil field. In all the media, anodic depolarization was always observed when a ferric iron reducing bacterium (Pseudomonas sp.) was present. In produced water, both anodic and cathodic depolarization of the mild steel could occur, although transiently. Addition of sodium lactate as a substrate to boost the total available energy in the produced water caused the anodic depolarization to be sustained while cathodic depolarization was not abolished. Anodic depolarization was accompanied by the bacterial reduction of ferric to ferrous compounds. In the absence of the ferric iron reducing bacterium, both the anode and the cathode were polarized. A mechanism for the bacteria induced anodic depolarization is suggested.

Journal ArticleDOI
TL;DR: In this paper, the mechanism underlying the removal of four micropollutants by mineral coagulants, namely, basic aluminum polychloride and ferric chloride, was investigated.

Journal ArticleDOI
TL;DR: The flash photolysis study on the NO-ferric HRP C complex clearly indicates that the iron is pentacoordinated inHRP C while it is hexacoordinate in metmyoglobin.
Abstract: Horseradish peroxidase C (HRP; ferric) reacts with H2O2 to form Compound I, with an equilibrium constant of about 10(14) M-1. Two-step reduction of Compound I to Compound II and further to the ferric enzyme occurs reversibly at Eo' values of 0.90 and 0.93 V (pH 7.0), respectively. The pH dependence of Eo' values for each one-electron step, ferrous leads to ferric leads to Compound II leads to Compound I indicates that presence of redox-linked ionization at pKa values of 7.3 in the ferrous state, 11.0 in the ferric and 8.6 in Compound II. Zinc-substituted HRP C is oxidized to its free-radical form at an Eo' value of 0.74 (pH 6.0) Comparison of oxidized zinc HRP C with Compound I shows that Compound I contains a porphyrin pi-cation radical. The flash photolysis study on the NO-ferric HRP C complex clearly indicates that the iron is pentacoordinated in HRP C while it is hexacoordinated in metmyoglobin. From the kinetic analysis of the acid-alkaline conversion of HRP C, the second-order rate constants of the reactions with H+ and HO- are estimated to be 1.5 X 10(10) and 6.7 X 10(4) M-1s-1, respectively. The latter rate constant greatly varies with the kind of hemoproteins. In the presence of HRP C and O2, indole-3-acetate is oxidized to its hydroperoxide form, which reacts effectively with HRP C to form Compound I and further converts Compound I to a verdohemoprotein.

Journal ArticleDOI
TL;DR: In this article, the reactions of methyl radicals with ferric, ferrous, and iron-free deuteroporphyrin have been investigated in neutral and alkaline water-2-propanol mixtures.
Abstract: The reactions of methyl radicals with ferric, ferrous, and iron-free-deuteroporphyrin have been investigated in neutral and alkaline water-2-propanol mixtures. Steady-state radiolysis of methyl chloride saturated solutions of ferric deuteroporphyrin, or chemical reduction of ferric deuteroporphyrin solutions containing methyl iodide, led to a new species which is spectrophotometrically characterized. The yield (G approx. = 3) and the redox titration (2 reducing equivalents) strongly suggest the structure DPFe/sup III/CH/sub 3/. This product is found to be stable under anaerobic conditions but is oxidized to the initial ferric form in the presence of air. Pulse radiolysis experiments show that methyl radicals react with both ferric and ferrous deuteroporphyrin with reaction rate constants of 2.3 x 10/sup 9/ and 3.9 x 10/sup 9/ M/sup -1/ s/sup -1/, respectively. The reaction with iron-free deuteroporphyrin is much slower, demonstrating that methyl radicals react with iron porphyrin at the iron center. The transient species resulting from the scavenging of a methyl radical by ferric porphyrin DPFe/sup IV/CH/sub 3/ undergoes a further slow reaction leading to ferrous deuteroporphyrin (t/sub 1/2/ approx. = 200 ..mu..s). Electron (or methyl) transfer between DPFe/sup IV/CHTnumber and ferrous deuteroporphyrin leading to DPFe/sup III/CH/sub 3/ and ferric deuteroporphyrin is shown to occur with a more » second-order rate constant k = 4 x 10/sup 8/ M/sup -1/ s/sup -1/. The reactions of ..cap alpha..-hydroxyisopropyl radicals with ferric, ferrous, and iron-free deuteroporphyrin and with methyl iodide were also studied. The reactions of methyl radicals are discussed in relation to the hypothesis of radical-mediated toxicity of halogenated compounds. « less

Journal ArticleDOI
TL;DR: Kinetic and thermodynamic features match each other, suggesting that a single ionizing group is responsible for all the observed effects.

Journal ArticleDOI
TL;DR: In this article, a ferric iron reducing bacterium was found to cause anodic depolarization of mild steel by removing or preventing the formation of a protective ferric covering.
Abstract: A protective ferric covering was formed on mild steel coupons by reaction with the oxidative inhibitor, nitrite. In cultures of a ferric iron reducing bacterium, there was a loss of passivity of coupons and polarization studies revealed intense depolarization of the anode, while in the absence of the bacterium, the metal remained passive. The anodic depolarization was accompanied by conversion of ferric to ferrous compounds and was marked by the change in color of the medium from dark brown to a greenish hue. Electron micrographs revealed that in the absence of the organism, a dense, crystalline surface deposit covered the metal, but was extensively removed in the presence of the bacterium. The bacterium caused anodic depolarization of mild steel by removing or preventing the formation of a protective ferric covering.

Journal ArticleDOI
P. C. DeKock1
TL;DR: The ratio of total phosphorus to total iron can be used as an index of iron status as mentioned in this paper, however, it is the form of nitrogen, whether as the ammonium or nitrate ion which determines the nutrient balances prevailing.
Abstract: Factors which affect the iron nutrition of plants are discussed. Phosphate plays an important role in determining the form of iron, whether ferric or ferrous, within the tissues. The ratio of total phosphorus to total iron can be used as an index of iron status. It is, however, the form of nitrogen, whether as the ammonium or nitrate ion which determines the nutrient balances prevailing.

Journal ArticleDOI
TL;DR: When the only substrate added to a solution of chloroperoxidase is a hydroperoxide, the reactions are: ferric enzyme + ROOH leads to compound I + ROH and compound I - ROH leads to ferric enzymes + O2 + R OH.
Abstract: When the only substrate added to a solution of chloroperoxidase is a hydroperoxide, the reactions are: ferric enzyme + ROOH → compound I + ROH and compound I + ROOH → ferric enzyme + O2 + ROH. When...

Journal ArticleDOI
TL;DR: In this article, the authors studied the kinetics of dissolution of chemical grade zinc sulphide powder in aqueous ferric chloride solutions of concentration range of 0.2 to 0.8 mol dm−3 at temperatures ranging from 30 to 70°C. The dissolution rate follows a parabolic equation which fits well with the model of Crank-Ginstling and Brounschtein.

Journal ArticleDOI
TL;DR: From the ligand- and the temperature-induced changes in the optical spectra of the ferric enzyme it is concluded that the band at 620-630 nm is an alpha band of the low-spin heme iron species, whereas the bands at 500 and 690 nm are probably 'charge-transfer' bands of the heme with the iron in the high-spin state.

Journal ArticleDOI
TL;DR: In this paper, it has been shown that quartz can be activated for polyacrylic acid adsorption by hydrolyzing metal cations, similar to activation in flotation.

Journal ArticleDOI
TL;DR: Since the intermediate haemoglobins such as (alpha 2+ beta 3+)2 and (alpha 3+ beta 2+)2 were found to be produced by the oxidation of haemoglobin by nitrite, the changes in oxyhaemoglobin, intermediate ha Hemoglobin tetramer and methaemobic changes during the reaction were well fitted to the simulation curves generated from the reaction model.
Abstract: The sigmoidal time course of haemoglobin oxidation by nitrite, involving an initial slow reaction accompanied by a subsequent rapid reaction, was extensively explored. The initial slow reaction was much prolonged by the addition of superoxide dismutase to the reaction mixture. On the other hand, in the presence of superoxide anion generated by xanthine oxidase systems, the slow phase disappeared and the reaction changed to first-order kinetics. The oxidation of intermediate haemoglobins [defined as haemoglobin tetramer in which different chains (alpha- or beta-) are in the ferric state and in the ferrous state] such as (alpha 2+ beta 3+)2 and (alpha 3+ beta 2+)2 also proceeded in a sigmoidal manner. Similar effects of superoxide anion on these reactions were observed. Since the intermediate haemoglobins such as (alpha 2+ beta 3+)2 and (alpha 3+ beta 2+)2 were found to be produced by the oxidation of haemoglobin by nitrite, the changes in oxyhaemoglobin, intermediate haemoglobins and methaemoglobin during the reaction were followed by isoelectric-focusing electrophoresis. The amounts of (alpha 2+ beta 3+)2 were larger than those of (alpha 3+ beta 2+)2 at the initial stages of the reaction, suggesting that there is a functional difference between alpha- and beta-chains in the oxyhaemoglobin tetramer. On the basis of these results, a reaction model of the haemoglobin oxidation by nitrite was tentatively proposed. The changes in oxyhaemoglobin, intermediate haemoglobins and methaemoglobin were well fitted to the simulation curves generated from the reaction model. Details of the derivation of the equations used for kinetic analysis have been deposited as Supplement SUP 50112 (5 pages) with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K. from whom copies may be obtained on the terms indicated in Biochem. J. (1978) 169, 5.

Journal ArticleDOI
TL;DR: In this article, the authors presented an analog of enterobactin, a microbial chelating agent with the greatest stability constant known for an Fe(III) complex, which exhibits: i) hydrolytic stability; ii) water solubility; iii) N-substitution to block peptidase hydrolysis.
Abstract: For treatment of chronic iron overload (as occurs in Cooley's anemia), ferric ion sequestering agents with specific properties are necessary. Two analogues of enterobactin [a microbial chelating agent with the greatest stability constant known for an Fe(III) complex] are reported which exhibit: i) hydrolytic stability; ii) water solubility; iii) N-substitution to block peptidase hydrolysis. The first compound, N,N',N"- trimethyl-N,N',N"-tris(2,3-dihydroxysulfobenzoyl)1,3,5-triaminomethyl- benzene, [Me{sub 3}MECAMS, 6] was prepared from the amide of trimesloyl chloride (1) and MeNH{sub 2}. The resulting amide was reduced to the triamine (3) and converted in three steps to the final product 6 in 6% overall yield. The proton-dependent formation constant (log K*) for the reaction: Fe{sup 3+} + H{sub 3}L{sup 6-} = FeL{sup 6-} + 3H{sup +} is 4.87, which gives an equilibrium concentration of [Fe{sup 3+}] at pH 7.4 of 2 x 10{sup -27} M for 10{sup -5} M L (6) and 10{sup -6} M total Fe{sup 3+}. The estimated formation constant (log {beta}{sub 110}) is 40. At low pH the FeL{sup 6-} complex undergoes a series of three, one-proton reactions which probably gives a tris-salicylate complex formed by the carbonyl and ortho-catechol oxygen of the 2,3~dihydroxybenzoyl units (the same reaction that occurs with ferric enterobactin). After six hours in the presence of 6 mM ascorbate, Me{sub 3}MECAMS (6.0 mM) removed 3.7% of the ferric ion initially sequestered by the iron storage protein, ferritin. The human iron transport protein transferrin goves up iron to Me{sub 3}MECAMS with a pseudo first-order rate constant of 1.9 x 10{sup -3}min{sup -1} (ligand concentration 2 X 10{sup -4} M). This rate is comparable to that of enterobactin and other catechoyl amide sequestering agents. and greatly exceeds that of desferrioxamine B (Desferal{reg-sign}). the current drug of choice in treating iron overload. Two related compounds have been prepared in which the catechol ring is attached to the amide nitrogen through a methylene group, with amide formation with an acetyl group. In N,N',N"-triacetyl-N,N' ,N"-tris(2,3- dihydroxysulfobenzoyl) -N,N',N"-triaminomethylbenzene [NAcMECAMS, 111... and its unsulfonated precursor, the amide linkage of the catechoyl amides such as Me{sub 3}MECAMS (6) has been shifted from an endo position relative to the benzene and catechol rings to an exo position in which the amide carbonyl is not conjugated with the catechol ring and cannot form a stable chelate ring in conjunction with a catechol oxygen. The preparation of 11 and 10 proceeded from the previously described precursor of TRIMCAM, 7. borane reduction to the tri.amine 8, and amide formation with acetyl chloride to 9, followed by deprotection of the catechol oxygens with BBr{sub 3}/CH{sub 2}Cl{sub 2} to give 10. Sulfonation of 10 to NAcMECAMS, 11, is carried out in fuming sulfuric acid. In comparison with Me{sub 3}MECAMS, the protonation of NAcMECAMS (11) proceeds by an initial two-proton step in contrast to the one-proton reactions typical of the catechoyl amides, which can form a salicylate mode of coordination involving the amide carbonyl group. Also as a result of the removal of the carbonyl group from conjugation with the catechol ring, the acidity of NAcMECAMS (11) is less than Me{sub 3}MECAMS (6). While the estimated log {beta{sub 110} is approximately the same as for Me{sub 3}MECAMS (40). the effective formation constant (log K*) and pM.(- log [Fe{sub aq}{sup 3+}] ) values are lower (4.0 and 25.0, respectively).

Journal ArticleDOI
TL;DR: In this article, the authors examined the equilibria among the species Fe3+, FeCl2+ and Cl− for ultraviolet absorption spectroscopy and showed that the formation of FeOHCl+ is much less significant than has been previously reported.
Abstract: The equilibria among the species Fe3+, FeCl2+, FeCl2+, FeOH2+ and Fe(OH)2+ have been examined by ultraviolet absorption spectroscopy. Our results indicate that previous workers have generally overestimated the stability constant of FeCl2+ and that the association of Fe3+ and Cl− is predominantly inner sphere. The formation constant of FeOH2+ obtained in 0.68 m NaCl is in good agreement with our earlier results obtained in 0.68 m NaClO4. Our results indicate that formation of FeOHCl+ is much less significant than has been previously reported. Molar absorptivities for the species Fe3+, FeCl2+, FeCl2+ and FeOH2+ are reported for wavelengths between 220 and 400 nanometers.

Journal ArticleDOI
TL;DR: In this paper, the effect of various metal ions on residual nitrite was studied in both a meat system and a model system, and the results showed that ferrous (Fe++), ferric (Fe+++), cupric (Cu++) and Zinc (Zn++) ions caused a depletion of nitrite while calcium (Ca++) and magnesium (Mg++) ions did not have any effect on residual Nitrite.
Abstract: The effect of various metal ions on residual nitrite was studied in both a meat system and a model system. Reducing ability, nitrosylhemochrome, nitrate and pH were determined in an attempt to explain the mechanism of loss of nitrite in the system. The control meat system contained 2% NaCl and 156 ppm of NaNO2 while treatment groups had 0.05% of metal ions (Fe++, Fe+++, Cu++, Zn++, Ca++, and Mg++ as chloride salts) substituted for the same amount of NaCl. The results showed that ferrous (Fe++), ferric (Fe+++), cupric (Cu++) and Zinc (Zn++) ions caused a depletion of nitrite while calcium (Ca++) and magnesium (Mg++) ions did not have any effect on residual nitrite. The model system consisted of 25 mM PIPES buffer at pH 5.8 to which was added 156 ppm NaNO2 with or without 0.05% metal ions. Only ferrous (Fe++) ion decomposed more nitrite than the control after heating. Considering the results from both systems, the mode of action for each ion appeared to be different; ferrous ion appeared to act directly as an electron donor, ferric ion acted like ferrous after being reduced, cupic ion appeared to act as an electron donor after being reduced and then as an electron acceptor, and zinc ion seemed to be effective only by decreasing the pH of the system.