Iron hydroxides buffer

The voltammograms for marmatite electrode in different pH buffer solutions are presented in Fig. 4.22. It can be seen from Fig. 4.22(a) that the first anodic peak occurred at about 200 mV, which may be due to the oxidation of dithiocarbamate to disulphide. The anodic oxidation peaks at higher potential may be attributed to the further oxidation of marmatite to form oxy-sulphur and zinc/iron hydroxide resulting in flotation descending. 

In the analysis of the deposition of iron sediments it has already been mentioned that quite likely both iron silicates and carbonates and amorphous iron hydroxide were formed, which could convert to other forms both during the formation of the sediment and in subsequent diagenesis. Reduction of hydroxide could have been controlled by external (atmospheric) or internal (organic matter, free carbon in the sediment) oxidation-reduction buffer systems. All these variants need additional consideration in the thermodynamic analysis of diagenetic processes. 

For example, if the amount of ferrous iron minerals present in repository backfill and fracture minerals (represented by FeC03(s) in Fig. 1(a)) is much greater than the amount of O2 remaining after closure, then with time, all O2 will be reduced to H2O by these minerals, producing iron hydroxide in the process. This would ensure that the reducing intensity would return to values at least as low as the redox potential of the Fe(0H)3(s)/FeC03(s) couple (near —0.05 V). This is below the threshold for corrosion of either copper or uranium oxide by O2. It is also shghtly above the threshold for sulphide production by sulphate reduction (—0.2 V). The presence of ferrous minerals thus buffers the redox intensity of the repository to conditions that are favourable for repository performance. 

FIGURE 13.7 Preparation and characterization of single-walled nanotube (SWNT) forest formed by metal-assisted deposition of oxidized, shortened SWNTs. (a) Schematic representation of SWNT forest preparation. Shortened, open-ended SWNTs with carboxyl-functionalized ends are produced by oxidation of SWNTs. A suspension of SWNTs is introduced to a metal surface functionalized with iron hydroxides. An SWNT forest results as SWNTs vertically align via self-assembly. (Part a adapted with permission from Chattopadhyay, D., Galeska, I., and Papadimitrakopoulos, F., Metal-assisted organization of shortened carbon nanotubes in monolayer and multilayer forest assemblies, J. Am. Chem. Soc., 123,9451-9452,2001. Copyright 2001 American Chemical Society.) Cyclic voltammograms (scan rate 300 mV s ) of (b) SWNT forest electrodes in pH 5.5 buffer with and without 0.2 mM HjOj. (Continued)... 

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

Calculate the solubility (g/100 mL) of iron(n) hydroxide in buffered solutions with the following pH s. 

Discussion. Minute amounts of beryllium may be readily determined spectrophotometrically by reaction under alkaline conditions with 4-nitrobenzeneazo-orcinol. The reagent is yellow in a basic medium in the presence of beryllium the colour changes to reddish-brown. The zone of optimum alkalinity is rather critical and narrow buffering with boric acid increases the reproducibility. Aluminium, up to about 240 mg per 25 mL, has little influence provided an excess of 1 mole of sodium hydroxide is added for each mole of aluminium present. Other elements which might interfere are removed by preliminary treatment with sodium hydroxide solution, but the possible co-precipitation of beryllium must be considered. Zinc interferes very slightly but can be removed by precipitation as sulphide. Copper interferes seriously, even in such small amounts as are soluble in sodium hydroxide solution. The interference of small amounts of copper, nickel, iron and calcium can be prevented by complexing with EDTA and triethanolamine. 

Cobalt in steel Discussion. An alternative, but less sensitive, method utilises 2-nitroso-l-naphthol, and this can be used for the determination of cobalt in steel. The pink cobalt(III) complex is formed in a citrate medium at pH 2.5-5. Citrate serves as a buffer, prevents the precipitation of metallic hydroxides, and complexes iron(III) so that it does not form an extractable nitrosonaphtholate complex. The cobalt complex forms slowly (ca 30 minutes) and is extracted with chloroform. 

Ke and Regier [71] have described a direct potentiometric determination of fluoride in seawater after extraction with 8-hydroxyquinoline. This procedure was applied to samples of seawater, fluoridated tap-water, well-water, and effluent from a phosphate reduction plant. Interfering metals, e.g., calcium, magnesium, iron, and aluminium were removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8). A buffer solution (sodium nitrate-l,2-diamino-cyclohexane-N,N,N. AT-tetra-acetic acid-acetic acid pH 5.5) was then added to adjust the total ionic strength and the fluoride ions were determined by means of a solid membrane fluoride-selective electrode (Orion, model 94-09). Results were in close agreement with and more reproducible than those obtained after distillation [72]. Omission of the extraction led to lower results. Four determinations can be made in one hour. 

Not all iron oxides are available for reduction. Some iron minerals are solid crystals or even entire iron grains, which makes them resistant to microbial reduction (Lovley, 1991 Postma, 1993 Heron et al., 1994b). Other iron oxides or hydroxides are amorphous and readily reducible. Over time, even some crystalline minerals such as goethite and hematite may be reduced in the complex environment in leachate (Heron and Christensen, 1995). This indicates that the importance of iron as a redox buffer controlling the size of plumes is not given just by the amount of iron oxides present. The composition and microbial availability of iron for reduction are key parameters. Methods for the actual quantification of the microbial iron reduction capacity have, however, not been developed. 

Re-solution of precipitated ferric hydroxide consumes hydrogen ions. These reactions provide a buffering effect against rapid pH fluctuations by maintaining the pH in the range 2.0—2.2 due to precipitation and resolubilization of ferric iron (Duncan and Walden, 1972). Other acid-generating reactions which involve ferric and other ions are the formation of jarosites (eqn (3)) ... 

The effects of pH on the binding of iron to various types of cellulose samples after 2k hours incubation at 30°C is shown in Table I. Carboxymethylcellulose bound as much as 70 of the ferrous iron at pH 7.0, compared to Whatman 3 filter paper which only bound 18 at pH = 7-0. Control samples of buffer and metal at each pH did not show any sign of metal hydroxide precipitation at the concentrations and temperature used in the study. The samples of pH 6.0 and 7.0 did however change to a faint yellow upon addition of iron. 

---