Iron dissolution methods

Basically, the literature provides two dissolution methods sample preparation with sample weights of 0.2—1 g and large dilutions, or smaller sample weights with less dilution (III.B). The relatively large dilution, in general after a fusion [51], for the determination of main and lesser components, as for example in silicate analysis [2], the determination of Al, Ca, Mg, Mn and Si in slags [4], Si [55], Pb and Mn [143], and also Cd, Ca, Cu, Pb, Mg and Si in ores or iron sinter [97, 147] and Cr, Mg in refractories [93] is presently used in routine analysis. 

An important oxide used in the reduction mechanisms discussed in the previous sections is Fe203. This oxide is one of the most common and low cost raw materials used in forming inexpensive CBPCs. This oxide is very stable and cannot be dissolved sufficiently in an acid solution to produce Fe " (aq) or Fe " (aq) to form CBPC by the conventional dissolution method. If, however, the Fe203 could be converted to more soluble FeO, or Fe304 that is a combination of FeO and Fe203, it can then be dissolved in the reduced state [3]. As will be discussed in this section, the reduction may be achieved simply by adding a small amount of elemental iron. We will now discuss how CBPC of Fc203 can be formed by this reduction mechanism. 

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

In situ X-ray absorption spectroscopy and X-ray diffraction, as well as the already mentioned STM studies, have shown the adsorption of anions at metal surfaces and their influence on metal dissolution. Adsorption and complexation by anions may be followed even by XPS if appropriate preparation of the metal surface and its transfer into the UHV is successful. Therefore, detailed studies with modern microscopic and spectroscopic methods are required to obtain detailed insight into the reaction steps of corrosion processes. However, even electrochemical corrosion studies give insight into their mechanism. As an example, the catalysis of iron dissolution according to Heus-ler is presented (Bonhoeffer and Heusler, 1956). 

Processes to clean nuclear power-generating equipment can be considered specialized applications of the iron oxide and copper dissolution methods considered earlier in this book. However, because of the radioactivity usually present in deposits that foul nuclear generating equipment, speciaiized procedures have been developed to clean some types of nuclear equipment. Three different types of processes are conducted (1) chemicai treatments are used to decontaminate parts of the units to allow maintenance or before decommissioning the plant (these are health physics issues) ... 

An amorphous precursor powder was desired to facilitate iron dissolution and geopolymer formation. However, when using the PVA method, a minimum calcination of 600 C was necessary to ensure PVA polymer removal. Shown in Fig. 1(a), the low angle peak near 10 20 was due to the presence of PVA, but was removed after the powder was heated to 600 C (Fig. 1(b)). The broadness of the peaks in Fig. 1(b) indicates that the calcined powder was poorly crystalline. In addition, calcination of the precursor powder caused its color change from a yellow-red to a deep ted color. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

This chapter reviews the various methods used to identify and characterize iron oxides. Most of these are non-destructive, i. e. the oxide remains unaltered while being examined. These methods involve spectroscopy, diffractometry, magnetometry and microscopy. Other methods, such as dissolution and thermal analysis destroy the sample being examined. Only the principle of each method is given here. The main weight is put on the information about Fe oxides which can be extracted from the analytical results obtained by the different techniques together with references to relevant studies. A detailed description of each technique can be found in the appropriate texts listed in each section. 

There are purely electrochemical methods for finding the amount of simple radicals such as H or O on noble metal electrodes. Basically, they rely upon die assumption that when some electrical variation in die state of die electrode is brought about, die only effect it has is to reduce or augment die H or the O on die electrode surface. Now of course this is not so if die substrate is, say, iron, or indeed all but the noble metals, for there may be a co-dissolution of die substrate, or competing oxide film formation, etc. Spectroscopic methods (e.g., FUR in a millisecond response version) or ellipsometiy are not affected by such difficulties. 

The simplest procedure, dissolution of metallic iron in the aqueous mineral acid, suffers from the risk of accidental oxidation. The following relatively simple procedure overcomes this difficulty. This method, with minor modifications, has also been used successfully by the author for the preparation of chromium-(II) halides. 

Some of the polycrystalline spin crossover systems of iron(II) described above retain their spin equilibrium property upon dissolution in appropriate solvents. The Evans NMR method of measuring the change of the paramagnetic shift with temperature is the most common technique to study the magnetic behaviour of such systems. The spin transition characteristics has been observed to depend on various chemical modi-... 

One of the methods used in corrosion control is anodic inhibition. The method applies in particular to iron and its steels. The electrode is moved in the anodic direction (at first stimulating the corrosion rate), but soon an oxide film forms and reduces the dissolution current. There are certain types of oxide film, passive films, that are particularly protective. Indeed, such films are involved in the way metals preserve themselves in nature. There is much to be found out about these films (why they are so protective) and some of the material that allows us to understand them and their eventual breakdown by aggressive ions such as chloride, has been given in this chapter. 

This method is for the determination of cadmium, cobalt, copper, iron, manganese, nickel, lead and zinc, which are solvent extracted and concentrated as their diethyldithiocarbamate chelates. After destruction of the organic complexes dissolution of the residue in dilute acid gives a solution suitable for atomic absorption analysis [13]. 

The quality control of pharmaceuticals is particularly important. Care must be taken to limit the levels of toxic metals in the final product. The acid dissolution. procedures described above (e.g. 6 M hydrochloric acid) are often equally applicable for the determination of impurities. Complete destruction of the matrix by wet oxidation or dry ashing may be necessary to obtain a completely independent method. Raw materials, catalysts, preparative equipment and containers are all possible sources of contamination. Lead, arsenic, mercury, copper, iron, zinc and several other metals may be subject to prescribed limits. Greater sensitivity is often required for lead and arsenic determinations and this can be achieved by electrothermal atomisation. Kovar etal. [112] brought samples into solution using 65% nitric acid under pressure at 170—180° C and, after adding ammonium and lanthanum nitrate, determined arsenic in the range 10—200 ng in a graphite... 

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