Iron Oxide and Copper Dissolution

Research into methods to remove iron oxide and copper deposits from powergenerating equipment constitutes a large portion of the activities of laboratories involved in chemical cleaning operations. With the exception of the work on corrosion inhibitor mechanisms, there is more published information on the chemistry of iron and copper removal than other techniques. A separate chapter is devoted to a review of this information. 

The purpose of this section is to review recent literature on the kinetics and mechanisms of iron oxide dissolution. Particular emphasis is placed on pure iron oxides and iron oxides on steel surfaces. The limited scope of this review precludes complete coverage of the vast amount of work on passive film formation and dissolution. However, some aspects of passivation that apply directly to chemical clening are reviewed in Chapter 4. 

The studies of Vermilyea and Engell provided some of the earliest theoretical bases for the dissolution of iron oxides. Vermilyea expressed the rate of oxide 

Other electrochemical phenomena must also be considered when the oxide is on an iron surface. Most authors agree that iron is released from the oxide only as the ferrous (Fe +) Ion. Hickling and Ives stated that an Fe +/Fe + redox system in the solid state is established and produces a potential determining exchange current. Therefore, 

Haruyama and Masamura stated that the reductive dissolution of magnetite occurs with 100% efficiency In the potential range from 0-900 mV (vs. SHE), as follows  

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CHAPTER 3 Mechanisms of Iron Oxide and Copper Dissolution... 

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) ... 

This is contrasted with the situation when ammonium citrate is used and the external oxidizer removes the copper (see the discussion in previous section). The continuing "air blowing" keeps the iron in the ferric oxidation state and also provides some of the motive force to the solvent that speeds passivation and copper dissolution. 

Total carbon in beryUium is determined by combustion of the sample, along with an accelerator mixture of tin, iron, and copper, in a stream of oxygen (15,16). The evolved carbon dioxide is usuaUy measured by infrared absorption spectrometry. BeryUium carbide can be determined without interference from graphitic carbon by dissolution of the sample in a strong base. BeryUium carbide is converted to methane, which can be determined directly by gas chromatography. Alternatively, the evolved methane can be oxidized to carbon dioxide, which is determined gravimetricaUy (16). 

As the overall concentration of copper and copper oxides in the boiler deposit increases, however, less thiourea is required. This is because, as ferric ions are generated during the iron oxide dissolution process, they oxidize the plated copper, which can then be removed from the boiler by forming a complex with thiourea. Conversely, if ferric ions are not generated, the plated copper remains and no complexing can take place. 

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... 

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

Atomic absorption spectroscopy has been used to determine the amount of impurities in talc samples based on the chemical composition [35]. The detection of calcium, iron, and aluminum gave an indication of the mineral and chemical purity of the talc, whereas, analyses for chromium, manganese, nickel, and copper were of toxicological interest. The sample preparation involved an acid extraction with dilute hydrochloric acid to remove magnesium and calcium carbonates. Total dissolution of the sample was achieved with nitric/hydrofluoric acid mixture, followed by nitric/perchloric acid mixtures. Calcium was determined in the nitrous oxide/acetyiene flame and the other elements were detected in the air/acetylene flame. 

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