Textbook of Quantitative Analysis

Abstract

at total solute concentrations above 150 g/1. increases in an apparently linear fashion with the total solute concentration. Changing acid anions from the chloride to sulfate or carbonate made little difference, but the presence of such an anion is necessary since there is no solubility of iron in an acid anion-free aqueous ammoniacal solution. The similarities of the three systems can be seen in Table II where the concentrations are shown in moles per liter and mole ratios. Apparently, the limitations on the iron solubility are: There must be sufficient acid anion present to combine with a divalent cationic complex; the NH4OH concentration must be greater than 50 wt % of the total solute concentration; and most importantly, the total solute concentration should be greater than about 150 g/1. (In the carbonate system an Fe(OH)2 solubility of 2% was measured at 138 g/1. total solute.) The requirement, not only of excess ammonia, but of high-strength solutions (4.5-8.5 moles of NH4OH per liter) indicates the nature of the equilibrium involved in the formation of the soluble iron species. These concentrations appear necessary to form the ammonia ligands. If the system is diluted, the ammonia is replaced by water, and the iron becomes insoluble. As the ammonia strength increases , the NHs/Fe mole ratio of all three systems decreases indicating a higher fraction of the ammonia involved in the soluble complex (Table II). In the sulfate system at 500 g/1., the NH3/Fe, SC>42_/Fe, and XH3/S042"" mole ratios are 6.3, 1.1, and 5.7, respectively, which compares with the theoretical values 6.0, 1.0, and 6.0 for an iron hexammine sulfate. Possibly , this ratio would be reached at higher solute contents. Dean (1952a), working with the systems Mn(OH)2-NH4-0H-H20-HC1, or H2SC>4 or H2C03, found a similar high solubility in high-strength ammoniacal solutions. He concluded that the manganese in these solutions existed as an anionic complex based on electrical conductivity and elec-trolytic transport measurements. Electrical conductivity measurements (Daniels et ah, 1962) on the iron system reported here showed that a change did occur as the ammonia to acid anion ratio was varied in solutions of constant iron content as reported by Dean. However, the plot of the change appeared parabolic rather than an intersection of two lines and might well result from the change in NH4C1 concentration. The results with the ion-exchange resins were felt to be a more reliable indication of the nature of the charge on the soluble species. The extent of the iron solubility in ammoniacal solutions suggests a possible application in the winning of iron from low-grade ores. A partial reduction of the iron oxides to the ferrous state could be followed by dissolution in a high-strength ammonia-ammonium salt solution. Filtering off this solution separates the iron from the gangue materials such as silica and alumina. The iron could then be recovered from solution by dilution, evaporation of ammonia, or by air oxidation which precipitates the iron as hydrated ferric oxide. The economics of such a system would depend on the recovery of the ammonia and the ammonium salt, particularly because of the high-strength solutions involved. The ammonia -ammonium carbonate system would appear to be attractive because of the ease of recovery of ammonia and carbon dioxide. Ruthenium is one of the most active elements for hydro-genolysis of paraffins-e.g., for Group VIII elements on silica in the hydrogenolysis of ethane (Sinfelt, 1969). For 5% ruthenium on silica, an ethane order of 0.8 and a hydrogen order of-1.3 have been reported (Sinfelt and Yates, 1967). These exponents were explained by a mechanism in which the ethane adsorbs reversibly to form an unsaturated surface species, and the overall rate is limited by the rupture of the carbon-carbon bond (Cimino etal., 1954). The hydrogenolyses of ethane and propane on ruthenium on alumina have been studied in a continuous stirred-tank reactor similar to that used in this study (Tajbl, 1969). The ethane hydrogenolysis was +1 order in ethane and-2 order in hydrogen; the pro-pane hydrogenolysis was +1 order in propane and-3/2 order in hydrogen. Propane reacted about 75 times faster than ethane. Examinations of the hydrogenolysis reactions of larger hydrocarbons have been principally concerned with the distribution of products.