Precipitation acidifying potential

Where e is the excess acid in mol/L and ionic concentrations are expressed as mol/L. While this more precise definition may apply in some strictly chemical responses such as soil erosion, Biydges and Summers 19) have considered the more complete reactions including biological ionic utilizations and have defined an "acidifying potential" of precipitation as ... 

For solvent extraction of a tetravalent vanadium oxyvanadium cation, the leach solution is acidified to ca pH 1.6—2.0 by addition of sulfuric acid, and the redox potential is adjusted to —250 mV by heating and reaction with iron powder. Vanadium is extracted from the blue solution in ca six countercurrent mixer—settler stages by a kerosene solution of 5—6 wt % di-2-ethyIhexyl phosphoric acid (EHPA) and 3 wt % tributyl phosphate (TBP). The organic solvent is stripped by a 15 wt % sulfuric acid solution. The rich strip Hquor containing ca 50—65 g V20 /L is oxidized batchwise initially at pH 0.3 by addition of sodium chlorate then it is heated to 70°C and agitated during the addition of NH to raise the pH to 0.6. Vanadium pentoxide of 98—99% grade precipitates, is removed by filtration, and then is fused and flaked. 

In this flow sheet, the aqueous raffinate from extraction is acidified to 5-6 mol dm with hydrochloric acid to optimize platinum extraction by the solvating extractant TBP. The coextraction of iridium is prevented by reduction with sulfur dioxide, which converts the iridium(IV) to the (III) species, which is not extractable. Once again, kinetics are a factor in this reduction step because, although the redox potentials are quite similar, [Ir(IV)/(III) —0.87 V Pt(IV)/(II) —0.77 V], iridium(IV) has a relatively labile configuration, whereas platinum(IV) has the inert arrangement. The species H2PtCl6 is extracted by TBP, from which platinum can be stripped by water and recovered by precipitation as (NH3)2PtCl2. 

The alkanimidamide hydrochloride (0.01 mol) was dissolved in a mixture of benzene (40 mL) and H20 (10 mL). Into this solution 1 M NaOH (10 mL) and the aroyl isothiocyanate 24 (R1 = aryl, X = O 0.01 mol) in anhyd benzene (10 mL) were dropped simultaneously with vigorous stirring. Then additional 2M NaOH (10 mL) was added within 10 min. After 1 h, EtOH (10-20 mL) was added to dissolve the potential precipitate, and the organic phase was extracted with 2 M NaOH (10 mL). The combined alkaline extracts were acidified to pH 4- 5 with 1 M H,S04, and the precipitated triazines were collected by suction. 

The recovery of actinides from such solutions by acidification with HNO3 followed by solvent extraction is hampered by the accumulation of H2MBP and HDBP in the solvent. This problem may be overcome by extracting the H2MBP and HDBP from the acidified carbonate solutions with 2-ethylhexanol, EHOH, prior to acidification. Laboratory trials indicated that such a process was potentially applicable to Purex waste streams. Problems identified were the formation of precipitates of H MBP or HDBP complexes on neutralization of the carbonate solutions and the transfer of some EHOH through the process into the actinide extractant used. The extraction of oxidation state (IV) and (VI) actinides could be accomplished using TBP while DHDECMP could be used to extract oxidation state (III) actinides and lanthanides also. 

The potential would appear to be great for particles lodged on leaf surfaces to interact chemically with either ambient gaseous pollutants or precipitation. Such particles could effectively increase the surface area of leaves available for absorption of gaseous pollutants and, depending on their source, either neutralize or further acidify rain water contacting the leaf surface. Such interactions certainly occur in nature, and may be important in the ultimate response of a plant to complex mixtures of biotic and abiotic stress factors. 

Certain Swedish scientists have expressed concern over the enhanced heavy metal concentrations, particularly cadmium, found in soils signficantly acidified by acid precipitation. It is thought these might be taken up in the foliage of certain vegetables with high metal uptake potential, e.g. lettuce and cabbage. 

Since CO2 gas can be used as an acidifying agent in chemical or biochemical processes, it can also be used to dissolve or precipitate metal carbonate salts through these common-ion effects. The chemical state of a system is described by means of Gibbs energy G, chemical potential /ij of species i, and mole amount [11-14]. 

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