Water timescale

The hydration of more inert ions has been studied by O labelling mass spectrometry. 0-emiched water is used, and an equilibrium between the solvent and the hydration around the central ion is first attained, after which the cation is extracted rapidly and analysed. The method essentially reveals the number of oxygen atoms that exchange slowly on the timescale of the extraction, and has been used to establish the existence of the stable [1 10304] cluster in aqueous solution. 

Archer, D., Peltzer, E. T. and Kirchman, D. (1997). A timescale for dissolved organic carbon production in equatorial Pacific surface waters. Glob. Biogeochem. Cycles 11,435-452. 

An ideal renewable resource will be one that can be replenished over a relatively short timescale or is essentially limitless in supply. The latter will include solar radiation, geothermal energy, oxygen, carbon dioxide and water. Nor should production or consumption of these resources contribute to the net atmospheric burden of carbon dioxide. Advantage can be taken of the fixation of atmospheric carbon dioxide into plant material by the process of photosynthesis. 

A useful analogy for understanding secular equilibrium is visualizing a decay chain as a series of pools of water (Fig. 2). These pools eventually lead to a continuously filling pool representing a stable isotope of lead (either ° Pb, ° Pb or ° Pb). Over the timescale... 

Thorium generally exists as a neutral hydroxide species in the oceans and is highly insoluble. Its behavior is dominated by a tendency to become incorporated in colloids and/or adhere to the surfaces of existing particles (Cochran 1992). Because ocean particles settle from the water column on the timescale of years, Th isotopes are removed rapidly and have an average residence time of = 20 years (Fig. 1). This insoluble behavior has led to the common assertion that Th is always immobile in aqueous conditions. While this is generally true in seawater, there are examples of Th being complexed as a carbonate (e.g.. Mono Lake waters, Anderson et al. 1982 Simpson et al. 1982) in which form it is soluble. 

An important issue that influences estuarine behavior is the determination of the riverine component. Variations in the riverine endmember may occur over timescales that are short compared to the residence times of water in the estuary, which result in nonlinear relationships between salinity and trace elements across the estuary as different riverine compositions progress through the estuary (Loder and Reichard 1981 Officer... 

There are two main sources of Rn to the ocean, (1) the decay of sediment-bound Ra and (2) decay of dissolved Ra in the water column. Radon can enter the sediment pore water through alpha recoil during decay events (see discussion in Porcelli and Swarzenski 2003). Since radon is chemically inert, it readily diffuses from bottom sediments into overlying waters. The diffusion of radon from sediments to the water column gives rise to disequilibrium between Rn and Ra, with ratios of ( Rn/ Ra) >1 due to the addition of excess Rn. This excess Rn is unsupported and so is rapidly diminished by decay. Therefore, the excess Rn signal is only resolvable where significant transport of Rn away from the sediment can occur over timescales that do not significantly exceed the half-life of Rn. 

The presence of a trace of acid and water however, causes collapse of the hydroxyl-OH to a singlet (at lower held), the proton can now protonate, and de-protonate the oxygen very rapidly, as the process is catalysed by the acid, i.e., exchange is said to be fast on the NMR timescale (less than about 10-6 s). 

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

During steady-state isotopic transient kinetic analysis, the 12CO was switched to 13CO and the carbon-containing adsorbed and gas phase species were monitored in the IR as they exchanged from the 12C to the 13C label. Particular attention was made to those species that exchanged on a timescale similar to that of the exchange of the product C02, as that species could be a likely intermediate to the water-gas... 

Cyclic ligands like DOTA or DOTAM are known to exist in two isomeric forms in solution, usually termed M for the major and m for the minor isomer (Fig. 11). They interconvert slowly on the NMR timescale (251-253). It was demonstrated by NMR that the m-isomer exchanges its 1st shell water much faster than the M-isomer, most probably due to steric effects (238,244). 

Femtosecond solvation dynamics experiments in water [147] clearly hint at the existence of a bimodal response of the solvent to a change in solute charge density that is produced by photon absorption for instance. Water appears to show an ultrafast component in the fl/ kT timescale and a slow component due to diffusive motions whose timescale would be in the 1/y range. 

Distance and timescales involved in the water activity concept... 

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