Dissolution experiments, laboratory

One additional aspect of laboratory dissolution experiments is the question of stoichiometric vs. non-stoichiometric dissolution. Many of the studies cited above analyzed only a few of the elements released by feldspar that is, although alkalis, alkaline earths, silica, and aluminum may be released during dissolution of feldspar, few studies report analyses for all elements. Often, only silica was analyzed. Where multiple elements are analyzed, they are often released to the solution in proportions which do not correspond to the bulk stoichiometry of the feldspar ( ] ). 

Dissolution of feldspars and hornblende is a slow process. If tunnels are formed by dissolution processes, laboratory experiments are not a promising tool to study the rate of tunnel formation. In that case, soil chronosequences can be of high value (Huggett, 1998). Tunnel formation has been studied in two chronosequences. 

Figure 11. Plot of the Peterson (41) and Berger (42) results for their water-column dissolution experiments in the Central Pacific Ocean, and the Morse and Berner (45) laboratory experiments as a function of equivalent depth. The depth of the lysocline was calculated from the data of Bramlette (49) (after Bef. 45).

Skidmore M. L., Sharp M. J., and Tranter M. Fractionation of carbon isotopes during the weathering of carbonates in glaciated catchments I. Kinetic effects during the initial phases of dissolution in laboratory experiments. Geochim. Cosmochim. Acta (in press). 

Suu.ivAN, P. J., A. A. Sober, and J. Rybarczyk, 1986. Evaluating mineral dissolution in laboratory weathering experiments using leaehate data. Soil Sci. Soc. Am. J. 50 251-54. 

The complexities involved in silicate weathering have caused difficulties for interpretation of laboratory dissolution data and have been the basis of heated controversies concerning the mechanisms of dissolution. This difficulty has been exacerbated by attempts to make direct comparisons of the results of dissolution experiments conducted in different pH regions. Because the extrapolation of rate and mechanistic data obtained from low or high pH weathering studies to more neutral conditions is a potential source of error, and because the majority of soils have pH values between 4 and 9, the interpretations made in this paper will be restricted to the pH range of 4 to 9, unless otherwise stated. 

A laboratory example of the above principle is given by the "strictly-congruent" dissolution experiment of Denis and Michard (14) on a 3.5% Sr-anhydrite. Analysis of their results shows that maximum SIceiestite SI... 

If the spreading is into a limited surface area, as in a laboratory experiment, the film front rather quickly reaches the boundaries of the trough. The film pressure at this stage is low, and the now essentially uniform film more slowly increases in v to the final equilibrium value. The rate of this second-stage process is mainly determined by the rate of release of material from the source, for example a crystal, and the surface concentration F [46]. Franses and co-workers [47] found that the rate of dissolution of hexadecanol particles sprinkled at the water surface controlled the increase in surface pressure here the slight solubility of hexadecanol in the bulk plays a role. 

The accuracy of a method should be assessed using a minimum of nine determinations conducted over a minimum range of three concentration levels (80%, 100%, and 120% of the target concentration) [37]. Experience from our laboratory has showed that by using at least five levels of concentrations in duplicate (i.e., 80%, 90%i, 100%i, 110%, and 120% of the target concentration), a better result can be achieved. For dissolution studies, the accuracy of the required profile should be tested at 40%, 75%, and 110% of the theoretical release) [20],... 

Rate constants for the dissolution and precipitation of quartz, for example, have been measured in deionized water (Rimstidt and Barnes, 1980). Dove and Crerar (1990), however, found that reaction rates increased by as much as one and a half orders of magnitude when the reaction proceeded in dilute electrolyte solutions. As well, reaction rates determined in the laboratory from hydrothermal experiments on clean systems differ substantially from those that occur in nature, where clay minerals, oxides, and other materials may coat mineral surfaces and hinder reaction. 

ABSTRACT Atmospheric carbon dioxide is trapped within magnesium carbonate minerals during mining and processing of ultramafic-hosted ore. The extent of mineral-fluid reaction is consistent with laboratory experiments on the rates of mineral dissolution. Incorporation of new serpentine dissolution kinetic rate laws into geochemical models for carbon storage in ultramafic-hosted aquifers may therefore improve predictions of the rates of carbon mineralization in the subsurface. 

Another important point to guarantee the long-term stability of the electrode is the procedure used to manage shutdowns. The experience gained through the laboratory tests shows that during shutdown the cell must be maintained under polarisation conditions to avoid the probable dissolution of the silver catalyst and its re-deposition... 

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

The Joint Panel proposed no in vivo requirements, but individual dissolution testing requirements were adopted in 12 compendial monographs. USP tests measured the time to attain a specified amount dissolved, whereas NF used the more workable test for the amount dissolved at a specified time. Controversy with respect to equipment selection and methodology raged at the time of the first official dissolution tests. As more laboratories entered the field, and experience (and mistakes ) accumulated, the period 1970-1980 was one of intensive refinement of official test methods and dissolution test equipment. 

Figures 6 and 7 are derived from laboratory experiments and illustrate that flow can become turbulent close to particle walls even when the bulk flow remains laminar. The turbulent vortices bore into the particle surface, magnifying cavitations and abrading protrusions, and hence accelerating the dissolution process [(10), Chapter 4.3.5]. However, irregulari-...

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