Chemical equilibrium buffer solutions

The chemistry of hydrothermal solutions from midoceanic ridges has been reasonably explained by the effect of buffering by alteration minerals (Seyfried, 1987 Bemdt et al., 1989). Therefore, it might be worth explaining the chemical composition of hydrothermal solutions from back-arc basins in terms of chemical equilibrium between hydrothermal solutions and alteration minerals. 

We know the initial concentration of NH3 in the buffer solution and can use the pH to find the equilibrium [OH ]. The rest of the solution is organized around the balanced chemical equation. Our first goal is to determine the initial concentration of NH/. 

In this section, you compared strong and weak acids and bases using your understanding of chemical equilibrium, and you solved problems involving their concentrations and pH. Then you considered the effect on pH of buffer solutions solutions that contain a mixture of acid ions and base ions. In the next section, you will compare pH changes that occur when solutions of acids and bases with different strengths react together. 

All membrane exposures were carried out by soak testing under equilibrium conditions at fixed concentrations and constant pH. Pretreatment chemicals were added to buffer solutions at pH 3.0, 5.8 and 8.6. These buffers, representing an arbitrary pH range were prepared according to directions given by Perrin and Dempsey... 

A more comprehensive analysis of the influences on the ozone solubility was made by Sotelo et al., (1989). The Henry s Law constant H was measured in the presence of several salts, i. e. buffer solutions frequently used in ozonation experiments. Based on an ozone mass balance in a stirred tank reactor and employing the two film theory of gas absorption followed by an irreversible chemical reaction (Charpentier, 1981), equations for the Henry s Law constant as a function of temperature, pH and ionic strength, which agreed with the experimental values within 15 % were developed (Table 3-2). In this study, much care was taken to correctly analyse the ozone decomposition due to changes in the pH as well as to achieve the steady state experimental concentration at every temperature in the range considered (0°C [Pg.86]

In general, buffer solutions contain a mixture of a weak acid and its salt or a weak base and its salt. The hydrogen-ion concentration can be calculated from considerations of the chemical equilibrium which exists in such solutions. Considering a buffer made up of a weak acid and its salt, the dissociation equilibrium... 

Since retention in RP systems depends on the interaction between hydrophobic groups of the solutes and the adsorbent surface, ionization of the solutes can result in severe peak distortion. Ions are very hydrophilic and are thus poorly retained in RP systems even if the non-ionized solute shows good retention. Because ionization reactions (e.g. dissociation of organic acids) are determined by their chemical equilibrium, solutes are present in both forms. This leads to broad peaks with poor retention. The ionization reaction must be suppressed by buffering the system. Thus, the stability of the adsorbent for the pH range must be checked. Especially for alkaline mobile phases, silica-based materials are problematic and the use of polymeric... 

Buffers, such as those used in so-called buffered aspirin (acetylsalicylic acid) for easier digestion, or those needed for maintaining a proper pH for enzyme function in living organisms, can be analyzed in terms of chemical equilibrium. A weakly dissociating acidic compound, for example, reaches equilibrium concentrations in solution. The following chemical equation describes the forward and reverse reactions for the hypothetical weak acid HX in the aqueous state. 

Measurements of the catalyzed reaction performed at chemical equilibrium starting from mixtures of C- 0-labeled HCO3 and C- 0-labeled CO2 have shown the transient formation of C- 0-labeled species (both CO2 and HC03 ) before 0-labeled water appears in solution. These experiments provided evidence that, at chemical equilibrium, an oxygen atom can pass from HCO3 to CO2 and vice versa several times before being released to water. Furthermore, maximal exchange rates are observed even in the absence of buffers. 

Skill 25.3 Explain the characteristics of buffered solutions in terms of chemical equilibrium of weak acids. 

Silica gel-chemically immobilized-Eriochrome black T was synthesized and the surface coverage was found to be 0.38 mmol/g. The stability toward hydrolysis of this silica gel phase in various buffer solutions (pH 1.0-10.0) was studied and evaluated. The applicability of silica gel phase-immobilized-Eriochrome black T as a solid-phase extractor for Zn(II), Mg(II), and Ca(ll) was performed by the batch equilibrium technique and was found to show an order similar to the formation constant values of the three metal ions with the indicator. The separation and selectivity of this modified silica gel for these metal ions, based on a column technique, were found to afford a reasonable performance of the three studied metal ions. The structure of silica gel-chemically immobilized-Eriochrome black T is given in Scheme 20. 

For the solid-state RE, the reference chemical potential is established by a mixture of two compounds whose equilibrium involves electrochemically active species, similar to the buffer solutions enabling fixing pH in the electrochemistry of aqueous electrolytes. In the case of oxygen sensors, these mixtures comprise... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

It is the chemical buffering system which contributes significantly to the carrying capacity of a soil [ 17]. In general, any soil cannot completely adsorb all the pollutants from the liquid solution. There is an equilibrium between solvent and solution phases. The amount left in solution gradually increases as the buffer capacity of the soil is approached. 

Equilibrium solubility This approach is considered a first attempt to characterize the true thermodynamic solubility of the compound. It is used to rank-order compounds and to extract a structure-solubility relationship within the chemical series. In this assay, compounds are usually equilibrated for 24 h before analysis. One can start from powder, but this is a quite labor-intensive step. In most cases one starts from DMSO stock solutions (usually 10 mM) because it is much more efficient from a compound logistics viewpoint. The solvent is then usually removed and the compound is dried before addition of the buffer medium [15, 16]. 

Now consider a primary standard buffer containing 0.025 0 m KH2P04 and 0.025 0 m Na2HP04. Its pH at 25°C is 6.865 0.006.4 The concentration unit, m, is molality, which means moles of solute per kilogram of solvent. For precise chemical measurements, concentrations are often expressed in molality, rather than molarity, because molality is independent of temperature. Tabulated equilibrium constants usually apply to molality, not molarity. Uncertainties in equilibrium constants are usually sufficiently great so that the 0.3% difference between molality and molarity of dilute solutions is unimportant. 

In this paper we have used the quantity (1 — vp0) in writing equations for sedimentation equilibrium experiments. Some workers prefer to use the density increment, 1000(dp/dc)Tfn, instead when dealing with solutions containing ionizing macromolecules. This procedure was first advocated by Vrij (44), and its advantages are discussed by Casassa and Eisenberg (39). Nichol and Ogston (13) have used the density increment in their analysis of mixed associations. The subscript p. means that all of the diffusible solutes are at constant chemical potential in the buffer... 

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