Adsorbable substances solutes

On account of the very great difficulty of measuring the extremely small amounts of adsorbed substance at a liquid/gas or liquid/liquid interface, very few experiments are available for testing Gibbs s equation. Zawidski(13) (1900) pointed out that the concentration of the foam of a solution should be different from that of the latter in bulk, and Miss Benson (14> (1903) by the analysis of a solution of amyl alcohol in water found... 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

Heterogeneous chemical reactions in which adsorbed species participate are not pure chemical reactions, as the surface concentrations of these substances depend on the electrode potential (see Section 4.3.3), and thus the reaction rates are also functions of the potential. Formulation of the relationship between the current density in the stationary state and the concentrations of the adsorbing species in solution is very simple for a linear adsorption isotherm. Assume that the adsorbed substance B undergoes an... 

It is very simple to determine the value of = T/Tm for a strongly adsorbed substance in electrolysis with a dropping mercury electrode. If a much smaller amount of substance is sufficient for complete electrode coverage than available in the test solution, then the surface concentration of the surface-active substance T is determined by its diffusion to the electrode. 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

Whether the adsorption of molecules at the surface of minerals is a curse or a blessing for the adsorbed substances depends on many parameters. Experiments showed very different adsorption behaviour of adenine on different minerals. Active minerals are of particular importance for hydrothermal processes (see Sect. 7.2). The surface concentration of adenine on pyrites is fifteen times, that on quartz five times, and on pyrrhotite three and a half times as high as in a starting solution whose concentration is 20 pM (Cohn, 2002). 

In 1952 Carsten (Cl) developed a method, which allowed him to isolate and characterize several lower peptides contained in normal and pathological urine. According to this procedure, urine was desalted on the Amberlite IR-100 column and the adsorbed substances washed out with 2 M ammonia solution. The eluate was then passed through the column of Amberlite IRA-400. This column retained the ampholytes and rejected the weak bases. The former were recovered by elution with 1 M hydrochloric acid and the eluate was subsequently fractionated on Dowex 50 resin with 2M and later 4M hydrochloric acid as the eluents. By applying two-dimensional paper chromatography to further analysis of... 

Substances, however, which are non-electrolytes as well as both ions and particles of the same sign as the gel fibrils penetrate at different speeds. The fibrils as we have npted are hydrated in solution and thus adsorb substances with polar groups whether ionised or not. 

The coating material (about 75ml per l(X)ml of column packing) is applied as a solution in a suitable solvent such as methylene chloride, acetone, methanol or pentane, which is then allowed to evaporate in air, over a steam-bath, or in a vacuum oven (provided the adsorbed substance is sufficiently non-volatile). The order in which a mixture of substances travels through such columns depends on their relative solubilities in the materials making up the stationary phases. 

The reversibility of the adsorption steps in mechanism (4) affects the total number of steady states. As can be seen from Table 1, if two adsorption steps are reversible, boundary steady-state points are absent. Irreversibility of one adsorption step leads to the appearance of one boundary steady-state point in which the concentration of the reversibly adsorbing substance is equal to zero and the irreversibly adsorbing substance occupies all active sites of the catalyst surface. In the case where both adsorption steps are irreversible, there exist two boundary steady-state points (x = 0, y = C2) and (x = Cz, y = 0). In the latter case, at equal kinetic orders of the adsorption steps (n = m) a multiplicity of steady-state solutions is possible, i.e. at pk2 = qk1 (non-rough case) there exists a singular line of steady states connecting two boundary steady-state points. It can manifest itself in the unreproducibility of experimental data in a certain range of the parameters. 

Fleszar and Sobkowiak [120] obtained less than 5% of catechol and hydroquinone from the hydroxylation of benzene and phenol on Hg, Pb, Cu, and Ag cathodes, even in the absence of Fe(II). It appears that the generation of H202 occurs even in electrodes where it may undergo catalytic decomposition if there are small quantities of adsorbable substances in solution. 

Concepts and terminology of adsorption processes on solid adsorbents are discussed by D. H, Everett, Reporting data on adsorption from solution at the solid/solution interface, Pure Appl. Chem. 58 967 (1986). See also D. H. Everett, Manual of Symbols and Terminology for Physicochemical Quantities and Units. Appendix II Definition, Terminology and Symbols in Colloid and Surface Chemistry, Butterworths, London, 1972 [published in Pure Appl. Chem. 31 577 (1972)]. The need for a relative definition of the amount of adsorbed substance stems from the fact that the actual position of an interface cannot be specified with absolute precision, even conceptually. 

This is the simplest isotherm equation, proposed in 1916 by I. Langmuir. The Langmuir equation is based on the fact that every active site in the surface acts the same way, and the maximum adsorption occurs at a saturated monolayer of the solutes on the surface. When an adsorbent is added to a solution containing a substance to be removed, the substance adsorbs on the surface of the adsorbent, and equilibrium is established. A fraction a of the adsorbing surface is occupied with adsorbed substance and a fraction (1 - a) of the adsorbing surface will remain unoccupied. The rate of adsorption is proportional to the availability of the adsorbing surface and the concentration [A] of the substance in solution ... 

If two substances adsorb onto given active sites of the surface of the adsorbent, the removal of substance A from the solution is highly dependent on the adsorption characteristics of substance B on the adsorbent surface. For example, the efficacy of activated carbon against an overdose of an active drug is subject to the presence of adsorbable substances in the gastrointestinal tract. Let us consider that substance A and substance B adsorbed the fractions of the adsorbent surface aA and aB, respectively. The remaining fractions, which can be used for adsorption, are l-aA-aB. Then, the rates of adsorption of A and B (i.e., rA and rB, respectively) are given by ... 

There are some important general relations for a substance adsorbed from solution on an electrode. These pertain to the equilibrium state and the kinetics of the process leading to equilibrium. Adsorption kinetics receives rather intermittent attention in the electrochemical literature. One of the clearest discussions is by Mohilner [403] see also Delahay [200], Bard and Faulkner [74]. 

At the interface between solids and organic solvents, however, specific attractions between the solid and the adsorbed substances may come into play and produce considerable adsorption. Some such cases are of importance in lubrication long-chain fatty acids, and some of their salts, are adsorbed from solution in hydrocarbon oils at the surface of many metals, and the result is a boundary lubricating layer (see Chap. VI). 

It has been found that closely related substances with almost similar physical and chemical properties which cannot be separated from one another by ordinary means, are adsorbed to different extents on the surface of adsorbents. This facilitates their separation and purification. If a solution containing different solutes is poured down a column filled with a finely divided adsorbent, the solute most readily absorbed is retained on the top layer along with smaller amounts of the other constituent while the less readily adsorbed constituents are held on lower portions of the column. A partial separation of the constituents of the mixture is thus easily achieved. A fuller separation is possible by repeating and modifying the process. 

The sorption efficiency of MC was determined as the ratio of the quantity of the adsorbed substance to its initial amount (w / w), expressed in % for a certain ratio (w / w) of adsorbent to substance. Optimal ratios of adsorbent to substance equal 15-20 for barbiturates, 20 - 25 for cyanocobalamin and bilirubin, and 40 - 50 for hemoglobin. The initial concentration of absorbed substances was 100 - 200 pg/ml. The substances were incubated for lmin with MC either in physiological solution or in donor plasma and donor blood at room temperature (pH 7.4). The concentration of substances in the solutions was measured by differential visual and UV-spectroscopy. Concentrations of substances in blood and plasma and adsorption of total plasma proteins was determined by thin-layer chromatography with a fluorescent label. 

The values of chemical shift for the substances adsorbed on the surface of the starting, reduced, and oxidized carbons are summarized in Table 3. The table lists also differences in chemical shifts for the adsorbed molecules and liquid substance (as regards methane its solution in chloroform was used). As in the case considered in the previous section, the 5 — 60 value determines the extent of the adsorbent surface effect on the chemical shifts of adsorbed substances. 

The latter is explained by the fact that the fulvic acid fraction concentrates the smaller molecules present in the original humic substance solution. The lower initial concentration can be responsible for a part of tbe former findings. Since the preparations of the 2 solutions was different some unknown influences may interfere however. Moreover, throughout the investigation lumped-parameters are used TOC, U.V. 254) and since the chemical structures of the 2 adsorbates are different, we may not interpret the numerical values obtained in exactly the same way. The substances giving higher U.V. 254 absorption seem to absorb more readily than would be expected from TOC measurements. For this fraction, the ratio U.V. 

Evaluation of carbons in dynamic adsorption is difficult and timeconsuming. Because of this evaluation of granular carbons is proposed to be carried out in static conditions. Determination of adsorption capacity in the presence of different indicators used for evaluation of powdered carbon is proposed to be carried out in the comparable studies of granular carbons. This method has both its followers and opponents. It is caused by differences in the nature of static and dynamic adsorption. Powdered carbons are added to water in the form of pulp and, after a short time (5—30 min), the adsorptive equilibrium between carbon and adsorbated substances in solution is fixed. The process is quick and its duration usually sufficient from the technological point of view, since static adsorption in most cases is connected with coagulation, where time of floculation (hence the time of contact of carbon with solution) reaches 30 min [15,16,40]. 

Most solutions used in the electrodeposition of metals and alloys contain one or more inorganic or organic additives that have a specific function in the deposition process. These additives (and impurities) affect the deposition and crystal-building processes as adsorbates (substances that are adsorbed) at the surface of the cathode. Adsorption and the factors that determine the adsorbate-surface interaction are discussed in detail in Volume 1 and Volume 2. 

Based on the rate equations of Plummer et al. (1979), we can compute that for groundwater pH s above 6 and 7 co, values less than 0.1 bar at 25°C and below, the solution rate of calcite far from equilibrium reduces toR = ky In other words, for these conditions (which are typical of many shallow groundwaters) the reaction is zero-order, as long as the surface area of the calcite is constant. This assumes no catalysis or inhibition of the rate by adsorbed substances. (Sc, Cu, and PO4 are strong inhibitors according to Sjoberg and Rickard 1984.) As equilibrium is approached, the rate equation becomes... 

There are other possible explanations when a model calculation indicates a water is supersaturated with respect to one or more carbonate minerals. They include (1) the use of inaccurate, inconsistent, or incomplete thermodynamic data for carbonate minerals and aqueous complexes (2) nonstoichiometry (i.e.. solid solution) and/or small (submicron) particle sizes of the carbonates, making them more soluble than the well-crystallized pure phases assumed in the calculation (cf. Busenberg and Plummer 1989) (3) different solution models used to define the mineral and in the calculation of saturation state in a natural water (4) inhibition of carbonate nucieation by adsorbed substances (cf. Inskeep and Bloom 1986) and (5) slow nucieation and precipitation rates that require times exceeding residence times of the water in the water-rock system (cf. Herman and Lorah 1987). The same possible explanations apply to model-computed supersaturations obtained for noncarbonate minerals. 

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