Solution, aqueous

2 Aqueous Solutions. - Over the years MD has been applied to investigate the solute structure and dynamics of alkali halides in water. It is well-known that in water at 25 °C the residence time of the water molecules around the ions decreases dramatically as the ion increases in size. MD simulations have been carried out to explore the hydration of ions in supercritical water solutions. They calculated a residence time correlation function. 

1 Aqueous Solutions. These samples can sometimes be introduced without any prior treatment. Among these are various water samples (natural, sea, and drinking water), wines, beers, and in some cases urine. It is of great help in the analysis, if the operator knows the approximate concentration of the analyte in the sample in order to decide whether or not some dilution is desirable. 

If the analyte concentration is high (more than about 200 times the reciprocal sensitivity), sensitivity should be reduced. In AAS this can be done by removal of the impact bead, by burner rotation, or use of less sensitive absorption lines. However, if these are not practicable, the solution should be diluted to bring it into the best range for measurement. On the other hand, if the concentration is too low, scale expansion or some chemical pretreatment are required. 

Sometimes it is necessary to add a spectroscopic buffer to aqueous samples in order to reduce interference effects. These buffers are normally used in concentrated form in order to avoid dilution of the samples. 

Typical of these samples are raw and treated waters, seawater, biological fluids, beer, wines, plating solutions, effluents, etc. With this type of sample very little preparation is usually required. If the solution is suitable for aspiration then its approximate concentration can be determined, to check whether dilution with water is necessary. Degassing may be required, and/or the addition of releasing agents, ionisation suppressants, complexing agents, etc., as required for interference compensation. Concentration methods will be described later. 

The electrostatic potential becomes more important in aqueous solutions or at interfaces, because the average bulk liquid properties 

Synthesis of fluoride compounds is performed in various media, such as aqueous solutions, non-aqueous systems and heterogeneous interactions. 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

Beakers and other related equipment made of polyethylene, polypropylene, Teflon or glassy carbon are usually used in the preparation of such solutions. 

Both tantalum and niobium dissolve in HF, but relatively slowly at first. The interaction is shown schematically in Equation (1), Me = Ta or Nb  

In some cases, a small amount of nitric acid, HN03, or hydrogen peroxide, H202, is added to the solution in order to accelerate the dissolution of the metal. Heating the solution increases dissolution rates as well. Taking into 

The surfaces of multicomponent glasses are significantly altered when exposed to aqueous solutions the extent of the alteration layer is dependent on a variety of parameters, including the glass composition, the duration of exposure, the temperature and pH of the solution, and the ratio of the glass surface area to the solution volume. This section is not intended to be a discussion of the aqueous corrosion of glass rather, several different studies are mentioned to provide an overview of the analytical tools that characterize such interactions. 

Because hydrogen often is incorporated into corroded glass surfaces, the characterization of H profiles is important to understanding corrosion behavior. SIMS and SIPS are useful for characterizing H in near surface ( 1 Jim) layers (e.g., see Reference 23). 

Ion scattering techniques are useful for characterizing corrosion layers. ISS provides information about the outermost monolayer. ERD (elastic recoil detection) is sensitive to light elements in a heavy matrix and thus provides useful H and Li profile information. Rutherford backscattering spectrometry (RBS), on the other hand, 

The majority of reactions studied by biochemists occur in solution. Consequently, it is appropriate to begin our mathematical survey by reviewing the various ways of expressing apd intercohverting concentrations of solutions. 

Concentrations based on the amount of dissolved solute per unit volume are the most widely used in biochemistry laboratories. The most common conventions are defined below. 

Molarity (M) = the number of moles of solute per liter of solution 

Molar concentrations are usually given in square brackets, for example, [H ] = molarity of H ion. To calculate M, we need lo know the weight of dissolved solute and its molecular weight, MW. 

Dilute solutions are often expressed in terms of millimolarity, micro molarity, and so on, where  

HER easily takes place in aqueous electrolytes by catlrodic polarization, usually competing with CO2 reduction. The rate is proportional to the proton activity in the electrolyte at a constant potential at various metal electrodes. HER is prevalent particularly in acidic solutions. whereas CO2 molecules do not exist in a basic solution as descibed in Section 11. Thus most of CO2 reduction studies were done with neutral electrolyte solutions. 

Many studies of electrochemical reduction of CO2 in early years were carried out in aqueous media with metal electrodes of high hydrogen overvoltage such as mercury and lead, aiming at suppression of HER. Eyring and his coworkers studied CO2 reduction at a Hg electrode in detail they showed that HCOO is exclusively produced with the faradaic efficiency 100% in neutral aqueous electrolytes. Hori and Suzuki revealed that the partial current of HCOO formation at a Hg electrode does not depend on pH at a constant potential, whereas HER is proportional to proton activity.  

Ito et al. studied CO2 reduction at Zn, Sn, In, Cd and Pb electrodes in hydrogen carbonate solutions with various alkali metal 

Hori and his coworkers carried out CO2 reduction at various metal electrodes in constant current electrolysis at 5 mA cm in 0.5 M KHCO3 aqueous solution purified with preelectrolysis. They applied full chemical analysis of the products and studied the faradaic balance. They revealed that CO2 reduction in aqueous media yields measurable amount of CO, CH4 and other hydrocarbons as well as formic acid at ambient temperature and pressure in a reproducible way, and the product selectivity depends greatly on the metal electrodes. The product distribution is tabulated in terms of faradaic efficiency with the current densities in Table 3, which contains the results revised in their later publications. The product selectivity is greatly affected by the purity of the electrode metals as well as that of the electrolyte solution. Tire results above were confirmed later by other workers.  

Metal electrodes are divided into 4 groups in accordance with the product selectivity indicated in Table 3. Pb, Hg, In. Sn, Cd, Tl, and Bi give formate ion as the major product. Au. Ag, Zn. Pd, and Ga, the 2nd group metals, form CO as the major product. Cu electrode produces CH4, C2H4 and alcohols in quantitatively reproducible amounts. The 4th metals, Ni, Fe, Pt, and Ti. do not practically give product from CO2 reduction continuously, but hydrogen evolution occurs. The classification of metals appears loosely related with that in the periodic table. However, the correlation is not very strong, and the classification such as d metals and sp metals does not appear relevant. More details of the electrocatalytic properties of individual metal electrodes will be discussed later. 

Five mixtures containing different concentrations of arsenocholine, arsenobe-taine, DMA, MMA, As(III) and As(V) were prepared in freshly boiled deionized water and distributed to the participants together with individual calibrant solutions. Twelve laboratories participated in this interlaboratory study on aqueous solutions, using LC-ICP-MS, GC-HAAS, LC-HAAS, LC-ICP, GC-HICP, CZE and LC-ETAAS. 

The five solutions were found to be stable [except for As(III)] for four months if kept in the dark at +4 °C. Storage in the dark at +40 °C led to the formation of As(III) in some solutions. Arsenobetaine resulting from the degradation of arsenocholine was observed to occur significantly when solutions were stored at +20 °C in daylight but no trace of degradation was detected at +4 °C in the dark. 

The presence of chloride ions may interfere with the As signal in ICP-MS M - 75 for As, as well as for the Ar Cl ion), but this interference is eliminated if chlorides are separated from As species by a pre-colunm or a properly selected analytical column [133]. 

Results of this interlaboratory study appeared satisfactory and it was decided to continue the evaluation with synthetic solutions before starting the exercises on real extracts. For a concentration of DMA of 5 pmol kg , the mean of the mean values was very close to the target value [(5.05 0.39) jjmol kg ] and the coefficient of variation of the mean of means was only 7.7%. 

Marcus [56-59] has shown that the free energy of activation AG, for an electrode reaction can be expected to be one-half of that AG ) for the corresponding selfexchange process, so that, from Eq. (5.4), we have for adiabatic electron transfer 

In sharp contrast, a plot of against AV for almost aU aqueous reactions for which both parameters are currendy available shows an excellent linear correlation of slope 0.50 0.02 and negligible intercept [16] (according to a recent reinvestigation [20], the Fe(CN)j case may be anomalous and should be excluded). For convenience, we refer to this relationship as the fifty-percent rule . The evident equivalence between and AV /2 implies that the pre-exponential factors Zei and Zex for aqueous systems are effectively independent of pressure. Such is known to be the case for Ze, from experience with the success of Eqs (5.5)-(5.8) for self-exchange reactions, and presumably it also applies to Z because the nature of a given electrode and its surface are virtually independent of pressure, even though the pre-exponential factor may differ widely from one electrode to another. 

Zei could, however, be influenced by solvent dynamics, but, as demonstrated in Sect. 5.4.2, this would not be apparent in AV ] values for dilute aqueous systems at near-ambient temperatures. 

Little is known about the effects of pressure on the electrical double layer [63], but compaction of the diffuse double layer by the high concentrations of supporting electrolytes used in our experiments means that double-layer contributions to AVgf can be expected to be small. In any event, the close equivalence between AV ] and AV jl, regardless of the medium, indicates that double layer effects in aqueous systems can be empirically neglected as far as pressure effects are concerned. 

It also implies that the metal center-electrode separation can indeed be taken to be simply one-half of a for the bimolecular electron transfer [56]. Thus, if there is an adsorbed layer of solute or solvent on the electrode, it either acts as a conducting surface or can otherwise be ignored for our purposes. 

A very important property of liquid water is its high dielectric comtanL The force of electrostatic attraction or repulsion between two charged bodies is inversely proportional to the dielectric constant of the medium. Water has a dielectric constant s of about 78 at ordinary temperatures. This means that, for example, a Na and a Cl ion are attracted to one another only l/78th as strongly in water as compared with a vacuum (where s = 1). Consequently, if we introduce solid sodium chloride into water the attractive forces between the Na and Cl ions- which hold the crystal together—are greatly reduced arid the salt goes into solution. 

Schematic representation of a protein molecule, showing different kinds of bonds which contribute to the folded structure  

Hydrogen bonds, which are so important in determining the structure of liquid water, also play a significant role in determining the conformations of proteins and other molecules. The most important hydrogen bonds found with proteins are between 

This matter is further considered later in this chapter. In addition, there is considerable hydrogen bonding between water molecules and the alcohol groups and carboxyl groups present in solute molecules  

Molecules containing a large proportion of such groups are soluble in water, whereas those containing mainly nonpolar groups are much less soluble. 

If an acid is added to water, Eq. 5.5 describes the reaction, because the base in solution is water. Further, if a base is added to water, Eq. 5.6 describes the reaction, because now water is the acid. These acid-base reactions are critical to life itself, since nature s solvent is water. Having a good understanding of the thermodynamics of these reactions is not only important for understanding organic reactions in water, but is of the upper most importance in understanding biochemical reactions, almost all of which have acid-base dependencies. The factors that control the thermodynamics of acid-base reactions are the strengths of the acids or bases and the pH of the solution, so these measurements of acidity need to be examined in detail. 

Since the Ka values reflect the relative stabilities of the species on the different sides of Eq. 5.5, we can use the Ka values to draw conclusions about the acid strength of HA relative to the strength of HaO. For example, we can first conclude that for Ka values larger than 1, HA is a stronger acid than This is because HA gives up its proton more efficiently than HjO does. Second, we can conclude that A is a weaker base than H2O, because a Ka greater 

As we will see, values range from very large (10 ) to extremely small (10 ). Hence, it 

Just as we expressed Eq. 5.5 in an equilibrium expression (Eq. 5.7), we can express Eq. 5.6 similarly (Eq. 5.9). The Ki, value gives insight into whether the base is a stronger base than hydroxide ion. Kt, values greater than 1 tell us that the added base is indeed stronger than hydroxide ion, while values less than 1 mean that the base is weaker. 

We do not want to rely on our own memory of pXaS, nor do we want to always go find pXa values in books to make our predictions about reactions such as Eqs. 5.5 and 5.6. Instead, our chemical intuition should be good enough such that the structures of HA and BHor the structures of A and B, will lead us to our predictions. Developing such a predictive ability is a large fraction of what this chapter addresses. However, we should also be able to predict, without resorting to calculations, what the protonation state of an acid or base will be when dissolved in water. This is determined by examining the pH and the pX.  

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Combustion in an incinerator is the only practical way to deal with many waste streams.This is particularly true of solid and concentrated wastes and toxic wastes such as those containing halogenated hydrocarbons, pesticides, herbicides, etc. Many of the toxic substances encountered resist biological degradation and persist in the natural environment for a long period of time. Unless they are in dilute aqueous solution, the most effective treatment is usually incineration. 

While incineration is the preferred method of disposal for wastes containing high concentrations of organics, it becomes expensive for aqueous wastes with low concentrations of organics because auxiliary fuel is required, making the treatment expensive. Weak aqueous solutions of organics are better treated by wet oxidation (see Sec. 11.5). 

Since free protons cannot exist, acidic properties can only be shown when the solvent can act as a proton acceptor, i.e. as a base. Thus aqueous solutions of acids contain the hydroxonium ion,... 

Benedict solution Aqueous solution of Na2C03, CuSO, and sodium citrate used for testing for reducing agents, particularly sugars, which give red-yellow colours or precipitates. 

Cd(OH) j. The hydroxide is precipitated from aqueous solution by OH", it does not dissolve in excess OH". Ignition of Cd(OH)2 or CdCO, gives CdO which varies in colour from red-brown to black because of lattice defects. 

Cr(02CCH3)2]2,2H20. Red insoluble compound formed from sodium ethanoate and CrC)2 in aqueous solution. The most stable Cr(II) compound contains a Cr —Cr bond, chromium fluorides... 

Cobalt chloride, C0CI2. Obtained as red crystals of CoCl2 6H20 from aqueous solution, CoCU HiO and C0CI2 are blue as is the (CoCU) " ion. No higher chloride is known although cobalt-haloammines, e.g. (Co(NH3)5C1) are stable. 

Copper(II) salts (blue in aqueous solution) are typical M(II) salts but generally have a distorted co-ordination (Jahn-Teller distortion, 4 near plus 2 far neighbours). Extensive ranges of complexes are known, particularly with /V-ligands. 

Isocroionic acid, -crotonic acid, cis-croionic acid. Colourless needles m.p. 14 C, b.p. 169 C. Prepared by distilling -hydroxy-glutaric acid under reduced pressure. Converted to a-crotonic acid by heating at 180 C, or by the action of bromine and sunlight on an aqueous solution. 

The -t-4 stale is stabilized in aqueous solution by fluoride ions. Cm02 and Cmp4 are formed by strong oxidation or the action of fluorine. 

CH2=CHC = CCH = CH2. a colourless liquid which turns yellow on exposure to the air it has a distinct garlic-like odour b.p. 83-5°C. Manufactured by the controlled, low-temperature polymerization of acetylene in the presence of an aqueous solution of copper(I) and ammonium chlorides. It is very dangerous to handle, as it absorbs oxygen from the air to give an explosive peroxide. When heated in an inert atmosphere, it polymerizes to form first a drying oil and finally a hard, brittle insoluble resin. Reacts with chlorine to give a mixture of chlorinated products used as drying oils and plastics. 

FenCon s reagent An aqueous solution of FeSO4 0rotherFe saltandhydrogen peroxide used for oxidizing polyhydric alcohols. 

Colourless crystals m.p. I25°C, soluble in water and alcohol. In aqueous solution forms equilibrium with its lactones. Gluconic acid is made by the oxidation of glucose by halogens, by electrolysis, by various moulds or by bacteria of the Acetobacter groups. 

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