Appearance Pure aqueous solutions

Analysis of the spectrum of CiT4 in TFE/D2O solution indicates differences in the secondary structure of the peptide when compared to that observed for the peptide in the pure aqueous solution. On addition of TFE, a more structured conformation of CiT4 is indicated. Notably, a-he ical structures and -structures appear to be formed in the less polar environment provided by the TFE. Thus, a change in environment, in this case the solvent, can cause notable changes to the peptide conformations which are observable when using infrared spectroscopy. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

Nicotinamide. Place 50 g. of pure ethyl nicotinate (Section V,23) in a 350 ml. bolt-head flask and add 75 ml. of concentrated aqueous ammonia saturated at 0°. Keep the flask loosely stoppered for 18 hours, after w)iich time the lower layer generally dissolves on shaking. Saturate the solution with ammonia and allow it to stand for a further 4 hours. Repeat the saturation with ammonia crystals of the amide commence to appear in the solution. Evaporate to drjmess in a dish on the steam bath and dry at 120°. The yield of nicotinamide, m.p. 130°, is usuallj quantitative. 

Hydrophobicity represented by AG° for the transfer of solute from the pure liquid to aqueous solution increases progressively with increasing temperature34>. There is, however, an extremum in the temperature—selectivity plot in some cases (e.g., R2 = i-CsHn, Ph, and p-MeC6H4) l4b,18). it appears that the observed selectivity cannot be explained in terms of hydrophobic interaction. 

First we write the balanced chemical equation for the reaction. Then we write the equilibrium constant expressions, remembering that gases and solutes in aqueous solution appear in the Kc expression, but pure liquids and pure solids do not. 

Pressures of gases and molarities of solutes in aqueous solution appear in thermodynamic equilibrium constant expressions. Pure solids and liquids (including solvents) do not appear. 

In writing the thermodynamic equilibrium constant, recall that neither pure solids (PbS(s) and S(s)) nor pure liquids (H20(1)) appear in the thermodynamic equilibrium constant expression. Note also that we have written H+(aq) here for brevity even though we understand that H30+(aq) is the acidic species in aqueous solution. 

About the same time Beutier and Renon (11) also proposed a similar model for the representation of the equilibria in aqueous solutions of weak electrolytes. The vapor was assumed to be an ideal gas and < >a was set equal to unity. Pitzer s method was used for the estimation of the activity coefficients, but, in contrast to Edwards et al. (j)), two ternary parameters in the activity coefficient expression were employed. These were obtained from data on the two-solute systems It was found that the equilibria in the systems NH3+ H2S+H20, NH3+C02+H20 and NH3+S02+H20 could be represented very well up to high concentrations of the ionic species. However, the model was unreliable at high concentrations of undissociated ammonia. Edwards et al. (1 2) have recently proposed a new expression for the representation of the activity coefficients in the NH3+H20 system, over the complete concentration range from pure water to pure NH3. it appears that this area will assume increasing importance and that one must be able to represent activity coefficients in the region of high concentrations of molecular species as well as in dilute solutions. Cruz and Renon (13) have proposed an expression which combines the equations for electrolytes with the non-random two-liquid (NRTL) model for non-electrolytes in order to represent the complete composition range. In a later publication, Cruz and Renon (J4J, this model was applied to the acetic acid-water system. 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

We have included here, for comparison, the results of a study of zirconolite-rich Synroc nominally composed of 80 wt% Ce- or Pu-doped zirconolite plus 10 wt% hollandite and 10 wt% rutile (Hart et al. 1998). Inclusion of this study in this section is significant because the two additional phases are both highly durable in their own right and the experiments were conducted at two different temperatures (90 and 200 °C) and in three different aqueous solutions (pure water, silicate, and brine). The authors found no major differences in the release rates of Ca, Ce, Hf, Ti, Zr, Pu, and Gd apart from those for Ce and Ti, which appeared to be somewhat higher in the brine. On average, for all elements, the increase in temperature caused the release rates to increase by a factor of approximately seven. Release rates were generally below 10 2 g/m2/d for Ca, 10 3 g/m2/d for Ce and Gd, and 10 4 g/m2/d for Ti, Zr, Hf, and Pu (except for the brine at 200 °C, which gave a Ti release rate of 2 x 10 3g/m2/d). Hart et al. (2000) also determined the release rate of Pu in an LLNL-type zirconolite ceramic. After nearly one year in pure water at 90 °C the release rate of Pu decreased from 2 x 10-3 g/m2/d to less than 10-5 g/m2/d (Fig. 7). 

The film deposition was carried out at room temperature from an aqueous solution of plumbous acetate, ammonium acetate, and ammonium persulphate, using NH4OH to bring the pH to 6. A trace of AgNOs was added as a catalyst for reaction 7.4 [29]. A film of PbOi ca. 50 nm thick was formed in an hour. Once this initial film was deposited, thicker films could be built up, usually at a somewhat higher pH, in the absence of the AgNOs. The initial film formation appears to be a pure CD reaction. However, electrochemical studies of further film buildup showed that an electroless deposition mechanism, involving two partial electrochemical reactions, was responsible for film formation. 

Even if crystals grow from the same aqueous solution, there are differences in Habitus. NaC103 crystals, for example, grow easily as polyhedral crystals, whereas NH Cl crystals always grow as dendrites, and NaCl crystals appear as hopper crystals. If Pb or Mn ions are added, cubic crystals of NaCl bounded by flat 100 faces may be obtained quite easily, but if NaCl is grown in pure solution all crystals take a hopper form, unless great care is taken to keep the supersaturation very low. These differences occur because the solute-solvent interaction energies, and, as a result, the values of Ap,/kT and A/x/kT, are different for different crystals. 

In aqueous iodide solution to which small amounts of iodine has been added, the I127 resonance for I- is broadened over that for pure iodide solutions. Degree of broadening here is related to the average lifetime of an iodine nucleus as Ir- These experimental conditions then appear to meet the conditions of slow exchange. The frequency separations between the I- and 12 and between the I and I3 resonances are not known, but presumably these are large with respect to the I line width and Eq. (38) is valid for this system. 

The substance so produced, even after repeated crystallisation from water, still contains sodium chloride, with which it appears to be iso-morphous. The pure chloride is best prepared by treating a solution of the crude salt with solid sodium iodide, whereby it is transformed into the iodide. The iodide is recrystallised, and is then converted into the chloride by shaking the aqueous solution with freshly precipitated silver chloride. It crystallises in transparent cubes or in small glistening needles, and loses 2J molecules of water on heating to 120° C.3... 

The Criss-Cobble correspondence principle is useful for aqueous solutions to about 200 °C. At higher temperatures, the heat capacities Cp of ionic solutes such as NaCl26 at constant pressure rapidly become strongly negative and appear to be headed toward infinite negative values on approaching the critical temperature (which, incidentally, is somewhat higher for aqueous electrolyte solutions than for pure water). If, however, we examine the heat capacities Cy of aqueous electrolytes at constant volume,... 

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