Solvents and Solution Concentration

The 5-mm tube held in the sample probe is indicative of the sample size used in an NMR instrument— typically some fraction of a milliliter. Typical analyte quantities dissolved in this volume range from 10 

A popular solvent for the sample is deuterated chloroform, CDC13. The D is the symbol for deuterium, the isotope of hydrogen that has one proton and one neutron. This solvent has no H nuclei and does not absorb RFs. It will therefore not interfere in the analysis. 

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Other experimental factors that must be considered attentively are temperature, nature of the solvent and solution concentration. A substantial improvement in S/N and sensitivity can be achieved by increasing the temperature of acquisition. Indeed, raising the temperature can lead to a decrease in the line width, due to faster molecular tumbling, and to an increase in the solubility or miscibility of the solute, if necessary. Low-viscosity solvents should be preferred, since molecular tumbling depends on the medium viscosity. 

The Effect of Solvent and Solute Concentration. The effect of solvent concentration on selectivity is qualitatively described by three types (2, 11) shown in Figure 3. 

The ultracentrifugation experiment was performed using a Beckman Model E Instrument with a capillary synthetic boundary cell at 30°C. Toluene was used as the solvent and solution concentrations were 1% (w/v). 

Price GJ, Smith PF (1993) Ultrasonic degradation of polymer-solutions. 3. The effect of changing solvent and solution concentration. Eur Polym J 29 419-424... 

Figure 1 shows how the solvent and solute concentrations are affected by the presence of the liquid-air interface The solvent (water) concentration decreases rapidly, and the solute (surfactant) exhibits a maximum concentration. 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

The crosscurrent scheme is not generally economically attractive for large commercial processes because solvent usage is high and solute concentration in the combined extract is low. 

In Chapter 7 we found it convenient to distinguish between proton transfers involving a solvent molecule and those involving only solute particles but this difference will lose its significance when the distinction between solvent and solute begins to break down. We recall that in Sec. 54 the mole fraction of the solvent did not differ appreciably from unity and could be omitted from (72). In investigating concentrated solutions, however, there is no question of extrapolating to infinite dilution the mole fraction of the solvent will differ from unity and will have to be retained in all formulas. At the same time each of the mole fractions will need to be multiplied by its activity coefficient. 

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

When working with solutions, one often talks about the various components as being the solvent or a solute. The solvent is the component in which the other components (solutes) are considered to be dissolved. The solvent is usually the component present in largest amounts, but this is not always so. Thus, water may be considered to be the solvent in concentrated sulfuric acid solution, although very little water is present. Water is usually considered to be the solvent in solutions in which it is present. Such solutions are designated as aqueous. Sometimes no one component plays a role that would justify calling it a solvent, in which case, the solvent and solute designation lose... 

Besides these special physical properties, hydrogen-bonded liquid water also has unique solvent and solution properties. One feature is high proton (H ) mobility due to the ability of individual hydrogen nuclei to jump from one water molecule to the next. Recalling that at temperatures of about 300 K, the molar concentration in pure water of H3O ions is ca. 10 M, the "extra" proton can come from either of two water molecules. This freedom of to transfer from one to an adjacent "parent" molecule allows relatively high electrical conductivity. A proton added at one point in an aqueous solution causes a domino effect, because the initiating proton has only a short distance to travel to cause one to pop out somewhere else. 

As far as crystallization is concerned, there are two components, solvent and solute, and F = C = 2. The solid phase is pure, and variables are concentrations, temperature, and pressure. Fixing one, the pressure, leaves either concentration or temperature as an independent variable. The relationship between temperature and concentration is the usual solubility curve. 

Heats of solution are dependent on concentration. The integral heat of solution at any given concentration is the cumulative heat released, or absorbed, in preparing the solution from pure solvent and solute. The integral heat of solution at infinite dilution is called the standard integral heat of solution. 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

Concentration problems are concerned with the definitions of the various units. It is possible to calculate the mass and/or volume of the solvent and solute by taking the difference between the final and initial measurements. The density, if not given, is calculated, not measured. It is important to recognize the difference between the values that must be measured and those that can be calculated. Moles are also calculated, not measured. 

The yield of crystals produced by a given degree of cooling may be estimated from the concentration of the initial solution and the solubility at the final temperature, allowing for any evaporation, by making solvent and solute balances. For the solvent, usually water, the initial solvent present is equal to the sum of the final solvent in the mother liquor, the water of crystallisation within the crystals and any water evaporated, or ... 

Crystallisation by freezing, or freeze crystallisation, is a process in which heat is removed from a solution to form crystals of the solvent rather than of the solute. This is followed by separation of crystals from the concentrated solution, washing the crystals with near-pure solvent, and finally melting the crystals to produce virtually pure solvent. The product of freeze crystallisation can be either the melted crystals, as in water desalination, or the concentrated solution, as in the concentration of fruit juice or coffee extracts. Freeze crystallisation is applicable in principle to a variety of solvents and solutions although, because it is most commonly applied to aqueous systems, the following comments refer exclusively to the freezing of water. 

Choosing the sample pre-treatment method is difficult. The most important consideration is the final condition of the target compounds. What kind of solution is obtained The type of solvent and the concentration of the target compounds are very important in the selection of the separation conditions. 

As a result, the plots of o- / vs. E for different bulk concentrations of the solute intersect at the same point (Umax. max). which corresponds to the adsorption maximum. This implies that at the adsorption maximum does not vary with increasing F. Consequently, the surface dipole potential due to the adsorbing molecules exactly matches that of the desorbed solvent molecules, " provided that the solvent and solute molecules assume only one orientation at the surface. In practice, however, a single point of intersection of the vs. E plot for different bulk solute concentrations is observed rather rarely. For example, a gradual inaease in the surface concentration of 2-propanol on mercury in aqueous solution brings about a small positive shift of a. A much bigger shift has been observed for hexafluoro-2-propanol (HFP). " It has been ascribed to the reorientation of the HFP molecules in such a way that both -CF3 groups are directed toward the electrode, which in turn results in... 

Because of the low rate of solution of the amine salt, the desired solution is obtained most rapidly by dissolving the salt in excess solvent and then concentrating the solution. 

Fig. 2.4 The vapor pressure diagram of a dilute solution of the solute B in the solvent A. The region of ideal dilute solutions, where Raoult s and Henry s laws are obeyed by the solvent and solute, respectively, is indicated. Deviations from the ideal at higher concentrations of the solute are shown. (From Ref. 3.)...

In ultrafiltration and reverse osmosis, in which solutions are concentrated by allowing the solvent to permeate a semi-permeable membrane, the permeate flux (i.e. the flow of permeate or solvent per unit time, per unit membrane area) declines continuously during operation, although not at a constant rate. Probably the most important contribution to flux decline is the formation of a concentration polarisation layer. As solvent passes through the membrane, the solute molecules which are unable to pass through become concentrated next to the membrane surface. Consequently, the efficiency of separafion decreases as fhis layer of concentrated solution accumulates. The layer is established within the first few seconds of operation and is an inevitable consequence of the separation of solvent and solute. 

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