Immersion in Dilute Solutions

The solution depletion procedure can be generalised to construct a simplified model of immersion experiment [64, 65]. This model is very useful to define the measurable enthalpy quantities, which does not necessarily mean that they can be easily obtained experimentally. The immersion process is schematically represented in Fig. 6.16. 

The balance of enthalpy in the initial (init) and final (fin) states of the model system can be written as follows (cf. Sect. 6.4.1)  

Taking into account the reference state for the solute defined in Sect. 6.4.1 and using Eqs.(6.40), (6.47), and (6.50), this expression may be transformed as follows  

Since the adsorption of the solvent is here considered negligible (i.e.,Mi = Mj), the first two terms on the right hand side of Eq. (6.54b) represents the enthalpy of immersion in a binary solution (1+2) with respect to the bulk solution at infinite dilution taken as the reference state [64] the third term corresponds to the enthalpy of dilution of S moles of the solute from molality m to infinite dilution in the solvent (cf., Eq. 6.44) and it is often incorporated into the previous enthalpy contribution to give the enthalpy of immersion H12 m of the solid in excess 

When the dilution term is evaluated independently in appropriate dilution experiment, the enthalpy of immersion AimmHn ( 2) determined experimentally by means of the same calorimetry equipment as that used to measure the enthalpy of immersion in a pure liquid (Sect. 6.3.2). However, great difficulties may be encountered when evaluating the usual correction terms in case of solutes which are volatile or surface-active (the composition of the vapour occupying the dead volume of the bulb is unknown). This method is also tedious. For systems containting electrified interfaces, the effects of EDL formation additionally contribute to the complexity of the adsorption phenomenon. Here the displacement experiment yields the enthalpy data easier to interpret. 

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The purer forms of iron (wrought iron and steel) appear to be much more susceptible to this kind of reaction than cast iron.3 If the attacking acid is readily reducible by hydrogen, many secondary reactions may take place. Thus with nitric acid, oxides of nitrogen and ammonia may be evolved, whilst with selenic acid a deposit of elementary selenium is obtained (see below). When iron is exposed to the action of acids that are also powerful oxidisers—such as, for example, fairly concentrated solutions of nitric and chromic acids,—it is frequently rendered inert or passive.4 Its surface may remain perfectly bright, but the metal does not show any appreciable solution in the acid, and if removed and immersed in dilute solutions of such salts as copper and silver sulphates, no reaction takes place, although ordinary active iron would cause an immediate precipitation of the more electronegative metal. 

Mention has already been made of the fact (see pp. 54, 69) that iron may be preserved from corrosion by immersion in dilute solutions of the alkali hydroxides. 

Chemical bath deposition is a technique in which thin semiconductor films are deposited on substrates immersed in dilute solutions containing metal ions and a source of hydroxide, telluride, sulfide, selenide, etc., ions. One of the first chemically deposited semiconductors, reported in 1869, was a PbS thin film [26]. During the ensuing 140 years, CBD has been used to deposit films of metal sulfides, selenides, and oxides, and various other compounds. While it is a well-known technique in a few specific areas (notably photoconductive lead salt detectors, photoelectrodes, and, more recently, thin-film solar cells), it is by and large an under-appreciated technique. 

The resistance of polyurethane elastomers to immersion in water has been identified as relatively poor and is directly applicable to immersion in dilute solutions of inorganic materials in water. Provided the inorganic substance has no catalytic effect the solution can be expected to behave as pure water. However, acidic or alkaline media accelerate hydrolytic attack and therefore solutions of salts of weak acids or bases are likely to degrade polyurethanes faster than water. As a generalization, it can be stated that, provided the pH of a solution lies between the values of 5-5 and 8, the action of the solution can be considered similar to the action of water. At higher acidities or higher alkalinities it is advisable to test the effect of the particular solution. As would be expected, strong acids and bases attack polyurethanes rapidly. 

Steel immersed in dilute chromate solutions does not mst. The exact mechanism of the inhibition is not known, although it is agreed that... 

Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

Silicone fluids containing Si—H groups are also used for paper treatment. The paper is immersed in a solution or dilute emulsion of the polymer containing either a zinc salt or organo-tin compound. The paper is then air-dried and heated for two minutes at 80°C to cure the resin. The treated paper has a measure of water repellency and in addition some anti-adhesive properties. 

FIGURE 27.30 Voltammogram of Ni in l.OM NaOH solution scan rate lOmV/s. Pretreatment polishing, immersion in dilute HCl solution and cathodic reduction at — I.IV vs. Hg/HgO in l.OM NaOH, for lOmin. Potentials where the ellipsometer spectra were obtained are indicated by arrows (1) -0.9 V, (2) -0.4 V, (3) +0.5 V, and (4) +0.2V. (From de Souza et ah, 1997, with permission from Elsevier.)... 

Electrolyte-concentration cells are based on electrolyte dilution, and have two identical electrodes that are immersed in two solutions of the same electrolyte containing ions of the electrode material at two different activities. Electrolyte concentration cells are classified as (i) cells without liquid junctions and (ii) cells with liquid junctions. 

Recently the wall-PRISM theory has been used to investigate the forces between hydrophobic surfaces immersed in polyelectrolyte solutions [98], Polyelectrolyte solutions display strong peaks at low wavevectors in the static structure factor, which is a manifestation of liquid-like order on long lengths-cales. Consequently, the force between surfaces confining polyelectrolyte solutions is an oscillatory function of their separation. The wall-PRISM theory predicts oscillatory forces in salt-free solutions with a period of oscillation that scales with concentration as p 1/3 and p 1/2 in dilute and semidilute solutions, respectively. This behavior is explained in terms of liquid-like ordering in the bulk solution which results in liquid-like layering when the solution is confined between surfaces. In the presence of added salt the theory predicts the possibility of a predominantly attractive force under some conditions. These predictions are in accord with available experiments [99,100]. 

Steel objects, when exposed to humid atmospheres or when immersed in electrolytes, corrode at a rapid rate. For example, abrasively polished, cold-rolled steel panels will show signs of rust within 15 minutes when immersed in dilute chloride solutions with pH in the range of 7-10. One of the methods used to control this rapid corrosion is to coat the metal with a polymeric formulation such as a paint. The role of the paint is to serve primarily as a barrier to environmental constituents such as water, oxygen, sulfur dioxide, and ions and secondarily as a reservoir for corrosion inhibitors. Some formulations contain very high concentrations of metallic zinc or metallic aluminum such that the coating provides galvanic protection as well as barrier protection, but such formulations are not discussed in this paper. 

The S S menbranes were immersed in dilute alcohol solutions during shipment and storage. The alcohol could be replaced by water, but the menbrane could not be allowed to -dry or it would shrink in an irreversible manner to become a useless membrane. Recalling the unsuccessful tests of Section II we heated the S S membranes under water to temperatures in the order of 80 -... 

A solution of 51.2 g. (0.2 mole) of potassium diphenylphos-phinate in 1500 ml. of methanol is prepared as follows A suspension of 43.64 g. (0.2 mole) of diphenylphosphinic acid in 500 ml. of methanol is slowly neutralized by adding a solution of 16.3 g. (0.25 mole) of potassium hydroxide (assay 86.0% KOH) in 500 ml. of methanol until a pH of 6.5-7.0 is reached. The solution is then diluted to 1500 ml. with methanol and deaerated by passing a stream of nitrogen for 15 minutes through a gas dispersion tube immersed in the solution. 

Following the Pitaevskii strategy for extracting small-particle van der Waals interactions for the interaction between suspensions, we specialize the general expression for ionic-fluctuation forces to derive forces between cylinders (Level 3). As with the extraction of dipolar forces between rods, consider two regions A and B, dilute suspensions of parallel rods immersed in salt solution interacting across a region of salt solution m (see Fig. L2.19). 

The electrical properties of polyelectrolyte complexes are more closely related to those of biologically produced solids. The extremely high relative dielectric constants at low frequencies and the dispersion properties of salt-containing polyelectrolyte complexes have not been reported for other synthetic polymers. Neutral polyelectrolyte complexes immersed in dilute salt solution undergo marked changes in alternating current capacitance and resistance upon small variations in the electrolyte concentration. In addition, their frequency-dependence is governed by the nature of the microions. As shown in... 

In conventional continuous wet finishing, a rather dilute, usually from 4 to 8% solids solution or dispersion of the chemicals Is prepared. The fabric Is then Immersed in this solution. The wet fabric Is passed through pad rollers to squeeze out the excess solution. Despite the squeezing, a cotton fabric usually retains 80 to 100% on the weight of the fabric of the finishing solution. For polyester-cotton blend fabrics, the wet pick-up Is normally 50 to 80% on the weight of the fabric. 

Iron dicarbide, FeC2.—A substance of this composition has been prepared 1 by allowing melts of iron, containing from 6 to 10 per cent, of carbon, to cool. Crystallisation begins at 2380° to 2000° C., a pale yellow carbide separating out, with a silvery reflex. It is slowly attacked by nitric acid, and, when immersed in dilute copper sulphate solution, becomes coated with a film of metallic copper. 

Activity Coefficients of Ampholytes.—The theoretical treatment of the electrical forces in a solution of an ampholyte containing free single ions, e.g., of added electrolytes, is very difficult it has, however, been carried out on the assumption that the amino-acid consists essentially of a spherical dipolar ion immersed in a continuous solvent medium. If R is the distance between the charges in the dipolar ion and a is the mean distance of closest approach of the other ions to this ion, the limiting value of the activity coefficient y of the dipolar ion in dilute solution is given by... 

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