Concentrations of ions

The conductivity of solutions depends, from A2.4.31. on both the concentration of ions and their mobility. Typically, for 1 M NaCl in water at 18°C, a value of 7.44 is found by contrast, 1 M H2SO4 has a... 

In principle, the effects of the concentration of ions can be removed by dividing A2.4.31 by the concentration. Taking Avagadro s constant as L and assuming a concentration of solute c mol m, then from the electroneutrality principle we have = A = z cL and clearly... 

These calculations have, as their aim, the generation of an adsorption isotherm, relating the concentration of ions in the solution to the coverage in the IHP and the potential (or more usually the charge) on the electrode. No complete calculations have been carried out incorporating all the above temrs. In general, the analytical fomi for the isothemr is... 

Because water is not protonated in these solutions, its addition reduces the concentration of ions, and therefore the electrical conductivity. The conductivity reaches a minimum in solutions containing 97% of acid, but rises on further dilution as a result of the formation of nitrate and hydroxonium ions. ... 

An unusual slurry process which works well with sodium hydroxide is based on diffusion of the aqueous reagent into pellets of acid polymer (28). The concentration of ions in the Hquid phase is preferably two to four times the stoichiometric level, and the temperature is maintained at 50—100°C. 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

At neutral or higher pH, the concentration of ions is too low for this reaction to contribute significantly to the overall corrosion rate. However, as pH decreases, this reaction becomes more important until, at a pH of about 4, it becomes the predominant cathodic reaction. 

Jons. Amelin [Theory of Fog Condensation, Israel Program for Scientific Translations, Jerusalem, (1967)] reports that ordinary air contains even higher concentrations of ions. These ions also reduce the required critical supersaturation, but by only about 10 to 20 percent, unless multiple charges are present. 

Whatever the water composition, corrosivity can be increased by evaporation, which may elevate pH by increasing concentrations of ions in the remaining liquid. This is the reason that cooling systems that experience boiling and/or large evaporative losses without sufficient makeup water additions may be especially prone to attack. Cycling may also increase dissolved species concentrations. 

Use of such constructions that can elongate arc-length and reduce the concentration of ions in the arc plasma and hence enhance the dielectric strength between the parting contacts. 

The oxidation of a particular metal in air is limited by the outward diffusion of metallic ions through an unbroken surface film of one species of oxide. Assume that the concentration of metallic ions in the film immediately next to the metal is Cj, and that the concentration of ions in the film immediately next to the air is C2, where and C2 are constants. Use Tick s First Law to show that the oxidation of the metal should satisfy parabolic kinetics, with weight gain Am given by... 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

For example, in the case of dilute solutions, the van t Hoff s equation may be used to piedict the osmotic pressure (jr = CRT) where n is the osmotic pressure of the solution, C is the molar concentration of the solute, ft is the universal gas constant and T is the absolute temperature, Fm dissociating solutes, the concentration is that of the total ions. For example, NaCI dissociates in water into two ions Na" " and Cl . Therefore, the total molar concentration of ions is hvice the molar concentration of NaCI. A useful rule of thumb for predicting osmotic pressure of aqueous solutions is 0,01 psi/ppm of solute (Weber, 1972). 

Substituting for UL by co and for the limiting case for the cathodic reduction process when = 0, the activity term Aff is then equal to (the concentration of ions in the bulk solution), and then... 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

Butler and Ison S have suggested that variation in corrosion rate can be influenced by surface roughness, which allows a large number of nuclei for steam bubble formation. In these circumstances they have suggested that concentration of ions in solution next to the surface will be greater, and their observations on corrosion damage indicate that the steam bubbles may provide crevices or at least enhanced conditions for dissolution at the triple interface (solution/metal/steam). 

When the Donnan equilibrium is operative the entry of ions into the membrane is restricted. Consequently as the concentration of ions in the solution increases the resistance of the membrane remains constant until the concentration of ions in the solution reaches that of the fixed ions attached to the polymer network. At this point their effect will be swamped and the movement of ions will be controlled by the concentration gradient. 

If Q > K, the solution contains a higher concentration of ions than it can hold at equilibrium. A precipitate forms, decreasing the concentrations until the ion product becomes equal to Ksp and equilibrium is established. 

If Q < Ksp, the solution contains a lower concentration of ions than is required for equilibrium with the solid. The solution is unsaturated. No precipitate forms equilibrium is not established. 

Equilibrium curve for silver chloride. Silver chloride (s) is in contact with Ag+ and Cl- ions in aqueous solution. The product Q of the concentration of ions [Ag+] X [Cl-] is equal to Ksp (curved line)when equilibrium exists. If 0 > K,p, AgCI(s) tends to precipitate out until equilibrium is reached. If 0 < Ksp, additional solid dissolves. 

In chemistry, the most important use of the Nemst equation lies in the experimental determination of the concentration of ions in solution. Suppose you measure the cell voltage and know the concentration of all but one species in the two half-cells. It should then be possible to calculate the concentration of that species by using the Nemst equation (Example 18.7). 

The ease with which an aqueous salt solution conducts electric current is determined by how much salt is dissolved in the water, as well as by the fact that ions are formed. A solution containing 0.1 moles per liter conducts much more readily than a solution containing 0.01 moles per liter. Thus the conductivity is determined by the concentration of ions, as well as by their presence. 

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