Common ions

Addition of excess H ions to this solution will cause the equilibrium to move towards undissociated acid thereby decreasing the concentration of Ac . This effect is known as the common-ion effect and is of considerable practical importance. Thus, e.g. in the precipitation of metal ions as insoluble sulphides. 

Sodium sulphate crystallises out in hydrated form (common ion effect) and is filtered off on concentration, sodium dichromate is obtained. For analytical purposes, the potassium salt. K2Cr20-. is preferred potassium chloride is added and the less soluble potassium dichromate obtained. 

The most common ion guide is the quadrupole, in which there are four parallel metal rods spaced evenly about a central axis. 

Sa.lts Salting out metal chlorides from aqueous solutions by the common ion effect upon addition of HCl is utilized in many practical apphcations. Typical data for ferrous chloride [13478-10-9] FeCl2, potassium chloride [7447-40-7] KCl, and NaCl are shown in Table 9. The properties of the FeCl2-HCL-H2 0 system are important to the steel-pickling industry (see Metal SURFACE TREATMENTS Steel). Other metal chlorides that are salted out by the addition of hydrogen chloride to aqueous solutions include those of magnesium, strontium, and barium. 

Eor salt-out, add solute which reduces effective solubihty of component to be crystallised (salt having common ion is usual choice) salting-out compound should be cheap, as may be difficult to recover. 

DecoloriZation. Filtration, often a refinery botdeneck, especially with poor-quaHty raw sugar, is foUowed by decolorization with bone char (traditional), granular activated carbon (now most common), ion-exchange resias, or any combination of these. Comparative merits and regeneration of these decolorizing systems are a frequent topic ia the Hterature (r6—r8,rll). 

Table 1 gives conductivity factors for common ions found in water supplies. 

The use of RBS concurrendy with ERS is necessary for the complete derivation of a hydrogen profile, and it offers some simplifications of analysis. For example, for thin-layer spectra that have been normalized for a common ion fluence Qand solid angle SI, the total yields Y (the areas under the spectral peaks) may be compared in order to derive the layer composition. For... 

The most common ions observed as a result of electron-stimulated desorption are atomic (e. g., H, 0, E ), but molecular ions such as OH", CO", H20, and 02" can also be found in significant quantities after adsorption of H2O, CO, CO2, etc. Substrate metallic ions have never been observed, which means that ESD is not applicable to surface compositional analysis of solid materials. The most important application of ESD in the angularly resolved form ESDIAD is in determining the structure and mode of adsorption of adsorbed species. This is because the ejection of positive ions in ESD is not isotropic. Instead the ions are desorbed along specific directions only, characterized by the orientation of the molecular bonds that are broken by electron excitation. 

Estimates of the heat of solvation of various species in DMSO as compared to water have been made and can be expressed as enthalpies of transfer. Some data for some common ions are given below. Discuss their significance. 

Table 5.8. Hardness and Softneis of Some Common Ions and Molecules...

The rate constant k is measured as a function of (added) concentration of the common ion X , and from the plot according to Eq. (4-76) the ratio k i/k2 = slope/intercept is evaluated. 

If the intermediate is very unstable, large rate constants may be measured in this way. Thus, Amyes and Jencks studied the hydrolysis of a-azidoethers (Ns", the azide ion, being the common ion), finding (because k i had been independently measured) k2 values in the range 10 to 10 ° M s for the reaction with water. 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

These substances accelerate the reaction, and their effectiveness increases in the order given. This suggestion was questioned by Pocker, who found that the effects of such added substances were not directly proportional to their concentrations and could easily be explained by macro effects on the solvent character. He also found that common-ion effects were small in the reaction, the effect of added 1-methylpyridinium bromide was negligible, and that there was no evidence for surface catalysis on the walls of the vessel. There is an exact parallel between the relative rates of the Finkelstein reactions... 

The predictions of the pH/potential diagram are generally fulfilled, but in very concentrated acid solutions, attack may diminish, owing to the relative insolubility of the relevant salt in the acid. Thus, lead nitrate, although soluble in water, has (owing to common ion effect) only slight solubility in concentrated nitric acid, and the corrosion rate is reduced. Similarly, lead chloride is less soluble in moderately concentrated hydrochloric acid than... 

A "common" ion comes from two sources such as BaS04 and Na2S04. 

The effect illustrated in Example 16.6 is a general one. An ionic solid is less soluble in a solution containing a common ion than it is in water (Figure 16.3, p. 438). 

Common ion effect The tube at the left contains a saturated solution of silver acetate (AgC2H302). Originally the tube at the right also contained a saturated solution of silver acetate. With the addition of a solution of silver nitrate (AgNOs), the solubility equilibrium of the silver acetate is shifted by the common ion Ag+ and additional silver acetate precipitates. 

Click Coached Problems for a self-study module on the common ion effect... 

Addition of sodium hydroxide, which contains the Na+ common ion, causes NaCl to precipitate from the solution. 

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