Freezing electrical effects

Moisture. Moisture can be present in the form of rain, dew, high humidity etc. It may be responsible for a number of forms of attack on polymeric materials. These include hydrolysis of the polymer chemically, the production of hydroxyl radicals in the presence of sunlight which lead to degradation reactions, erosion caused by freezing, and the leaching out of additives such as plasticizers and stabilizers, A further important action of moisture is to dissolve solid pollutant and this leads to destructive electrical effects. 

The underlying metal lattice structure also has a significant effect on the water structure. As pointed out by Spohr," although the Pt-Pt nearest-neighbor distance is ao/ Jl = 0.277 nm, which is very close to the 0-0 distance in ice, the cubic symmetry of the 100 surface is incompatible with the hexagonal symmetry of the ice lattice. As a result, the water molecules cannot form a uniform monolayer and occupy all adsorption sites. On the other hand, Berkowitz and co-workers showed that the hexagonal Pt (111) surface is able to support a more complete layer of adsorbed water molecules, and one can identify patches of an icelike structure in the first layer. This freezing is further enhanced by an external electric field, as will be discussed later. 

In the last section the method was illustrated by which per cent ionization could be calculated from freezing point on the assumption that ions and un-ionized molecules are of equal effect. In the following tables are given the values calculated for the ionization of various electrolytes in 0.1 equivalent solution on the basis of this assumption. These values we shall designate as the apparent ionizations. They come out approximately the same whether they are calculated from electrical-conductivity measurements or from freezing points. 

The e> planation of the fact that a strong electrolyte such as potassium bromide produces a smaller freezing-point lowering than calculated for complete ionization is that there are strong electrical forces operating between the ions, which decrease their effectiveness, so that the properties of their solutions are different from those of ideal solutions, except at extreme dilution. The interionic attraction reduces the activity of the ions to a value less than their concentration. 

The freezing points of biochemical products containing hygroscopic molecules (e.g. sugars, protein complexes) are well below zero by effect of some water molecules being tightly bound to molecules of the dissolved product. Eutectic points are usually determined from electrical resistivity measurements. 

Acids are classified as strong or weak, depending on the extent to which they are ionized in solution. In a weak acid the transfer of hydrogen ions to water does not proceed to completion. A weak acid such as acetic acid is thus also a weak electrolyte its aqueous solutions do not conduct electricity as well as a strong acid of the same concentration because fewer ions are present. A weak acid shows smaller values for colligative properties than a strong acid (recall the effect of dissolved acetic acid on the freezing point of water in Fig. 11.13). 

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