Dilute solutions, of acids and bases

When using dilute solutions of acids and bases, you can assume their density is close to the density of water. Therefore, you can easily measure the volume of the solutions and calculate their mass. 

Furthermore, the advent of FT NMR allows the routine study of these nuclei, as well as the use of relatively dilute solutions of acids and bases, thus reducing the importance of concentration effects. 

Since a chemist should be familiar with the taste of hydrogen and hydroxyl ions, we have set the very bad precedent here of giving directions to taste the dilute solutions of acids and bases. In no other case should any laboratory chemical be taken in the mouth. 

Silicones are resistant to water between 0°C and 100°C. Steam, at higher temperatures in prolonged attack, destroys the system. Normally, silicone rubbers are resistant to salt solution and to dilute solutions of acids and bases. However, they are not resistant to organic solvents. In this case a reversible swelling takes place. 

In dilute solutions of acids and bases and in pure water, the activities of H and OH" may be considered to be the same as their concentrations. 

Concentrations of acids and bases are conventionally expressed in terms of molarity when the concentrations are greater then 0.1 M. However, for very dilute solutions of acids and bases where exponential numbers are required to describe [H" ] and [OH"] in a solution, it is more convenient to use a compressed logarithmic scale for [H" ] and to express pH mathematically as... 

The potential for harm from strong acids and bases, however, does not mean all acids and bases are to be avoided. The dilute solutions of acids and bases that are in everyday use would be sorely missed. Orange juice, vinegar, soda pop, household ammonia, and most of our soaps and determents fall in this category of dilute solutions. 

When solutions of acids and bases are sufficiently dilute or when other electrolytes are present, the activity, rather the concentration of hydrogen ions, should be substituted in the pH equation. 

The rest of this chapter is a variation on a theme the use of equilibrium constants to calculate the equilibrium composition of solutions of acids and bases. We begin with solutions of acids, bases, and salts, explore the contribution of the autoprotolysis of the solvent to the pH, which is significant in very dilute solutions, and see how to handle the complications of acids that can donate more than one proton. Although the applications are varied, the techniques are all very similar and are based on the material in Chapter 9. 

The solutions of acids and bases that are sold commercially are too concentrated for most laboratory uses. We often dilute these solutions before we use them. We must know the molar concentration of a stock solution before it is diluted. This can be calculated from the specific gravity and the percentage data given on the label of the bottle. 

Using the equations developed so far, we can examine the interrelations of the species present at equilibrium in pure water and in solutions of acids and bases. Clearly equation (2-9) requires that the product of the two variables H and OH be constant. As one rises, the other must fall. For the moment, consider roughly, using molarities for activities, the possible ranges of H and OH. Neither can be zero and satisfy equation (2-9). At the other extreme, one liter can contain a limited amount of solute. One hundred moles, or 4000 g of NaOH will not fit in a liter of solution. Let us take 10 mol/liter as about the maximum possible. Now we look at the total range between 10 M strong acid and 10 M strong base. In 10 M HCl solution we take H as approximately 10 M. (This hardly qualifies as a dilute aqueous solution.) What is OH Equation (2-9) requires... 

The logarithmic pH and pOH scales reduce the extremely wide variation in concentrations of hydrogen ions, H (aq), and hydroxide ions, OH (aq), in dilute aqueous solutions of acids and bases (typically 1 to 10 ) to a narrower range of pH (typically 1 to 14) (Figure 18.8). pH values of solutions are measured with a pH meter or narrow-range indicator paper (Figure 18.9). 

The evolution of hydrogen and oxygen are well-known phenomena during electrolysis of dilute aqueous solutions of acids and bases between inert metal electrodes. If bright platinum electrodes are used for the electrolysis, it is found that in most cases the minimum potential difference which must be applied between them before gas bubbles appear is close to 1 -7 V. That a similar value s)iould usually be observed is hardly surprising since the same overall chemical process is occurring, viz. the decomposition of water. 

Decide what type and concentration of acid and base you will add to your systems. Dilute solutions of strong acids and bases work well. 

Figure 6. Experimental data on the effect of pK and degree of dissociation of acids and bases on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes ( 5, 16, 17,1...

Human use of acids and bases dates back thousands of years. Probably the first acid to be produced in large quantities was acetic acid, HC HjO. Vinegar is a diluted aqueous solution of acetic acid. This acid is an organic acid that forms when naturally occurring bacteria called acetobacter aceti convert alcohol to acetic acid. Ancient Sumerians used wine to produce vinegar for... 

Pyrrole is a colorless liquid, boiling point 131°C, insoluble in water, soluble in alcohol or ether. Pyrrole dissolves slowly in dilute acids, being itself a very weak base rcsiniflcation lakes place readily, especially with more concentrated solutions of acids and on warming with acid a red precipitate is formed. Pyrrole vapor produces a pale red coloration on pine wood moistened with hydrochloric acid, which color rapidly changes to intense carmine red. Pyrrole may be made (1) by reaction of succmimide... 

The extreme values of acids and bases indicate some extra high mobility of ions H+ and OH. The effect of temperature manifests itself in such a way that with increasing temperature the transference numbers approach a limit value of 0.5 this means that the transference numbers above 0.5 decrease and the ones below that value increase. The transference numbers do not depend on the current passing through the electrolyte, yet change somewhat with the concentration. The values which are high in a diluted solution usually increase with increasing concentration while the lower ones decrease. 

Where both the acid and the base are strong electrolytes, the neutralization point will be at pH = 7 and the end point break will be distinct unless the solutions are very dilute (< 10" 3 mol dm"3). The composition of the titrand at any point in the titration may W computed from the total amount of acid and base present. However, when one of the reactants is a weak acid or base the picture is less clear. The incomplete dissociation of the acid or base and the hydrolysis of the salt produced in the reaction must be taken into account when.calculations of end points and solution composition are made. These points have been considered in chapter 3 and are used in the indicator selection procedure outlined in the preceding section of this chapter. 

The Electrolytic Dissociation Theory. —From his studies of the conductances of aqueous solutions of acids and their chemical activity, Arrhenius (1883) concluded that an electrolytic solution contained two kinds of solute molecules these were supposed to be active molecules, responsible for electrical conduction and chemical action, and inactive molecules, respectively. It was believed that when an acid, base or salt was dissolved in water a considerable portion, consisting of the so-called active molecules, was spontaneously split up, or dissociated, into positive and negative ions it was suggested that these ions are free to move independently and are directed towards the appropriate electrodes under the influence of an electric field. The proportion of active, or dissociated, molecules to the total number of molecules, later called the degree of dissociation, was considered to vary with the concentration of the electrolyte, and to be equal to unity in dilute solutions. 

In dilute solutions, strong acids and strong bases are completely ionized or dissociated. In the following sections we consider dilute aqueous solutions of salts. Based on our classification of acids and bases, we can identify four different kinds of salts. 

The degree of dissociation, or strength, of acids and bases has a profound influence on their aqueous chemistry. For example, vinegar (a 5% [w/v] solution of acetic acid in water) is a consumable product aqueous hydrochloric acid in water is not. Why Acetic acid is a weak acid and, as a result, a dilute solution does no damage to the mouth and esophagus. The following section looks at the strength of acids and bases in solution in more detail. 

CAUTION Dilution of a concentrated solution, especially of a strong acid or base, frequently liberates a great deal of heat. This can vaporize drops of water as they hit the concentrated solution and can cause dangerous spattering. As a safety precaution, concentrated solutions of acids or bases are always poured slowly into water, allowing the heat to be absorbed by the larger quantity of water. Calculations are usually simpler to visualize, however, by assuming that the water is added to the concentrated solution. 

Discussion Neutralization is the interaction of an acid and a base, as the result of which a salt and water are formed. From the standpoint of the theory of electrolytic dissociation, in neutralization the hydrogen ions furnished by the acid unite with the hydroxyl ions furnished by the base. If the solutions are sufficiently dilute, both the acid and the base are completely dissociated, and the only change that occurs when they are mixed is the formation of undissociated water from its ions. It follows that equal volumes of equivalent solutions of acids and of bases should produce the same amount of heat when neutralization takes place. The experiment described below is designed to test this conclusion. Equal quantities of normal solutions of several acids and bases are mixed and the heat developed in each case is measured. This is done by determining the rise in temperature that occurs when the solutions are mixed. Since dilute solutions are used, it is assumed in the calculations that the specific heat of the resulting salt solutions is equal to that of water, which is 1. If the volume of the solution is multiplied by the rise in temperature, the product is the number of calories set free. 

The heat of neutralisation is the amount of heat evolved when one gram equivalent of an acid is neutralised by one gram equivalent of a base to give one gram equivalent of a neutral salt. Thus, in dilute solution, an acid or base is considered to be completely ionised, and the neutralisation reaction is Na + ) + OH(-) + H( + ) + Q(-) ==> Na(+) + a(-) + P... 

---