The Common Ion Effect and Buffer Solutions

In laboratory reactions, in industrial processes, and in the bodies of plants and animals, it is often necessary to keep the pH nearly constant despite the addition of acids or bases. The oxygen-carrying capacity of the hemoglobin in your blood and the activity of the enzymes in your cells are very sensitive to the pH of your body fluids. A change in blood pH of 0.5 units (a change in [H3O+] by a factor of about 3) can be fatal. Our bodies use a combination of compounds known as a buffer system to keep the pH within a narrow range. 

The operation of a buffer solution depends on the common ion effect, a special case of LeChatelier s Principle. 

When a solution of a weak electrolyte is altered by adding one of its ions from another source, the ionization of the weak electrolyte is suppressed. This behavior is termed the common ion effect. 

Many types of solutions exhibit this behavior. Two of the most common kinds are 

a solution of a weak acid plus a soluble ionic salt of the weak acid (e.g., CH3COOH plus NaCH3COO), and 

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The Common Ion Effect and Buffer Solutions 19-2 Buffering Action 19-3 Preparation of Buffer Solutions 19-4 Acid-Base Indicators... 

The common-ion effect is quite general. The chemistry of buffer solutions is another important application of this principle. A buffer solution relies on the common-ion effect to suppress the concentration of hydronium ions and maintain a steady pH ... 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

Weak acid and its salt, or weak base and its salt (see p 356). Here the common-ion effect is involved the solutions are buffers. The equilibria involved are the same as in type 4 or 5 above, except that for tljff j-idfg Juljgp... 

In ammoniacal solutions of copper salts, the oxidation products are likely to contain nitrogen thus, hexoses give oxalic acid, imidazoles, hydrogen cyanide, and urea. Kinetic studies have been reported for the reaction of Cu(II) in the presence of ammonia with maltose, lactose, melibiose, and cellobiose.190 For the oxidation by tetraamminecopper(II) in ammoniacal and buffered media the rate of reaction is first order in disaccharide concentration, order one-half in ammonia concentration, but it is independent of Cu(II) concentration. The reaction rate is decreased by the addition of ammonium chloride, because of the common ion effect. These kinetics suggested mechanisms involving an intermediate enediolate ion, with the rate of reaction being equal to the rate of enolization.191 A similar mechanism has been proposed for the oxidation of D-fructose by a copper-pyridine complex in an excess of pyridine.192... 

The correct answer is (C). HC1 is a very strong acid, which means that the chloride ion is a very weak conjugate base. This means the solution is not a buffer solution. The potassium chloride is really a distracter in this question. Although CP is present in each salt, the common-ion effect is only seen in equilibrium systems. The reverse reaction for the dissociation of either of these two would be so slight it would be negligible. However, because KC1 does nothing to neutralize the acid, the solution would be both acidic and not a buffer. 

On the AP exam, acid-base questions appear on a regular basis, and buffer calculations are among the most common of those questions to pop up. One of the things you are expected to be able to do is calculate the pH of a buffer solution, and though you may not realize it yet, you have already done this calculation. In the previous example of the common ion effect, when you calculated the pH of the solution made with the 0.30 M acetic acid and the 0.30 M sodium acetate, that was a buffer solution. That is the first type of buffer calculation you are expected to know. The second type of calculation is to determine the effect of adding a strong acid or base to a buffer, and the... 

As you can see, the two reactions that characterize this buffer system are identical to those for the common ion effect described in Example 16.1. The buffering capacity, that is, the effectiveness of the buffer solution, depends on the amount of acid and conjugate base from which the buffer is made. The larger the amount, the greater the buffering capacity. 

The formation of a buffer solution is an example of the common-ion effect. Explain how a buffer works with reference to a solution containing acetic acid and sodium acetate. 

Direct synthesis is perhaps the single most widely used method of preparation. This method involves nucleating and growing the metal hydroxide layer by mixing an aqueous solution containing the salts of two metal ions, in the presence of the desired anion, and a base 50% sodium hydroxide is particularly useful for this purpose, since the common ion effect keeps it relatively free of carbonate (see later). It has been demonstrated that LDH materials form in preference to a mixture of the individual metal hydroxides (68) and that, in the case of aluminum as the trivalent cation, they generally do so through an aluminum hydroxide intermediate. Variations of this method include titration at constant or varied pH and buffered precipitation. 

A solution of a strong acid and its salt (e.g., HCI/NaCi) has the acid nearly completely Ionized, so the strong acid cannot be present In significant amounts.Therefore such a solution does not exhibit the common Ion effect, nor can It act as a buffer solution. A similar reasoning applies to a solution of a strong base and Its salt (e.g., NaOH/NaCI). 

The pH of human blood is normally 7.40. If the pH were to fall below 7.0 or rise above 7.8, death would be likely, for the efiFectiveness of enzymes is very sensitive to pH. How can the pH be kept so constant during ingestion of sauerkraut (acid) followed by baking soda (base) A solution with the property that the addition of acids or bases causes only a relatively small change in pH is known as a BUFFER SOLUTION. The way to make a buffer solution is to dissolve an acid and its conjugate base, both moderately weak. Buffer solutions are an application of the common-ion effect as illustrated in Example 20. 

Samarium, europium and ytterbium can readily be removed from lanthanon mixtures by reductive extraction from a buffered acidic solution into a dilute (0.5% or less) sodium amalgam, Marsh (1957), or by electrolyzing an alkaline citrate solution with a lithium amalgam cathode, Onstott (1955, 1956). Europium can be obtained especially pure from such amalgams by treatment with cold concentrated HCl, which causes precipitation of sparingly soluble EuC. 2H2O, Hulet et al. (1972), by the common-ion effect. Both Sm(II) and Yb(II) are rapidly oxidized by hydronium ion to the very soluble trihalides, but oxidation of Eu(II) in the absence of oxygen proceeds slowly. 

Commercially available buffer solutions can be purchased for virtually any desired pH. A buffer solution commonly used to calibrate pH meters contains 0.025 m Na2HP04(aq) and 0.025 M KH2P04(aq) and has pH = 6.87 at 25°C. However, the method demonstrated in Example 11.1 would give pH = 7.2 for this solution. Because these calculations interpret activities as molarities, not effective molarities, they ignore ion—ion interactions so the values calculated are onl) approximate. 

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