Worked examples, buffers buffer

Example 14.4 shows, among other things, how effectively a buffer soaks up H+ or OH ions. That can be important Suppose you are carrying out a reaction whose rate is first-order in H+. If the pH increases from 5 to 7, perhaps by the absorption of traces of ammonia from the air, the rate will decrease by a factor of 100. A reaction that should have been complete in three hours will still be going on when you come back ten days later Small wonder that chemists frequently work with buffered solutions to avoid disasters of that type. 

While asymmetric approaches are certainly important, other synthetically significant epoxidation protocols have also been reported. For example, buffered two-phase MCPBA systems are useful for epoxidations in which the alkenes and/or resultant epoxides are acid-sensitive. Bicarbonate works quite well for cinnamate derivatives (e.g., 55) <96SC2235> however, 2,6-di-t-butyl-pyridine was shown to give superior results in the case of certain allyl acetals (e.g., 57) <96SC2875>. 

Worked Example 6.12 We need to prepare a buffer of pH 9.8 by mixing solutions of ammonia and ammonium chloride solution. What volumes of each are required Take the Ka of the ammonium ion as 6 x 10 l0. Assume the two solutions have the same concentration before mixing. 

We want a buffer solution because its pH stays constant after adding small amounts of acid or base. Consider the example of adding hydrochloric acid to a buffer, as described in the following Worked Example. 

Worked Example 6.13 Consider the so-called acetate buffer , made with equal volumes of sodium ethanoate and ethanoic acid solutions. The concentration of each solution is 0.1 mol dm-3. A small volume (10 cm3) of strong acid (HC1 of concentration 1 mol dm 3) is added to a litre of this buffer. The pH before adding HC1 is 4.70. What is its new pH ... 

SAQ 6.14 Consider the ammonia-ammonium buffer in Worked Example 6.12. Starting with 1 dm3 of buffer solution containing 0.05 mol dm-3 each of NH3 and NH4CI, calculate the pH after adding 8 cm3 of NaOH solution of concentration 0.1 mol dm 3. 

Worked Example 3.12. A pH electrode is immersed in a solution of buffer at pH 4, and the emf is 129 mV. What is the pH when the pH electrode is subsequently immersed in cider vinegar and the emf is 150 mV Assume the behaviour to be wholly nernstian, and that I is the same for both measurements. 

Elaboration of new electrophoretic methods for PolyP separation is continuing. For example, capillary electrophoretic separations of sodium PolyPs with chain lengths of 5 to 44 has been reported. In this work, a buffer containing pyromellitic acid, triethanolamine and hexamethonium hydroxide gives high-resolution separation of linear and cyclic PolyPs (Stover, 1997 Wang and Li, 1998). 

To keep our stomach at a constant working pH, buffer solutions can be used. Give one example. What materials do the body tissues and cells use to keep the pH constant... 

Example 8.2 demonstrates the ability of a buffered solution to absorb hydroxide ions without a significant change in pH. But how does a buffer work Suppose a buffered solution contains relatively large quantities of a weak acid HA and its conjugate base A-. Since the weak acid represents the best source of protons, the following reaction occurs when hydroxide ions are added to the solution ... 

Working examples of practical algorithms used in the author s laboratory are presented in Chapter 12. The statistical data used to construct them were gathered over a period of several years using the following specimens that were fixed routinely in 10% neutral-buffered formalin primary antibody incubations at 4°C for 16 to 18 hours the Elite avidin-biotin-peroxidase complex method of immunodetection (Vector Laboratories, Burlingame, Calif) and the antibody reagents listed in Table 11.1. 

Buffer systems work by neutralizing H or OH ions that come from the dissociation of acids or bases added to a system. As an example of buffers in action, consider the fate of lactic acid (HLac) entering the bloodstream. When lactic acid dissolves in the blood, it dissociates to produce H and lactate ions (Lac ). The liberated H ions drive the pH of the blood down. The bicarbonate ions of the bicarbonate buffer system react with the excess H and return the pH to the normal range ... 

Certain commonly used buffers react with the components of glass electrodes. One example is that of Tris(tris-(hydroxymethyl)aiminomethane] bufifer. This buffer can cause lai ge changes in pH readings. One should read the manual supplied along with the electrodes carefully. The buffers that can cause changes in the electrode are usually listed there. One should then avoid working with these buffers as far as possible. 

The calculation of pH in buffer solutions will only be assessed in Options B and D (see Chapters 23 and 25 on the accompanying website), but the derivation of the relevant equations and worked examples of calculations are presented here for completeness. 

In preparing amine buffer solutions maintained at constant ionic strength, /, by added NaCl, it is convenient to use two stock solutions. One of these contains the amine hydrochloride at a concentration A moles 1 . and sodium chloride at I — A) moles 1 . The other solution contains sodium hydroxide at a convenient concentration (say 4A—5A moles 1 ) and sodium chloride at / moles/1. Mixing the two solutions in any ratio (so long as the amount of sodium hydroxide does not exceed that of the amine hydrochloride) gives a final solution of constant ionic strength, /. (For a worked example, see (iv) below.)... 

However, there are very many substances that could be, or have been, of use as buffers, and Appendix III lists some of these. Chapter 5 shows how new pH-buffer tables can be constructed from the thermodynamic values, and some simple computer programmes are included to facilitate the necessary calculations. Tables and worked examples are given for use if a computer is not available. 

The first four steps of the seven-step strategy are identical to the ones in Example. In this example, addition of a strong acid or base modifies the concentrations that go into the buffer equation. We need to determine the new concentrations (Step 5) and then apply the buffer equation (Step 6). In dealing with changes in amounts of acid and base, it is often convenient to work with moles rather than molarities. The units cancel in the concentration term of the buffer equation, so the ratio of concentrations can be... 

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