Acetic acid-acetate ion buffer

The irreversible A-Se2 mechanism was observed in dilute hydrochloric acid solution when there was no substituent at C-7 and when both syn- and 3f n -isopropyl and -butyl groups were at carbon-7. The reversible mechanism was only found when an isopropyl or /-butyl group was syn to the diazo group and the reaction was carried out in the acetic acid-acetate ion buffer. 

All of the previous acid-base examples dealt with situations in which an acid and its conjugate base were close to equilibrium. In the following example, we consider the addition of a strong acid to an acetic acid/acetate ion buffer. Hydro-nium ion reacts with the conjugate base (acetate ion) in the buffer, so that the con-centtations of species in solution change markedly from their initial value. (If you add a strong base to this buffer, hydroxide ion reacts with acetic acid in the buffer.)... 

In the case of our acetic acid (1.0 A/) and sodium acetate (1.0 A/) buffer, the concentrations of weak acid and conjugate base are equal. When this is true, the log term in the Henderson-Hasselbalch equation is zero and the pH is numerically equal to the pAa. In the case of an acetic acid-acetate ion buffer, pAa = -log 1.8 X 10 = 4.74. 

Q How does this acetic acid/acetate ion buffer maintain pH ... 

Suppose you take two flasks one containing pure water and the other a buffer solution mam tamed at a pH of 7 0 If you add 0 1 mole of acetic acid to each one and the final volume m each flask IS 1 L how much acetic acid is present at equi librium How much acetate ion In other words what IS the extent of ionization of acetic acid m an unbuffered medium and m a buffered one ... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Self-Test 11.4A Calculate the ratio of the molarities of acetate ions and acetic acid needed to buffer a solution at pH = 5.25. The pKa of CH3COOH is 4.75. 

As a result, both acetate ions and acetic acid molecules are present as major species in solution. The presence of an acid and its conjugate base means that in this region of the titration, the solution is buffered, so the pH changes slowly as hydroxide ions are added to the solution. 

Acetic anhydride is the only monocarboxylic acid anhydride that is important in modification reactions. The acetylation of the amino groups of proteins can be made relatively specific if the reaction is done in saturated sodium acetate, since the o-acetyltyrosine derivative is unstable to an excess of acetate ions (Fraenkel-Conrat, 1959). The tyrosine derivative rapidly hydrolyzes in alkaline reaction conditions, even in the absence of added acetate buffer (Uraki et al., 1957 Smyth, 1967). Treatment with hydroxylamine also cleaves any o-acetyltyrosine modifications, forming acetylhydroxamate, which can be followed by its purple complex with Fe3+ at 540 nm... 

All that is necessary to form a buffer is to have approximately equal amounts of a weak acid and its conjugate base in solution. This will be achieved if we add an amount of HC1 equal to approximately half the original amount of acetate ion. 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

For sufficient retention of these very polar sulfonated carboxylates on RP columns, the addition of an ion-pairing (IP) agent such as tetraethylammonium acetate (TEAA) to the LC buffer was compulsory [13]. To maintain the compatibility of the eluent with the MS interface, the use of such an involatile cationic additive entailed a cation exchanger to be placed between the column and the interface [13]. Alternatively, equimolar amounts (5 mM) of acetic acid and triethyl-amine, which form the volatile IP agent triethylammonium, were added to the mobile phase in order to improve the retardation of very polar SPC [14]. While the first approach with TEAA was effective in retaining even the very short-chain C3- and C4-SPC (Fig. 2.10.4), the weaker IP agent triethylammonium notably increased the retention of C5-SPC and higher, whereas C4-SPC elutes almost with the dead volume of the LC (Fig. 2.10.5). Addition of commonly used ammonium acetate as buffer component led to the co-elution of the short-chain SPC ([Pg.322]

Adding an acid to a buffer Acetate ions react with the hydronium ions added to the solution. 

Recall, from Chapter 8, that a buffer consists of a weak acid/conjugate base mixture or a weak base/conjugate acid mixture. One buffer that you examined previously contains acetic acid and sodium acetate. The common-ion effect applies to this buffer. The equilibrium of the acetic acid is affected by the common acetate ion from sodium acetate. 

Addition of an acid such as HCl to the buffer solution provides H" ", which combines with the acetate ion to give acetic acid. This has a twofold effect it reduces the amount of acetate ion present and, by so doing, also increases the amount of undissociated acetic acid. Provided the amount of acid added is small relative to the original concentration of base in the buffer, the alteration in base acid ratio in the Henderson-Hasselbalch equation is relatively small and has Mttle effect on the pH value. 

Similar considerations apply if a base such as NaOH is added to the buffer solution. This will decrease the amount of undissociated acid, and increase the amount of acetate ion present. 

Hine has demonstrated that simple amino acids, such as glycine and p-alanine, are not capable of intramolecular deprotonation in the reaction with isobutyraldehyde-2-d (Scheme 8) [62], Apparently, the carboxylate moiety in the iminium ion intermediate 29 is a relatively weak base and, as such, external bases, present in the buffer used (e.g. acetate ions), are largely responsible for the formation of the enamine intermediate 30. 

To illustrate how a buffer works, consider the simple buffer consisting of acetic acid and the acetate ion HC2H3O2/ C2Hj02 . This buffer is prepared by combining acetic acid and sodium acetate. In solution the two components give the following reactions ... 

A buffered solution may contain a weak acid and its salt, e.g. acetic acid and acetate ion, or a weak base and its salt, e.g. NH3 and NH4CI. By choosing the appropriate components, a solution can be buffered at virtually any pH. The pH of a buffered solution depends on the ratio of the concentrations of buffering components. When the ratio is least affected by adding acids or bases, the solution is most resistant to a change in pH. It is more effective when the acid-base ratio is equal to unity. The pK of the weak acid selected for the buffer should be as close as possible to the desired pH, because it follows the following equation ... 

The acetic acid helps to buffer the solution and maintain a low hydrogen-ion concentration, which is favorable for a good yield. 

Add the strong acid HC1 to an acetic acid—sodium acetate buffer solution, however, and the H ions produced by the HC1 do not stay in solution to lower the pH because they react with the acetate ions, C2H302-, of sodium acetate to form acetic acid, as shown in Figure 10.19. (Remember that acetic acid, being a weak acid, stays mostly in its molecular form, HC2H302, and so does not contribute hydronium ions to the solution.) Add the strong base NaOH to the acetic acid—sodium acetate buffer solution, and the OH-ions produced by the NaOH do not stay in solution to raise the pH because they combine with H+ ions from the acetic acid to form water, as shown in Figure 10.20. 

When hydrolysis was carried out in 50% dioxan-water 1 M in acetic acid-acetate buffer of pH 5.5, it was found that some 75% of the 180 derived from one of the oxygen atoms of the labelled acetate ion appeared in the benzoic acid produced. 

Now let s consider what happens when we add H30+ or OH- to a buffer solution. First, suppose that we add 0.01 mol of solid NaOH to 1.00 L of the 0.10 M acetic acid-0.10 M sodium acetate solution. Because neutralization reactions involving strong acids or strong bases go essentially 100% to completion (Section 16.1), we must take account of neutralization before calculating [P130+]. Initially, we have (1.00 L)(0.10 mol/L) = 0.10 mol of acetic acid and an equal amount of acetate ion. When we add 0.01 mol of NaOH, the neutralization reaction will alter the numbers of moles ... 

Now suppose that we add 0.01 mol of HC1 to 1.00 L of the 0.10 M acetic acid-0.10 M sodium acetate buffer solution. The added strong acid will convert 0.01 mol of acetate ions to 0.01 mol of acetic acid because of the neutralization reaction... 

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