Sodium acetate-acetic acid system

A buffer system is preferred for the control of pH, the most common one being the relatively cheap acetic acid/sodium acetate system, although a simple addition of acetic acid may be adequate with water that does not show a significant pH shift on heating. 

There are many different buffer systems useful for maintaining particular pH values. The acetic acid—sodium acetate system is good for maintaining a pH around 4.8. Buffer solutions containing equal mixtures of a weak base and a salt of that weak base maintain alkaline pH values. For example, a buffer solution of the weak base ammonia, NH3, and ammonium chloride, NH4C1, is useful for maintaining a pH about 9.3. 

The buffer process is an equilibrium reaction and is described by an equilibrium constant expression. For acids, the equilibrium constant is represented as K, the subscript a impl)ring an acid equilibrium. For example, the acetic acid/sodium acetate system is described by... 

Note that in these three examples involving hydrogen peroxide, chromium trioxide and sodium nitrite, dangerous reactions have been described for carboxyiic acids (see on p.316-317). They all referred to the three following systems acetic acid-/hydrogen peroxide, acetic acid/chromium trioxide and o-phthalic acid/sodium nitrite. One can ask oneself whether the same reactions did not take place after the acetic and phthalic anhydride hydrolysis. 

This can be seen from the titration curve for phosphoric acid [6] shown in Figure 10.2. In practice the mono- and di-sodium salt system is used most extensively, since this covers the pH range over which precise control is most often needed. These phosphate buffers are more resistant than the acetate systems to temperature-induced changes. 

Buffers contain mixtures of weak acids and their salts (i.e., the conjugate bases of acids), or mixtures of weak bases and their conjugate acids. Typical buffer systems used in pharmaceutical dosage forms include mixtures of boric acid and sodium borate, acetic acid and sodium acetate, and sodium acid phosphate and disodium phosphate. The reason for the buffering action of a weak acid, HA (e.g., acetic acid) and its ionized salt, A" (e.g., sodium acetate) is that A" ions from the salt combine with the added hydrogen ions, removing them from solution as undissociated weak acid. 

The relatively complex cyclic system camphidine has been converted into IV-nitrosocamphidine with the acetic acid/sodium nitrite reagent [22]. This... 

Two comments. First, notice the pH is on the acidic side of the pKa, because the concentration of the weak acid (NH4+) was greater than the concentration of its conjugate base (NH3). Second, notice how different the pH of an ammo-nia/ammonium chloride buffer system is from the pH of an acetic acid/sodium acetate buffer system. The pH of a buffer depends on the acid strength of the weak acid from which it is composed. 

Melamine resins are used from this group of thermosets for the manufacture of food contact materials. The melamine can be used in mixtures with urea and in some applications with phenol (< 1 %). The polymerization process is catalyzed in the presence of organic acids (e.g. acetic acid, lactic acid, tartaric acid, citric acid), hydrochloric acid, sulfuric acid, phosphoric acid, sodium and potassium hydroxide, ammonia, calcium or magnesium hydroxide as well as salts of these substances (total < 1 %) which cause the elimination of water and lead to a cured resin system. Stearic acid can be used as a lubricant as can zinc, calcium and magnesium salts, esters of montanic acid with ethandiol and 1,3-butandiol, as well as silicone oil (total < 1 %). 

A reversed-phase HPLC post-column ion-pair extraction system was developed by Kim and Stewart [71, 72] for the analysis of carboxylic acid drugs and their salts (sodium formate, sodium acetate, 3-bromopropionic acid, 6-aminocaproic acid, 11-bromoundecanoic acid, 1-heptanesulfonic acid, / -n i t rophcny 1 acetic acid, sodium benzoate, sodium salicylate, valproic acid, probenecid, naproxen, ketoprofen, ibuprofen, mefenamic acid, flufenamic acid, and cefuroxime sodium) using a-(3,4-dimethoxy-phenyl)-4,-trimethylammoniummethylcinnamonitrile methosulfate... 

What you saw in the last section was an example of how the presence of certain ions in solution can regulate the pH of a system. In the acetic acid/sodium acetate example, we started with a weak acid and then added a salt containing the conjugate base of the weak acid to the solution. This pairing of the weak acid and the conjugate base modified the pH of the solution. This is the principle behind buffered solutions. If an acid was added to the solution containing the acetic acid/acetate conjugates, the hydronium ions from the acid would combine with the acetate ions to form more acetic acid. So, what should have amounted to an increase in hydronium ions in the solution was foiled by the acetate ions. 

The cobalt(II) acetate/acetic acid/sodium bromide (CAB) combination is well known for the autoxidation of alkyl benzenes.287 It is normally employed for the production of terephthalic acid from 1,4-dimethylbenzene.281 However, the use of hydrogen peroxide, as mentioned earlier, means the systems are generally more selective, and can be operated at ambient pressure and relatively low temperatures. 

One is the pH sensitivity of many systems, particularly lAere there is a third continent. Thus whereas the potassiiun persulfC ate - sodium bisulfitu system is accelerated by Pe in acidic media, acceleration oi polymerisation occurs with Cu in an alkaline medium. Unusually with Cu, a second peak occurs at alkaline pH if both methyl methacrylate and vinyl acetate are added, one of the few examples where a vinyl acetate copolymer is formed in the presence of a copper compound. (2). Conplex formation is indicated, producing active intermediates vdiich... 

Sapogenins Reactions of the Spiro-acetal System.—Nitrous acid, provided by sodium nitrite and boron trifluoride in acetic acid, readily nitrosates the C(23) position in a spirostan (271), giving the oxime (272), from which the 23-oxo-deriva-tive is available in high yield. Paraformaldehyde and acetic acid gave a... 

Acetic acid in this system may be considered as slightly basic since it combines with hydrogen ions. Salts such as sodium acetate become strong bases. Acids may vary from their corresponding strengths in aqueous solutions in a manner dependent upon the relative tendencies of water and acetic acid to take up hydrogen ions. Measurements have... 

A buffer solution functions in the following manner to resist changes in acidity or alkalinity. In an acetic acid/sodium acetate buffer system, the species present in solution are CH3COOH, CH3COO", Na+, and H2O. Amounts of H+ and OH" are initially assumed to be small. 

A buffer maintains the pH of a solution at a fairly constant value. Compare what happens when acid and base are added to an acetic acid/sodium acetate buffer system at pH = 5 (top) to what happens when the same amount of acid and base are added to an unbuffered solution of pH = 5 (bottom). 

For the acetic acid/sodium acetate buffer system,... 

Organizations such as the Chemical Abstacts Service and the BeUstein Institute that prepare indexes to the chemical literature must adopt a system for selecting unique names in order to avoid excessive cross referencing. Chemical Abstracts Service uses a system which groups together compounds derived from a single parent compound. Thus most index names are inverted (e.g.. Benzene, bromo rather than bromobenzene Acetic acid, sodium salt rather than sodium acetate). 

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