Of weak acids

By way of caution it should be noted that free alkali or the alkali salts of weak acids will redden the reagent like an aldehyde. It is also, of course, reddened by heat or when exposed in small quantities to the air for some time. Mineral acit greatly reduce the sensitivity of the test. 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

Equation 6.44 is written in terms of the concentrations of CH3COOH and CH3COO- at equilibrium. A more useful relationship relates the buffer s pH to the initial concentrations of weak acid and weak base. A general buffer equation can be derived by considering the following reactions for a weak acid, HA, and the salt of its conjugate weak base, NaA. 

If the initial concentrations of weak acid and weak base are greater than [H3O+] and [OH ], the general equation simplifies to the Henderson-Hasselhalch equation. 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

Thompson, R. Q. Identification of Weak Acids and Bases by Titration with Primary Standards, /. Chem. Educ. 1988, 65, 179-180. 

In addition to the Hquid—Hquid reaction processes, there are many cases in both analytical and industrial chemistry where the main objective of separation is achieved by extraction using a chemical extractant. The technique of dissociation extraction is very valuable for separating mixtures of weakly acidic or basic organic compounds such as 2,4-dichlorophenol [120-83-2] and 2,5-dichlorophenol [583-78-8] which are difficult to separate by... 

Acetate and triacetate are essentially unaffected by dilute solutions of weak acids, but strong mineral acids cause serious degradation. The results of exposure of heat-treated and untreated triacetate taffeta fabrics to various chemical reagents have been reported (9). Acetate and triacetate fibers are not affected by the perchloroethylene dry-cleaning solutions normally used in the United States and Canada. Trichloroethylene, employed to a limited extent in the UK and Europe, softens triacetate. 

Wea.kA.cid Cation Exchangers. The syathesis of weak acid catioa exchangers is a one-step process when acryHc acid or methacrylic acid is copolymetized with DVB. If an acryHc ester is used as the monomer iastead of an acryHc acid, the ester groups must be hydrolyzed after polymerization usiag either an acid or base (NaOH) to give the carboxyHc acid functionaHty, or the sodium salt (4) of it. 

The acryHc weak base resias are syathesized from copolymers similar to those used for the manufacture of weak acid cation-exchange resias. For example, uader appropriate temperature and pressure conditions, a weak acid resia reacts with a polyfuactioaal amine, such as dimethylaminopropylamine [109-55-7] (7) to give a weak base resia with a tertiary amine fuactioaaHty. 

Process Licensors. Some of the well-known nitric acid technology licensors are fisted in Table 3. Espindesa, Grande Paroisse, Humphreys and Glasgow, Rhfyne Poulenc, Uhde, and Weatherly are all reported to be licensors of weak acid technology. Most weak acid plant licensors offer extended absorption for NO abatement. Espindesa, Rhfyne Poulenc, Weatherly, and Uhde are also reported (53,57) to offer selective catalytic reduction (SCR) technology. 

In the presence of amine salts of weak acids, the nitro olefin is formed direcdy. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Diorganotin esters of strong acids are relatively stable to hydrolysis under neutral conditions, but generally, diorganotin compounds ate more reactive chemically than the triorganotins. Diorganotin esters of weak acids are somewhat susceptible to hydrolysis, even under neutral conditions, but this reactivity is moderated somewhat by their hydrophobicity. 

Bismuth Salts. Bismuth trioxide dissolves in concentrated solutions of strong oxyacids to yield bismuth salts. In more dilute solutions of strong acids or in solutions of weak acids, the oxide reacts to form bismuthyl or basic salts. The normal salts are very susceptible to hydrolysis. 

Ceric salt solutions are strongly acidic, basic salts tend to form readily, and there are no stable simple salts of weak acids. 

Astringents are designed to dry the skin, denature skin proteins, and tighten or reduce the size of pore openings on the skin surface. These products can have antimicrobial effects and are frequendy buffered to lower the pH of skin. They are perfumed, hydro-alcohoHc solutions of weak acids, such as tannic acid or potassium alum, and various plant extracts, such as bitch leaf extract. The alcohol is not only a suitable solvent but also helps remove excess sebum and soil from the skin. After-shave lotions generally function as astringents. 

The concentration of acid impurities is an important indication of the quality of petroleum products and the purity of organic solvents, plasticizers, mineral oils, food fats, and polymers. Methods are used to detect organic acids in such compounds have many disadvantages the alkalimetry - low sensitivity, especially in the determination of weak acids, the extraction-photometric method is laborious, instmmental methods are expensive. In addition, most of methods are commonly unsuitable for direct analysis. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

The most common method of purification of inorganic species is by recrystallisation, usually from water. However, especially with salts of weak acids or of cations other than the alkaline and alkaline earth metals, care must be taken to minimise the effect of hydrolysis. This can be achieved, for example, by recrystallising acetates in the presence of dilute acetic acid. Nevertheless, there are many inorganic chemicals that are too insoluble or are hydrolysed by water so that no general purification method can be given. It is convenient that many inorganic substances have large temperature coefficients for their solubility in water, but in other cases recrystallisation is still possible by partial solvent evaporation. 

Concentration by simple distillation of weak acid is only possible up to about 68%. Higher concentrations of 80%-90% can be obtained by dehydrating weak acid with sulfuric acid. 

Mix strong acidic gases with weak ones to facilitate production of sulfuric acid from sulfur oxides, thereby avoiding the release of weak acidic gases. 

The proportion of ionized and unionized forms of a chemical compound can be readily calculated according to the above equation. It can be easily seen that pK is also a pH value at which 50% of the compound exists in ionized form. The ionization of weak acids increases as the pH increases, whereas the ionization of weak bases increases when the pH decreases. As the proportion of an ionized chemical increases, the diffusion of the chemical through the biological membranes is greatly impaired, and this attenuates toxicokinetic processes. For example, the common drug acetosalicylic acid (aspirin), a weak acid, is readily absorbed from the stomach because most of its dose is in an unionized form at the acidic pH of the stomach. 

Ketals bearing the double bond at the 4,5-position are prepared by reaction with glycol in the presence of weak acids such as adipic acid. In this case ketal formation proceeds via the intermediate 2,4-dien-3-ol ether. Hemithioketals (71) of A -3-ketones are prepared by reaction with mer-... 

Aluminium compounds of weak acids are extensively hydrolysed to [A1(H20)3(0H)3] and the corresponding hydride, e.g. AI2S3----->... 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

A -Methylation of the NH of heterocycles using 1 is also known as exemplified by the methylation of indole/ The interesting mechanism is delineated below. O-methylation of weak acids such as phenols, carboxylic acids and oximes as well as 5-methylation such as A -phenylisorhodanine, certain thioketones, and dithiocarboxylic acids have also been reported." ... 

Direct titration [119,120] In order to know the number of exchangeable hydrogen ions at different dissociation stayes, various salts of weak acids were used. Data observed in Table 8 shows an increase in the exchange capacity with an increase in the pH of the solution, indicating the presence of weak acid capacity [118]. 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

The reasons why some anions exhibit strong inhibitive properties while others exhibit strong aggressive properties are not entirely clear. The principal distinction seems to be that inhibitive anions are generally anions of weak acids whereas aggressive anions are anions of strong acids. Due to hydrolysis, solutions of inhibitive anions have rather alkaline pH values and buffer capacities to resist pH displacement to more acid values. As discussed... 

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