Indicators, acid-base acidic

The use of methyl-orange as an indicator is based on the fact that the sodium salt in aqueous solution furnishes a yellow anion, which when treated with acids (except weak acids such as H2CO3 and H SOj) apparently gives rise to a red... 

Fischer esterification is reversible and the position of equilibrium lies slightly to the side of products when the reactants are simple alcohols and carboxylic acids When the Fis cher esterification is used for preparative purposes the position of equilibrium can be made more favorable by using either the alcohol or the carboxylic acid m excess In the following example m which an excess of the alcohol was employed the yield indicated IS based on the carboxylic acid as the limiting reactant... 

Indicator Acid Color Base Color pH Range P/Ca... 

A double end point, acid—base titration can be used to determine both sodium hydrosulfide and sodium sulfide content. Standardized hydrochloric acid is the titrant thymolphthalein and bromophenol blue are the indicators. Other bases having ionization constants in the ranges of the indicators used interfere with the analysis. Sodium thiosulfate and sodium thiocarbonate interfere quantitatively with the accuracy of the results. Detailed procedures to analyze sodium sulfide, sodium hydro sulfide, and sodium tetrasulfide are available (1). 

Acid-base indicator Acid (or base). Neutralization is complete as determined by color change of indicator. 

KEY TERMS acid-base indicator conjugate base ionization... 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

Another problem is that different workers make their calculations of second-order rate constants in the micelle in different ways. For example, the surface potential of a micelle may be specifically included in the calculation in order to estimate ion binding, but there is a circularity in this argument because surface potentials are often estimated from micellar effects upon indicator acid-base equilibria which themselves depend upon ion-binding to micelles (Fernandez and Fromherz, 1977). 

Deuterium exchange studies on decaborane and other boranes indicate acidic character of bridge hydrogens. They react with bases undergoing proton abstraction reactions ... 

In organic solvents the acidity functions H corresponding to hydrogen dissociation from neutral indicator acids were reported for solutions of alkali metal alkoxides in various alcohols (2), using nitroanilines (21), aminobenzenecarboxylic acids (22), or indols (23) as indicators. For addition reactions of methoxide and ethoxide ions to neutral indicator acids, acidity functions J (also denoted as Hr) based on use of nitrobenzenes (21) and a-cyanostilbenes (18) as indicators in methanol and dimethylsulfoxide-methanol and -ethanol mixtures were reported. Recently (24) the acidity function J- (denoted as Jm) was derived for methoxide ion solutions in methanol using substituted benzaldehydes as indicators. These scales involve arbitrary choice of water as the solvent for determination of the dissociation constant of the anchoring acid. 

K. Spectrophotometry with indicators Acid-base indicators are themselves acids or bases. Consider an indicator, Hln, which dissociates according to the equation... 

The non-linear plot in Fig. 11.2A indicates acid-base catalysis (i.e. when both acid and base are involved in the same catalytic mechanism with rate = 1cha.b[HA] [B] [HS]), and the Icha.b parameter can be determined from the slope of a plot of k0bs/(l - FB)( [acid] + [base])2 versus FB (not shown). 

A pH indicator molecule exhibits an equilibrium between the indicator acid InA and the indicator base InB [Eq. (1)]. 

Hammett (on quantitative study of very weak bases,409 reaction rates and indicator acidities,410 and rate-equilibrium relationships411) Hey and Waters (on free radicals) 412 Mayo and Walling (on free radicals) 413 and Dippy (on the strengths of organic acids)414... 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

The McReynolds data were standardized and subjected to principal component analysis by several groups of workers who were able to reduce the data to three statistical components. Burns and Hawkes42 further refined the calculations to produce four quasi-theoretical indices that measure dispersion, polarity, acidity, and basicity. Hawkes has described this process in a more recent paper43 in which his group confirmed and refined these calculations with spectroscopic measurements. In addition to justifying their approach, they provide four indices for each of the 26 common liquid phases that were identified earlier as being the most important.36 The dispersion index is calculated from refractive indices, but the other three indices are based at least partially on chromatographic data. 

An acid-base titration is a procedure that is used where a base of known concentration is added to an acid of unknown concentration (or vice-versa) in order to determine the concentration of the unknown. In addition, it is possible to determine the Ka of the acid being titrated (or Kb of the base) as well as an appropriate indicator. Acid-base titrations are often the topic of AP test questions and are frequently used in the laboratory questions. You should know about titrations from a conceptual level, be able to perform calculations for titrations, and know how to properly perform one in the laboratory. We ll begin with the conceptual explanation of titrations. 

Because they vary by many orders of magnitude, base-dissociation constants are usually listed as their negative logarithms, or pATb values. For example, if a certain amine has Kb = 10-3, then pKb = 3. Just as we used pKa values to indicate acid strengths (stronger acids have smaller pKa values), we use pKb values to compare the relative strengths of amines as proton bases. 

Reactions with indicators. Acids and bases provide characteristic reactions with indicators that change colors during neutralization. 

A. The use of indicators and indicator test papers An indicator is a substance which varies in colour according to the hydrogen-ion concentration. It is generally a weak organic acid or weak base employed in a very dilute solution. The undissociated indicator acid or base has a different colour to the dissociated product. In the case of an indicator acid, Hind, dissociation takes place according to the equilibrium... 

Figure 3 Proposed CODH mechanism. This mechanism proposes that the active state is Credl, consistent with recent electrochemical studies,which contains a hridghig hydroxide at the hinuclear NiFe center that serves as the nucleophile to attack a Ni-bound carbonyl forming a Ni-carboxylate. This OH, which is formed by acid-hase catalysis by indicated acid-base residues, is in the position of the bridging sulfide in the C. hydrogenoformans structure. The enzyme is proposed to remain in the Credl redox state until formation of CO2 when it becomes two-electrons reduced to the Cred2 state. Conversion of Credl to Cred2 occurs faster than electron transfer from Cred2 to the FeS clusters, which in turn reduce external electron acceptors. The electron transfer reactions are proposed to occur through a diamagnetic Cint state. ...

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