Acid-base stoichiometry

The quantitative aspects of acid-base chemistry obey the principles Introduced earlier in this chapter. The common acid-base reactions that are important in general chemistry take place in aqueous solution, so acid-base stoichiometry uses molarities and volumes extensively. Example Illustrates the essential features of aqueous acid-base stoichiometry. 

Ion-protein combinations which do not specifically involve dissociable groups shed no light on acid-base stoichiometry. The number of sites... 

In contrast to boron-centered Lewis acids, tin and titanium derivatives prefer a 1 2 (acid base) stoichiometry in complexation with carbonyls. and H NMR spectra of an equimolar solution of SnCU and 4-f-butylbenzaldehyde showed signds only for free ligand and a 1 2 (acid base) complex over a wide temperature range. Only with excesses of SnCU was any 1 1 adduct observed and even in the presence of 10 equiv. of Lewis acid, only 25% of the 1 1 complex could be detected. Intermolecular exchange in this case was shown to be slow below -40 C and no synr-anti isomerization could be detected. 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

Where Is the Equivalence Point In discussing acid-base titrations and com-plexometric titrations, we noted that the equivalence point is almost identical with the inflection point located in the sharply rising part of the titration curve. If you look back at Figures 9.8 and 9.28, you will see that for acid-base and com-plexometric titrations the inflection point is also in the middle of the titration curve s sharp rise (we call this a symmetrical equivalence point). This makes it relatively easy to find the equivalence point when you sketch these titration curves. When the stoichiometry of a redox titration is symmetrical (one mole analyte per mole of titrant), then the equivalence point also is symmetrical. If the stoichiometry is not symmetrical, then the equivalence point will lie closer to the top or bottom of the titration curve s sharp rise. In this case the equivalence point is said to be asymmetrical. Example 9.12 shows how to calculate the equivalence point potential in this situation. 

The discussion of acid-base titrations in Chapter 4 focused on stoichiometry. Here, the emphasis is on the equilibrium principles that apply to the acid-base reactions involved. It is convenient to distinguish between titrations involving—... 

Poly A form a complex with a 4 1 stoichiometry. The apparent hypochromicities of various mixtures are listed in Table 4. The mixtures of A12 with Poly U and of T12 with Poly A showed large hypochromicities compared with other mixtures, which suggests the importance of the hydrogen-bonding formation between complementary nucleic acid bases such as A-U and T-A. 

Based on the fact that pi-acids interact with the trinuclear gold] I) pi-bases, TR(carb) and TR(bzim), the trinuclear 3,5-diphenylpyrazolate silver(I) complex was reacted with each. Mixing [Au3(carb)3] or [Au3(bzim)3] with [Ag3(p,-3,5-Ph2pz)3] in CH2CI2 in stoichiometric ratios of 1 2 and 2 1 produced the mixed metal/mixed ligand complexes in the same gold-silver ratios. The crystalline products were not the expected acid-base adducts. It is suspected that the lability of the M-N bond (M=Au, Ag) in these complexes results in the subsequent cleavage of the cyclic complexes to produce the products statistically expected from the stoichiometry of materials used [74]. As a result of the lability of Au-N and Ag-N bonds, and the stability of... 

By plotting i versus the ratio R = (CHX)t/(CB)t during the titration, they determined simultaneously the extent of acid-base interaction, the stoichiometry of that interaction and the degree of association of the acid-base adduct. Fig. 4.13 shows hypothetical titration curves line ABC corresponds to the interaction between B and HX as monomers without further reaction between BHX and HX, and the subsequent occurrence of the latter reaction to a small extent is indicated by the line ABC and to the full extent by line ABDE, when no more HX can react with BHX HX line AFDE arises when formation of BHX HX starts right away in the case of previous partial dimerization of B, the various lines will begin at A instead of A. 

Acids are selected based on the nature of the well treatment and the mineralogy of the formation. The critical chemical factors in properly selecting an acid are stoichiometry (how much formation material is dissolved by a given amount of acid), the equilibrium constant (complete reaction of the acid is desired), and reaction rate between the acid and the formation material (106). 

Phenol methylation to 2,6-xylenol has been widely studied for the past few deeades owing to the room for improvisation from the viewpoint of product selectivity. Generally during phenol methylation to 2,6-xylenol, occurs via sequential methylation of phenol to o-cresol to 2,6-xylenol, various reaction parameters mediate the selectivity between the two. For instance, when the reaetants stoichiometry of methanol to phenol molar ratio > 2, and significant residence time of o-cresol may favor 2,6-xylenol selectivity. However, excess methanol is often used, sinee some amount of methanol tend to undergo oxidation into various reformate produets [71] under vapor phase condition. Similarly, reaction temperature, catalyst acid-base property, and space velocity of the reaetant are the parameters that govern the selectivity to 2,6-xylenol. 

In a related application, polyelectrolyte microgels based on crosslinked cationic poly(allyl amine) and anionic polyfmethacrylic acid-co-epoxypropyl methacrylate) were studied by potentiometry, conductometry and turbidimetry [349]. In their neutralized (salt) form, the microgels fully complexed with linear polyelectrolytes (poly(acrylic acid), poly(acrylic acid-co-acrylamide), and polystyrene sulfonate)) as if the gels were themselves linear. However, if an acid/base reaction occurs between the linear polymers and the gels, it appears that only the surfaces of the gels form complexes. Previous work has addressed the fundamental characteristics of these complexes [350, 351] and has shown preferential complexation of cationic polyelectrolytes with crosslinked car-boxymethyl cellulose versus linear CMC [350], The departure from the 1 1 stoichiometry with the non-neutralized microgels may be due to the collapsed nature of these networks which prevents penetration of water soluble polyelectrolyte. 

Mass intensity measures the amount of material needed to synthesize the desired product. It takes into account yield, reaction stoichiometry, solvents, and reagents in a reaction mixture, and this covers everything that is put into a reaction vessel. It also includes all mass used in acid, base, salt and organic solvent washes, and organic solvents used in extractions, crystallizahons, or solvent switching. 

In titrations involving 1 1 stoichiometry of reactants, the equivalence point is the steepest point of the titration curve. This is true of acid-base, complexometric, and redox titrations as well. For stoichiometries other than 1 1, such as 2Ag+ + CrO —> Ag2Cr04(s), the curve is not symmetric near the equivalence point. The equivalence point is not at the center of the steepest section of the curve, and it is not an inflection point. In practice, conditions are chosen such that titration curves are steep enough for the steepest point to be a good estimate of the equivalence point, regardless of the stoichiometry. 

Two complications can prevent a simple determination or the concentration of each species from a measurement of absorbance at a chosen frequency. Although most of the acid-base reactions of interest result in a one-to-one stoichiometry, one cannot assume this a priori, and two-to-one and three-to-one adducts might also be present. Fortunately, this is usually an easy point to Tesolve. The presence of an isosbeslic point or point of constant absorbance (see Fig. 9.1) is usually a reliable criterion that only two absorbing species (the free acid or base and a single adduct) are present.26... 

As indicated by the flow diagram in Figure 3.5, using molarity is critical for carrying out stoichiometry calculations on substances in solution. Molarity makes it possible to calculate the volume of one solution needed to react with a given volume of another solution. This sort of calculation is particularly important in acid-base chemistry, as shown in Worked Example 3.14. 

A pH titration curve is a plot of the pH of a solution as a function of the volume of base (or acid) added in the course of an acid-base titration. For a strong acid-strong base titration, the titration curve exhibits a sharp change in pH in the region of the equivalence point, the point at which stoichiometri-... 

When a strong acid or base undergoes a complete ionization in solution, the concentrations of the newly formed ions can be understood using basic stoichiometry principles. This is because essentially all of the acid is converted to ions. With weaker acids and bases, equilibrium is established between the ions, much like the equilibria studied in the last chapter. The concentrations of the ions must be determined by using an equilibrium constant, K. The equilibrium constants used to describes acid-base equilibria are in the same form as Kc from the last chapter. Well use the dissociation of acetic acid to begin our description of the new equilibrium constant. 

Titration — A process for quantitative analysis in which measured increments of a - titrant are added to a solution of an - analyte until the reaction between the analyte and titrant is considered as complete at the - end point [i]. The aim of this process is to determine the amount of an analyte in a -> sample. In addition, the determination can involve the measurement of one or several physical and/or chemical properties from which a relationship between the measured parameter/s and the concentration of the analyte is established. It is also feasible to measure the amount of a - titrand that is added to react with a fixed volume of titrant. In both cases, the -> stoichiometry of the reaction must be known. Additionally, there has to be a means such as a -> titration curve or an - indicator to recognize that the -> end point has been reached. The nature of the reaction between the titrant and the analyte is commonly indicated by terms like acid-base, complexometric, redox, precipitation, etc. [ii]. Titrations can be performed by addition of measured volume/mass increments of a solution,... 

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