Weak Base versus Strong Acid

At the beginning of the titration, we have 0.1 M NH3, and the pH is calculated as described for weak bases in Chapter 7. See Table 8.2. As soon as some acid is added, some of the NH3 is converted to ISnETj , and we are in the buffer region. At the midpoint of the titration, equals [NH3], and the pH is equal to (14 — 

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Hydrolysis resistance is generally good versus hot water, weak acids and bases. Versus strong and concentrated acids and bases, PPE has a good to limited resistance. 

A base is any material that produces hydroxide ions when it is dissolved in water. The words alkaline, basic, and caustic are often used synonymously. Common bases include sodium hydroxide (lye), potassium hydroxide (potash lye), and calcium hydroxide (slaked lime). The concepts of strong versus weak bases, and concentrated versus dilute bases are exactly analogous to those for acids. Strong bases such as sodium hydroxide dissociate completely while weak bases such as the amines dissociate only partially. As with acids, bases can be either inorganic or organic. Typical reactions of bases include neutralization of acids, reaction with metals, and reaction with salts ... 

Case C, the titration of a weak acid with a weak base and vice versa, has in fact already been illustrated in Fig. 2.18 by the curves BB and B B are fully valid and for characteristic (3) the initial point is still dependent on the original concentration c however for the further main part of the curve we see a clean symmetry versus the equivalence point, which has become a true inflection point, independent of the concentration and simply determined by the mean value of pKg and pKb, i.e., (p/ia + pKh)/2 or (pifa + pifw - pKa.)/2. It also means that in the simultaneous titration of a polyvalent acid or a series of weak acids of different strength with a strong base and vice versa, (1) the stronger the acid the earlier it is titrated within the series, (2) the initial point and the final end-point of the series are still influenced by the concentration, but (3) the intermediate steps are only determined at the pH of the inflection point by the mean value of the pifas of the subsequent acids and in its steepness by the difference between these pKgs. Therefore, consultation of pKa tables provides the most suitable way of predicting the results of such simultaneous titrations. 

Figure 4.1 Solubility versus pH for a monoprotic weak base. A constant amount of weak base is considered and the pH is adjusted with a strong acid or strong base.

Roughly sketch the titration curve (pH versus volume) that you would expect if you titrated a weak base with a strong acid. Mark the equivalence point. 

The equivalent volume is readily determined in the laboratory by using an indicator that changes color near pH 8.7, the pH at the equivalence point of the acetic acid titration. A suitable choice would be phenolphthalein, which changes from colorless to red over a pH range from 8.2 to 10.0. The slope of pH versus volume of strong base is less steep near the equivalence point for a weak acid than it is for a strong acid, so determination of the equivalent volume—and of the unknown weak acid concentration—is somewhat less accurate. 

Standard solutions of strong acids and strong bases are used e.xtensively for determining analytes that are themselves acids or bases or that can be converted to such species by chemical treatment. This chapter introduces neutralization titrations, deals with the principles of titration, and discusses the common indicators that are used. In addition, titration curves that are plots of pH versus volume of titrant are explored, and several examples of pH calculations are presented. Titration curves for strong and weak acids and bases are described. 

Figure 6.3 shows plots of pH versus % ionized for a strong acid and weak base with pKa 2.5 and weak acid and strong bases with pKa 9.5, respectively. 

WEAK ACIDS VERSUS STRONG BASE—A BIT LESS STRAIGHTFORWARD... 

The titration of a weak base with a strong acid is completely analogous to the above case, but the titration curves are the reverse of those for a weak acid versus a strong base. The titration curve for 100 mL of 0.1 M ammonia titrated with 0.1 M hydrochloric acid is shown in Figure 8.8. The neutralization reaction is... 

It might appear that peak width merely serves as range extension. Yet, since peak width is related to a time span, which in turn is related through linear flow velocity to volumetric rate, peak width measurements allow flow titrations to be performed in a novel way. Therefore, similarly to classical batch titrations, FIA titrations encompass a domain of determinations, which cannot be performed in any other way, because they are based on consumption of an equivalent amount of reagent and, therefore, titrations yield different information than a direct measurement (pH measurement versus titration of a mixture of a weak and strong acid). 

This chapter will show that the potent ionizing ability of water is a key to predicting bimolecular versus unimolecular reactions. Aprotic solvents favor bimo-lecular reactions. It is assumed that water is required for ionization. Primary alkyl halides rmdergo Sn2 reactions under most conditions, whereas tertiary alkyl halides never undergo Sn2. If a nucleophile is classified as the conjugate base of a strong acid, it is a weak base and will probably not induce an E2 reaction. 

If the acid being titrated is a weak acid, then there are equilibria that will be established and accounted for in the calculations. (See the Utterly Confused section at the end of the chapter.) Typically, a plot of pH of the weak acid solution being titrated versus the volume of the strong base added (the titrant) starts at a low pH and gradually rises until close to the equivalence point in which the curve rises dramatically. After the equivalence point region, the curve returns to a gradual increase. We can see this in Figure 16-1. 

As illustrated in Section 2.1.2, the titration curve of a weak acid with a strong base can be constructed in terms of pH versus volume of the strong base. Consider a weak acid (e.g., HA) of volume Va its concentration Ca is titrated with a strong base (e.g., NaOH) of volume Vb and concentration Cb. The mass balances and electroneutrality equation can be given by ... 

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