Neutralization Reactions and Titration Curves

As we learned in the discussion of the stoichiometry of titration reactions (Section 5-7), the equivalence point of a neutralization reaction is the point at which both acid and base have been consumed and neither is in excess. 

A graph of pH versus volume of titrant (the solution in the buret) is called a titration curve. Titration curves are most easily constructed by measuring the pH during a titration with a pH meter and plotting the data with a recorder. In this section we will emphasize calculating the pH at various points in a titration. These calculations will serve as a review of aspects of acid-base equilibria considered earlier in this chapter and in the preceding chapter. 

In a typical titration, the volume of solution delivered from a buret is less than 50 mL (usually about 20-25 mL). The molarity of the solution used for the titration is generally less than 1 M. The typical amount of OH (or H3O ) delivered from the buret during a titration is only a few thousandths of a mole—for example, 5.00 X 10 mol. In calculations it is often easier to work with millimoles instead of moles. The symbol mmol stands for a millimole, which is one thousandth of a mole, or 10 mol. 

Recall from Chapter 4 that molarity is defined as the number of moles per liter. We can use an alternative definition of molarity by converting from moles to millimoles and from liters to milliliters. 

the expression from Chapter 4 that the amount of solute is the product of molarity and solution volume (page 123) can be based either on mol/L X L = mol or on mmol/mL X mL = mmol. 

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Buffer Solutions 17-3 Acid-Base Indicators 17-4 Neutralization Reactions and Titration Curves... 

Neutralization Reactions and Titration Curves—As described in Chapter 5, the concentration of an acidic or basic solution of unknown concentration... 

PART III Classical Methods of Analysis 311 Chapter 12 Gravimetric Methods of Analysis 314 Chapter 13 Titrimetric Methods Precipitation Titrimetry 337 Chapter 14 Principles of Neutralization Titrations 368 Chapter 15 Titration Curves for Complex Acid/Base Systems 395 Chapter 16 Applications of Neutralization Titrations 428 Chapter 17 Complexation Reactions and Titrations 449... 

As the titration begins, mostly HAc is present, plus some H and Ac in amounts that can be calculated (see the Example on page 45). Addition of a solution of NaOH allows hydroxide ions to neutralize any H present. Note that reaction (2) as written is strongly favored its apparent equilibrium constant is greater than lO As H is neutralized, more HAc dissociates to H and Ac. As further NaOH is added, the pH gradually increases as Ac accumulates at the expense of diminishing HAc and the neutralization of H. At the point where half of the HAc has been neutralized, that is, where 0.5 equivalent of OH has been added, the concentrations of HAc and Ac are equal and pH = pV, for HAc. Thus, we have an experimental method for determining the pV, values of weak electrolytes. These p V, values lie at the midpoint of their respective titration curves. After all of the acid has been neutralized (that is, when one equivalent of base has been added), the pH rises exponentially. 

Below is the titration curve for the neutralization of 25 mL of a monoprotic acid with a strong base. Answer the following questions about the reaction and explain your reasoning in each case, (a) Is the acid strong or weak (b) What is the initial hydronium ion concentration of the acid (c) What is K, for the acid (d) What is the initial concentration of the... 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

A titration curve for sperm whale myoglobin has been reported by Bres-low and Gurd (1962). The most striking feature is that it exhibits a time-dependent acid denaturation, which resembles that observed for the similar protein hemoglobin. To elucidate the physical nature of this reaction, emphasis was placed on the back titration to neutral pH of denatured protein. As in the case of hemoglobin (mentioned earlier), there are two major differences between the titration curves of native and denatured myoglobin, as shown by the data of Table XV. 

In generating titration curves, care should be taken that any reactions are allowed to go to completion (check for pH stabilization over about ten minutes if slow reactions are observed). The material to be titrated should be at the same temperature as the effluent to be treated, and care should be taken to avoid the loss of volatile components from samples to the atmosphere. The reagent used should be the main reagent to be used for neutralization. If problems are experienced in metering small quantities of lime-based reagents, caustic (NaOH) can be used near neutral to get sufficient resolution of the titration curve—at least one point per unit pH change should be generated. 

This reaction is characterized by a pK value of — 2.04 0.4 and the buffer number equal to (3 — 0.12. The equivalence point of this neutralization step can be detected more sharply using the differential potentiometric titration curve. 

Various approaches have been used to classify the components of soil acidity. As a carryover from the titration curves used to characterize soil acidity in earlier studies, a common category is titratable acidity or total acidity. This is the quantity of a strong base (NaOH or Ca(OH)2) required to raise soil pH to a predetermined level. Time, method of stirring, and period between additions of base must be specified because the neutralization of soil acidity is highly dependent on reaction conditions. The values are also meaningless unless the initial and final pH values are specified, because more base is consumed if the reaction is carried out ova a wider pH range. 

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