Titration curves, acid-base

Titration of a Strong Acid with a Strong Base 

The addition of a strong base to a strong acid (or the reverse) is the simplest type of titration. The chemical reaction is the neutralization  

FIGURE 15.13 A titration curve for the titration of a strong acid by a strong base. The curve shown is for 100.0 ml of 0.1000 M HCI titrated with 0.1000 M NaOH. 

This reacts with (and neutralizes) an equal number of moles of the HjO ion present initially and reduces mhjO to 

In addition—and this is very important to remember—the volume of the titration mixture has increased from 100.0 to 130.0 mL (that is, from 0.1000 to 0.1300 L). The concentration of HaO at this point in the titration is 

The familiar curves of pH versus volume of titrant added are displayed and discussed in many texts. Here we shall examine only a few mathematical aspects which follow from our previous treatment and which bear upon the reliability of the end point determination. 

An acid-base titration is a procednre for determining the amount of acid (or base) in a solntion by determining the volnme of base (or acid) of known concentration that will completely react with it. An acid-base titration curve is a plot of the pH of 

The following example shows how to calculate a point on the titration curve of a strong acid and a strong base. 

Curve for the titration of a strong acid by a strong base 

Here 25.0 ml of 0.100 M HCI is titrated by 0.100 M NaOH.The portions of the curve where indicators bromcresol green and phenolphthalein change color are shown. Note that both indicators change color where the pH changes rapidly (the nearly vertical part of the curve). 

Calculating the pH of a Solution of a Strong Acid and a Strong Base 

The theoretical study of titration curves is necessary because it allows us, among other things, to choose the optimal conditions for detecting the equivalence point in order to minimize the titration error. 

In Chapter 4, we discussed the acid-base titration as an analytical method. Let s re-examine it, this time tracking the change in pH with an acid-base titration curve, a plot of pH vs. volume of titrant added. The behavior of an acid-base indicator and its role in the titration are described first. To better understand the titration process, we apply the principles of the acid-base behavior of salt solutions (Section 18.7) and, later in the section, the principles of buffer action. 

The two common devices for measuring pH in the laboratory are pH meters and acid-base indicators. (We discuss the operation of pH meters in Chapter 21.) An acid-base indicator is a weak organic acid (denoted here as HIn) that has a different color than its conjugate base (In ), with the color change occurring over 

Animation Acid-Base Titration Online Learning Center 

Strong Acid-Strong Bose Titration Curves 

Features of the Curve There are three distinct regions of the curve, which correspond to three major changes in slope  

Animation Acid-Basa Titration Onlina Ltarning Cantor 

In the overview to this chapter we noted that the experimentally determined end point should coincide with the titration s equivalence point. For an acid-base titration, the equivalence point is characterized by a pH level that is a function of the acid-base strengths and concentrations of the analyte and titrant. The pH at the end point, however, may or may not correspond to the pH at the equivalence point. To understand the relationship between end points and equivalence points we must know how the pH changes during a titration. In this section we will learn how to construct titration curves for several important types of acid-base titrations. Our 

Titrating Strong Acids and Strong Bases For our first titration curve let s consider the titration of 50.0 mb of 0.100 M HCl with 0.200 M NaOH. For the reaction of a strong base with a strong acid the only equilibrium reaction of importance is 

The first task in constructing the titration curve is to calculate the volume of NaOIi needed to reach the equivalence point. At the equivalence point we know from reaction 9.1 that 

Before the equivalence point, HCl is present in excess and the pH is determined by the concentration of excess HCl. Initially the solution is 0.100 M in HCl, which, since HCl is a strong acid, means that the pH is 

The equilibrium constant for reaction 9.1 is Ky,y, or 1.00 X 10. Since this is such a large value we can treat reaction 9.1 as though it goes to completion. After adding 10.0 mb of NaOH, therefore, the concentration of excess HCl is 

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Sketching an Acid—Base Titration Curve To evaluate the relationship between an equivalence point and an end point, we only need to construct a reasonable approximation to the titration curve. In this section we demonstrate a simple method for sketching any acid-base titration curve. Our goal is to sketch the titration curve quickly, using as few calculations as possible. 

Now that we know something about EDTA s chemical properties, we are ready to evaluate its utility as a titrant for the analysis of metal ions. To do so we need to know the shape of a complexometric EDTA titration curve. In Section 9B we saw that an acid-base titration curve shows the change in pH following the addition of titrant. The analogous result for a titration with EDTA shows the change in pM, where M is the metal ion, as a function of the volume of EDTA. In this section we learn how to calculate the titration curve. We then show how to quickly sketch the titration curve using a minimum number of calculations. 

In overall form this equation resembles that for the glass electrode (Chapter 6) and a pM-EDTA curve resembles an acid-base titration curve. The mercury electrode is most usefully employed when coloured or turbid solutions are being titrated, or when dilute solutions and weak complexes lead to poor colour changes. 

FIGURE 5.1 Acid-base titration curves (a) 0.10 M HCI (strong acid) titrated with 0.10 M NaOH (strong base), (b) 0.010 M HCI titrated with 0.010 M NaOH, and (c) 0.10 M acetic acid (weak acid) titrated with 0.10 M NaOH. 

FIGURE 5.2 A family of acid-base titration curves for a 0.10 M strong acid (HC1) and three weak acids, as indicated (0.10 M each), titrated with 0.10 M NaOH (strong base). HAc is a representation of acetic acid. 

In Investigation 8-A, you performed a titration and graphed the changes in the pH of acetic acid solution as sodium hydroxide solution was added. A graph of the pH of an acid (or base) against the volume of an added base (or acid) is called an acid-base titration curve. 

When an indicator is used in a titration, the range of pH values at which its endpoint occurs must include, or be close to, the equivalence point. Some representative acid-base titration curves, shown in Figures 8.11, 8.12, and 8.13, will illustrate this point. 

In this section, you examined acid-base titration curves for combinations of strong and weak acids and bases. You may have noticed the absence of a curve for the reaction of a weak acid with a weak base. A weak acid-weak base titration curve is difficult to describe quantitatively, because it has competing equilibria. You may learn about this curve in future chemistry courses. 

O OD In this section, you examined acid-base titration curves. 

If the profile of the observed or the intrinsic rate constant plotted against pH resembles the profile for an acid-base titration curve, this strongly suggests that one of the reactants is involved in an acid-base equilibrium in that pH range. Such behavior is ftiirly common and is illustrated by the second-order reaction between the Co(II)-trien complex and O2 (Fig. 1.12). The limiting rate constants at the higher and low acidities correspond to the acidic and basic forms of the Co(II) reactant, probably. 

The combination of Equations 15.7 and 15.8 represents the net retention factor as a function of pH and results in a sigmoid curve with the same form as an acid-base titration curve. 

From an acid-base titration curve, we can deduce the quantities and pK.d values of acidic and basic substances in a mixture. In medicinal chemistry, the pATa and lipophilicity of a candidate drug predict how easily it will cross cell membranes. We saw in Chapter 10 that from pKa and pH, we can compute the charge of a polyprotic acid. Usually, the more highly charged a drug, the harder it is to cross a cell membrane. In this chapter, we learn how to predict the shapes of titration curves and how to find end points with electrodes or indicators. 

Why does an acid-base titration curve (pH versus volume of titrant) have an abrupt change at the equivalence point ... 

A difference plot, also called a Bjerrum plot, is an excellent means to extract metal-ligand formation constants or acid dissociation constants from titration data obtained with electrodes. We will apply the difference plot to an acid-base titration curve. 

To extract acid dissociation constants from an acid-base titration curve, we can construct a difference plot, or Bjerrum plot, which is a graph of the mean fraction of bound protons, H, versus pH. This mean fraction can be measured from the quantities of reagents that were mixed and the measured pH. The theoretical shape of the difference plot is an expression in terms of fractional compositions. Use Excel SOLVER to vary equilibrium constants to obtain the best fit of the theoretical curve to the measured points. This process minimizes the sum of squares [nH(measured) -nH( theoretical) 2. 

Figure 3-2 Acid-base titration curve for hen lysozyme at 0.1 ionic strength and 25°C. O, initial titration from the pH attained after dialysis , hack titration after exposure to pH 1.8 A, hack titration after exposure to pH 11.1. The solid curve was constructed on the basis of "intrinsic" pKa values based on NMR data. From Kuramitsu and Hamaguchi5...

By plotting the chemical shift variation against the acidity, one observes a typical acid-base titration curve (Figure 1.5) and the p/fBH+ of the indicator can be determined this way. This NMR method, which was first proposed by Grunwald et al.,41 has been applied by Levy et al42 using various ketones and a-haloketones for the determination of ketone basicity and evaluation of medium acidity. 

When the equivalence point is reached, the Fe2+ will have been totally consumed (the large equilibrium constant ensures that this will be so), and the potential will then be controlled by the concentration ratio of Ce3+/Ce4+. The idea is that both species of a redox couple must be present in reasonable concentrations for a concentration to control the potential of an electrode of this kind. If one works out the actual cell potentials for various concentrations of all these species, the resulting titration curve looks much like the familiar acid-base titration curve. The end point is found not by measuring a particular cell voltage, but by finding what volume of titrant gives the steepest part of the curve. 

The amphoteric nature of wool was demonstrated in the early studies of Speakman and Hirst (1933), Elod (1933), and in particular by the complete acid-base titration curve obtained by Speakman and Stott (1934). Even earlier attempts had been made to determine the isoelectric point of wool by the methods indicated in Table XXIII. Some variation in the isoelectric point is to be expected because the pH at which the net charge, including bound ions, is zero depends on the nature and concentrations of ions in the aqueous environment. For example, Sookne and Harris (1939) have shown that the early electrophoretic value of Harris (1932) was affected by the absorption of phthalate ions from the buffer solutions. With acetate buffer they obtained values of 4.2 and 4.5 for powdered wool and cortical cells, respectively. The isoelectric points listed in Table XXIII are... 

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