Chemistry
Polyprotic Acid Titration
Polyprotic acid titration involves the process of determining the concentration of a polyprotic acid by adding a strong base to it. Since polyprotic acids can donate multiple protons, their titration curves exhibit multiple equivalence points. Each equivalence point corresponds to the neutralization of a different proton, and the pH at each point can be used to calculate the acid's dissociation constants.
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8 Key excerpts on "Polyprotic Acid Titration"
eBook - ePub- James F. Pankow(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
7 Titrations of Acids and Bases7.1 Introduction
In Chapter 5 , we discussed how to determine the one equilibrium pH and other concentration values in a particular solution of a chemical like HA, NaA, H2 B, NaHB, or Na2 B. Then in Chapter 6 , we discussed how, if we add strong acid or strong base, a whole range of pH values becomes accessible in a solution with a specific value of AT (or BT ), not just the one pH value for the solution of HA, or NaA, etc. In this chapter, we consider how the incremental addition of a strong base or acid causes incremental changes in pH, focusing on monoprotic (AT ) systems. Chapter 9 will consider diprotic systems, in the context of CO2 chemistry.Many readers will recognize the concept of “incremental addition of strong base or acid” as relating to titration . In an alkalimetric titration, successive increments of strong base are added to a solution while recording the resulting successively higher equilibrium pH values. In an acidimetric titration, successive increments of strong acid are added to a solution while recording the resulting successively lower equilibrium pH values. Alkalimetric titration and acidimetric titration are negative analogs of one another, just like going west is a negative analog of going east, and going west negatively amounts to going east. So, if we add some strong base (e.g. , NaOH), we can negate that by adding the equivalent amount of strong acid (e.g. , HCl): we end up where we started, plus some dissolved salt (e.g. , NaCl).A titration of a given volume of a sample solution is usually carried out by adding small volume increments of a concentrated (e.g. , 0.1 N ) strong base or acid solution (the titrant). For alkalimetric and acidimetric titrations, the titration curve is usually plotted as pH vs. amount (e.g. , volume ) of strong base or strong acid added. Here, however, since we want to discuss things in a general context, for the x -axis rather than volume, we will mostly use the value of C B − C A that is produced in the solution (or the related parameter f , defined below). If the concentration in the titrant is sufficiently large relative to the concentration of what is being titrated (say 20×), then near-constancy in the value of AT (or BT , etc.) in the solution can be assumed over the course of the titration: the volume of titrant added does not significantly increase the volume of solution being titrated so that AT (or BT
eBook - ePub- Clyde Frank(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
An assumption in this calculation is that the hydrolysis of acetate ion does not contribute to the pH. However, this must be considered for a precise calculation.A plot of the approximate pH values against percent neutralization was shown in Fig. 8-2 . The pH break at the stoichiometric point is not well defined and, therefore, an accurate end point can not be detected by ordinary means.End-point detection by ordinary means in this case implies potentiometry or the use of indicators. Actually, end points for weak acid–weak base titrations are readily and accurately determined by measuring change in conductance, heat change, or change in absorption of radiant energy. Also, by appropriately choosing a solvent other than water many weak acid–weak base titrations can be followed by potentiometry or indicators. Consequently, it is possible to use a weak acid–weak base titration for analysis.POLYFUNCTIONAL ACIDS AND BASES
Many acids and bases are capable of furnishing more than one hydronium or hydroxide ion per molecule and are called polyprotic acids or polyfunctional bases. The removal of each proton or hydroxide ion, if the substance is a weak acid or base, respectively, constitutes a separate equilibrium step with its corresponding equilibrium constant. Consequently, any precise calculations dealing with these kinds of systems must include a consideration of all ionization steps. Whether approximations are possible will be determined by the magnitude of each ionization constant, the difference between the stepwise constants, and the purpose for doing the calculation. In general, reasonable answers can often be obtained if the polyprotic systems are considered to be a series of individual steps. Hence, each system is examined and the main equilibrium step is identified and used for the calculation. As the stepwise constants approach each other in value, the error in the calculation becomes larger. Thus, it should be recognized that the answers are not exact and the degree of error can be estimated by inserting the approximate answers into equations which are a more exact description of the multistep equilibrium system.*
No longer available |Learn more- Steven Boone, Drew H. Wolfe(Authors)
- 2011(Publication Date)
- Collins Reference(Publisher)
acid-base titration is a volumetric laboratory procedure for determining the concentration or number of moles of an unknown acid or base, using either a standard base or acid. In an acid-base titration, the acid is usually poured into an Erlenmeyer flask. Then, the base is carefully added until the acid is neutralized. If a few drops of an acid-base indicator, such as phenolphthalein, are present in the solution, a color change will occur, signaling the equivalence point of the titration. A titration curve is plotted to best understand what happens during a titration. These curves are plots of pH of the solution versus the volume of the titrant added.Titration of a Strong Acid with a Strong Base The net ionic equation for the reaction of a strong acid and strong base is as follows.H+ (aq) + OH- (aq) → H2 O(l)The strong acid is the source of H+ ions and the strong base of the OH- ; ions. At the equivalence point, the moles of H+ equal the moles of OH- ; thus, the pH at the equivalence point is 7 because the reaction products are water and a neutral salt.Characteristics of a Titration of a Strong Acid and BaseThe titration curve for the titration of a strong acid by a strong base begins at a low pH value because a strong acid is present before base is added. The initial addition of base produces a small change in pH.Figure 17.1: Graph of strong acid and strong base titration (pH versus mL NaOH).A significant change in pH does not occur until the equivalence point, pH 7, is approached. An added drop of strong base near the equivalence point changes the pH by as much as five to six pH units, which means the H+ ion concentration changes 105 to 106 times. Beyond the equivalence point, the pH initially rises rapidly, and then levels off. Due to the steep segment of the curve near the equivalence point, acid-base indicators that change color between pH 4 and 10 may be used to detect the equivalence point. Figure 17.1 shows an example of such a curve.Exercise 17.6Consider the titration of 25.00 mL 0.1000 M HCl with a standard 0.1000 M NaOH solution. (a) What is the initial pH of the solution before the addition of NaOH(aq)? (b) What is the pH after the addition of 15.00 mL NaOH? (c) What is the pH after the addition of 24.00 mL NaOH? (d) What is the pH after the addition of 24.99 mL NaOH? (e) What is the pH after the addition of 25.00 mL NaOH? (f) What is the pH after the addition of 26.00 mL NaOH?
eBook - ePub- John Kenkel(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
For a titrimetric analysis to be successful, the equivalence point must be easily and accurately detected; the reaction involved must be fast; and the reaction must be quantitative. If an equivalence point cannot be detected (i.e., if there is no acceptable indicator or other detection method), then the correct volume of titrant cannot be determined. If the reaction involved is not fast, then the end point cannot be detected immediately upon adding the last fraction of a drop of titrant and there would be some doubt if the end point has been reached. If the reaction is not quantitative, meaning that if every trace of reactant in the titration flask is not consumed by the titrant at the end point, then again the correct volume of titrant cannot be determined. This latter point means that equilibrium reactions that do not go essentially to completion immediately are not acceptable reactions for this type of analysis. Thus, not all reactions are acceptable reactions.In this chapter, we investigate individual types of reactions that meet all the requirements. We will also discuss “back titrations” and “indirect” titrations in which some of the limitations that we may encounter are solved. Our discussions in Chapter 4 involved acid–base reactions. Acid– base reactions will be discussed here, but we will see that there are others that are applicable reactions.5.2 Acid–Base Titrations and Titration Curves
Various acid–base titration reactions are discussed in this section, including a number of scenarios of base in the buret and acid in the reaction flask and vice versa and also various monoprotic and polyprotic acids titrated with a strong base and various weak monobasic and polybasic bases titrated with strong acids. A monoprotic acid is an acid that has only one hydrogen ion (or proton) to donate per formula. Examples are hydrochloric acid, HCl, a strong acid, and acetic acid, HC2 H3 O2 , a weak acid. A polyprotic acid is an acid that has two or more hydrogen ions to donate per formula. Examples include sulfuric acid, H2 SO4 , a diprotic acid , and phosphoric acid, H3 PO4 , a triprotic acid.A monobasic base is one that will accept just one hydrogen ion per formula. Examples include sodium hydroxide, NaOH, a strong base, ammonium hydroxide, NH4 OH, a weak base, and sodium bicarbonate, NaHCO3 , also a weak base. A polybasic base is one that will accept two or more hydrogen ions per formula. Examples include sodium carbonate, Na2 CO3 , a dibasic base , and sodium phosphate, Na3 PO4 , a tribasic base .5.2.1 Titration of Hydrochloric Acid
A graphic picture of what happens during an acid–base titration is easily produced in the laboratory. Consider again what is happening as a titration proceeds. Consider, specifically, NaOH as the titrant and HCl as the substance titrated. In the titration flask, the following reaction occurs when titrant is added:
No longer available |Learn more- Brian M. Tissue(Author)
- 2013(Publication Date)
- Wiley(Publisher)
For acetic acid, the halfway point is equal to for an initial concentration of approximately and higher, but not for lower concentrations. 6.4 Polyprotic Acids Polyprotic acids are acids that possess more than one acidic proton, for example,,,, and EDTA,. Besides being common in environmental and biological systems, polyprotic acids are quite useful in analytical applications because they can serve as pH buffers over extended pH ranges. Because there are multiple acidic protons, the and have an additional numerical subscript to indicate that the constant is for the equilibrium involving the first proton, second proton, etc. The values decrease as protons are removed from a polyprotic acid because each subsequent proton is more difficult to remove as the molecule becomes more electronegative overall. Figure 6.4 shows the three forms of the diprotic phthalic acid: o -phthalic acid, ; hydrogen phthalate ion, ; and phthalate ion, (from left to right). The and p values of phthalic acid are is the equilibrium constant for phthalic acid;, for the hydrogen phthalate ion. To obtain the for the phthalate ion,, the appropriate value to use in the calculation is. Figure 6.4 Structures of the three forms of o -phthalic acid. When trying to predict equilibrium concentrations of the predominant species of a polyprotic acid in water, there are several different cases that we will encounter. We can usually treat addition of the fully protonated form of a polyprotic acid to water as a weak acid equilibrium problem, as we did for carbonic acid in Example 6.1: Similarly, when placing the salt of the fully deprotonated form in water, we can usually treat the problem as a weak base: I say “usually” for these cases because I am assuming that only the one equilibrium, involving two species, is important. Knowing if this is the case is where alpha plots can be very useful. Figure 6.5 shows the alpha plots for the o -phthalic acid species as a function of pH
- Stig Pedersen-Bjergaard, Bente Gammelgaard, Trine G. Halvorsen(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
Compared to indicator endpoint detection, potentiometric endpoint detection has a slightly slower response time. That is, there is often a slight delay from the addition of titrant until the reading of the voltmeter has stabilized. For this reason, the titrant is added slowly when approaching the endpoint.5.3 Aqueous Acid–Base Titrations
Most titrations of pharmaceutical ingredients are acid–base titrations. Acid–base titrations are used to assay basic or acidic pharmaceutical ingredients. In aqueous solution, strong acids and bases are entirely dissociated and the reaction for titration of a strong acid (titrate) with a strong base (titrant) is therefore(5.6)When a strong acid is gradually titrated with a strong base, pH changes according to Figure 5.6 . This curve is termed a titration curve. As appears from the titration curve, pH is very low in the strong acid titrate at the onset. At this point, the titrate comprises an aqueous solution of a strong acid. As titrant is added gradually, pH rises slightly due to the acid–base reaction. Just before the equivalence point, pH increases sharply, and in the equivalence point the stoichiometric reaction is complete. If the titration is continued after the equivalence point, the curve levels off at high pH values. Now, the titrate comprises an aqueous solution of a strong base, because the acid has been neutralized and the base is in excess. The equivalence point is located at the point of maximum slope of the titration curve and is the inflection point (point where curvature changes) of the curve.Figure 5.6Titration curve for titration of a strong acid (50 mL of 0.1 M HCl) with a strong base (0.2 M NaOH)To detect the endpoint of an acid–base titration visually, a colour indicator can be added to the titrate to perform indicator endpoint detection. Colour indicators of acid–base titration are acidic or basic substances, which change colour as they are transformed to their basic or acidic form, respectively. An example of an acid–base indicator is phenolphthalein, which is shown in Figure 5.7 . Phenolphthalein has a pKa value of 9.4. Thus, at pH 9.4, 50% of the indicator is in acidic form and 50% is in basic form. At this pH, the indicator is coloured pink. At pH 8.4 only 10% of the indicator is in basic form and the pink colour is very weak, while at pH 10.4 about 90% of the indicator is in basic form and the pink colour is very intense. In practice, the colour change of phenolphthalein is seen clearly between pH 8.4 and 10.4. From the titration curve in Figure 5.6 , it appears that pH rises very rapidly from about pH 4 to 12 around the equivalence point. With the presence of phenolphthalein in the titrate, the colour changes from colourless to pink at the equivalence point. This colour shift serves as endpoint detection, the addition of titrant is terminated, and the volume of titrant is read. Based on this volume, the result of the assay is calculated as exemplified in Box 5.1
- Mitsuru Nagasawa, Stuart A. Rice, Aaron R. Dinner, Mitsuru Nagasawa(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
The breaking point corresponds to the maximum in Figure 8. Figure 13 Modified Henderson–Hasselbalch plots of poly(methacrylic acid) (circles) and poly(acrylic acid) (squares) in pure aqueous solutions. Concentrations: (circles) base mol/l and (squares). (Reproduced with permission from Ref. [43]. Copyright Wiley.) IV. Applications of Potentiometric Titration to The Study of Conformational Transition of Macromolecules It was explained in Section “Theory of Ionization Equilibrium” that, if the polyion is a homopolymer of a monovalent acid monomer, such as PAA, the electrostatic free energy of a macro-ion can be calculated by charging up the molecule from the uncharged state to the actual state of ionization degree, such as 74 where. This may be written as 75 Equation (75) shows that the electrostatic free energy of a macro-ion having the degree of ionization,, is given by the area under the graph of versus. The electrostatic free energy of a macro-ion can thus be obtained from potentiometric titration data. The potentiometric titration method is particularly important for the study of the conformational change of macromolecules or biopolymers. In the previous section, we have already observed that various transition phenomena of polyelectrolytes and proteins can be observed in potentiometric titration curves (Figures 5, 6, and 8). A. Helix–Coil Transition of Ionic Polypeptides Let us take poly(l-glutamic acid) (PGA) as an example. Titrating a solution of PGA with NaOH, pH of the solution is measured as a function of the degree of neutralization and shown in the form of versus. The entire potentiometric titration curves are already shown in Figure 11. One example of them is shown in Figure 14 A. The original plot of versus is also shown in Figure 14 B. The curves are independent of the molecular weight of the sample if the molecular weight is not too low. Figure 14 Method for determining the degree of helix
eBook - ePubChemistry
With Inorganic Qualitative Analysis
- Therald Moeller(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Titration of an acid and a base is a technique used to determine accurately the concentration of an acidic or an alkaline solution. The practical need for such information arises frequently in medical laboratories and in industrial laboratories.Suppose we have 50 ml of a 0.10M solution of HCl. The number of moles of HCl present is(21)From the stoichiometry of the reaction between HCl and a base, say NaOH,we know that when exactly 0.005 mole of NaOH has been added to the original solution containing 0.005 mole of HCl, the acid will be exactly neutralized. The point of exact neutralization in an acid-base reaction is called the equivalence point. For any type of reaction the equivalence point is the point at which chemically equivalent amounts of the reactants have been mixed. For the HCl solution we started with, the equivalence point could be reached by the addition of 50 ml of 0.1M NaOH, or 25 ml of 0.2M NaOH, or 100 ml of 0.05M NaOH, or any other combination of concentration and volume that equals 0.005 mole of NaOH.Titration is the measurement of the volume of a solution of one reactant that is required to react completely with a measured amount of another reactant. Frequently both reactants are in solution, and the titration is the measurement of the volume of one solution that must be added to a known volume of the other solution. Usually the concentration of one solution—a standard solution—is known.For example, if the HCl solution described above was of unknown concentration, we could take exactly 50 ml of that solution and gradually add a standard 0.10M NaOH solution until the equivalence point was reached. This would occur when 50 ml of the base was added, which would indicate that the original solution contained an equal molar amount of the acid and was therefore 0.10M .In Chapter 4
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