Titration curves and buffers

Buffer Intensity and Neutralizing Capacity 137 Table 3.8. Titration Curve and Buffer Intensity of a Two-Protic Acid (H2C)"... 

FIGURE 2.15 The relationship between the titration curve and buffering action in H2PO4". 

Barbosa J, Bosch E, Cortina JL, and Roses M (1992) Ionic equilibria in amphiprotic solvents of low dielectric constants. Titration curves and buffer capacity of bases in anhydrous acetic acid. Analytica Chimica Acta 256 211-220. 

Fig. 2. Titration curve and buffer capacity of a wine. The total acidity is equal to about 107 meq. (after Vergnes from Jaulmes, 1961).

Before the equivalence point, and for volumes of titrant in the titration curve s buffer region, the concentrations of HA and A are given by the following equations. 

Calcium and magnesium influence the titration curves of milk because as the pH is raised they precipitate as colloidal phosphates, and as the pH is lowered, colloidal calcium and magnesium phosphates are solubilized. Since these changes in state are sluggish and the composition of the precipitates depends on the conditions (Boulet and Marier 1961), the slope of the titration curves and the position of the maximum buffering depend upon the speed of the titration. 

Figure 6.11 shows the titration curve for adipic acid, hexane-1,6-dioic acid with p Ti = 4.41 and pK2 = 5.28 at 25°C. Here the curve resembles that of a monobasic acid with one extended buffer region, and only one end-point. There is thus a clear distinction between this titration curve and those for malonic and phosphoric acids. The two dissociations for this acid cannot be treated as independent of each other, and analysis of the curve must consider both equilibria simultaneously. 

The total acidity of must or wine takes into account all types of acids, i.e. inorganic acids such as phosphoric acid, organic acids including the main types described above, as well as amino acids whose contribution to titratable acidity is not very well known. The contribution of each type of acid to total acidity is determined by its strength, which defines its state of dissociation, as well as the degree to which it has combined to form salts. Among the organic acids, tartaric acid is mainly present in must and wine as monopotassium acid salt, which still contributes towards total acidity. It should, however, be noted that must (an aqueous medium) and wine (a dilute alcohol medium), with the same acid composition and thus the same total acidity, do not have the same titration curve and, consequently, their acid-alkaline buffer capacity is different. 

Dilute 69 cm of purest syrupy phosphoric acid to 1 dm to obtain 1 M solution and by 10-fold dilution, prepare 0.1 M solution. Pipette 20.0 cm of the acid into a 250 cm conical flask. Calibrate the pH meter with a combined glass electrode and two buffer solutions at pH 4 and at 7. Dip the rinsed electrode into the flask, introduce a magnetic bar and add from a burette 0.1 M KOH solution in 2 cm aliquots, reading the pH on the meter when it settles. When the pH change becomes more rapid as the titration proceeds, decrease the volume down to 0.5 cm and then to 0.1 cm. After noting the first inflection point, continue with 2 cm additions of KOH and eventually to 0.1 cm addition until the second inflection point is reached. If a facility is available record the titration curve and preferably the derivative titration curve. 

Figure 9.8b shows a titration curve for a mixture consisting of two weak acids HA and HB. Again, there are two equivalence points. In this case, however, the equivalence points do not require the same volume of titrant because the concentration of HA is greater than that for HB. Since HA is the stronger of the two weak acids, it reacts first thus, the pH before the first equivalence point is controlled by the HA/A buffer. Between the two equivalence points the pH reflects the titration of HB and is determined by the HB/B buffer. Finally, after the second equivalence point, the excess strong base titrant is responsible for the pH. 

It was indicated that the original method can be extended on systems where two or three analytes can be determined from a single titration curve. The shifts DpH affected by j-th PT addition should be sufficiently high it depends on pH value, a kind and concentration of the buffer chosen and its properties. The criterion of choice of the related conditions of analysis has been proposed. A computer program (written in MATLAB and DELPHI languages), that enables the pH-static titration to be done automatically, has also been prepared. 

A biologically important point is revealed by the basic shape of the titration curves of weak electrolytes in the region of the pK, pH remains relatively unaffected as increments of OH (or H ) are added. The weak acid and its conjugate base are acting as a buffer. 

FIGURE 2.15 A buffer system consists of a weak acid, HA, and its conjugate base, A. The pH varies only slightly in the region of the titration curve where [HA] = [A ]. The unshaded box denotes this area of greatest buffering capacity. Buffer action when HA and A are both available in sufficient concentration, the solution can absorb input of either H or OH, and pH is maintained essentially constant. 

Buffers are solutions that tend to resist changes in their pH as acid or base is added. Typically, a buffer system is composed of a weak acid and its conjugate base. A solution of a weak acid that has a pH nearly equal to its by definition contains an amount of the conjugate base nearly equivalent to the weak acid. Note that in this region, the titration curve is relatively flat (Figure 2.15). Addition of H then has little effect because it is absorbed by the following reaction ... 

Initial hydrolysis would therefore lead to further hydrolysis and pH drop in storage tanks, resulting in a product that is difficult to recover and may cause irreparable damage (corrosion) in tanks, pipelines, and pumps. Therefore the pH must be kept high (9-11) to avoid acid material entering bulk storage (steep titration curve). If for product formulation requirements a product of pH 6-7 is essential, the use of buffers, e.g., phosphoric acid or citric acid, is recommended. 

The titration curve does not give directly. However, at the midpoint of the titration the concentration of Ep and EpH+ are identical. Use this information in the buffer equation to show that at the midpoint of the titration, the pH of the solution equals the p for EpH ... 

Point A iies aiong the section of the titration curve known as the buffer region. Buffering action comes from the presence of a weak acid and its conjugate base as major species in solution. Moreover, Point A iies beyond the midpoint of the titration, which teiis us that more than half of the weak acid has been consumed. We represent this soiution with two moiecuies of H four ions of A, and four H2 O moiecuies ... 

When plotted on a graph of pH vs. volume of NaOH solution, these six points reveal the gross features of the titration curve. Adding additional calculated points helps define the pH curve. On the curve shown here, the red points A-D were calculated using the buffer equation with base/acid ratios of 1/3 and 3/1. Point E was generated from excess hydroxide ion concentration, 2.00 mL beyond the second stoichiometric point. You should verify these additional five calculations. 

C18-0020. Glycolic acid (HOCH2 CO2 H), a constituent of sugar cane juice, has a p Zg of 3.9. Sketch the titration curve for the titration of 60.0 mL of 0.010 M glycolic acid with 0.050 M KOH. Indicate the stoichiometric point, the buffer region, and the point of the titration where pH- p. S a. Sketch the curve qualitatively without doing any quantitative calculations. 

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