Passage
Oxytocin is a naturally occurring peptide hormone that is used clinically to prevent postpartum hemorrhage, one of the leading causes of maternal mortality globally. Because oxytocin is heat and light sensitive, it requires refrigeration at temperatures below 8 °C. However, in underdeveloped regions with hot climates (such as sub-Saharan Africa) , refrigeration is often unavailable for pharmaceutical storage and transport.In an effort to improve the availability of oxytocin in regions with limited refrigeration, researchers conducted two experiments to investigate the use of buffers and divalent alkaline-earth metal salt additives as a means to produce an aqueous oxytocin formulation that is stable when stored at temperatures up to 50 °C.Experiment 1To evaluate effects from buffer concentration, two sets of solutions were prepared containing 0.3 mg/mL oxytocin monoacetate salt in 5, 10, and 50 mM citrate buffer solution made from a mixture of citric acid (a weak acid) and sodium citrate (a salt of the conjugate base) . An unbuffered aqueous solution served as a control sample. Using 1.0 M NaOH, the buffered solutions were adjusted to pH 4.5 (the pH at which oxytocin is more stable) and then stored at 5 °C (set 1) and 50 °C (set 2) for 4 weeks in a dark area. After storage, the amount of undegraded oxytocin in each sample was determined (Figure 1) using reverse-phase, high-performance liquid chromatography (RP-HPLC) .
Figure 1 Oxytocin recovery in a citrate buffer (pH 4.5) and water (pH 6.8) Experiment 2To examine the effect of divalent alkaline-earth metal cation additives, Experiment 1 was repeated, but the solutions were modified to contain 0.3 mg/mL of oxytocin with 5, 10, or 50 mM of a divalent cation (from a chloride salt) dissolved in 10 mM citrate buffer. The results are shown in Figure 2.
Figure 2 Oxytocin in 10 mM citrate buffer with added Ca²⁺, Mg²⁺, or Zn²⁺ salts
-Suppose that citric acid (H₃C₆H₅O₇) is titrated with 0.1 M NaOH to form a citrate buffer solution with a pH of 4.5. What is the pH at the first equivalence point? (Note: pK_a1 = 3.13, pK_a2 = 4.76, pK_a3 = 6.40)

Question

Passage
Oxytocin is a naturally occurring peptide hormone that is used clinically to prevent postpartum hemorrhage, one of the leading causes of maternal mortality globally. Because oxytocin is heat and light sensitive, it requires refrigeration at temperatures below 8 °C. However, in underdeveloped regions with hot climates (such as sub-Saharan Africa) , refrigeration is often unavailable for pharmaceutical storage and transport.In an effort to improve the availability of oxytocin in regions with limited refrigeration, researchers conducted two experiments to investigate the use of buffers and divalent alkaline-earth metal salt additives as a means to produce an aqueous oxytocin formulation that is stable when stored at temperatures up to 50 °C.Experiment 1To evaluate effects from buffer concentration, two sets of solutions were prepared containing 0.3 mg/mL oxytocin monoacetate salt in 5, 10, and 50 mM citrate buffer solution made from a mixture of citric acid (a weak acid) and sodium citrate (a salt of the conjugate base) . An unbuffered aqueous solution served as a control sample. Using 1.0 M NaOH, the buffered solutions were adjusted to pH 4.5 (the pH at which oxytocin is more stable) and then stored at 5 °C (set 1) and 50 °C (set 2) for 4 weeks in a dark area. After storage, the amount of undegraded oxytocin in each sample was determined (Figure 1) using reverse-phase, high-performance liquid chromatography (RP-HPLC) .

Figure 1 Oxytocin recovery in a citrate buffer (pH 4.5) and water (pH 6.8) Experiment 2To examine the effect of divalent alkaline-earth metal cation additives, Experiment 1 was repeated, but the solutions were modified to contain 0.3 mg/mL of oxytocin with 5, 10, or 50 mM of a divalent cation (from a chloride salt) dissolved in 10 mM citrate buffer. The results are shown in Figure 2.

Figure 2 Oxytocin in 10 mM citrate buffer with added Ca²⁺, Mg²⁺, or Zn²⁺ salts
-Suppose that citric acid (H₃C₆H₅O₇) is titrated with 0.1 M NaOH to form a citrate buffer solution with a pH of 4.5. What is the pH at the first equivalence point? (Note: pK_a1 = 3.13, pK_a2 = 4.76, pK_a3 = 6.40)

Quizplus · Accepted Answer

11ecba2d_1246_6344_a3bf_31127693b2d2_MD0008_00 During the titration of a weak polyprotic acid with a strong base, the first stoichiometric equivalence point occurs when the moles of added base are equal to the moles of the initial acid species. Prior to the equivalence point, a buffering region forms in which both the weak acid and its conjugate base are present and the pH is determined by the pK_a and the ratio of the acid and base species concentrations. When the concentrations of the acid and base species are equal, pH = pK_a. At the first stoichiometric equivalence point, the initial acid species has been fully consumed and converted into its conjugate base. For a polyprotic acid, the first conjugate base species formed is amphoteric and can act as either an acid (by donating an additional proton) or a base (by accepting a proton and regenerating the original acid). Consequently, the pH at the first equivalence point results from pK_a1 and pK_a2 and must be between them. In this question, the pH of the triprotic citric acid solution at the first equivalence point must be greater than 3.13 (pK_a1) but less than 4.76 (pK_a2) because the two equilibrium reactions occurring in solution are: 11ecba2d_1246_6345_a3bf_9f4ca8b2414d_MD0008_00 and: 11ecba2d_1246_6346_a3bf_67d530e46fd4_MD0008_00 Although unnecessary for this question, the exact pH can be found by averaging the pK_a values: 11ecba2d_1246_6347_a3bf_39691bbbea54_MD0008_00 (Choice A) The pH will be greater, not less, than pK_a1 (3.13) because at the equivalence point, all of the H₃C₆H₅O₇ species have been deprotonated once by OH⁻. (Choice C) The pH at the half-equivalence point for a monoprotic acid is equal to pK_a1 (3.13). However, with polyprotic acids, the pH at the first equivalence point is determined by both pK_a1 and pK_a2. (Choice D) The pH at the first equivalence point will be less than pK_a2 (4.76) because at the first equivalence point, additional acidic protons are present within the structure and can dissociate into solution. Educational objective: In a titration of a weak polyprotic acid and a strong base, the pH at a stoichiometric equivalence point is determined by the pK_a values of the acid and the conjugate base species present in the equilibrium reaction.

Passage Oxytocin Is a Naturally Occurring Peptide Hormone That Is Used