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Titration of a Polyprotic Acid02:08

Titration of a Polyprotic Acid

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A polyprotic acid contains more than one ionizable hydrogen and undergoes a stepwise ionization process.  If the acid dissociation constants of the ionizable protons differ sufficiently from each other, then the titration curve for such polyprotic acid generates a distinct equivalence point for each of its ionizable hydrogens. Therefore, titration of a diprotic acid results in the formation of two equivalence points, whereas the titration of a triprotic acid results in the formation of three...
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Titration of Polyprotic Acids with a Strong Base01:23

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Titration of a polyprotic acid, which contains multiple ionizable protons, involves distinct dissociation steps, each with its own dissociation constant (Ka). Each successive Ka is weaker than the previous one. In the titration of a polyprotic acid like sulfurous acid with a strong base such as sodium hydroxide, the base first neutralizes the initial ionizable proton, forming an intermediate species (e.g., hydrogen sulfite ions). This step's titration curve resembles that of a weak...
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Polyprotic Acids

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Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:
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Acid and Bases: Ka, pKa, and Relative Strengths02:35

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This lesson delves into a critical aspect of the relative strengths of acids and bases. The strength of an acid is evaluated by the acid dissociation into its conjugate base and a hydronium ion in water. The complete dissociation of a strong acid is confirmed with a very high concentration of hydronium ions. As a result, an incomplete dissociation process affirms a weak acid. Therefore, the equilibrium is in the forward direction for strong acids and backward for weak acids in these reactions.
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Consider a neutral form of an amine, B, with a partition coefficient, K, in a liquid mixture containing organic and aqueous phases. The pH of the aqueous phase affects the charge on acidic and basic solutes, and the charged form is usually more soluble in the aqueous phase. Suppose the conjugate acid form of the amine is soluble only in the aqueous phase while the base form is soluble in both phases. Then the distribution coefficient, D, can be given as the ratio of amine concentration in the...
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Titration of a Weak Acid with a Weak Base01:08

Titration of a Weak Acid with a Weak Base

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Weak acids and bases do not undergo dissociation completely, and titrations between these two are rarely studied. When such studies are performed, say, for the titration of a weak acid with a weak base, the titration curve plots the change in pH as a function of the volume of base added. Take the titration of acetic acid with ammonia, for instance. During the titration, these two species form ammonium acetate and water, but the pH change is slow and gradual.
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Quantifying Acidity in Heterogeneous Systems: Biphasic pKa Values.

Andre Leesment1, Sigrid Selberg1, Merili Tammiste1

  • 1Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia.

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Summary

Aqueous pKa values inaccurately estimate acidities in biphasic systems. This study introduces biphasic pKa (pKaow) for accurate acidity measurements in lipophilic environments, crucial for catalysis and sensors.

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Area of Science:

  • Physical Chemistry
  • Analytical Chemistry

Background:

  • Aqueous pKa values are often used to approximate acidities of lipophilic compounds in biphasic systems.
  • This approximation is flawed because H+ ions concentrate in the aqueous phase, while acids and their anions partition into the organic phase.

Purpose of the Study:

  • To introduce and validate the biphasic pKa concept for accurate acidity determination in biphasic systems.
  • To provide a more reliable method for quantifying acidities of lipophilic compounds relevant to various chemical applications.

Main Methods:

  • Determined biphasic pKa (pKaow) values for 35 diverse acids in a 1-octanol:water system.
  • Utilized UV-vis and NMR spectroscopy for direct pKaow measurements.
  • Applied Debye-Hückel theory to extrapolate apparent, concentration-dependent pKaow values to concentration-independent results.

Main Results:

  • Demonstrated that aqueous pKa values lead to significantly biased acidity estimations in biphasic systems.
  • Successfully determined concentration-independent biphasic pKa values for a range of acids.
  • Validated the biphasic pKa concept as a superior method for acidity quantification in such systems.

Conclusions:

  • Aqueous pKa is not justified for assessing acidities of lipophilic species in biphasic environments.
  • The biphasic pKa concept offers a more accurate and reliable approach.
  • This work provides essential data and methodology for fields utilizing lipophilic acids in multiphase systems.