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Composition of Polyprotic Acid Solutions as a Function of pH01:19

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Polyprotic acids of the type H2M constitute two ionizable protons. As a result, on titration with a base, they exhibit two equivalence points in the titration curve. During titration, the species H2M, HM−, and M2− will be present in the solution at different points. The fractions of H2M, HM−, and M2− present at the various instances of the titration are denoted by α0, α1, and α2, respectively.
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Different monodentate and polydentate ligands are used as complexing agents in complexometric titration reactions. The formation of complexes by mono- and bidentate ligands involves two or more intermediate steps, limiting their use as complexing agents. In comparison, polydentate ligands can form complexes with metal ions in a single-step process, facilitating sharper end points. This means polydentate ligands, such as amino carboxylic acid derivatives, are most commonly employed in...
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Polymorphism refers to the existence of a drug substance in multiple crystalline forms, known as polymorphs. Recently, this term has been expanded to include solvates (forms containing a solvent), amorphous forms (non-crystalline forms), and desolvated solvates (forms from which the solvent has been removed).
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Polydentate ligands are most widely used in complexometric titrations because they form more stable complexes with the metal ions than mono- or bidentate ligands due to the chelate effect. Examples of polydentate ligands are ethylenediaminetetraacetic acid (EDTA), crown ethers, and cryptands. The most important feature of optimal polydentate ligands is the ability to form 1:1 complexes in a single-step process. Amino carboxylic acid derivatives are frequently used as complexing agents. EDTA is...
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Polyamorphism of D-mannitol.

Men Zhu1, Lian Yu1

  • 1Department of Chemistry and School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, USA.

The Journal of Chemical Physics
|July 3, 2017
PubMed
Summary
This summary is machine-generated.

Polymorphism, the ability of a substance to exist in multiple forms, is now observed in D-mannitol

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Area of Science:

  • Materials Science
  • Physical Chemistry
  • Chemical Physics

Background:

  • Polymorphism is common in crystalline solids but rare in liquid or glassy states.
  • Water is a known example exhibiting polymorphism with low-density amorphous (LDA) and high-density amorphous (HDA) ices.
  • The phenomenon of polyamorphism in molecular liquids is not well-established.

Purpose of the Study:

  • To investigate the existence of polyamorphism in D-mannitol, an extensively hydrogen-bonded system.
  • To compare the observed polyamorphic transition in D-mannitol with that of water.
  • To explore the underlying mechanisms driving polyamorphism in hydrogen-bonded networks.

Main Methods:

  • Experimental observation of phase transitions in supercooled D-mannitol.
  • Analysis of volume, energy, and hydrogen bonding characteristics of different D-mannitol phases.
  • Comparative analysis with known polyamorphic systems like water.

Main Results:

  • D-mannitol's supercooled liquid spontaneously transforms into a lower-energy amorphous phase with expanded volume (2.1%) and stronger hydrogen bonds.
  • This transition mirrors water's high-density amorphous (HDA) to low-density amorphous (LDA) ice transition.
  • The transition exhibits an anomalous coupling of heat release and volume expansion.

Conclusions:

  • Polyamorphism is demonstrated in D-mannitol, a system with extensive hydrogen bonding.
  • The findings suggest polyamorphism may be more widespread in hydrogen-bonded systems than previously thought.
  • Competing hydrogen bonds and van der Waals forces are proposed as the driving forces for polyamorphism in both D-mannitol and water.