Ammonium Purpurate

Ammonium purpurate, commonly known as murexide, is a fascinating organic compound with a rich history and specific applications in analytical chemistry. Here's a more detailed description:

Chemical Properties and Characteristics:

• Chemical Formula: C₈H₈N₆O₆ (or sometimes presented as the ammonium salt of purpuric acid)
• Appearance: Dark purple crystals or powder.
• Color Change: Exhibits a distinct color change depending on pH and the presence of metal ions, particularly calcium.
• Solubility: Slightly soluble in cold water, more soluble in hot water.
• Chemical Nature: It's an ammonium salt of purpuric acid, a complex organic molecule.
• Complexometric Indicator: It forms colored complexes with metal ions.

Applications and Uses:

• Complexometric Titrations:
  • Murexide is primarily used as a complexometric indicator, especially in the titration of calcium ions.  
  • It's valuable in determining the hardness of water.
  • The color change provides a clear endpoint in titrations.
• Analytical Chemistry:
  • It is a reagent used in analytical chemistry.
• Dyeing:
  • Historically, it was used as a dye, although this application is less common today.

Color Changes and Mechanism:

• In alkaline solutions, murexide exhibits a purplish-red color.
• When calcium ions are added, the color changes to a pink or orange-red.
• This color change is due to the formation of a metal-murexide complex.
• The color of the unbound murexide molecule is dependant on pH.

Safety Considerations:

• Irritant:
  • Murexide can be an irritant to the skin, eyes, and respiratory tract.
• Handling:
  • It's essential to handle this chemical with care, using appropriate protective equipment.
  • Avoid inhalation of dust.

Key Points:

• Ammonium purpurate (murexide) is a valuable indicator in complexometric titrations.
• Its ability to change color in the presence of calcium ions makes it particularly useful in water hardness determination.
• While not highly toxic, care should be taken when handling it.