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Precipitation and Co-precipitation01:17

Precipitation and Co-precipitation

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Precipitation and coprecipitation methods can be used to separate a mixture of ions in a solution. In qualitative inorganic analysis, ions that form sparingly soluble precipitates with the same reagent are separated based on the differences in solubility products. For example, consider the separation of Cu(II) and Fe(II) ions by precipitation as insoluble sulfides. First, copper(II) sulfide is precipitated by the addition of acidic H2S, where the dissociation of H2S is suppressed. Adding H2S...
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Precipitation Processes01:12

Precipitation Processes

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The experimental conditions in a gravimetric analysis should be optimized to maximize the particle size and purity of the obtained precipitate. Ideally, the concentration of the precipitating reagent should be low with effective stirring to maintain low relative supersaturation for the growth of large crystals. In homogeneous precipitation, the precipitant is slowly generated by a chemical reaction in the solution to avoid local reagent excesses. For example, urea decomposes gradually to...
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Washing, Drying, and Ignition of Precipitates00:52

Washing, Drying, and Ignition of Precipitates

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After filtration, the precipitate is washed to remove coprecipitated impurities and any remaining mother liquor. Colloidal precipitates, such as silver chloride, are washed with an electrolyte (such as dilute nitric acid) to prevent the peptization of the precipitate. In the case of slightly soluble precipitates, the wash solution contains a common ion to reduce solubility. Lead sulfate, which is slightly soluble in water, is washed with dilute sulfuric acid. Similarly, wash solutions may be...
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Precipitate Formation and Particle Size Control01:16

Precipitate Formation and Particle Size Control

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In precipitation gravimetry, the precipitating agent should react specifically or selectively with the analyte. While a specific reagent reacts with the analyte alone, a selective reagent can react with a limited number of chemical species.
The obtained precipitate should be either a pure substance of known composition or easily converted to one by a simple process, such as ignition or drying. In addition, the precipitate should be insoluble and easily filterable. In general, filterability...
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Precipitation Reactions03:10

Precipitation Reactions

66.0K
In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound – the precipitate. The reaction occurs when oppositely charged ions in solution overcome their attraction for water and bind to each other, forming a precipitate that separates out from the solution. Since such reactions involve the exchange of ions between ionic compounds in aqueous solution, they are also referred to as double displacement, double replacement, exchange reactions, or...
66.0K
Colloidal precipitates01:09

Colloidal precipitates

6.6K
The high insolubility of some precipitates can result in an unfavorable relative supersaturation. This can lead to colloidal particles with a large surface-to-mass ratio, where adsorption is promoted. For instance, in the precipitation of silver chloride, silver ions are adsorbed on the surface of the colloidal particles, forming a primary layer. This layer attracts ions of opposite charge (such as nitrate ions), forming a diffuse secondary layer of adsorbed ions. This electric double layer...
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Transformation Cascades in Iron Oxides: Quantitative Resolution of Sequential Precipitation Using the

Nour Abi Aad1, Mazen Al-Ghoul1

  • 1Department of Chemistry, American University of Beirut, Riad El-Solh 1107 2020, Beirut, Lebanon.

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This study reveals the sequential formation of iron oxides (goethite, green rust, magnetite) through diffusion-controlled reactions in hydrogels. These findings clarify mineral formation pathways obscured by traditional methods.

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Stable Aqueous Suspensions of Manganese Ferrite Clusters with Tunable Nanoscale Dimension and Composition
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Area of Science:

  • Materials Science
  • Geochemistry
  • Chemical Engineering

Background:

  • Mineral formation and corrosion are governed by sequential phase transformations under transport limitations.
  • Traditional methods like equilibrium phase diagrams and well-mixed experiments often fail to capture transient intermediates, spatial segregation, and kinetic hierarchies.
  • Understanding these complex processes is crucial for fields ranging from materials synthesis to environmental remediation.

Purpose of the Study:

  • To resolve the transformation cascade of iron oxides using a precipitation-diffusion system in agar hydrogels.
  • To quantitatively track the kinetics of spatially separated reaction fronts.
  • To establish a link between front kinetics, hydroxide consumption, and the observed phase transformation sequence.

Main Methods:

  • Utilized precipitation-diffusion experiments in 1.0 wt% agar hydrogels.
  • Introduced hydroxide (1.0-3.0 M NaOH) into Fe2+/Fe3+-loaded gels to induce iron oxide formation.
  • Employed quantitative tracking of reaction front positions and analyzed kinetics using power-law models.
  • Applied Stefan moving-boundary analysis to correlate front propagation with hydroxide consumption.
  • Characterized mineral products using microscopy and spectroscopy.

Main Results:

  • Observed three distinct, spatially separated reaction fronts: goethite (α-FeOOH), green rust (Fe2+-Fe3+ LDH), and magnetite (Fe3O4).
  • Front positions followed power-law kinetics (d(t) = αtβ) with high goodness-of-fit (R2 ≥ 0.97).
  • Determined an alkalinity-demand hierarchy (ΛG/ΛGR/ΛM ≈ 1:1.3:1.9) that explains the transformation sequence and goethite region widening.
  • Front propagation (3-12 mm after 96 h) was accelerated by increasing external hydroxide concentration and slowed by increased iron loading.
  • Identified a solution-mediated dissolution-reprecipitation pathway and observed a transition to Liesegang banding under Fe2+-rich conditions.

Conclusions:

  • The study elucidates the kinetic hierarchy and spatial segregation governing iron oxide phase transformations under diffusion-limited conditions.
  • The findings demonstrate that diffusion-reaction dynamics control both the cascade formation and potential for periodic precipitation (Liesegang banding).
  • This research provides a more nuanced understanding of mineral synthesis and transformation processes compared to equilibrium-based models.