Precipitation from Homogeneous Solution
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Questions and Answers

What is the purpose of dissolving polluted precipitate in a solvent?

To reduce inaccuracy by minimizing foreign material in the second precipitation, resulting in minimal precipitate entrainment.

What is the technique called when a precipitant is not added directly, but is slowly generated by a homogeneous chemical reaction within the solution?

Precipitation from Homogeneous Solution.

How can the physical appearance of a precipitate be altered?

By varying the chemical reaction rate, with slower reactions forming larger crystals.

Why is it crucial to regulate pH in precipitation reactions?

<p>To prevent excessive co-precipitation and ensure the formation of a compact, filterable precipitate.</p> Signup and view all the answers

What is the purpose of urea hydrolysis in precipitation reactions?

<p>To slowly generate a precipitant and regulate pH, allowing for the formation of a compact precipitate.</p> Signup and view all the answers

How do certain anions, such as succinate and sulphate, influence the physical characteristics of a precipitate?

<p>These anions can alter the size, shape, and properties of the resulting precipitate.</p> Signup and view all the answers

What is the primary role of the 'appropriate anion' in precipitation reactions?

<p>To produce a compact precipitate by creating a basic salt.</p> Signup and view all the answers

Why is it important to adapt the starting solution's pH in precipitation reactions?

<p>To ensure optimal precipitation conditions and prevent excessive co-precipitation.</p> Signup and view all the answers

What is the outcome of digestion on rapidly formed crystals?

<p>Rapidly formed crystals become more regular in character and dense, resulting in a decrease in surface area and adsorption.</p> Signup and view all the answers

Why is it important to precipitate in a diluted solution?

<p>To minimize CO-precipitation inaccuracies.</p> Signup and view all the answers

What is the effect of heating to slightly below boiling on crystallization?

<p>It increases solubility, causing a drop in supersaturation, coagulation, sol formation, and crystallization velocity, resulting in more perfect crystals.</p> Signup and view all the answers

Why should crystalline precipitates be broken down overnight?

<p>To minimize CO-precipitation impact and filter easier.</p> Signup and view all the answers

What is the purpose of washing the precipitate with a dilute solution of an electrolyte?

<p>To prevent peptization.</p> Signup and view all the answers

Under what condition is precipitation typically done, and why?

<p>Hot liquids, because solubility normally rises with temperature</p> Signup and view all the answers

What is the effect of slow mixing of reagents on crystal formation?

<p>It forms large crystals and maintains low supersaturation.</p> Signup and view all the answers

Why is it important to control supersaturation during precipitation?

<p>To prevent co-precipitation and promote crystal growth.</p> Signup and view all the answers

What is the purpose of adding a reagent gradually to a diluted solution during precipitation?

<p>To induce precipitation and serve as nuclei for further material to precipitate</p> Signup and view all the answers

What is the outcome of reducing the extent of CO-precipitation?

<p>Easier filtration.</p> Signup and view all the answers

How does precipitation from homogeneous solution help to control supersaturation?

<p>By creating the precipitating agent in the solution through a homogeneous reaction, preventing supersaturation</p> Signup and view all the answers

What is CO-precipitation, and what is an example of it?

<p>Contamination of the precipitate by substances normally soluble in the mother liquor, such as adsorption at the surface of particles</p> Signup and view all the answers

How does the nature of the precipitate affect its tendency to exhibit adsorption?

<p>Gelatinous precipitates exhibit high levels of adsorption, while macrocrystalline precipitates exhibit low levels</p> Signup and view all the answers

According to the Fajans-Hahn adsorption rule, which ion is most firmly adsorbed by an ionic material?

<p>The ion that forms the least soluble salt</p> Signup and view all the answers

What is an example of the second type of CO-precipitation?

<p>Occlusion of foreign substances during crystal growth from primary particles</p> Signup and view all the answers

What is the purpose of thorough stirring during precipitation?

<p>To ensure uniform distribution of the reagent and induce precipitation</p> Signup and view all the answers

Study Notes

Precipitation Reaction

  • Aims to separate a solid phase into a compact, filterable form through gradual addition of a diluted precipitating agent solution.
  • Minimal precipitate entrainment is achieved by reducing concentration effects.

Precipitation from Homogeneous Solution

  • Precipitant is slowly generated by a homogeneous chemical reaction within the solution.
  • Conditions eliminate concentration effects, making the precipitate dense and filterable.
  • CO-precipitation is minimized.
  • Physical appearance of the precipitate can be altered by varying the chemical reaction rate.
  • Slower reactions form larger crystals.

pH Regulation

  • pH must be carefully regulated to achieve compact precipitation.
  • Urea hydrolysis is used to regulate pH, breaking down into carbon dioxide and ammonia.
  • Urea is easily regulated and soluble in water, with minimal basic characteristics.

Anions in Precipitation

  • The presence of certain anions (e.g. succinate, sulphate, formate, oxalate, benzoate, chloride, chlorate, perchlorate, nitrate, sulphate, and chromate) significantly influences the physical characteristics of the precipitate.
  • Succinate is the favored anion in the precipitation process for metal aluminum.

Precipitation Conditions

  • Precipitation is typically done in hot liquids.
  • A diluted solution is used to induce precipitation, and the reagent is added gradually while being thoroughly stirred.
  • Initial particles to precipitate serve as nuclei and increase as more material precipitates.

Purity of the Precipitate: CO-Precipitation

  • A precipitate that separates from a solution is not always pure and may contain contaminants.
  • CO-precipitation is the contamination of the precipitate by substances normally soluble in the mother liquor.
  • Two types of CO-precipitation: adsorption at the surface of the particles and occlusion of foreign substances during crystal growth.

CO-Precipitation Reduction

  • Digestion can reduce CO-precipitation by making particles more regular and dense, resulting in a decrease in surface area and adsorption.
  • Rapidly formed crystals have irregular shape and large surface area, but upon digestion, they become more regular and dense, reducing CO-precipitation.

Conditions of Precipitation

  • Precipitation should be done in a diluted solution to minimize CO-precipitation inaccuracies.
  • Slowly mix reagents while stirring to form large crystals and maintain low supersaturation.
  • Excess may be needed in rare cases, and mix sequence may affect precipitation.
  • Precipitation can occur in heated liquids if solubility and stability allow.
  • Heating to slightly below boiling increases solubility, causing a drop in supersaturation, coagulation, sol formation, and crystallization velocity.
  • Crystalline precipitates should be broken down overnight, preferably on a steam bath, to minimize CO-precipitation impact and filter easier.
  • Digestive treatment is preferred for amorphous or gelatinous precipitates.
  • The precipitate should be washed with the appropriate dilute solution of an electrolyte to prevent peptization.

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Description

Learn about precipitation from homogeneous solution, a technique to separate a solid phase into a compact, filterable form through the gradual addition of a diluted precipitating agent solution. This method reduces inaccuracy by minimizing foreign material in the second precipitation.

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