24 Questions
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?
To prevent excessive co-precipitation and ensure the formation of a compact, filterable precipitate.
What is the purpose of urea hydrolysis in precipitation reactions?
To slowly generate a precipitant and regulate pH, allowing for the formation of a compact precipitate.
How do certain anions, such as succinate and sulphate, influence the physical characteristics of a precipitate?
These anions can alter the size, shape, and properties of the resulting precipitate.
What is the primary role of the 'appropriate anion' in precipitation reactions?
To produce a compact precipitate by creating a basic salt.
Why is it important to adapt the starting solution's pH in precipitation reactions?
To ensure optimal precipitation conditions and prevent excessive co-precipitation.
What is the outcome of digestion on rapidly formed crystals?
Rapidly formed crystals become more regular in character and dense, resulting in a decrease in surface area and adsorption.
Why is it important to precipitate in a diluted solution?
To minimize CO-precipitation inaccuracies.
What is the effect of heating to slightly below boiling on crystallization?
It increases solubility, causing a drop in supersaturation, coagulation, sol formation, and crystallization velocity, resulting in more perfect crystals.
Why should crystalline precipitates be broken down overnight?
To minimize CO-precipitation impact and filter easier.
What is the purpose of washing the precipitate with a dilute solution of an electrolyte?
To prevent peptization.
Under what condition is precipitation typically done, and why?
Hot liquids, because solubility normally rises with temperature
What is the effect of slow mixing of reagents on crystal formation?
It forms large crystals and maintains low supersaturation.
Why is it important to control supersaturation during precipitation?
To prevent co-precipitation and promote crystal growth.
What is the purpose of adding a reagent gradually to a diluted solution during precipitation?
To induce precipitation and serve as nuclei for further material to precipitate
What is the outcome of reducing the extent of CO-precipitation?
Easier filtration.
How does precipitation from homogeneous solution help to control supersaturation?
By creating the precipitating agent in the solution through a homogeneous reaction, preventing supersaturation
What is CO-precipitation, and what is an example of it?
Contamination of the precipitate by substances normally soluble in the mother liquor, such as adsorption at the surface of particles
How does the nature of the precipitate affect its tendency to exhibit adsorption?
Gelatinous precipitates exhibit high levels of adsorption, while macrocrystalline precipitates exhibit low levels
According to the Fajans-Hahn adsorption rule, which ion is most firmly adsorbed by an ionic material?
The ion that forms the least soluble salt
What is an example of the second type of CO-precipitation?
Occlusion of foreign substances during crystal growth from primary particles
What is the purpose of thorough stirring during precipitation?
To ensure uniform distribution of the reagent and induce precipitation
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.
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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