Aging refers to keeping the solution still under certain conditions for a period of time after the precipitation is complete.The purpose is to make the components inside fully react or make the suspended solids settle.For example, wine making is different from aged wine because after a long time, a small amount of higher alcohols react with esters to form new onesFatty acid esterDifferent fatty acid esters have different specific smells.Another example is perfume. After being placed for a period of time, transesterification reaction will take place inside, forming new fatty acid esters, making the smell more mellow and fragrant.
Chinese name
Aging
Foreign name
Ageing effect
Alias
ageing、Ripening
Meaning
Let the precipitate be placed together with the mother liquor for a period of time
After the precipitation is complete, let the initially generated precipitation and mother liquor be placed together for a period of time. This process is called "aging", and its purpose is to:
1. Remove the impurities contained in the sediment.
2. Let settlecrystal growthIncrease the crystal particle size and make its particle size distribution more uniform.[1]
Grain growth
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Recondensation
The solubility of small particles of the same substance is greater than that of large particles, and the dissolution of small particles promotes the growth of large particles.
According to the relevant form of Kelvin formula, In (c/c *)=2 σ M/prRT, the finer the particles, the greater the solubility.When large particles and fine particles exist at the same time, if the large particles are saturated, the small particles must be unsaturated. As a result, the dissolution of small particles increases the concentration of solution.As the concentration of solution exceeds the saturation concentration of large particle sediment, solute can precipitate on the surface of large particles, so that large particles continue to grow.
This phenomenon of material transfer from small particles to the surface of large particles through dissolution and re precipitation to make the precipitated particles grow is called re condensation, also known as Ostwald aging.Ostwald aging also occurs in the same crystal.Due to the difference in particle size, Ostwald ripening is induced.The particles with smaller particle size have greater solubility, and after dissolution, they are deposited on the particles with larger particle size, and the total number of particles decreases.Recoagulation or Ostwald aging occurs not only after the precipitation process, but also during the precipitation process.It promotes the growth of sediment particles, so it is conducive to the formation of large particle sediment.[2]
Coalescence
It refers to particles contacting each other, merging and growing up.For example, when drying wet materials, materials are transferred from the convex surface of spherical particles to the concave meniscus formed by particle contact, and finally large particles are formed.The particle growth rate depends on the solubility. When there are impurities in the solution, the solubility increases, and it is easy to get large particles.
Perfection and transformation of crystal form
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Example: In preparation and reformingcatalyzerCarrier adoptionAluminium trichlorideProduction of Al2O3 and Al2O3 mixed crystal phase Al (OH) with ammoniathreeThe X-ray diffraction pattern of the newly formed sediment does not show obvious peaks, and only after aging for 14 h can we get the fully crystallized Paixite and Nowoxite, as shown in Figure 1:
Figure 1 Aging Process
That is, the primary precipitation does not necessarily have a stable structure. For example, calcium oxalate precipitates at room temperature to obtain CaCtwoOfour·2HtwoO and CaCtwoOfour·3HtwoThe mixed precipitates of O, which are placed together with mother liquor at high temperature, will become stable CaC2 04 - H2O.
It is also possible for some fresh amorphous or colloidal precipitates to gradually transform and crystallize during aging.E.g. molecular sieveHydrated aluminaThe aging of etc. is the most typical example of this transformation.imageAluminium hydroxide、Ferric hydroxideThis kind of polycrystalline precipitates often use different aging conditions to obtain materials with different crystal forms.For example:
Aging process
In this way, aging time, temperature and pH value of mother liquor will become several factors that should be considered for aging.In addition, the pore structure and surface area of the sediment will also change during aging.
Related extensions
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Grain aging
Physiological changes: the physiological changes of grain aging, whether with or without embryo, are mainly manifested in the changes of enzyme activity and metabolic level.During grain storage, physiological changes are mostly carried out under the action of various enzymes.If the activity of enzymes in grain is weakened or lost, its physiological function will also be weakened or stopped.With the aging process, the viability of grain gradually loses, and the enzymes related to respiration, such ascatalaseIts activity tends to decrease, so does its respiratory function;The activities of hydrolases such as phytase, protease and phospholipase increased.
1. Changes in chemical composition: the changes in chemical composition of grains, whether with or without embryos, are generally characterized by rapid changes in fat, followed by protein, and weak changes in starch.
2. Change of fat: During grain storage, fat is easy to hydrolyze,free fatty acidsIt first appeared in grain.Especially when the environmental conditions are suitable, the stored grain mold begins to reproduce, secretes lipase, participates in fat hydrolysis, increases the fatty acid in the grain, and deepens the grain aging.
3. Changes in protein: during grain storage, protein hydrolysis and denaturation are affected by external physical, biological and other factors.After protein hydrolysis, free amino acids and acidity increased.After protein denaturation, the spatial structure is loose, peptide bonds are extended, non-polar groups are exposed, hydrophilic groups are stored inside, protein changes from sol to gel, solubility is reduced, and grain aging is deepened.
4. Change of starch: during grain storage, maltose and dextrin hydrolyzed from starch continue to hydrolyze, reducing sugar increases, dextrin decreases relatively, viscosity decreases, and grain begins to age.
5. Changes in physical properties: physical properties change greatly when grain is aged.
It is manifested as: grain tissue hardens, flexibility and toughness become weak, rice quality becomes brittle, rice grains stiffen, body bone shrinks, starch cells become hard, cell membrane permeability increases, gelatinization and water absorption rate decrease, water retention rate also decreases, rice is broken, viscosity is poor, and taste is "old".
