"Cooling heat saturated solution crystallization method" is also called "cooling heat saturated solution method"“Cooling crystallization”。Cooling hot saturated solution is a method of separating or purifying solid substances according to the different solubility of substances at different temperatures (generally high solubility at high temperature and low solubility at low temperature).This method is mainly used for substances whose solubility decreases significantly with the decrease of temperature.Evaporate, concentrate, and then cool down to make S change greatly with T (such as potassium nitrate), evaporate, concentrate, cool, crystallize, filter, wash, and dry.[1]
It is a method for crystallization of saturated solution.Essentially, the solution is separated from the solution in the form of crystals by lowering the temperature (applicable to substances whose solubility increases significantly with the increase of temperature).Generally, it refers to the process of reducing the temperature of saturated solution at a relatively high temperature to precipitate crystals.
principle
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Temperature affects the solubility of substances. Generally, when the temperature rises, the solubility will rise (or when the temperature rises, the solubility will fall, for examplecalcium hydroxide、carbon dioxideEtc.).The solution is then saturated at high temperature.At this time, the content of dissolved matter in the unit solution is very high, and then it is cooled to reduce the solubility, so that the content of dissolved matter in the unit solution should be reduced, and the excess dissolved matter has to be separated from the solution in the form of solid (crystal).This is the cooling heat saturated solution crystallization method.It is called cooling heat saturated solution method for short.Soluble substances have different solubility at different temperatures.The solubility of most substances increases with the increase of temperature.The cooling heat saturated solution method is also applicable to such substances.
The method is: at a higher temperature, make the solution saturated, so that after the temperature decreases, because the solubility of the substance decreases, the solution will precipitate crystals of this substance.
The cooling heat saturated solution method is used for crystallizationsoluteSubstances whose solubility changes significantly with temperature（Evaporative solvent crystallization methodIt is used to crystallize substances whose solubility changes little or basically unchanged with the change of temperature).
When the solution is cooled, the temperature of the solution will gradually decrease. At this time, only the "material whose solubility changes greatly with temperature" will have a smaller solubility. At this time, it will crystallize. Of course, this also happens to the material whose solubility increases with temperature.On the contrary, it is a method for crystallization of substances whose solubility decreases with the increase of temperature.
In industry, this method is often compared withconcentrateCombined use: first concentrate the solution, then use the cooling heat saturated solution crystallization method to obtain the solute crystallization, and then centrifuge to obtain the solute.
matters needing attention
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1: Difference between cooling heat saturated solution crystallization method (cooling crystallization method for short) and evaporative crystallization method:
Cooling crystallization: heating before cooling, applicable to substances whose solubility increases with temperature and decreases with temperature.For example, KNO3. Evaporative crystallization: it means heating until crystals are precipitated, and it is used for substances whose solubility changes little with temperature.Such as NaCl.
2: The solute to which this method is applicable must meet the requirements of "solutesolubilityIt is proportional to the temperature.That is, the solubility of solute increases with the increase of temperature ".
When the temperature is high and the solution has reached saturation, when the solution is cooled, the temperature of the solution decreases, the solubility of the solute decreases, and the solute will grow crystals and crystallize.
In industry, this method is often used in combination with concentration. First concentrate the solution, then useCooling heat saturated solution crystallization methodThe solute is crystallized, and then centrifuged to obtain this solute.
It is widely used in industry, but an important condition must be considered in industrial application, that is, the solubility of solute must be very sensitive to temperature.That is, when the temperature drops, the solubility of the solute must have a relatively large change, so that a large number of crystals can be obtained and benefits can be generated.Otherwise, it cannot be applied.
Example: purification of crude salt
Step 1: Weigh a certain mass of crude salt with a tray balance
Step 2 Dissolve the weighed crude salt into a dry beaker, add water to dissolve it, add a little to dissolve it, and stir it with a glass rod
Step 3 filtration: fold the filter paper into four folds, stretch it out, and place it in the funnel. Fold it three times at one time and fold it one time at the same time. The edge of the filter paper must be lower than the edge of the funnel. Place the glass rod against the edge of the three layers of filter paper. Place the beaker against the glass rod, and pour the liquid slowly. Remember that the liquid height should not exceed the edge of the filter paper (Note: the funnel mouth is against the beaker wall)
Step 4 evaporation: pour the filtrate into the evaporating dish, light the alcohol lamp, wait until the liquid is slightly boiling, stir with a glass rod while heating to prevent the liquid from splashing, and stop heating when there is more solid precipitation in the evaporating dish.[2]
Other purification methods
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Recrystallization
Recrystallization is the process of dissolving crystals in solvent or melting and then recrystallizing from solution or melt.Recrystallization can purify impure substances or separate mixed salts from each other.
The separated solution with high solubility obtained by recrystallization may be impurities or solutes needed.It has a wide range of applications.The cooling heat saturation method can only use the property of low solubility at low temperature to obtain the desired solute.
extraction
Extraction device diagram
Extraction ---- using substances in two incompatible solventssolubilityDifferent from the separation method of extracting substances from one solvent to another to separate solute and solvent.[3]
distillation
Distillation is a thermodynamic separation process, which uses the components of mixed liquid or liquid-solid systemboiling pointDifferent, the unit operation of evaporating the low boiling component and then condensing to separate the whole componentprocess, is the combination of evaporation and condensation.Compared with other separation methods, such as extraction, filtration crystallization, etc., its advantage is that it does not need to use other than system componentssolventTo ensure that no newimpurity。
application
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It is widely used in industry, but an important condition must be considered in industrial application, that is, the solubility of solute must be very sensitive to temperature.That is, when the temperature drops, the solubility of the solute must have a relatively large change, so that a large number of crystals can be obtained and benefits can be generated.Otherwise, it cannot be applied.In order to obtain larger complete crystals in the laboratory, it is often used to slow down the temperatureCrystallization rateMethod.
