Solubility curve

Solubility curve of the same substance at different temperatures

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The solubility curve is the curve drawn by the solubility of the same substance at different temperatures. Because the solubility of solid substances changes with temperature, and it is certain with temperature, this change can be expressed by the solubility curve. We use the ordinate to represent the solubility, and the abscissa to represent the temperature, and draw the curve of the solubility of solid matter changing with temperature. This curve is called the solubility curve. The solubility curve generally increases with the increase of temperature, but a few substances will decrease with the increase of temperature.

Chinese name: Solubility curve
Foreign name: Solubility curve
Variable: temperature

Discipline: Chemistry
Substantive: Gas-liquid equilibrium curve
Definition: The curve drawn by the solubility of the same substance at different temperatures

catalog

five Extraction equilibrium
six application

essential information

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The relationship curve of gas-liquid phase equilibrium drawn with two-dimensional coordinates is called solubility curve.

Fig. 1 Curve (1 sheet)

Any point on the solubility curve indicates the composition of gas and liquid in equilibrium, indicating that to make a gas reach a certain concentration in the solution, a certain equilibrium partial pressure of the gas must be maintained above the liquid level. For the same system, at the same temperature, the gas solubility increases with the increase of the partial pressure of the component in the gas phase; At the same equilibrium partial pressure, the gas solubility decreases with the increase of temperature.

At the same temperature and the same partial pressure, the equilibrium compositions of different gases in the same solvent are very different. At a certain temperature, the gas reaches a certain composition in the solution, and the dissolved gas presents a certain partial pressure. From this point of view, it can be seen that there are three kinds of gases: easily soluble gas (ammonia), moderately soluble gas (sulfur dioxide), and slightly soluble gas (oxygen). When the slightly soluble gas contacts with the liquid, the partial pressure above the liquid surface is larger, while the partial pressure above the liquid surface of the easily soluble gas is smaller. According to the change rule of solubility curve, it can be seen that pressurization and cooling are conducive to the absorption process; On the contrary, temperature rise or pressure reduction is conducive to the desorption process.^[1]

significance

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1. Represents the solubility of the same substance at different temperatures or the solubility changes with temperature;

2. It refers to the solubility of different substances at the same temperature. You can compare the solubility of different substances at the same temperature. If the solubility curves of two substances intersect, the solubility of two substances is equal at this temperature;

3. According to the solubility curve, the method of separating and purifying crystals from saturated solution or mixture can be determined;

4. Relevant calculations can be carried out according to the solubility curve.

Meaning of points on the curve

Solubility curve

1. The point on the solubility curve represents the solubility of the substance at the temperature shown at this point, and the state of the solution is Saturated solution 。 The point under the solubility curve represents the solubility of the substance at the temperature shown at this point, and the state of the solution is Unsaturated solution 。

2. The point on the area below the solubility curve indicates that the solution is in an unsaturated state, and the solution prepared according to its data is an unsaturated solution at the corresponding temperature.

3. For the point on the area above the solubility curve, the solution prepared according to its data is the saturated solution at the corresponding temperature, and the solute is residual.

4. The intersection of two solubility curves indicates that the solubility of two substances is equal at the temperature shown at this point.

Meaning of curve curve

The solubility curve shows the solubility of a substance at different temperatures or the solubility changes with temperature. The greater the slope of the curve, the greater the influence of temperature on solubility; On the contrary, it indicates that the temperature is less affected. The solubility curve also has three applications: 1. According to the solubility curve, it can be seen that the solubility of substances changes with temperature. 2. According to the solubility curve, compare the solubility of substances within a certain temperature range. 3. According to the solubility curve, select the method of separating some soluble mixtures.

Meaning above the curve

For any point on the lower area of the curve, the solution prepared according to its data is the unsaturated solution at the corresponding temperature; At the point on the upper area of the curve, the solution prepared according to its data is the saturated solution at the corresponding temperature, and there is residual solute. If the unsaturated solution (a point at the bottom of the curve) is to be changed into a saturated solution at the corresponding temperature, there are two methods: the first method is to add solute to the solution to make it reach the curve; The second method is to evaporate a certain amount of solvent.

Meaning of curve intersection

The intersection of the two solubility curves indicates that the solubility of the two substances is the same at the temperature shown at this point, and the solute mass fraction of the saturated solution of the two substances is also the same.

change rule

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1. The solubility of most solid substances increases with temperature, and the curve is "steep rise", such as potassium nitrate 。

2. The solubility of a few solid substances is little affected by temperature, and the curve is "slowly rising", such as sodium chloride 。

3. The solubility of a few solid substances decreases with the increase of temperature, and the curve is "descending", such as calcium hydroxide 。

4. The solubility of gaseous substances decreases with the increase of temperature (the ordinate represents the volume), and the curve is also "descending", such as oxygen 。

Determination of curve

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When the solute atoms dissolve into the solvent to form a solid solution, the intrinsic lattice of the original solvent remains, but the lattice parameters increase (or decrease) with the increase of the solution mass.

Take a series of alloys with known compositions with different contents of A and B, heat them until B is completely dissolved in A to form the same solution, quench them to room temperature, and accurately measure the lattice parameter a of each alloy. Draw the relationship curve between the amount of solution and the lattice parameter a. When the amount of solution is small, the result is a straight line.

If the relationship curve is made in advance, the solute content can be obtained according to the lattice parameters of a solid solution measured.

In order to measure the solubility curve, take the alloy with sufficient B content t one The lattice parameters of the solid solution can be accurately measured by holding for a long time a one , make in the established diagram a one The horizontal line intersects the straight line at a point, and the component corresponding to the point is the component of the solid solution, that is, at the temperature of t one Lower the solubility of B in A. It can also be measured t two Solubility at other temperatures. The solubility curve of solid solution can be obtained by connecting the solubility at various temperatures. This method is called external standard method.^[2]

Extraction equilibrium

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For the extraction system with complete mutual solubility of A and B, S, and partial mutual solubility of B and S, the equilibrium relationship can be determined by experiment. The specific steps are as follows: under the condition of constant pressure and temperature, mix B and S in proper proportion, and the total composition is represented by point M. After full contact and standing, two mutually balanced liquid phases are obtained,

Solubility curve and connecting line

Its composition is as shown in the figure E zero Points and R zero Point. These two mutually balanced liquid phases are called Conjugate phase Its corresponding composition is called conjugate composition. Add a small amount of A into the mixed solution and mix it fully to reach a new balance. After standing, a pair of conjugate phases are obtained by layering. The composition point is E one and R one 。 Then continue to add solute A, repeat the above operation, and several pairs of conjugate phase composition points can be obtained E i and R i , until the amount of A added makes the mixed liquid just change from two phases to one phase, and its composition point is P express. Add A again, and the mixed liquid will remain in a single liquid phase state. P The point is called critical miscibility point. The solubility curve of the ternary system at the experimental temperature can be obtained by connecting the points representing the composition of the equilibrium liquid phase.

The solubility curve divides the ternary system into two regions: the region bounded by the solubility curve and the bottom edge is a two-phase region, and the region outside the solubility curve is a homogeneous region. In the two-phase region, mutually balanced extraction phases E And raffinate phase R It is called conjugate phase, which is the limit composition of extraction separation.

Obviously, the extraction and separation should be controlled in the two-phase zone. Due to insufficient contact in the actual extraction process, the composition of the extraction phase and the raffinate phase is lower than the conjugate composition, but its relative amount conforms to the lever rule.

At the same temperature, the smaller the mutual solubility of extractant and component B, the larger the area of the two-phase region and the larger the composition of the extracted phase; vice versa. Therefore, extractants with smaller mutual solubility with component B should be selected as far as possible to facilitate the separation of solute A.

If component B and component S are completely immiscible, point R zero And E zero Coincident with triangle fixed point B and fixed point S respectively.^[3]

application

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1. Find the solubility of the substance at the specified temperature, and judge the solubility according to the solubility.

2. Compare the solubility of different substances at the same temperature (or within a certain temperature range).

3. Compare and determine the extent to which the solubility of the substance is affected by temperature, and determine the method of substance crystallization or mixture separation and purification accordingly.

4. Determine the state of the solution (saturated and unsaturated).

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