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precipitate

The process of separating solid matter from solution

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Sediment, at Chemistry It refers to the process of separating solid matter from solution; It also refers to the solid matter precipitated in the precipitation process. In fact, most of the precipitates are insoluble (solubility<0.01g at 20 ° C). Precipitation is widely used in chemical experiments and production to separate substances.

Chinese name: precipitate
Foreign name: precipitation
Pinyin: chén diàn wù

Interpretation: Hard to dissolve solid matter released from solution
Reaction: chemical reaction
Principle: Precipitated from supersaturated solution Insoluble substance

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three reaction
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identification

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Generally, precipitation will be marked with "↓" in the chemical reaction equation, such as:

Ca(OH) two + CO two → CaCO three ↓+ H two O

The process of producing separable solids from the liquid phase.^[1]

Scientific terminology

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principle

The process of producing a separable solid phase from the liquid phase or separating from the supersaturated solution Insoluble substance 。 Precipitation indicates the formation process of a new condensed phase, or due to the addition of Precipitant The process of depositing some ions into insoluble compounds. The chemical reaction that produces precipitation is called Precipitation reaction 。 The precipitation and dissolution of substances is an equilibrium process, usually using Solubility product constant Ksp to judge whether the insoluble salt is precipitated or dissolved. The solubility product constant refers to the value of Saturated solution The product of the concentration of the ions that make up the precipitation is a constant. This relationship is often used in analytical chemistry to reduce the precipitation solubility by adding the same ions, so that the measured components remaining in the solution can be ignored.^[1]

classification

Precipitation can be divided into Crystalline precipitation And amorphous precipitation. barium sulfate Is a typical crystalline precipitation, Fe two O three ·nH two O is a typical amorphous precipitate. The internal arrangement of crystalline precipitation is regular, the structure is compact, the particles are large, and it is easy to settlement And filtration; Amorphous precipitation particles are very small, without obvious lattice, disorderly arrangement, loose structure, large size, easy to absorb impurities, difficult to filter, and difficult to clean.

Preparation

Experiments have proved that the type of precipitation and particle size depend on both the nature of the substance and the conditions of precipitation. In actual work, different precipitation conditions must be selected according to different precipitation types to obtain satisfactory precipitation. For crystalline precipitation, dilute precipitant shall be added slowly in hot dilute solution under stirring for precipitation. After precipitation, the sediment will be mixed with mother liquor Place them together“ Aging ”To make incomplete grain The transformation becomes more complete, and small grains are transformed into large grains. For amorphous precipitation Concentrated solution And add a large amount of electrolyte to accelerate the precipitation particulate Coagulation to prevent formation Colloidal solution 。 After sedimentation, filter immediately without aging.^[1]

effect

In classical qualitative analysis, almost half of the detected reactions are precipitation reactions. stay quantitative analysis Medium, it is Gravimetric method and Precipitation titration The foundation of. Precipitation reaction is also a common separation method, which can not only separate the components to be measured, but also remove other coexisting interfering components.

type

Silver chloride precipitation;

Cu(OH) two Blue flocculent precipitation;

BaSO four White precipitate;

Mg(OH) two White precipitate;

Al(OH) three White precipitate;

BaCO three White precipitate;

CuO black precipitate;

Cu two O Red precipitate;

Cu two (OH) two CO three Dark green precipitate;

CaCO three White precipitate;

Fe(OH) three Reddish brown precipitate;

Fe(OH) two It is white flocculent precipitation (quickly turns grayish green in the air, and then becomes Fe (OH) three Reddish brown precipitate);

Fe two O three Reddish brown precipitate;

FeO Black precipitate;

FeCO three Gray precipitation;

FeS two Yellow precipitate;

AgBr Yellowish precipitate;

Ag two CO three Yellow precipitate;

AgBr light yellow sediment;

AgCl White precipitate;

Ag three PO four Yellow precipitate;

PbS Black precipitate.^[1]

PH value

Metal Ions	PH value
Metal Ions	Start sedimentation	Complete precipitation
Fe3+	two point seven	three point seven
Al3+	three point seven	four point seven
Cu2+	four point four	six point four
Zn2+	six	eight
Co2+	six point seven	eight point seven
Fe2+	seven point six	nine point six
Mn2+	seven point eight	nine point eight
Mg2+	nine point three	ten point eight

reaction

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CuSO four +2NaOH=Cu(OH) two ↓+Na two SO four Blue precipitation is generated, and the upper part is clarify solution Law of conservation of mass experiment;

Ca(OH) two +CO two = CaCO three ↓+ H two O Clarified lime water change muddy Application CO two Inspection and Lime slurry Paint the wall;

HCl+AgNO three = AgCl↓+HNO three Produce white precipitate, insoluble in Dilute nitric acid Test Cl - Principle of;

Ba(OH) two + H two SO four =BaSO four ↓+2H two O generates white precipitate and does not dissolve in dilute nitric acid to test SO four 2- Principle of;

BaCl two + H two SO four =BaSO four ↓+2HCl generates white precipitate and does not dissolve in dilute nitric acid to test SO four 2- Principle of;

Ba(NO three ) two +H two SO four =BaSO four ↓+2HNO three Produce white precipitate and do not dissolve in dilute nitric acid to test SO four 2- Principle of;

FeCl three +3NaOH=Fe(OH) three ↓+3NaCl solution yellow fades and reddish brown precipitates are formed;

AlCl three +3NaOH=Al(OH) three ↓+3NaCl has white precipitation;

MgCl two +2NaOH = Mg(OH) two ↓+2NaCl has white precipitation;

CuCl two +2NaOH = Cu(OH) two ↓+2NaCl solution blue fades and blue precipitates are formed;

CaO+ H two O = Ca(OH) two White lumpy solid becomes powder quick lime Preparation of lime slurry;

Ca(OH) two +SO two =CaSO three ↓+ H two O There is white precipitate generated, which is generally not used in junior high school;

Ca(OH) two +Na two CO three =CaCO three ↓+2NaOH has white precipitate to generate industrial production caustic soda . The laboratory makes a small amount of caustic soda;

Ba(OH) two +Na two CO three =BaCO three ↓+2NaOH has white precipitation;

Ca(OH) two +K two CO three =CaCO three ↓+2KOH has white precipitation;

AgNO three +NaCl = AgCl↓+NaNO three White insoluble in Dilute nitric acid Sedimentation of (Others chloride Similar reaction) applied in the test solution Chloride ion;

BaCl two + Na two SO four = BaSO four ↓+2NaCl white precipitate insoluble in dilute nitric acid (others sulfate Similar reaction) shall be used to test sulfate ion;

CaCl two +Na two CO three = CaCO three ↓+2NaCl has white precipitation;

MgCl two +Ba(OH) two =BaCl two +Mg(OH) two ↓ There are white precipitates.^[1]

type

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According to the concentration, nature and flocculation Due to different properties, precipitation can be divided into the following types.

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one Free precipitation 。 suspended particles The concentration of is low, it is discrete in the precipitation process, does not adhere to each other, does not change the shape, size and density of particles, and completes independent precipitation process. This type is mainly manifested in Grit chamber 、 Primary sedimentation tank Initial stage.

2. Flocculation and sedimentation. The concentration of suspended particles is relatively high (50~500mg/L), which can agglomerate or Flocculation , so that suspended particles collide with each other and condense, and the particle mass gradually increases, Settling velocity Gradually accelerate. Sedimentation of particles in coagulated water, later stage of primary sedimentation tank Biofilm method Secondary sedimentation tank Activated sludge process Secondary sedimentation tank at the initial stage belongs to flocculent settling 。

3. Congestion and sedimentation. The concentration of suspended particles is very high (more than 500mg/L) settlement During the process, particles interfere with each other, forming an obvious interface (mixed liquid surface) between clear water and muddy water, and gradually moving downward, so it is also called Stratified precipitation 。 Later stage of activated sludge secondary sedimentation tank Concentration tank The upper part belongs to this sedimentation type.

4. Compression sedimentation. suspended particles The concentration is extremely high (so that it is no longer called the concentration of particles in water, but the water content in solids). During the sedimentation process, particles contact each other, and the lower layer particles are compressed by gravity, so that the liquid in the lower layer particle gap is squeezed out of the interface and flows upward, and the solid particle group is concentrated. The sludge thickening process in the sludge bucket and thickener of the activated sludge method secondary sedimentation tank belongs to this type.^[1]

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