沉淀的类型及沉淀条件的选择（The type of precipitation and the selection of precipitation conditions）

沉淀的类型及沉淀条件的选择（The type of precipitation and the selection of precipitation conditions） 沉淀的类型及沉淀条件的选择(The type of precipitation and the selection of precipitation conditions) 5.3 the type of precipitation and the selection of precipitation conditions 5.3.1 type of precipitation According to the difference of particle size, the precipitation can be divided into two broad categories: one is crystalline precipitation; Another is amorphous precipitation, or amorphous precipitation or gelatinous precipitation. The grain diameter of crystalline precipitates is approximately 0.1-1mm, and the structure of the amorphous ion arrangement is compact, such as BaSO4. The diameter of amorphous precipitate particles is less than 0.02 mm, and the precipitation particles have no regular accumulation, and the precipitation is porous and water is large, such as Fe2O3 ? nH2O. Between the crystalline precipitate and amorphous precipitate, the coagulated emulsion precipitates, and its particle size is between the above, such as AgCl. Precipitation belongs to what type, it is determined by precipitation character, but precipitation condition also plays a big role. The concentration of particles in precipitation is related to the concentration of the crystalline ions. The formation of 5.3.2 precipitation In the process of precipitation, the crystallization of the crystalline ions is first formed in the supersaturated solution, and then the crystallization is further developed into a lattice arranged by a certain lattice. 1. Formation of crystal nucleus Nucleation of nucleation and heteromorphous nucleation are the two conditions of nucleation. (1) nucleation: the nucleus formed by the association of crystalline ions. The crystal nucleus of barium sulfate is Ba2 + and SO42 - association, forming BaSO4, (Ba2SO4) 2 + and [Ba (SO4) 2] 2 +, etc. These are the germ of the crystal. The basic condition of the nucleation is that the solution is in supersaturated state, that is, the concentration Q of the solution is greater than the solubility of the substance. (2) heterogeneous nucleation: the presence of fine particles in the solution, such as dust, impurity, etc., in the deposition process, they play the role of nucleation and induce precipitation formation. 2. Aggregation and orientation process After the formation of crystal nucleus, solution of crystal ion diffuses to the surface of the crystal nucleus, and deposition on the surface of crystal nucleus, the nucleus gradually grew up and became precipitation particles and precipitation particles can gather again to a larger aggregate, this process is called aggregation process. At the same time, the crystal ions are arranged in a lattice to form a crystal, which is called the orientation process. The sedimentation type is related to the speed of the aggregation process and the orientation process. If the aggregation speed is greater than the directional velocity, the crystals are not aligned and the crystals are stacked together, thus resulting in amorphous precipitation. If the orientation velocity is greater than the aggregation velocity, the crystal ions can be aligned to form a crystalline precipitate. The concentration velocity is mainly related to the relative degree of saturation of the solution, and the orientation velocity is mainly related to the properties of the sediment, such as the highly polar salt, which generally has a large orientation velocity. 3. Influence of oversaturation on crystal growth and crystal growth Predecessors have done a lot of research on the precipitation process, but there is still no mature theory. According to the experimental phenomenon, Von Weimarn combines the experience of precipitation dispersion with the relative degree of saturation of solution. That is, the dispersion is equal to In the formula, the solubility of precipitate concentration of precipitate after adding the precipitant, the solubility of s precipitation, is to precipitate the transient oversaturation of the precipitate, and the relative oversaturation of the beginning of precipitation is the constant. For each nucleation, the greater the degree of supersaturation, the more the nucleation number formed, the higher the dispersion degree; In the case of crystal growth, the greater the supersaturation, the faster the aggregation velocity, which is not conducive to the orientation of the amorphous ion, so it is not conducive to the growth of the crystal. In conclusion, the higher the relative saturation of the solution, the greater the dispersion degree of precipitation. The data of the right table shows the relationship between the saturation size of barium sulfate precipitation and the sedimentation form formed. 5.3.3 factors that affect precipitation purity Precipitation weight analysis not only requires the precipitation to be small but also pure. But when precipitation precipitation in the solution, more or less the other components of the mixed solution, deposition of defiled, influence the cause of the purity of precipitation can be roughly divided into coprecipitation and following the precipitation category. 1. The coprecipitation In the case of precipitation, some of the soluble impurities are precipitated, It's called co-precipitation. For example, BaSO4 is the precipitant, such as Fe3 + in the test solution, and the original soluble binder is deposited in the precipitate, which makes the precipitate burn and then mixed with yellowish brown. This is the coprecipitation phenomenon. There are three main reasons for coprecipitation: (1) surface adsorption co-precipitation The surface of precipitate adsorbs impurities to produce a coprecipitation, called surface adsorption co-precipitation. The reason for this phenomenon is due to the incomplete equilibrium of ion charge on the surface of the crystal. For example, in the sediment, each of the crystalline ions is surrounded by ions with opposite charges and is in static equilibrium. But on the surface of the precipitate, Ba2 + or at least one side of the ion is surrounded by ions that are uncharged, which has the ability to attract the opposite charge ions. The precipitate surface adsorbs an excess of the crystal ions in the solution to form the first adsorption layer. Such as BaCl2 solution gradually join the barium salt solution, precipitation first adsorption excess Ba2 + adsorption layer formation, precipitation positively charged at this time, to remain neutral, adsorption layer outside also negatively charged ions, such as Cl - (referred to as the counter ion), form the second adsorption layer, or layer diffusion, adsorption precipitation layer and diffusion layer on the surface of the electric double layer. The ions in the double layer can be deposited together with precipitation, thus contaminating the precipitation. Surface adsorption is selective, and the law of selecting adsorption is: (A) the first adsorption layer first adsorbs the crystallization ion. For example, BaSO4 precipitation is easy to adsorb Ba2 +. Secondly, the ions with the same charge are easy to be adsorbed, such as BaSO4 precipitation, which is easy to adsorb Pb2 +. (B) the secondary adsorption layer is selectivity: the higher the price, the easier it is to be adsorbed, such as Fe3 + than Fe2 +. Ions of a compound that is less soluble in solubility and less dissociation are also easily adsorbed. In addition, the larger the total surface area, the larger the adsorption quality. The amorphous precipitation is more adsorbed impurities than the crystal precipitate, and the small crystalline precipitates are more adsorbed impurities. The greater the concentration of impurities in solution, the larger the absorption. Adsorption is the exothermic process, so the adsorption of impurity decreases when the temperature increases. In the precipitation weight method, the adsorption impurity is reduced by the method of washing precipitation, so that the precipitation is pure. (2) mixed or solid solution Each crystal shape precipitation, have a certain crystal structure, if the impurity ions similar to compose crystal ion radius, shell structure is the same, and the formation of the crystal structure is the same, they can generate mixed crystal. Common crystals are BaSO4 and PbSO4, AgC1 and AgBr, MgNH4PO4, 6H2O, and MgNH4AsO4, 6H2O, etc. There are also some impurities with different crystal structures such as the NaCl and tetrahedral Ag2CrO4 crystal structure of the cube, which can also generate mixed crystals. This kind of mixed crystal shape is often incomplete, when put together with the solution, the impurity ions will be out gradually, slowly becoming complete crystal shape, the resulting precipitation is more pure. (3) the retention and the inclusion of the package Precipitation reaction occurs, the precipitation generated too quickly, the precipitation surface adsorption of impurities or too late to leave, mother liquor was covered by the generated precipitation and precipitation doubled-teamed on internal, this phenomenon is called the occluded and double coprecipitation. Occluded and double degree is also in line with the rules of adsorption, such as precipitation, BaSO4 Ba (NO3) 2 doubled-teamed quantity is greater than the BaCl2, because the former solubility is small and easy to adsorb, and double to precipitation, double crystal shape is the main cause of precipitation. Or to reduce or remove impurities by precipitating or recrystallization, as the contents of the package are inside the crystal, so the washing cannot be removed. 2. Subsequent precipitation Refers to some component in the solution after precipitation precipitation, another was difficult to precipitation precipitation component, or to form stable supersaturated solution and precipitation can't separate the material, on the surface of the precipitation and precipitation precipitation phenomenon, and the amount of precipitation increased with the extension of storage time. For example, In the acidic solution containing the Cu2 +, Zn2 + plasma, the initial CuS precipitation was not mixed with ZnS. But if precipitation and mother liquor contact over a long period of time, due to the CuS precipitation from a solution on the surface adsorption of S2 -, and make the deposit on the surface of S2 - concentration greatly increased, and the precipitation of CuS on the surface, lead to ZnS precipitate out administered. After the precipitation, the amount of impurity contamination is more than the co-precipitation, especially after long-term placement. The time to shorten precipitation and the co-location of the mother solution is the method to reduce the precipitation. After coprecipitation and precipitation is a negative factor, but sometimes also can be transformed into positive factors, for example, total precipitation separation is the phenomenon of using coprecipitation precipitate the enrichment of trace components in a solution. 5.3.4 the selection of precipitation conditions According to the type of precipitation, different precipitation conditions are applied to obtain the precipitation of pure and easy to wash. 1. Precipitation conditions of crystalline precipitates (1) precipitation should be carried out in an appropriate dilute solution and add dilute solution of precipitant. The precipitation in the dilute solution is to reduce the relative saturation of the solution and get larger particles precipitation. The concentration of impurities in dilute solution is relatively low, which can reduce the amount of coprecipitation. (2) continuously stir and slowly add the precipitant to prevent the concentration of the solution, so as not to generate a large number of crystal nuclei. (3) deposition, should be in the hot solution, the precipitation solubility increases slightly, so that we can reduce the relative degree of supersaturation of solution, to help generate little and big crystal particles, at the same time, also can reduce the adsorption effect of impurities. (4) after the precipitation has been completed, the precipitation and the mother liquid should be put together for a period of time, so that the precipitation crystal is complete and pure. This process is called aging. Because the small grain S is relatively large, the small crystal is dissolved and transferred to the big grain to deposit, which can make the crystal small grain become a big grain. Incomplete grain can become more complete grain; "Metastable" precipitates into a "steady state precipitation". Because the small grain is dissolved, the impurities of the adsorption and inclusion are reentered into the solution, so the quality can be reduced. However, the process of aging may not be effective for the co-precipitation of mixed crystals. The precipitate conditions of crystalline precipitates can be simply summarized as: "thin, hot, slow, stirring, Chen". 2. Precipitation conditions of amorphous precipitation Amorphous precipitation solubility is very small, can't by controlling the relative degree of supersaturation method to change the nature of the precipitation, precipitation and more loose, water cut, bulky, easy adsorption impurities, peptizing, even so when the precipitation mainly consider to accelerate precipitation particles condensed and reduce the adsorption of impurities, colloid, prevent the problem such as peptizing. (1) the precipitation should be carried out in the stronger solution, and the precipitant will be added quickly. After the precipitation has been completed, a large amount of hot water will be added to dilute and stir to transfer some of the absorbed impurities into solution. (2) the precipitation action should be carried out in a hot solution in order to obtain the precipitation with low water content and close structure, while heating it can promote the condensation of precipitate particles and prevent the formation of colloid. (3) appropriate electrolytes are added to prevent the formation of colloidal solutions. It is also necessary to add electrolytes in the washing solution to prevent the gelatine penetrating filter paper during washing and deposition, and usually the volatile ammonium salt electrolyte is used when burning. (4) don't age. Because aging does not improve precipitation shape, but makes precipitation more bond, impurities are difficult to wash away. (5) reprecipitation if necessary. Amorphous precipitation usually contains more impurities. If the accuracy requirement is higher, reprecipitation should be carried out. Uniform precipitation Homogeneous precipitation method is to control the certain conditions, to join the precipitant immediately and not be detect ion generated precipitation, but by a chemical reaction, make the precipitant from slowly and evenly produced in the solution, so that the precipitation in the whole solution precipitate slowly and evenly. So you can avoid the local overconcentration of the solution. The precipitate is a crystallization precipitation which is larger in particle size, less adsorption impurities and easy to filter and wash. For example, when determining Ca2 +, add the precipitation agent (NH4) 2C2O4 in neutral or alkaline solution to produce fine crystalline precipitate CaC2O4. If the solution is acidified and then added (NH4) 2C2O4, the oxalic acid in the solution is mainly in the form of HC2O4 - and H2C2O4, which does not produce CaC2O4 precipitation. When urea is added and boiled, the urea gradually hydrolyzes, producing NH3: CO (NH2) 2 + H2O = CO2 write + 2 nh3 The H + in the generated NH3 neutralizes the solution, making the acidity gradually decrease, and the concentration of C2O42 is gradually increasing. Finally, CaC2O4 precipitation is evenly and slowly precipitated out. The resulting CaC2O4 precipitation is a large crystalline precipitate.