What Is Oxalosis?

Subsequent precipitation (pastprecipitation) is also called post-precipitation, which refers to the phenomenon in which impurities ions are slowly precipitated onto the original precipitate after being precipitated by some groups in the solution. This phenomenon generally occurs in a supersaturated solution of the component. The longer the storage time is, the more impurities are introduced by the secondary precipitation, and the higher the temperature is, the more severe the secondary precipitation phenomenon is. Therefore, the main method to avoid or reduce the secondary precipitation is to shorten the coexistence time of the precipitate and the mother liquor. After the precipitation is stirred for a certain time, it is quickly filtered and separated.

In general, the amount of impurities introduced by secondary precipitation is

For example, when H ₂ S gas is passed into a 0.15 mol·L ^-1 HCl solution containing 0.01 mol·L ^-1 Zn ² +, ZnS is precipitated slowly due to the formation of a supersaturated solution. However, if Cu ²⁺ is added to the solution, CuS precipitates after passing H ₂ S. At this time, the amount of ZnS precipitates carried by the precipitate is not significant. If the precipitates are left for a period of time, the ZnS precipitates. Continuous precipitation on CuS precipitation surface.

The reason for the secondary precipitation phenomenon may be that the concentration of the precipitant on the surface of the precipitation is higher than that in the solution body due to surface adsorption; for the above example, it may also be that S ²⁺ is adsorbed on the surface, as the counter ion H ⁺ and the Zn in the solution ²⁺ ion exchange occurs, so that the ion product of the secondary precipitation component is much larger than the solubility product, and precipitates out. ^[1]

Co-precipitation is a phenomenon that causes some soluble substances to precipitate together when the precipitate is precipitated from the solution. For example, when barium chloride is used to precipitate barium sulfate, if K ⁺ and Fe ³⁺ are present in the solution, the potassium sulfate and iron sulfate that were originally soluble under the precipitation conditions will also be entrained by the barium sulfate precipitation as Impurities are mixed in the main precipitate.

The difference between co-precipitation and subsequent precipitation is as follows:

1. The amount of impurities introduced in the subsequent precipitation increases with the increase of the precipitation time in the test solution, while the coprecipitation is less affected by the time. Therefore, the main method to avoid or reduce the secondary precipitation is to shorten the time of coexistence of the precipitation and the mother liquor.

2. Whether impurities are added before precipitation or added after precipitation, the amount of impurities introduced after precipitation is basically the same.

3. As temperature rises, the phenomenon of secondary precipitation is sometimes more serious.

4. The degree of impurities introduced by subsequent precipitation is sometimes much more severe than co-precipitation. The amount of impurities introduced may be similar to the amount of the component being measured.

In the precipitation gravimetric method, the influence of co-precipitation and subsequent precipitation on the measurement results should be analyzed according to specific conditions.