Laboratory chemical analysis instrument operation skills summary (2)

D.Solution and precipitation separation

Solution and precipitation can be separated by three methods: decanting, filtration and centrifugation.

1. The decanting method

When the density of precipitation or crystallization of larger particles can settle to the bottom of the container after standing, can be used for precipitation separation and washing decantation.

The specific method is to pour the solution on the top of the precipitation into another container, and then add a small amount of washing liquid to the container containing the precipitation, after full stirring, settling, pouring to the washing liquid. Repeat the operation for more than 3 times, the precipitation can be washed, so that the precipitation and solution separation.

2. The filtering method

The most common method of operation for separating solution from precipitation is filtration. During filtration, the precipitation remains on the filter and the solution passes through the filter into the container. The resulting solution is called the filtrate. There are three kinds of filtration methods: atmospheric filtration, decompression filtration and thermal filtration.

(1) Atmospheric filtration

This method is the most simple and common, using glass funnel and filter paper for filtration.

According to the pore size, filter paper can be divided into fast, medium and slow speed three. Rapid filter paper has the largest pores.

When filtering, first fold round filter paper or square filter paper into 4 layers (square filter paper should be cut into a fan after folding). Then tear off a corner of the filter paper and place it in a funnel. The edge of the filter paper should be slightly lower than the edge of the funnel. Moisten the filter paper with water and press it against the inner wall of the glass funnel. If there are still bubbles between the filter paper and the wall of the funnel, you should press the filter paper gently with your finger to remove the bubbles, and then add distilled water to the funnel until it almost reaches the edge of the filter paper. At this time, the funnel neck should be fully filled with water, and when all the water on the filter paper has been drained, the water column in the funnel neck can still be retained. If it does not form a water column, you can plug the lower mouth of the funnel with your fingers, slightly lift one side of the filter paper, and add water between the filter paper and the funnel until the funnel neck and most of the cone are fully filled with water, and the bubbles in the neck are completely discharged. Then press the edge of the paper tightly, and then release the finger blocking the exit below, at this time the water column can form. During the entire filtration process, the funnel neck must always be filled with liquid so that filtration can be rapid.

The following points should be paid attention to when filtering: adjust the height of the funnel frame so that the end of the funnel is close to the inner wall of the receiver. Pour the solution before transferring the precipitate. Stir rod should be used for transfer. When pouring the solution, the stirring rod should be pointed at the 3-layer filter paper. The liquid level in the funnel should be less than 2/3 of the height of the filter paper.

If the precipitation needs washing, should wait for the solution transfer is completed, pour a small amount of detergent into the precipitation, and then fully stir with the stirring rod, rest for a period of time, after the precipitation sinks, pour the upper clear liquid into the funnel, so repeated washing two or three times, and finally transfer the precipitation to the filter paper.

(2) The reduced pressure filter

This method can accelerate filtration, and the precipitation suction is dry, but it is not suitable to filter the gelatinous precipitation and particles too small precipitation, because the gelatinous precipitation is easy to penetrate the filter paper, particles too small precipitation is easy to form a layer of dense precipitation on the filter paper, the solution is not easy to penetrate.

The circulating water vacuum pump reduces pressure in the filter bottle, and the filtration speed is accelerated due to the pressure difference between the bottle and the liquid surface of the Brynner funnel. When installing, attention should be paid to the oblique mouth of the funnel and the branch pipe of the filter bottle.

There are many small holes in the Brinner funnel. The filter paper should be cut to a size that is slightly smaller than the inner diameter of the funnel, but can cover all the porcelain holes. Moisten the filter paper with a small amount of water, open the pump, reduce the pressure so that the filter paper is close to the funnel, and then start filtering.

When stopping filter suction, first remove the rubber pipe connecting the filter bottle and pump, and then close the pump to prevent back suction. In order to prevent the reverse suction phenomenon, generally between the suction filter bottle and the pump, a safety bottle.

(3) Hot filtering

In order to ensure that the filter paper and the funnel close, the second fold first do not fold dead, the filter paper into a cone, with the index finger on the filter paper on the glass funnel (funnel should be clean and dry) wall, slightly change the folding degree of the filter paper, until the filter paper and the funnel close, at this time can fold the second fold dead.

Some substances are easy to crystallize and precipitate when the temperature of the solution is reduced. In order to filter out other insoluble impurities contained in this kind of solution, hot filter funnel is usually used for filtration to prevent the crystallization of solute. For filtration, a glass funnel is placed inside a copper thermal filter funnel, which is filled with hot water to maintain the temperature of the solution.

3. Centrifugal separation

When the amount of precipitation to be separated is very small, the precipitation and solution can be placed in the centrifugal tube, into an electric centrifuge for centrifugation. When the centrifuge is used, the centrifuge tube containing precipitation is put into the tube sleeve of the centrifuge, and the tube containing the same volume of water is also put into the opposite tube sleeve of the other tube, and then slowly start the centrifuge, gradually accelerating. When stopping centrifugation, the centrifuge should be allowed to stop naturally.