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Preparation and operation of volumetric flask
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Preparation and operation of volumetric flask

Views: 239     Author: Site Editor     Publish Time: 2020-10-08      Origin: Site

The volumetric flask is a thin-necked pear-shaped flat-bottomed glass bottle with a glass ground stopper or a plastic stopper. There is a marking line on the neck, which indicates the volume of liquid in the bottle when the liquid is full to the marking line at the indicated temperature (generally 20°C). The volumetric flask is mainly used to prepare accurate concentration solutions. Usually there are several specifications such as 5mL, 10mL, 25mL, 50mL, 100mL, 250mL, 500mL, 1000mL, 2000mL, etc.

(1) Preparation of volumetric flask

Before using the volumetric flask, check first: ① Whether the volume of the volumetric flask is consistent with the required; ②If you prepare a solution of easily decomposable substances in light, you should choose a brown volumetric flask; ③ Whether the glass ground stopper or plastic stopper leaks. The leak detection method is: add tap water to the vicinity of the marking line, and close the bottle stopper. Hold the stopper with your index finger, and stand the bottle upside down for 2 minutes. Use a piece of dry filter paper to check the gap between the bottle mouth to see if there is any water seeping. If there is no water leakage, put the bottle upright, rotate the stopper 180°, stop it tightly, and stand upside down for 2 minutes. If it still does not leak water, it can be used.

The qualified volumetric flask should be washed clean. The washing method and principle are the same as washing the burette. The inner wall of the washed volumetric flask should be evenly moistened without water droplets, otherwise it must be rewashed.

The stopper must be kept matching with the bottle, and it must be marked or tied to the bottle neck with a string or rubber band to prevent it from being broken or messing with other stoppers.

Volumetric flaskVolumetric flask

(2) Operation of volumetric flask

When preparing a solution from a solid substance, accurately weigh a certain amount of the solid substance, place it in a small beaker, add water or other solvents to dissolve it (if it is difficult to dissolve, cover it with a watch glass and heat it to dissolve, but it must be cooled before Transfer), quantitatively transferred into the volumetric flask. When transferring, extend the glass rod into the volumetric flask so that the lower end is against the inner wall of the bottleneck, and the upper end does not touch the mouth of the bottle. The mouth of the beaker is close to the glass rod so that the solution flows along the glass rod and the inner wall, as shown in Figure 2-42.

After all the solution is transferred, lift the glass rod slightly upwards while making the beaker upright, and put the glass rod back into the beaker. Blow the glass rod and the inner wall of the beaker with bottle-washing distilled water, and transfer the washing liquid to the volumetric flask. Repeat the washing several times (at least 3 times). After the quantitative transfer is completed, when adding water to about 3/4 of the volume of the volumetric flask, shake the volumetric flask for a few weeks (do not invert) to mix the solution initially. Then put the volumetric flask flat on the table, slowly add water to about 1cm close to the marking line, and wait 1 to 2 minutes to allow the solution adhering to the inner wall of the bottleneck to flow down. Use a slender dropper to extend into the neck of the neck near the liquid level, look straight at the marking line, and add water until the lowest point of the lower edge of the meniscus is tangent to the marking line. Immediately plug the dry cork, hold the volumetric flask as shown in Figure 2-43 (for volumetric flasks with a volume of less than 100mL, only use your left hand to operate), turn the volumetric flask upside down to make the bubbles rise to the top. After the bottle is upright, shake it upside down again, repeat this 10-20 times to make the solution evenly mixed. Finally, put the volumetric flask upright, open the stopper to make the solution around it flow down, re-plug the stopper, and shake it upside down for 1 or 2 times to mix the solution thoroughly.

Be careful not to hold the bottle with your hands, so as not to cause the body temperature to expand the liquid and affect the accuracy of the volume. The hot solution should be cooled to room temperature before being poured into the volumetric flask, otherwise it may cause volume errors. Volumetric flasks cannot store solutions for a long time, especially alkaline solutions, which will corrode the glass and make the stopper sticky and cannot be opened. If the prepared solution needs to be stored, it should be transferred to the reagent bottle. After the volumetric flask is used, rinse it with water. If it is not used for a long time, wash and dry the grinding mouth and put a piece of paper on it. The volumetric flask must not be heated, and must not be baked in an oven. If you rush to use it after washing, you can shake it with organic solvents such as ethanol and then dry it, or blow it dry with cold air from a hair dryer.


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