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Precautions for the use of volumetric flasks
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Precautions for the use of volumetric flasks

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The volumetric flask is a thin-necked pear-shaped flat-bottomed volumetric vessel with a ground glass stopper. There is a marking on the neck, which indicates the volume of the solution when the liquid meniscus is tangent to the marking of the bottleneck of the volume at the specified temperature Exactly equal to the volume marked on the bottle. The volumetric flask is marked with: temperature, capacity, scale line.


1. The method of preparing solution using volumetric flask is:  


(1) Check whether the stopper is leaking before use. Pour 2/3 of the volume of water into the bottle and stopper the bottle. Hold the cork with your fingers, and hold the bottom of the bottle with your other hand, and stand the bottle upside down for a while. After repeating it several times, observe whether there is water seeping around the cork. The volumetric flask can only be used if it is not leaking.

Volumetric flask

(2) Put the accurately weighed solid solute in a beaker and dissolve it with a small amount of solvent. Then transfer the solution along the glass rod to the volumetric flask. In order to ensure that all the solutes can be transferred to the volumetric flask, the beaker should be washed with solvent several times, and all the washing solution should be transferred to the volumetric flask (see Figure a). 

 

(3) When the liquid level added to the volumetric flask is about 1 cm away from the marking line, use a dropper to carefully drip it, and finally make the meniscus of the liquid exactly tangent to the marking line.


(4) Tighten the bottle stopper, and use the method of inverting and shaking to mix the liquid in the bottle evenly. Before using the volumetric flask, you must first check the following two: ① Whether the volume of the volumetric flask is consistent with the requirements. ②To check whether the stopper is tight and does not leak water. Put water in the bottle near the marking line, close the stopper, make it stand upside down for 2 minutes, and check with a piece of dry filter paper along the seam of the bottle mouth to see if there is any water droplets oozing out. If there is no leakage, then rotate the stopper 180°, plug it tightly, turn it upside down, and test for leakage in this direction. It is necessary to do two inspections, because sometimes the stopper and the bottle mouth are not close at any position. 

Volumetric flask

2. When preparing a standard solution with a volumetric flask, first place the accurately weighed sample in a small beaker, add a small amount of solvent, and stir to dissolve it (if it is difficult to dissolve, cover it with a watch glass and heat it slightly, but keep it cold Can be transferred later). Move the solution quantitatively into the washed volumetric flask with the operation of transferring the sediment along the stir bar, then use the washing bottle to flush the wall of the beaker 2 to 3 times, and transfer it into the volumetric flask in the same way. When the solution is added to 2/3 of the flask, shake the flask horizontally for a few weeks (do not turn it upside down) to make the solution roughly mixed. Then, put the volumetric flask flat on the table, slowly add water to about 2-3cm from the marking line, wait for 1~2min to let the solution adhering to the inner wall of the bottleneck flow down, and use a plastic dropper to reach the bottleneck near the liquid level , Keep your eyes level at the marking line, and add water until the bottom of the meniscus of the solution is tangent to the marking line. Cover the cork immediately, hold the cork with your palm, and hold the bottom of the bottle with the fingers of the other hand. 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 (for the volume less than 100mL) Bottle, do not need to hold the bottom of the bottle). Then turn the volumetric flask upside down to make the bubbles rise to the top, at this time the flask can be shaken several times. Turn it upside down and still make the bubbles rise to the top. Repeat this more than 10 times to mix evenly.

   

The solution needs to return to normal temperature before filling the volumetric flask. Because the solute will absorb or release heat when it is dissolved in the beaker, the volumetric flask must be used at room temperature (20 degrees Celsius).

   

Volumetric flasks cannot store solutions for a long time, especially alkaline solutions will erode the walls of the flask and make the stopper sticky and cannot be opened. The volumetric flask cannot be heated.


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