Views:0 Author:Site Editor Publish Time: 2020-09-08 Origin:Site
Measuring flasks are mainly used to accurately prepare solutions of a certain concentration. It is a slender-necked, pear-shaped, flat-bottomed glass bottle with a ground stopper. The bottleneck is engraved with a marking line. When the liquid in the bottle reaches the marking line at the specified temperature, its volume is the volume number indicated on the bottle.
1. The method of using a volumetric flask to prepare a solution is:
(1) Check if there is water leakage at the stopper before use (check after cleaning when newly purchased). The specific operation method is: fill a half bottle of water in the volumetric flask, close the cork, hold the cork with the index finger of your right hand, and hold the bottom of the volumetric flask with the five fingers of the other hand, stand it upside down (bottle mouth down), and observe the volume Whether the bottle is leaking. If there is no water leakage, stand the bottle upright and turn the stopper 180°, and then stand upside down again to check for water leakage. If no water leaks around the stopper of the volumetric flask after two operations, it means that the volumetric flask does not leak. The volumetric flask can only be used if it is not leaking.
(2) Put the accurately weighed solid solute in a beaker and dissolve it with a small amount of solvent. Then transfer the solution to a volumetric flask. In order to ensure that all the solutes can be transferred to the volumetric flask, the beaker must be washed with solvent several times and the washing solution should be transferred to the volumetric flask. Use a glass rod to drain when transferring. The method is to lean one end of the glass rod against the inner wall of the volume bottleneck, and be careful not to let other parts of the glass rod touch the mouth of the volumetric flask to prevent the liquid from flowing to the outer wall of the volumetric flask. After adding an appropriate amount of solvent, shake for initial mixing.
(3) When the liquid level added to the volumetric flask is about 0.5 to 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. If the water exceeds the mark, it needs to be re-constituted.
(4) Tighten the bottle stopper, and mix the liquid in the bottle evenly by inverting and shaking. If the liquid level is found to be lower than the graduation line after standing still, this is because a very small amount of solution in the volumetric flask is lost by wetting at the neck of the bottle, so it does not affect the concentration of the prepared solution, so do not add water to the bottle, otherwise it will cause The concentration of the prepared solution decreases.
(5) Uncap and reflux: After mixing, carefully open the cap of the volumetric flask to allow the solution at the cap and the mouth of the bottle to flow back into the bottle, then close the cap, and then use the method of inversion and shaking to mix the liquid in the bottle evenly . This is very important when dealing with small samples.
2. Pay attention to the following points when using volumetric flasks:
(1) The volumetric flask must be cleaned and calibrated after purchase, and can be used only after the calibration is qualified.
(2) Substances that are easy to dissolve and do not generate heat can be directly dissolved in a volumetric flask. Other substances can hardly dissolve the solute in the volumetric flask. The solute should be dissolved in the beaker and transferred to the volumetric flask.
(3) Pay attention to the solution that emits heat, absorbs heat, or changes in volume after mixing water with organic solvents (such as methanol, etc.). For those that generate heat, add an appropriate amount of solvent (about 0.5 cm from the bottle marking line) and let it cool. Bring to room temperature and then dilute to the mark; for volume changes, add an appropriate amount of solvent (do not add to the neck for easy shaking), shake, and then add to the bottle mark about 0.5 cm away, and leave it for a while Then dilute to the mark.
(4) The total amount of solvent used to wash the beaker cannot exceed the marking line of the volumetric flask.
(5) The volumetric flask cannot be heated. If the solute emits heat during the dissolution process, wait for the solution to cool before transferring, because the general volumetric flask is calibrated at a temperature of 20℃. If the solution with a higher or lower temperature is injected into the volumetric flask, the volumetric flask will It will expand with heat and contract with cold, and the measured volume will be inaccurate, resulting in inaccurate solution concentration.
(6) The volumetric flask can only be used to prepare the solution, and the solution cannot be stored for a long time, because the solution may corrode the bottle body (especially the alkaline solution), which will affect the accuracy of the volumetric flask.
(7) The volumetric flask should be washed clean after use.