Volumetric flasks are mainly used to accurately prepare solutions of a certain concentration. They are a thin, pear-shaped, flat-bottomed glass bottle with a ground stopper. There is a mark on the neck of the bottle. When the liquid in the bottle reaches the mark at the specified temperature, its volume is the volume indicated on the bottle.
How to prepare solutions using volumetric flasks
1. Check whether the cork is leaking before use (check after cleaning the newly purchased one). The specific operation method is: fill half a bottle of water in the volumetric flask, plug the cork tightly, hold the cork with the index finger of your right hand, and hold the bottom of the volumetric flask with the five fingers of your other hand, turn it upside down (the bottle mouth faces down), and observe whether the volumetric flask is leaking. If it is not leaking, stand the bottle upright and rotate the cork 180°, then turn it upside down again to check whether it is leaking. If there is no water leakage around the cork of the volumetric flask after two operations, it means that the volumetric flask is not leaking. The volumetric flask that has been checked for leaking can be used.
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. To ensure that the solute can be fully transferred to the volumetric flask, wash the beaker with solvent several times and transfer all the washing solution to the volumetric flask. Use a glass rod to drain during transfer. The method is to place one end of the glass rod against the inner wall of the bottleneck of the volumetric flask, 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 it for initial mixing.
3. When the liquid level in the volumetric flask is about 0.5 to 1 cm away from the mark, you should use a dropper to carefully add the liquid so that the meniscus of the liquid is exactly tangent to the mark. If the water added exceeds the scale line, it needs to be re-prepared.
4. Close the bottle stopper tightly, and mix the liquid in the bottle evenly by inverting and shaking. If the liquid level is found to be below the scale line after standing, it is because a very small amount of solution in the volumetric flask is lost by wetting at the bottleneck, so it does not affect the concentration of the prepared solution. Therefore, do not add water to the bottle, otherwise, the concentration of the prepared solution will be reduced.
5. Open the cap and reflux. After mixing, carefully open the cap of the volumetric bottle, let the solution at the cap and the bottle mouth 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 point is very important when dealing with small volume samples .
The following points should be noted when using volumetric flasks
1. After purchasing, the volumetric flask must be cleaned and calibrated , and can only be used after passing the calibration.
2. Substances that are easily soluble and do not generate heat can be directly transferred into a volumetric flask for dissolution. Other substances basically cannot be dissolved in a volumetric flask. The solute should be dissolved in the beaker and then transferred to the volumetric flask.
3. Pay attention to solutions that release heat, absorb heat or change in volume after mixing water and organic solvents (such as methanol). For those that generate heat, add an appropriate amount of solvent (about 0.5 cm from the mark on the bottle), cool to room temperature, and then dilute to the scale; for those that change in volume, add an appropriate amount of solvent (do not add to the narrow neck to facilitate shaking), shake, then add to about 0.5 cm from the mark on the bottle, leave for a while, and then dilute to the scale.
4. The total amount of solvent used to wash the beaker must not exceed the mark on the volumetric flask.
5. The volumetric flask cannot be heated. If the solute releases heat during the dissolution process, the solution must be cooled before transfer, because the general volumetric flask is calibrated at a temperature of 20°C. If a solution with a higher or lower temperature is injected into the volumetric flask, the volumetric flask will expand and contract due to heat and cold, and the measured volume will be inaccurate, resulting in inaccurate concentration of the prepared solution.
6. Volumetric flasks can only be used to prepare solutions and cannot store solutions for a long time, because the solutions may corrode the bottle (especially alkaline solutions), thus affecting the accuracy of the volumetric flask.
7. The volumetric flask should be cleaned promptly after use.
Post time: Aug-01-2024