How to use high school chemical volumetric flasks

Volumetric flasks are mainly used to accurately prepare solutions of a certain molar concentration. It is a slender-necked, pear-shaped, flat-bottomed glass bottle with a grinding stopper. The neck of the bottle is engraved with a reticle, and when the liquid in the bottle reaches the reticle at the specified temperature, its volume is the volume indicated on the bottle.

A volumetric flask of one size can only be measured. Commonly used volumetric flasks are available in milliliters and other sizes.

The method of preparing the solution using a volumetric flask is:

1) Check the stopper for water leakage before use. The specific operation method is: fill half a bottle of water in the volumetric flask, plug the cork tightly, hold the stopper with the index finger of the right hand, and hold the bottom of the volumetric flask with the five fingers of the other hand, and turn it upside down (the mouth of the bottle is facing down) to observe whether the volumetric flask is leaking.

If there is no water leakage, the bottle is upright and the bottle stopper is rotated 180°, then stand upside down again to check whether there is water leakage, if there is no water leakage around the volumetric flask stopper during two operations, it means that the volumetric flask is not leaking. Only volumetric flasks that have been inspected for non-leakage can be used.

2) Put the accurately weighed solid solute in a beaker and dissolve it with a small amount of solvent. The solution is then transferred to a volumetric flask. In order to ensure that all solutes can be transferred to the volumetric flask, the beaker should be washed several times with solvent and all the washing solution should be transferred to the volumetric flask.

Drainage with a glass rod is used during transfer. The method is to place one end of the glass rod against the inner wall of the volumetric bottleneck, and be careful not to let other parts of the glass rod touch the mouth of the volumetric flask, so as to prevent the liquid from flowing to the outer wall of the volumetric flask.

3) When the liquid level of the liquid added to the volumetric flask is about 1 cm away from the reticle, it should be carefully added with a dropper, and finally the meniscus surface of the liquid is exactly tangent to the reticle. If the water is added beyond the scale mark, it needs to be reconstituted.

4) Tightly close the cork, and use the method of inverting and shaking to mix the liquid in the bottle evenly. If the liquid level is found to be lower than the scale mark after standing, this 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, so do not add water to the bottle, otherwise, the concentration of the prepared solution will be reduced.

The following points should be taken into account when using volumetric flasks:

1) The volume of the volumetric flask is specific, and the scale is not continuous, so one type of volumetric flask can only prepare the same volume of solution. Before preparing the solution, it is necessary to figure out the volume of the solution to be prepared, and then choose a volumetric flask of the same size.

2) The substances that are easy to dissolve and do not generate heat can be directly dissolved in the volumetric flask with a funnel, and other substances can not be dissolved in the volumetric flask, and the solute should be dissolved in the beaker and transferred to the volumetric flask.

3) The total amount of solvent used to wash the beaker should not exceed the reticle of the volumetric flask.

4) The volumetric flask cannot be heated. If the solute is exothermic during the dissolution process, it is necessary to wait for the solution to cool down 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 expand and contract, and the measured volume will be inaccurate, resulting in the inaccurate concentration of the prepared solution.

5) The volumetric flask can only be used to prepare the solution, and the solution can be stored everywhere, because the solution may corrode the bottle body, so that the accuracy of the volumetric flask is affected.

Let me illustrate this with the example of a prepared sulfuric acid standard solution.

Preliminary preparation: 1) First determine the volume of the prepared solution (this data is generally given in the title), such as 90ml.

2) Select an appropriate large number of volumetric flasks, and 100ml volumetric flasks should be selected for this question (bottle selection principle: rather a little larger than a small one). Check the volumetric flask for leaks. Wash the volumetric flask.

3) Calculate the required volume of concentrated sulfuric acid. If 98% dense sulfuric acid is used, and the required volume is vml, then according to the equal column formula of solute mass before and after preparation, it is as follows: v*, and the solution is v=

Preparation process: 1) Measuring solution: Concentrated sulfuric acid measured with a 5ml graduated cylinder.

2) Dilution: Choose a clean beaker of appropriate size, add about 20ml of distilled water (about 1 5 of the total volume of preparation), and then slowly pour the good amount of concentrated sulfuric acid into the water, stirring while pouring. Cool to room temperature. (Note that the graduated cylinder does not have to be washed).

3) Pipetting: Carefully pour the prepared concentrated sulfuric acid into the volumetric flask along the glass rod, wash the beaker and glass rod 2-3 times (about 15ml of distilled water each time), and inject the washing solution into the volumetric flask along the glass rod each time. Shake and shake.

4) Volume: Inject distilled water into the volumetric flask along the glass rod with a beaker to 2-3cm below the engraving line, and use a rubber head dropper to add distilled water dropwise to the lowest part of the concave liquid surface and tangent to the engraving line (head-up). Shake again.

5) Transfer the prepared solution to the fine-mouth reagent bottle, and label it with the type and concentration of the reagent.

Ready.

The knowledge points of high school chemical volumetric flasks are as follows:

1. The volumetric flask is a thin-necked, pear-shaped, flat-bottomed glass bottle with a grinding mouth and a grinding plug. The bottleneck is marked with a ring marking, and the solution reaches the accurate rest of the reticle when the bottleneck is marked at 20 degrees Celsius. Commonly used volumetric flasks are 25ml, 50ml, 100ml, 250ml ml, 1000ml and other specifications.

2. In order to ensure the tightness, the bottle and the stopper should be used together, and the stopper generally cannot be exchanged or matched with each other. Since the volumetric flask cannot be used after the stopper is broken, the grinding plug should be bound to the neck of the volumetric flask with a thin rope before use to prevent Hanzi from accidentally breaking, breaking or mixing with the stoppers of other volumetric flasks. The remaining length of the tether does not need to be large, 2 3cm can be, so that the plug can be opened.

3. Check whether there is water leakage before using the volumetric flask. Leak test operation: add tap water to the volumetric flask above the scale line, plug the cork, hold the cork with one hand, hold the bottom of the bottle with the other hand, and then turn the volumetric flask upside down for a few minutes, and carefully observe whether there is water seeping out of the bottle mouth.

If there is no leakage, rotate the plug 180°; Continue to stand upside down for a few minutes to see if the bottle mouth is leaking.

4. Volumetric flasks can be used to prepare accurately weighed solid reagents into solutions of accurate concentration. When preparing, it is necessary to stir and dissolve the accurately weighed solid substance in a beaker with a small amount of water, and transfer the solution to the volumetric flask along the glass rod, paying attention to the contact point between the glass rod and the volumetric flask below the bottleneck marking.

The volumetric flask is an important experimental instrument in high school chemistry, and the teacher must have said its importance when entering the school, so let's see how to use the volumetric flask.

01 The volumetric flask is a measuring instrument, not a reaction instrument, so we cannot add reactants to the volumetric flask in parts, nor can it be used to store liquids for a long time.

02 The volumetric flask generally has a working temperature, generally 20 degrees, and it must be used at the operating temperature, otherwise the measurement of megadispersion will not be allowed to be accurate.

03 The liquid should be drained with a glass rod when pouring the liquid into the volumetric flask, and the glass rod should be kept above the scale mark.

04 When shaking, the sedan should be bent to hold the bottle belly with the left hand, hold the glass stopper with the right hand, shake it a few times and turn the glass stopper to release the gas.

Before using a volumetric flask, the following two inspections should be performed:

1) Whether the volume of the volumetric flask is consistent with the requirements.

2) To check whether the cork is tight and does not leak.

Specific operation: Put water in the bottle near the marking line, plug the cork tightly, make it stand upside down for 2min, and check it along the seam of the bottle mouth with a dry filter paper to see if there are water beads seeping out. If there is no leakage, rotate the plug 180°, plug it tightly, turn it upside down, and test whether there is leakage in this direction.

This double check is necessary because sometimes the stopper is not sealed in any position. The closure must be properly protected, preferably tied to the neck of the bottle with a rope to prevent it from breaking or getting mixed up with other volumetric flasks.