The basic process of chemical experiment includes: weighing, dissolution, fixed volume, filtering, dilution, determination.
Weighing
Select the appropriate balance according to the requirements of weighing accuracy. 1/10000 balance can be used for samples above 50mg. The sample with less equal quantity of reference substance and high accuracy is generally 1/100000 balance, and the weighing quantity had better not be less than 5mg.
The electronic balance needs to be warmed up for 30 minutes before use. It is possible to open the balance before preparing to weigh, and then prepare to weigh the required items.
Before weighing, first check whether the balance is leveled. After leveling, the balance cannot be moved, otherwise it should be re-leveled.
When weighing, handle it gently to avoid bumping against the balance tray or other parts affecting the accuracy of the balance.
When reading, the cabin door must be closed tightly, and the upper cover shall not be opened for weighing and reading.
Avoid spilling powder on the balance tray or other parts.
Dissolution
Take the volumetric flask as an example, first add a small amount of solvent, 1/3~1/2, shake it slightly, and observe the dissolution situation, if it is easy to dissolve, add solvent to 3/4 of the full scale, and shake to make dissolve. If the solute is not easy to dissolve, add the solvent to the full scale 3/4, dissolve by ultrasonic, cool to room temperature after ultrasonic completion, and then fix the volume.
The accurately weighed solid solute can also be placed in a beaker and dissolved with a small amount of solvent. Then transfer the solution along the glass rod to the volumetric bottle. In order to ensure that all the solute can be transferred to the volumetric bottle, the beaker should be washed many times with solvent, and all the washing solution should be transferred to the volumetric bottle.
When adding liquid to the volumetric flask, when the liquid level is about 1cm away from the marking line, use a dropper to carefully drop it, and finally make the meniscus of the liquid exactly tangent to the marking line.
Stopper the bottle tightly, and mix the liquid in the bottle evenly by inverting and shaking.
Fixed Volume
Fixed volume can not hold the volume bottle body by hand, in case the temperature rise affects the solution volume, use the thumb and index finger of the left hand to gently pinch the bottleneck (the part on the scale line), lift it away from the table (make the bottle body hang naturally), and the eye line of sight is level with the scale. Use an eyedropper to slowly drip the solvent along the mouth of the bottle to the lowest point of the concave liquid surface, which is exactly tangent to the scale.
Filtering
Whether it is filter paper filtration or membrane filtration, the initial filtrate should be discarded first, generally 2-3 times to ensure that the adsorption of the filter paper or filter membrane reaches saturation, and the container receiving the filtrate should be moistened and washed at the same time to prevent the adsorption of the container wall.
Dilution
If the sampling volume is an integer volume such as 1, 2, 5, 10ml, it is generally drawn with a calibrated pipette. If it is other volumes, such as 3, 4, 8ml, etc., it can also be drawn with a graduated pipette.
Hold the earwash ball in the left hand, the pipette in the right hand, and hold the neck mark with the thumb and middle finger of the right hand.
First absorb a small amount of solution to be taken and wash the pipette twice, gently suck the solution with an ear-washing ball in the left hand, and pay attention to the position of the rising page. When the liquid level in the tube rises above the scale mark, quickly block the orifice with the right index finger, remove the pipette, dry the outer wall of the pipette with filter paper, and make it perpendicular to the ground, release the right index finger slightly, and make the page drop slowly. At this time, the line of sight should be flat. Until the meniscus is tangent to the mark, immediately press the index finger so that the liquid no longer flows out, then move the pipette into the container that is ready to receive the solution, make the vertical tip of the tube lean against the inner wall of the container, release the index finger of the right hand, and let all the solution in the tube flow down the wall naturally. After waiting for another 15 seconds, take out the pipette and do not blow out the solution that remains in the tip of the tube.
When using the same pipette to measure solutions of different concentrations, pay attention to the full flushing. First measure the thinner solution, and then measure the thicker solution. When absorbing the first solution, it is better not to be more than 1cm higher than the marking line, and then put it down to the marking line, so that when absorbing the second solution with different concentrations, the inner wall can be swabbed higher to eliminate the impact of the first solution.
Determination
The determination methods include volumetric analysis, gravimetric analysis and instrumental analysis, such as UV, HPLC, GC, etc. Appropriate methods can be adopted according to the sample.
