E.Pipette, volumetric flask and buret
1. Pipette
A pipette is used to measure a certain volume of solution accurately. It is a glass tube with an enlarged part (called the ball) in the middle and a line marked on the upper part of the neck. The line is calibrated according to the volume released. There are 5, 10, 25 and 50mL pipettes commonly used, and the most commonly used is 25mL pipettes.
The other pipette has a scale, called a pipette, to measure the volume of the test fluid within the pipette.
Pipette suction steps:
(1) The thumb and middle finger hold the pipette above the line;
(2) the lower end of the pipette is properly extended into the liquid level, too deep or too shallow will make the outer wall with too much test liquid or easy to suck air;
(3) Align the ear ball with the top of the pipette, inhale the test solution to about 2cm above the mark line, and quickly replace the ear ball with the index finger to plug the pipe mouth;
(4) Take out the pipette and lean against the inner wall of the liquid container, and then slowly turn the pipette, so that the test liquid above the line flow to the line;
(5) the pipet rapid acceptance into the container.
Pipette discharge steps:
(1) Tilt the receiving vessel and erect the pipette;
(2) The outlet tip contacts the container wall;
(3) Loosen the index finger, so that the test fluid freely flow out;
(4) when the pipette is blowing, it is necessary to blow out all the test liquid in the pipe.
2. The volumetric flask
The volumetric bottle is a thin – necked, pear-shaped, flat – bottomed bottle with a grinding stopper. A line on the neck indicates that at the indicated temperature (usually 20℃), when the liquid is filled to the line, the volume of the liquid in the bottle is exactly the same as the volume indicated on the bottle.
Volumetric bottles are used to prepare solutions of precise concentration. Often used with pipettes. To divide a substance into equal parts. Usually, there are 25,50,100,250,500,1000 ml and other specifications. In this experiment, 100 and 250mL volumetric bottles are commonly used.
Before using a volumetric flask, check the following two things:
(1)The volume of the volumetric bottle is consistent with the required.
(2)To check whether the cork is tight and watertight. Put water in the bottle near the marking line, close the bottle stopper tightly, and make it stand upside down for 2min. Check with dry filter paper along the bottle mouth seam to see if there is any water leakage. If there is no leakage, then rotate the plug 180°, plug tight, inverted, test whether there is leakage in this direction. It is necessary to do this twice, because sometimes the cork does not fit the bottle in all places.
The stopper must be properly protected, preferably tied to the neck of the bottle with a string to prevent it from breaking or being confused with other volumetric bottles.
To prepare a standard solution with a volumetric flask, place the accurately weighed sample in a small beaker, add a small amount of solvent, and stir to dissolve it (if it is insoluble, cover the plate and heat a little, but must be cooled before transfer). The solution was transferred to the cleaned volumetric bottle by transferring precipitation along the stirring rod, and then the flask wall was blown and washed 5 ~ 6 times, and transferred to the volumetric bottle by the same method. When the solution has been added to 2/3 of the flask, turn the volumetric flask horizontally for several weeks (do not reverse) until the solution is roughly mixed. Then, put the volume bottle flat on the table, slowly add water to about 1cm away from the mark line, wait for 1 ~ 2min, make the solution adhering to the inner wall of the bottleneck flow down, use the dropper to reach the bottleneck near the liquid level, eye level mark line, add water to the lower part of the meniscus and the mark line. Cover the cork immediately, press the cork with the index finger of one hand and hold the bottom of the bottle with the finger of the other hand. Be careful not to hold the bottle with the palm of your hand, in case the temperature makes the liquid expand and affects the accuracy of the volume (for the volumetric bottle with the volume less than 100mL, it is not necessary to hold the bottom of the bottle). Then the volumetric flask is reversed to make the bubbles rise to the top, and the flask can be oscillated several times. Reverse it, and the bubble still rises to the top. Do this more than 10 times to mix well.
Volumetrical bottles should not be stored for a long time, especially alkaline solutions, which will erode the bottle walls and make the cork stick and cannot be opened. Volumetric bottles cannot be heated.
3. The burette
Burette is a volumetric instrument for accurate discharge of uncertain quantity of liquid. It’s made of a thin, uniform glass tube with a scale on it, a tip at the bottom, and a sluice gate in the middle to control the rate of titration.
The burette is divided into two kinds: acid type and basic type. The former is used to measure liquid reagent which has the corrosive effect on the rubber tube. The latter is used to measure liquids that have a corrosive effect on glass.
The capacity of burette is generally 50mL, and each large cell of the scale is 1mL. Each large cell is divided into 10 smaller cells, so each small cell is 0.1ml.
The bottom end of the acid buret is a glass piston. When the piston is opened, the liquid drops from the tube. Before use, remove the piston, wash it, blot or blow dry it with filter paper, and then apply a very thin layer of petroleum jelly to both ends of the piston (do not block the plug hole). Install the piston and rotate, so that the piston and the plug groove contact is transparent state, and finally fill the water test whether the leakage.
The bottom end of the basic buret is connected with a rubber tube to a small glass tube with a sharp nose. Inside the rubber tube is a glass ball. Use your left thumb and forefinger to gently press the rubber tube on the outside of the glass ball to one side, creating a gap in the tube and allowing the liquid to drip out of the dropper. Place your hand slightly above the glass ball as you press. If placed under the ball, when released, bubbles will appear in the tip of the glass tube.
Care must be taken not to have bubbles at the bottom of the burette. Rapid liquid discharge can drive away the bubbles in the acid buret; Air bubbles can be removed from a basic buret by gently lifting the beaded glass tube and squeezing the glass ball with your finger.
Acid burets should not be used for alkaline solution, as the grinding part of the glass is easily eroded by alkaline solution, making the stopper unable to rotate.
Basic burets should not be filled with solutions that are corrosive to rubber tubes, such as iodine, potassium permanganate and silver nitrate.
4. Instrument washing
Pipette, volumetric bottle and burette require accurate volume. Generally, it is not necessary to wash mechanically with a brush. It is best to clean the oil on the inner wall with concentrated sulfuric acid-potassium dichromate lotion.
Pipette: in the cover on top of a rubber tube, with washing ears ball will wash the scribe lines more than part of the suction pipe, with tongs clamp, upright soaking time (can also be used to wash ears ball will wash suction pipe, a finger block snappy, with the flat pipette, constantly turning, until all the lotion infiltrates the lining), is due to return to the original lotion bottle.
Volumetric bottle: small volumetric bottle can be filled with lotion and soaked for a certain time. The volumetric bottle of large capacity does not need to be filled, inject about 1/3 volume of lotion, plug the bottle stopper tightly, shake for a while, and then shake several times at intervals.
Buret: 10mL of lotion can be injected into the buret. Hold the buret in both hands and turn it continuously until the lotion has soaked all the tubes, and then pour the lotion back to the original storage bottle through the upper mouth or beak. If the upper method cannot be cleaned, the lotion should be filled with burets and soaked.
After the above instruments are soaked with lotion, they need to be washed off with tap water first. At this time, it should be bright to check whether the oil has been washed, and whether the inner wall water film is uniform. If it is found that there is still water, it should be soaked with lotion and then check, until thoroughly washed.
Finally, wash the tap water with deionized water (or distilled water). The dosage of deionized water is about 1/3 of the volume of the instrument to be washed each time, generally 2 ~ 3 times.
For pipettes and burets, wash them two to three times with the solution to be loaded.
5.Reading
Take burette as an example to illustrate below. The solution in the buret forms a meniscus due to adhesion and cohesion. There is often a shadow under the meniscus, which is independent of the reading. When reading, the line of sight should be at the same level as the lowest lower edge of the meniscus to avoid parallax. Because the liquid level is spherical, you get different readings depending on where the eye is. For a commonly used 50mL buret, the reading should be up to 0.01mL.
The color is too deep solution, such as iodine solution, potassium permanganate solution, meniscus is very difficult to see, and the highest point of the liquid surface is clearer, so often read the highest point of the liquid surface, read the position of the eye should be adjusted, so that the highest point of the liquid surface before and after the same level position.
For pipette, volumetric flask (including cylinder) reading method, can be analogized from the buret reading method, no longer introduced one by one.
Post time: Sep-14-2021
