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Operation errors and solutions of commonly used chemical measuring utensils

Views: 0     Author: Site Editor     Publish Time: 2023-08-02      Origin: Site

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Operation errors and solutions of commonly used chemical measuring utensils

Teflon series utensils have been used in the laboratory for long-term storage

Ⅰ. Graduated Cylinder

Common problems:
A. Reading with graduated cylinder in hand;

B. When reading, some look down and some look up;
C. What have cannot select the graduated cylinder of suitable measuring range according to needing to measure the volume of liquid;
D. If add too much liquid, then need to use the dropper to suck it out again.

Correct method: When using a measuring cylinder, you should choose the smallest size measuring cylinder that can be measured at one time according to the volume of liquid to be measured. The operation essentials are “measure the liquid, keep the cylinder stable; keep the mouth open to avoid outflow; change to dripping to the scale; when reading, keep the line of sight and the lowest point of the liquid level at the same level”. If the amount of liquid added accidentally exceeds the scale, you should hold a measuring cylinder and pour a small amount into the designated container, and then drop it to the scale with a dropper.

PFA graduated cylinder

Ⅱ. Volumetric flask

Common problems:
A. Do not test for leaks in advance, do not clean the volumetric flask; During the preparation of the standard solution, some students held the volumetric flask with their hands to prevent the volumetric flask from falling down.

Correct method: Check whether the volumetric flask is leaking before use. The method is as follows: add tap water to the vicinity of the marking line, cover the bottle stopper, press the stopper with the index finger, hold the part above the marking line of the flask neck with the other fingers, hold the bottom of the flask with the fingertips of another hand, stand upside down for two minutes, if not leakage, stand the bottle at attention, turn the stopper 180°, and try again upside down. If the volumetric flask and cork are stained, the cleaning method is similar to that of the burette. If it is not cleaned with tap water, you can soak it with lotion, then rinse the flask with tap water, and then wash it with distilled water three times. When constant volume, do not hold the volumetric flask by hand to avoid thermal expansion of the glass, affecting the volume of the volumetric flask, and then affecting the concentration of the solution.

B. Transfer the not cooled solution into the volumetric flask; incomplete transfer of solute.

Correct method: When preparing the standard solution in the volumetric flask, if it is a solid substance, it should be dissolved in a small beaker first (if there is a thermal effect, it should be cooled thoroughly before operation), then transfer the solution to a volumetric flask. To transfer all the solutes to the volumetric flask, it should be: put the mouth of the beaker close to the glass rod extending into the volumetric flask, the upper part of the glass is not against the bottle mouth, and the lower end is against the wall of the bottle, but do not use force (to prevent the volumetric flask from falling down) , let the solution flow into the bottle along the glass rod. After the solution flows in, move the beaker upright along the glass rod and then move the glass rod up along the wall of the bottle, take it out and put it back into the beaker, and rinse the glass rod and beaker with a washing bottle. Press the glass rod with the tiger’s mouth (to prevent the glass rod from shaking during washing), grasp the mouth of the beaker with five fingers, wash the glass rod first when washing the bottle, and then rinse around the beaker. 

Transfer the solution to the volumetric flask in the same way, repeat two or three times until it is washed. For the preparation of standard solutions with liquids (such as concentrated sulfuric acid), attention should also be paid to thermal effects, and other operating steps are similar to the preparation of solutions with solid substances.

PFA volumetric flask

Ⅲ. Burette

A. Do not check (or solve) whether the burette is blocked or leaking before use.

To check whether the burette is plugged, the correct method is: fill it with tap water, clamp it on the titration stand, let it rest for about 2 minutes, and observe whether there is any water drop seeping out. Rotate the piston 180 degrees and check it again. If there is no water seepage in the front and back two times, and the piston rotates flexibly, it can be used. Leakage should be re-applied with Vaseline, and the school with good conditions can be replaced; also check whether it is blocked, the blockage should be wiped off with Vaseline and the Vaseline in the small hole should be removed with filter paper and filaments. The method of applying Vaseline is: Take out the piston and use Use filter paper and a clean cloth to wipe off the water in the piston and piston groove. Dip a little vaseline with your fingers, apply a thin layer on both ends of the piston, and apply less on both sides away from the piston hole, so as not to block the plug hole with vaseline, or on the inner wall of the thick end of the piston and the thinner end of the plug groove Apply a thin layer of Vaseline (this method is better for beginners), insert the piston into the piston groove and tighten it, and at the same time rotate the piston in the same direction until the oil film in the piston is uniform and transparent; if the piston does not match, it needs to be replaced; If it is found that the rotation is not working or there are lines on the piston, it means that the Vaseline is not applied enough; if the Vaseline is squeezed out from the piston gap, or the piston hole is blocked, it means that the Vaseline is applied too much. In this case, the plug groove and the piston must be wiped clean and then re-coated with Vaseline; after the Vaseline is coated, the piston is fastened with a rubber ring to prevent the piston from falling off and being broken.

B. When washing, block the test tube by hand and wash it upside down several times; after washing with distilled water, do not rinse the burette with the operating solution, and use a funnel, beaker and other instruments when loading the operating solution; do not check whether there are bubbles in the burette; Wash the buret.

Correct method: Do not block the mouth of the burette with your hands when washing, so as to prevent the oil on your hands from contaminating the operating fluid in the burette.

Rinse the burette with the operating solution for 2~3 times in advance, use about 10ml for the first time, and about 5ml for the second and third times, otherwise the distilled water remaining in the burette will dilute the solution. For this reason, inject about 10ml of the operating solution, and then Hold the burette with both hands flat and turn it slowly so that the solution flows through the entire tube, then stand up the burette, open the cock of the burette, and let the solution flow out from the lower end of the open tube.

After rinsing for 2-3 times, the operating solution can be loaded. Note that the operating solution to be loaded should be poured directly from the storage bottle into the burette instead of relying on other instruments. For the inspection of air bubbles in rubber tubes, in addition to visual inspection, light inspection is also required. The burette can be washed with tap water first, and then washed with distilled water for 3 times. The washing liquid should not flow out from the lower end, but poured out from the upper mouth of the burette.

Although the requirements for washing the alkaline burette are the same as those of the acid burette, when it needs to be washed with lotion, it should be clamped upside down on the titration stand, the nozzle should be inserted into the bottle of lotion, the glass tip should be removed, and the latex tube Connect the ear wash ball, lightly press the latex tube at the glass ball with your hand, and suck up the lotion until it fills the whole tube (do not suck the lotion into the latex tube).

Soak for a few minutes, release the lotion, and rinse with tap water or distilled water. Note: No matter what kind of burette, every time it is washed (except for the operating liquid rinse), the outer wall should be wiped dry after pouring out the water, and the inner wall should be observed. If there are no water drops, it means that it is washed.

C. When the reading is close, the dripping speed cannot be controlled; the fingers are improperly operated, the liquid leaks or the glass ball in the alkaline burette is displaced.

Correct method: When using an acid burette, bend the ring finger and little finger of the left hand against the pointed mouth, and control the palm not to touch the small end of the piston. Be careful not to put the palm against the small end of the piston to prevent water leakage caused by the piston pushing out. Hold the piston handle gently with the thumb, index finger, and middle finger in parallel.

When turning the piston, do not straighten the index or middle finger to prevent the force that will pull the piston out. Operation movements should be gentle and gestures should be natural. The “T” head of the piston is perpendicular to the burette, which means it is closed, and the parallel is the maximum flow rate.

Therefore, the dripping speed is determined by the angle between the “T” head and the burette, the small angle, the fast dripping speed, as the angle increases, the dripping speed gradually decreases to zero. When using an alkaline burette, put the thumb of your left hand in front and the index finger behind, and squeeze the latex tube (the upper part of the glass ball) to form a gap between the glass balls of the latex tube. The dripping speed is controlled by the squeeze force. Control the size, and be careful not to pinch the lower part of the glass ball, otherwise the tip of the rubber tube will produce air bubbles when you let go.

D. Looking up or looking at the reading, the meniscus is not accurate. Bubbles in the buret cannot be removed.

Correct method: read again 1-2 minutes after loading or releasing the solution.

When reading, the burette can be clamped on the titration table, or the upper part of the burette without scale can be held by hand, and the burette should be absolutely vertical (do not hold it on the part containing the liquid, so as not to expand the liquid and the glass tube, nor tilt it). For colorless or light-colored solutions, the lowest point of the solid line at the lower edge of the meniscus should be read. The line of sight should be on the same level as the lowest point, preferably facing the light source. Black and white cardboard can also be used as an aid when reading, so that the meniscus boundary is very clear.

If the color of the solution is dark, the uppermost edge of the liquid surface should be read, but the initial reading and the final reading should be consistent, otherwise errors will easily occur. Bubbles in the burette: For the acid burette, the piston can be quickly turned to make the solution rush out quickly, and the bubbles will be taken away. If the bubbles cannot be taken away, the teacher will treat it with acidic potassium dichromate solution.

For alkaline burettes, the rubber tube can be bent upwards, and the glass beads can be squeezed by using the principle of bubbles floating up, so that the solution can be sprayed out from the tip, and the bubbles can be eliminated. Note that readings can only be taken after air bubbles have been removed.

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