Measuring cup and measuring cylinder are a kind of measuring instruments for measuring liquid volume, and their accuracy requirements are not high. General capacity is 5mL, 10mL, 25mL, 50mL, 100mL, 250mL, 500mL, 1000mL, etc. , you can choose according to your needs. Don't use large-capacity measuring cups and measuring cylinders to measure small volumes, which will reduce the accuracy. When measuring liquid, let the measuring cylinder be placed stably for more than 15 seconds. When the liquid level is calm, keep the line of sight flush with the lowest meniscus of the liquid in the measuring cylinder (cup). Too high or too low will cause large errors due to inaccurate reading (see figure 1). Generally speaking, measuring cylinders are more accurate than measuring cups.

2. Pipes and pipets

Pipets and pipets are both precision measuring instruments for accurately measuring a certain volume of liquid. The pipette is a pot-bellied tube with constant volume, with only one scale line and no scale line, so the scale line is the specified volume at constant temperature; General capacity is 1mL, 2mL, 5mL, 10mL, 20mL, 25mL, 50mL, 100mL.

The pipette is a linear pipette with a graduated scale, and the scalar on the pipette is the maximum capacity. Generally, there are 0. 1mL, 0.2mL, 0.5mL, 1mL, 2mL, 5mL, 10mL and other specifications. For example, the maximum capacity of a 5mL pipette is 5.00mL, and its scales are 5.00, 4.50, 4.00...0; Therefore, it can move any volume of liquid within 0 ~ 5ml, and its accuracy is higher than that of measuring cylinder.

(1) First, use the ear washing ball to absorb 1/4 pipette volume of sulfuric acid-potassium dichromate lotion, then hold it by hand, keep the pipette level, hold it with both hands and rotate it, so that the lotion can wet all the pipe walls, and pour out the lotion from the upper mouth. Then rinse the residual lotion with tap water, rinse it with distilled water for several times, and finally rinse it with the removed liquid for three times, each time the dosage is about 1/4 of the pipette volume.

(2) pipetting operation:

Insert the tip of the pipette into the removed solution, place the thumb and middle finger of the right hand (or left hand) above the neck label, hold the ear washing ball with the left hand (or right hand), insert the tip of the ear washing ball into the neck opening, seal it, and slowly let the ear washing ball recover naturally until the liquid rises above the neck label, quickly remove the ear washing ball, and immediately hold the neck opening with the index finger of the right hand (or left hand). Hold the container (beaker, volumetric flask, etc.). ) Put the removed solution in the left hand (or right hand) to vertically lift the pipette until the neck marking line is horizontal with the line of sight, connect the container mouth held by the left hand to the tip of the pipette, loosen the index finger of the right hand (or left hand) or gently rotate the pipette with the thumb and middle finger, so that the liquid level drops slowly and steadily until the meniscus on the cushion surface is tangent to the marking line, and immediately press the index finger to prevent the liquid from flowing down; If there is half a drop at the tip, it should be against the inner wall of the original blood vessel.

(3) If the experiment requires higher accuracy, the pipette needs to be corrected.

Step 3 measure the bottle

A volumetric flask is a liquid measuring instrument for preparing accurate concentration solution. At a certain temperature, its internal volume is the specified volume: the general capacity is 10mL, 25mL, 50mL, 100mL, 200mL, 250mL, 500mL, 1000mL, 2000mL and other specifications.

There are two kinds of volumetric bottles: grinding plugs and plastic plugs. The usage method is as follows:

(1) Check whether the cork leaks?

Add some water to the bottle, plug it, hold the bottle in your left hand and the plug in your right hand, and turn the bottle upside down to see if the plug leaks or seeps. If there is no leakage, rotate the cork 180, then plug it tightly and repeat the above operation. If there is no leakage or leakage, the bottle can be used.

(2) washing:? Clean the volumetric flask and stopper with sulfuric acid-potassium dichromate lotion, tap water and distilled water.

(3) solution preparation operation:

①? To prepare a solution with a solid substance:

Put the weighed solid into a beaker, pour some water to dissolve it, transfer it to a volumetric flask, and then rinse the beaker with a small amount of distilled water for 3-5 times. Combine the washing liquid into the volumetric flask, add distilled water to about 3/4, shake well first, then add water to the scale, plug the stopper, hold the cork with one index finger and the bottom of the flask with the other hand (small volumetric flask only needs one hand), and shake the flask horizontally.

②? The liquid solution is diluted to another precise concentration:

Transfer a certain volume of solution to a volumetric flask with a pipette, and then add water. The following operations are the same as ①.

4? burette

There are two kinds of burettes: acid burette and alkaline burette. All the solutions are carried out in an acidic burette except that the alkaline solution is put in an alkaline burette.

The lower end of the acid burette is a glass piston, which can be used as follows:

(1) Vaseline daubing operation:

Remove the piston, dry it with filter paper, and then dry the piston groove; Coat a thin layer of vaseline on the big end of the piston, and coat a thin layer of vaseline on the small end of the piston groove. Plug the piston tightly in the piston groove and rotate the piston until the contact between the piston and the plug groove is transparent and the piston rotates flexibly. If there is opaque wire drawing at the contact, it needs to be oiled again. Put on a rubber ring to prevent the piston from slipping out of the plug groove.

(2) Check for water leakage:

Add water to the burette near the zero scale, stand vertically on the titration stand, and observe whether the burette mouth drops water or whether the gap between the piston and the plug groove leaks water. If there is no leakage, rotate the piston before inspection180; If there is no leakage, the next operation can be carried out; If there is any leakage, it needs to be reapplied with vaseline before inspection.

(3) Washing method:

Pour in sulfuric acid-potassium dichromate lotion with a volume of about 1/4, slowly tilt and rotate the burette to make the tube wall completely stained with lotion, then open the piston to make the lotion fill the lower end, then close the piston, pour the lotion back into the storage bottle from the nozzle, and open the piston to let all the lotion return to the storage bottle. Rinse the residual lotion with tap water, rinse it with distilled water for 3 ~ 4 times, and wash away Ca2+, Mg2+ and Cl-? Plasma, finally washed with titrant for 3 times, can be loaded with titrant.

(4) filling the solution and expelling bubbles:

Add solution above the zero scale, open the piston to the maximum, release part of the solution, and rely on gravity to make the solution fill the lower end of the piston, drive away bubbles and close the piston. Check whether there are bubbles at the lower end? If there are still bubbles, reopen the piston, accelerate the burette from top to bottom, drive away the bubbles and close the piston.

(5) Reading operation:

The burette is a precise liquid measuring instrument, so reading is a very important operation.

The burette should be installed vertically on the titration stand. When reading, the line of sight should be kept on the same level as the lowest point of the solid line at the lower part of the liquid meniscus. Too high or too low will bring errors. If the titration stand is too high, take off the burette, gently hold the upper part of the burette with the thumb and forefinger of the left hand (or right hand), let the burette be naturally vertical under the action of gravity, move to the lowest point of the solid line at the lower part of the meniscus, and read flush with the line of sight. 50 ml and 25 ml burettes can read two decimal places.

In order to facilitate reading, you can make a "reading card". Stick a piece of 3× 1.5cm black paper (or black ink) on the white card to make a reading card. When reading, put the hand-held reading card behind the burette so that the black part is about 1mm below the meniscus. At this time, the meniscus will reflect black, and read the lowest point of this meniscus.

For a dark solution like potassium permanganate, read the highest point of the liquid level.

There is also a blue line burette. There is a whole piece of white opaque glass in the burette with a blue line in the middle, so the liquid has two meniscus intersecting at a certain point of the blue line of the burette. When reading, the line of sight should be on the same level with this point. If it is a colored solution, the line of sight should be tangent to the highest points on both sides of the liquid surface.

(6) Titration operation:

When titrating, it is best to start with a scale of 0.00mL or close to 0.00mL each time to reduce the error caused by uneven calibration of burette. The burette is best carried out in a conical flask and in a beaker if necessary.

The burette is vertically clamped on the burette clamp, and the lower end extends into the mouth of the conical bottle about 1cm. The left hand controls the burette piston, with the thumb in front and the index finger and middle finger behind. Fingers slightly bent, palms empty, gently buckle the open piston inward to prevent the piston from loosening or even ejecting. Hold the conical bottle in your right hand, shake it while dripping, and make a circular rotation in one direction. Don't shake it back and forth or up and down, so it's easy to spill the solution. Start titration quickly, generally controlled at about 10mL per minute, about 3 ~ 4 drops per second, that is, drop by drop, or drop by drop. When the titration is near the end point, it should be added drop by drop or half drop, that is, after adding one drop or half drop, a small amount of water is blown into the conical bottle wall by washing the bottle, and then shaken evenly, and then another drop or half drop is added and shaken evenly until the indicator color changes no longer, and the end point can be considered as reached.

The lower end of the basic burette is connected with a rubber tube, which is filled with a glass ball and a small glass tube with a sharp mouth instead of a glass piston. The use method is the same as that of the acid burette, but there are the following differences.

(1) washing method:

Because rubber will be corroded by oxidant, when cleaning with lotion, invert the upper mouth of burette in a beaker filled with lotion and connect the sharp mouth to the water pump. Turn on the water pump, gently hold the glass ball, and let go of the glass ball until the lotion slowly rises to the place close to the rubber tube. After soaking the lotion for a period of time, leave the water pump, unplug the rubber tube, let the lotion flow out, then rinse with tap water, rinse with distilled water for several times, install the rubber tube, clean glass ball and glass tube with sharp mouth, and rinse with titration solution for three times.

(2) Liquid filling and bubble driving:

Fill the titration solution above the zero scale, bend the rubber tube upward, and gently pinch the glass ball, so that the liquid slowly rises to the communication device to drive away bubbles, and fill the rubber tube and the glass tube with a pointed mouth.

(3) Pinch burette posture:

With the left thumb in front and the index finger behind, hold the middle of the rubber tube slightly higher than the glass ball, and squeeze the rubber tube to the right to form a gap between the rubber tube and the glass ball, so that the solution can flow out. But be careful not to clip the glass tube under the glass ball, otherwise air will enter and bubbles will easily form. The answer comes from