First, the structure and accuracy of the burette Â
( 1 ) Construction of the burette is the most basic measuring instrument in the capacity analysis. It consists of an elongated glass tube and a switch with an accurate scale. The burette is the most basic measuring instrument in volumetric analysis and is used to determine the volume of solution flowing out of the tube at the time of titration. Â
( 2 ) The burette for accuracy constant analysis is 50ml or 25ml , the scale is as small as 0.1ml , the reading can be estimated to 0.01ml , and the reading error is generally ±0.02ml , so the volume of the solution used for each titration is preferably above 20ml . If the volume used for titration is too small, the influence of the burette scale reading error increases.
Second, the type of burette Â
( 1 ) Acid burette (Boise burette) Â
The glass piston of the acid burette is fixedly fitted to the burette, so it cannot be replaced at will. Pay attention to whether the cork rotates freely, usually take out the piston, wipe it dry, wipe a thin layer of Vaseline as a lubricant on both ends of the piston, then insert the piston, tighten it tightly, rotate it a few times to make the Vaseline evenly distributed (almost transparent) Then, put a rubber band on the end of the piston to fix it. Note that Vaseline should not be painted too much, otherwise it will easily block the small hole in the piston or the tip of the lower end of the burette. Test leaks before use. The acid burette can be used for the general standard solution. However, the alkaline titration solution often causes the glass plug to adhere to the glass hole, so that it is difficult to rotate. Therefore, the alkaline titration solution should be a basic burette. However, if the alkaline titration solution is not used for a long time, it can be rinsed with water immediately after use, and an acid burette can also be used. Â
( 2 ) Basic burette Â
The bottom of the tube end of the basic burette is connected with a rubber tube, and a glass bead control switch is installed in the tube, which is generally used as a titration of an alkaline standard solution. It is not as accurate as an acid burette, as long as the liquid level changes due to the elasticity of the rubber tube. An oxidizing solution or other solution that is easy to work with rubber, such as potassium permanganate, iodine, silver nitrate, etc., cannot be used with a basic burette. Before use, check whether the rubber tube is broken or aged and the size of the glass beads is suitable. It can be used without leakage. Â
Third, preparation before use (using acid burette as an example)
1 titration tube washing
1.1 No obvious oily burette, rinse directly with tap water or wash with soapy water or washing powder, but do not use decontamination powder to avoid scratching the inner wall, affecting the accurate measurement of volume.
1.2 When the oil is not easy to wash, wash it with chromic acid washing solution. When washing, remove the water in the tube as much as possible, close the piston, pour 10 ~ 15 ml of washing liquid into the burette, hold the burette with both hands, and tilt to the tube while rotating until the washing liquid fills all the walls. After standing up, open the piston and put the lotion back into the original bottle.
1.3 When the oil is serious, it needs to be immersed in the burette with more washing liquid for ten minutes or more, even with warm washing liquid for a period of time. After the washing liquid is released, it is first rinsed with tap water and then rinsed with distilled water for 3-4 times. The inner wall of the washed burette should be completely wetted by water without hanging water drops.
2 , the oil of the burette
The method of oiling is: put the burette on the table, remove the rubber ring of the fixed piston, take out the piston, and wipe the inner wall of the piston and the plug with clean paper or cloth (if there is old grease plug in the piston hole) Block, use wire to gently remove, if the tip of the pipe is blocked by grease, first fill the whole pipe with water, then place the pipe tip in hot water to dissolve, suddenly open the piston and wash it away). Use your fingers to apply a small amount of Vaseline (or vacuum grease) to the circumference of the piston hole and apply a thin layer on the circumference of the piston hole. Do not apply Vaseline close to the piston hole to avoid blocking the piston hole. After painting, put the piston back into the sleeve and turn the piston in the same direction until it is completely transparent when viewed from the outside. Then use a rubber band to secure the piston in the sleeve to prevent slipping out. The oil-coated acid burette piston and plug should be tightly sealed and watertight, and the rotation should be flexible. 3 , acid burette test leak
Close the burette piston, fill the distilled water to a certain line, and stand the burette for 2 minutes. Carefully observe whether the liquid level on the reticle is lowered, whether there is any water leakage at the lower end of the burette, and there is no water oozing out in the gap of the piston. Then rotate the piston 180° and wait for 2 minutes before observing. If there is water leakage, dry the oil again.
4 , the acid burette loading solution and bubble
Shake the standard solution in the bottle, mix the water condensed on the inner wall of the bottle into the solution, and then rinse the burette with the standard solution. Use about 10ml each time , release about 1/3 from the lower mouth to wash the tip part, then close the piston. Hold the burette and rotate it slowly to make the solution contact with the inner wall of the tube. Finally, pour the solution out of the tube, but do not open the piston to prevent the grease on the piston from rushing into the tube. Wash it as much as possible and then wash it a second time. Each time you need to rinse the tip of the mouth, so wash 2-3 times, you can remove the residual moisture in the burette to ensure the standard solution concentration. When filling the liquid, inject the burette directly from the reagent bottle, and do not pass through other containers such as the funnel. When the standard solution is filled into the burette, the outlet tube is not filled with the solution. At this time, the acid burette is tilted by about 30° , and the left hand quickly opens the piston to flush the solution, so that all the outlet tubes can be filled . After the bubbles are removed, the standard solution is added . Above the “0†scale, wait for 30 s , then turn the piston and adjust the liquid level to the mark of 0.00 ml to titrate.
Fourth, the operation notes Â
1. After the burette is filled with the standard solution, the solution on the outer wall of the tube should be dried to avoid the flow of the solution or the evaporation of the solution to cool the solution (especially in summer). When holding the burette, also avoid the palm of the hand holding the wall of the solution part, so as to avoid the hand temperature rising above room temperature (especially in winter) and the volume of the solution to expand, resulting in reading error. Â
2. When using the acid burette, fix the burette on the burette clamp, the piston handle is to the right, the left hand is extended to the right from the middle, the thumb is in front of the tube, the index finger and middle finger are behind the tube, and the three fingers are gently held in parallel. The piston handle, the ring finger and the small pointing palm are bent. The index finger and the middle finger are pressed from the bottom to the end of the piston handle, and the thumb cooperates with the upper side. When turning, do not straighten the middle finger and index finger. It should be slightly bent and gently buckled to the left. This is easy to operate and prevents the piston from being ejected. Â
3. Each titration must start from the scale zero, so that each measurement can offset the calibration error of the burette. Â
4. After filling the standard solution, the “ first reading †zero point before titration should be set to stand for 1 to 2 minutes and read again. If the liquid level reading has not changed, it is still zero and can be titrated. The titration should not be too fast, it is better to release 3 to 4 drops per second , and it should not be flowed into the liquid column. Especially when it is close to the metering point, it should be added drop by drop (the titration can be accelerated before the metering point). ). After titration to the end point, and the like to be 1 to 2 minutes, so that the standard solution to the inner wall of the flow down again after reading, if rather slow release titrant, like half a minute after the reading can "always read" is also read at least two Times. Â
5 , the burette reading can be vertically clamped on the titration tube holder or the upper end of the hand-held burette to freely read the scale vertically. The reading should also pay attention to the position of the eye and the liquid level at the same level, otherwise it will cause errors. Â
The reading should be at the lowest point on the lower edge of the meniscus, but when the standard solution is too dark, you can read the highest point on both sides of the liquid when you can't observe the lower edge. The “ first reading †and “ final reading †apply the same standard. Â
6. The burette is colorless or brown. Generally, it is necessary to avoid the light titration solution (such as silver nitrate standard solution, sodium thiosulfate standard solution, etc.), and a brown burette is required.
( 1 ) Construction of the burette is the most basic measuring instrument in the capacity analysis. It consists of an elongated glass tube and a switch with an accurate scale. The burette is the most basic measuring instrument in volumetric analysis and is used to determine the volume of solution flowing out of the tube at the time of titration. Â
( 2 ) The burette for accuracy constant analysis is 50ml or 25ml , the scale is as small as 0.1ml , the reading can be estimated to 0.01ml , and the reading error is generally ±0.02ml , so the volume of the solution used for each titration is preferably above 20ml . If the volume used for titration is too small, the influence of the burette scale reading error increases.
Second, the type of burette Â
( 1 ) Acid burette (Boise burette) Â
The glass piston of the acid burette is fixedly fitted to the burette, so it cannot be replaced at will. Pay attention to whether the cork rotates freely, usually take out the piston, wipe it dry, wipe a thin layer of Vaseline as a lubricant on both ends of the piston, then insert the piston, tighten it tightly, rotate it a few times to make the Vaseline evenly distributed (almost transparent) Then, put a rubber band on the end of the piston to fix it. Note that Vaseline should not be painted too much, otherwise it will easily block the small hole in the piston or the tip of the lower end of the burette. Test leaks before use. The acid burette can be used for the general standard solution. However, the alkaline titration solution often causes the glass plug to adhere to the glass hole, so that it is difficult to rotate. Therefore, the alkaline titration solution should be a basic burette. However, if the alkaline titration solution is not used for a long time, it can be rinsed with water immediately after use, and an acid burette can also be used. Â
( 2 ) Basic burette Â
The bottom of the tube end of the basic burette is connected with a rubber tube, and a glass bead control switch is installed in the tube, which is generally used as a titration of an alkaline standard solution. It is not as accurate as an acid burette, as long as the liquid level changes due to the elasticity of the rubber tube. An oxidizing solution or other solution that is easy to work with rubber, such as potassium permanganate, iodine, silver nitrate, etc., cannot be used with a basic burette. Before use, check whether the rubber tube is broken or aged and the size of the glass beads is suitable. It can be used without leakage. Â
Third, preparation before use (using acid burette as an example)
1 titration tube washing
1.1 No obvious oily burette, rinse directly with tap water or wash with soapy water or washing powder, but do not use decontamination powder to avoid scratching the inner wall, affecting the accurate measurement of volume.
1.2 When the oil is not easy to wash, wash it with chromic acid washing solution. When washing, remove the water in the tube as much as possible, close the piston, pour 10 ~ 15 ml of washing liquid into the burette, hold the burette with both hands, and tilt to the tube while rotating until the washing liquid fills all the walls. After standing up, open the piston and put the lotion back into the original bottle.
1.3 When the oil is serious, it needs to be immersed in the burette with more washing liquid for ten minutes or more, even with warm washing liquid for a period of time. After the washing liquid is released, it is first rinsed with tap water and then rinsed with distilled water for 3-4 times. The inner wall of the washed burette should be completely wetted by water without hanging water drops.
2 , the oil of the burette
The method of oiling is: put the burette on the table, remove the rubber ring of the fixed piston, take out the piston, and wipe the inner wall of the piston and the plug with clean paper or cloth (if there is old grease plug in the piston hole) Block, use wire to gently remove, if the tip of the pipe is blocked by grease, first fill the whole pipe with water, then place the pipe tip in hot water to dissolve, suddenly open the piston and wash it away). Use your fingers to apply a small amount of Vaseline (or vacuum grease) to the circumference of the piston hole and apply a thin layer on the circumference of the piston hole. Do not apply Vaseline close to the piston hole to avoid blocking the piston hole. After painting, put the piston back into the sleeve and turn the piston in the same direction until it is completely transparent when viewed from the outside. Then use a rubber band to secure the piston in the sleeve to prevent slipping out. The oil-coated acid burette piston and plug should be tightly sealed and watertight, and the rotation should be flexible.
Close the burette piston, fill the distilled water to a certain line, and stand the burette for 2 minutes. Carefully observe whether the liquid level on the reticle is lowered, whether there is any water leakage at the lower end of the burette, and there is no water oozing out in the gap of the piston. Then rotate the piston 180° and wait for 2 minutes before observing. If there is water leakage, dry the oil again.
4 , the acid burette loading solution and bubble
Shake the standard solution in the bottle, mix the water condensed on the inner wall of the bottle into the solution, and then rinse the burette with the standard solution. Use about 10ml each time , release about 1/3 from the lower mouth to wash the tip part, then close the piston. Hold the burette and rotate it slowly to make the solution contact with the inner wall of the tube. Finally, pour the solution out of the tube, but do not open the piston to prevent the grease on the piston from rushing into the tube. Wash it as much as possible and then wash it a second time. Each time you need to rinse the tip of the mouth, so wash 2-3 times, you can remove the residual moisture in the burette to ensure the standard solution concentration. When filling the liquid, inject the burette directly from the reagent bottle, and do not pass through other containers such as the funnel. When the standard solution is filled into the burette, the outlet tube is not filled with the solution. At this time, the acid burette is tilted by about 30° , and the left hand quickly opens the piston to flush the solution, so that all the outlet tubes can be filled . After the bubbles are removed, the standard solution is added . Above the “0†scale, wait for 30 s , then turn the piston and adjust the liquid level to the mark of 0.00 ml to titrate.
Fourth, the operation notes Â
1. After the burette is filled with the standard solution, the solution on the outer wall of the tube should be dried to avoid the flow of the solution or the evaporation of the solution to cool the solution (especially in summer). When holding the burette, also avoid the palm of the hand holding the wall of the solution part, so as to avoid the hand temperature rising above room temperature (especially in winter) and the volume of the solution to expand, resulting in reading error. Â
2. When using the acid burette, fix the burette on the burette clamp, the piston handle is to the right, the left hand is extended to the right from the middle, the thumb is in front of the tube, the index finger and middle finger are behind the tube, and the three fingers are gently held in parallel. The piston handle, the ring finger and the small pointing palm are bent. The index finger and the middle finger are pressed from the bottom to the end of the piston handle, and the thumb cooperates with the upper side. When turning, do not straighten the middle finger and index finger. It should be slightly bent and gently buckled to the left. This is easy to operate and prevents the piston from being ejected. Â
3. Each titration must start from the scale zero, so that each measurement can offset the calibration error of the burette. Â
4. After filling the standard solution, the “ first reading †zero point before titration should be set to stand for 1 to 2 minutes and read again. If the liquid level reading has not changed, it is still zero and can be titrated. The titration should not be too fast, it is better to release 3 to 4 drops per second , and it should not be flowed into the liquid column. Especially when it is close to the metering point, it should be added drop by drop (the titration can be accelerated before the metering point). ). After titration to the end point, and the like to be 1 to 2 minutes, so that the standard solution to the inner wall of the flow down again after reading, if rather slow release titrant, like half a minute after the reading can "always read" is also read at least two Times. Â
5 , the burette reading can be vertically clamped on the titration tube holder or the upper end of the hand-held burette to freely read the scale vertically. The reading should also pay attention to the position of the eye and the liquid level at the same level, otherwise it will cause errors. Â
The reading should be at the lowest point on the lower edge of the meniscus, but when the standard solution is too dark, you can read the highest point on both sides of the liquid when you can't observe the lower edge. The “ first reading †and “ final reading †apply the same standard. Â
6. The burette is colorless or brown. Generally, it is necessary to avoid the light titration solution (such as silver nitrate standard solution, sodium thiosulfate standard solution, etc.), and a brown burette is required.
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