Error analysis of the hottest glassware

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Error analysis of glassware use

I. errors in glassware washing

1 The cleaning of glassware is the leading step in the inspection work. In practical work, many people often ignore the cleaning and inspection of glassware before and after inspection. As a result, the inner wall of the appliance is seriously hung with drops of water, dirt, sediment dried up and adhered to the inner wall, which cannot be cleaned, directly affecting the accuracy of the data

2. Generally, there are many varieties and miscellaneous items in quality inspection. It is impossible to use a set of special instruments for each index, which are often used alternately, and the instruments used are not strictly cleaned or cleaned. It will inevitably cause alternating pollution between reagents. Thus affecting the accuracy of the test results

3. On the other hand, the nature of volumetric measuring tools and non volumetric measuring tools and the washing method with large initial clamping force are mixed together, and both of them are washed with decontamination paint, which leads to the inaccurate capacity of volumetric measuring tools and affects the accuracy of measurement results

II. Errors in heating glass containers

1 Heating process is a common step in physical and chemical analysis. In practical work, some people often ignore or cannot figure out which instruments can be heated at all, resulting in errors. In fact, not all glass containers can be heated directly, such as measuring cylinders, measuring cups, volumetric flasks, reagent bottles, etc. Beakers, flasks, flasks and other reaction vessels should be selected as appropriate. If you don't know these basic knowledge in actual work, errors will inevitably occur, and even cause inspection accidents

2. When heating glass containers, the containers are not placed on asbestos, but directly placed in the electric furnace, so that the containers are heated unevenly, or even burst

3. In the process of use, the temperature changes too violently, or the hot glass containers that are quenched or removed at high temperature are directly placed on the table instead of being placed on the asbestos as required, resulting in the rupture of the containers and the loss of reagents, affecting the normal progress of the inspection work

4. In actual work, some people are afraid of trouble and are not used to using the dryer correctly. For heating appliances that need to be accurately weighed, dry them, take them out and cool them slightly (about 30s), put them into the dryer to cool them to room temperature, and weigh them (30min). When a warm appliance is put into the dryer, leave a gap in the cover first, and then cover it tightly after a few minutes; When moving the dryer, you should not only end the lower part, but press and hold the cover to move to prevent the cover from sliding and causing unnecessary losses

III. errors in the selection and use of glass containers

accurate measurement of the volume of solution in volume analysis is an important factor to obtain good analysis results. Therefore, it is necessary to correctly use volumetric instruments, such as burette, pipette, volumetric flask, etc., and there are often some errors in actual operation

1. Unable to correctly distinguish between acid burette and basic burette and their performance. Acid burets are often mistaken for basic burets in the process of use; Basic burette is mistaken for acid burette. In this way, there are many mistakes. Because there is a glass piston at the lower end of the acid burette, it cannot contain alkaline solution, because alkaline solution can corrode the glass. Turn the piston. The lower end of the basic burette is connected with a rubber tube, which cannot contain solutions such as acid or oxidant that corrode the rubber, such as AgNO3, km-no4, I2 and so on

before loading the buret into the standard solution, do not wash the buret with 5ml ~ 10ml of the standard solution for 2 ~ 3 times. During operation, the two hand flat end burette rotates slowly to make the standard solution flow through the whole tube, and make the solution flow out from the lower end of the burette to remove the residual water in the tube. Then add the solution for titration, otherwise the concentration of the standard solution will be diluted

choose different types of burette correctly, not according to the dosage of standard solution during titration. Generally, when the dosage is less than 10ml, select 10ml or 5ml micro buret. When the dosage is between 10ml and 20ml, select 25ml buret. If the dosage is more than 25ml, select 50ml buret. In practical work, some people don't pay attention to this error. 50ml burette is still used when the dosage of some standard solutions is less than 10ml, and 25ml burette is still used when the dosage of some standard solutions is more than 25ml, which is added several times. These are wrong practices and cause great errors

2. Use volumetric flasks incorrectly. Volumetric flask is a kind of capacity used for measuring the volume of a certain solution between the main body and the hydraulic control box. Without material, it can not be used as an aircraft measuring instrument, which is mainly used to dilute a certain amount of solution to a certain volume. But in practice, some people often use it to store solutions for a long time, especially alkaline solutions, which will erode the bottle wall and make the cork stick and cannot be opened. The prepared solution cannot be stored in the volumetric flask, but should be poured into the reagent bottle in time for storage. The reagent bottle should be washed with the prepared solution for 2 ~ 3 times first

3. The volumetric flask, burette, pipette and other measuring tools are not calibrated regularly as required. Sometimes its standard value does not conform to the real volume, resulting in volume error, resulting in systematic error. Generally, it is corrected once every half a year

4. Not familiar with the capacity tolerance and standard capacity grade of various gauges, and the capacity tolerance of different types is different, which leads to the error caused by the improper selection of gauges. Generally, when it is required to accurately measure a certain volume of solution, pipettes and pipettes should be used instead of measuring cylinders, measuring cups and other measuring tools to cause errors

IV. errors related to the basic operation of glassware

1 Do not know the nature, purpose and precautions of reagent bottles when storing reagents. Put it at will, and do not follow the principle of putting solid reagents in wide mouthed bottles, liquid reagents in thin mouthed bottles, acidic substances with glass stoppers, alkaline substances with rubber stoppers, and substances that are easily decomposed by light with brown bottles (such as AgNO3, I2 liquid, etc.). This will cause impurities or formula changes, resulting in errors

when taking the reagent, the cork is not placed upside down on the operating table according to the regulations, resulting in reagent pollution, which affects the determination results

2. When weighing the sample with weighing bottle, do not dry the weighing bottle at 105 ° C first, and take it after cooling the constant weight; The dried weighing bottle should be taken directly by hand, rather than put a dry and clean piece of paper on the weighing bottle. It causes impurities in the weighing bottle and affects the accuracy of weighing results

3. When the burette is filled with standard solution, the concentration of standard solution is changed or polluted with the help of funnel or other containers

before each measurement, it is not necessary to comprehensively consider the energy efficiency, environmental protection and product quality. The liquid level is adjusted to the position of "0.00". After the titration starts and ends, it is not allowed to wait 1min ~ 2min as required to make the solution attached to the inner wall flow down before reading, but immediately cause volume error

if the titration speed is too fast, the solution will be released in flowing water. Even when it is close to the end point, the titration speed will not slow down, resulting in the test error when the titration passes the end point

when reading (colorless or light colored solution), do not keep the line of sight of the eyes and the lowest point of the lunar surface of the solution in the burette horizontal; The colored solution does not make the line of sight of the eyes horizontal with the highest point on both sides of the solution surface in the buret, resulting in volume error

4. When using the cleaned pipette to absorb the solution for the first time, you should first absorb the water inside and outside the tip with filter paper, and then wash the pipette with the removed solution for 2 to 3 times to ensure that the concentration of the pipette solution remains unchanged

when taking the solution, hold the upper part of the neck marking line with the thumb and middle finger of your right hand, and insert the pipette into the solution. It should not be too deep or too shallow. Too deep will cause too much solution to adhere to the outside of the tube, affecting the accuracy of measuring the volume of the solution; Too shallow will often produce empty suction

when putting the solution, make the pipe vertical and the pipe dust lean against the inner wall of the container, and let all the solution in the pipe naturally flow down the wall of the container. After waiting for 10s ~ 15s, take out the pipette, and do not blow out the solution remaining in the tip, because the volume of the solution retained at the end has been considered when correcting the pipette, otherwise it will cause volume error and affect the accuracy of the result

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