Errors in glassware washing
1. The cleaning of glassware is a step in the inspection work. In actual work, many people often neglect to clean the glassware used immediately before and after the inspection, or the cleaning and inspection of the tools, and the inner wall of the device is seriously polluted with water droplets and dirt. The precipitate dries up and adheres to the inner wall, etc., and cannot be cleaned, which directly affects the accuracy of the data
2. There are many varieties of general quality inspections, and it is impossible to use a set of special instruments for each indicator, which is often used alternately, and the instruments used are not strictly cleaned or cleanliness inspections will inevitably lead to alternate contamination between reagents. Affect the accuracy of test results
3. On the other hand, when the properties and washing methods of the capacity measuring tool and the non-volume measuring tool are mixed together, they are all washed with decontamination powder, which will cause the capacity of the capacity measuring tool to be inaccurate and affect the accuracy of the measurement results.
Two errors in the heating of glass containers
1. The heating process is a common step in physical and chemical analysis. In actual work, some people often ignore or simply do not know which instruments can be heated, and even errors occur. In fact, not all glass containers can be directly heated, such as measuring cylinders, measuring cups, and volumetric reagents. Bottles, etc. cannot be directly heated, and reaction vessels such as beakers, flasks, triangular flasks, etc. should be used as appropriate. If these basic knowledge are not understood in actual work, errors will inevitably occur, and even inspection accidents will occur.
2. When heating the glass container, do not put the container on the asbestos mesh, but directly place the container in the electric furnace, so that the container is heated unevenly or even burst.
3. During use, the temperature changes too drastically, or the hot glass container that is quenched or removed at high temperature is placed directly on the table instead of placing it on the asbestos net as required, resulting in the rupture of the container 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 the heating appliance that needs to be accurately weighed, it should be dried and taken out after a little cooling (about 30s), put in the dryer to cool to room temperature, and then weighed ( 30min) When the warm appliance is put into the dryer, leave a gap in the cover first, wait for a few minutes, and then cover it tightly; when moving the dryer, you should not only end the lower part, but press the cover to move it to prevent the cover slip, causing unnecessary losses
Three errors in the selection and use of glass containers
Accurately measuring the volume of the solution in volumetric analysis is an important factor in obtaining good analytical results. Therefore, it is necessary to use volumetric instruments correctly, such as burettes, pipettes, volumetric flasks, etc. There are often some errors in actual operation.
1. It is not possible to correctly distinguish between acid burettes and basic burettes and their performance. During use, acid burettes are often mistaken for basic burettes; basic burettes are mistaken for acid burettes. The glass piston cannot hold alkaline solution, because the alkaline solution can corrode the glass to make the piston rotate, and the bottom end of the alkaline burette is connected with a rubber tube, and cannot hold the solution that corrodes the rubber such as acid or oxidant, such as: AgNO3KM-nO4I2 and other solutions
Before loading the standard solution into the burette, wash the burette 2 to 3 times without first washing the burette with 5 mL to 10 mL of the standard solution. During the operation, the burette with both hands is rotated slowly to make the standard solution flow through the whole tube, and the solution flows out from the lower end of the burette to remove the inside of the burette. The residual water should be loaded into the solution for titration, otherwise the concentration of the standard solution will be diluted
Regardless of the amount of standard solution used in the titration, the correct selection of different types of burettes is generally less than 10mL, and 10mL or 5mL microburettes are used. The amount is between 10mL and 20mL, and the 25mL burette is selected. If the amount exceeds 25mL, the 50mL burette is used for practical work. Some people do not pay attention to the error in this regard. Some standard solution dosage is less than 10mL and still use 50mL burette, and some standard solution dosage exceeds 25mL, still use 25mL burette, add in several times, etc. These situations are all wrong practices and cause large error
2. Do not use the volumetric flask correctly according to the rules Volumetric flask is a commonly used volumetric device for measuring the volume of a certain solution. It is mainly used to dilute a certain amount of solution to a certain volume of volumetric devices, but in practice, some people often use it for long-term use. The storage solution, especially the alkaline solution, will corrode the bottle wall and make the bottle stopper stick. The prepared solution cannot be stored in the volumetric flask, but should be poured into the reagent bottle in time for preservation. The reagent bottle should be prepared before use. The solution was washed 2 to 3 times
3. The volumetric flask, burette, pipette and other measuring quantities are not regularly calibrated according to the regulations. When the standard value does not match the actual volume, the volume error is caused, and the system error is generally corrected every six months.
4. Not familiar with the capacity tolerances and standard capacity grades of various measuring instruments. Different types of capacity tolerances are different, resulting in the error caused by the improper selection of the measuring instruments. Usually, it is required to accurately measure a certain volume of solution. Pipettes and pipettes, but not measuring cylinders, measuring cups and other measuring tools cause errors
4 Errors in the basic operation of glass instruments
1. When storing reagents, do not understand the nature, use and precautions of the reagent bottle. Store it at will, do not follow the rules of storing solid reagents in wide-mouth bottles, liquid reagents in narrow-mouth bottles, glass stoppers for acidic substances, and rubber for alkaline substances. Plug, see the principle of brown bottle for substances that are easily decomposed by light (such as AgNO3I2 liquid, etc.)
When taking the reagent, the stopper is not placed on the operating table according to the regulations, resulting in the contamination of the reagent, thus affecting the measurement result.
2. When using a weighing bottle to weigh the sample, do not dry the weighing bottle at 105°C first, and then use it after cooling to constant weight; take the dried weighing bottle directly by hand, instead of using a dry and clean one. The paper strip is put on the weighing bottle to get it, which will cause impurities in the weighing bottle, which will affect the accuracy of the weighing result.
3. When the standard solution is loaded into the burette, the concentration of the standard solution is changed or contaminated with the help of a funnel or other container
Do not adjust the liquid level to 0.00 before each measurement, and do not wait for 1 to 2 minutes after the start and end of the titration to allow the solution attached to the inner wall to flow down before reading, and the volume error is caused by the reading immediately.
When the titration speed is too fast, the solution is released in the form of running water, and even when it is close to the end point, the titration speed does not slow down, resulting in inspection errors when the titration passes the end point.
When reading (colorless or light-colored solution), the line of sight of the eye and the low point of the concave surface of the solution in the burette are not kept horizontal; the colored solution does not make the line of sight of the eye and the high point on both sides of the solution surface in the burette level, etc., cause volume error
4. When sucking the solution with a washed pipette, it is not necessary to use filter paper to absorb the water inside and outside the top, and then wash the pipette 2 to 3 times with the pipetted solution to ensure the pipetting solution. no change in concentration
When pipetting the solution, use the thumb and middle finger of the right hand to hold the upper part of the neck marking line, and insert the pipette into the solution, neither too deep nor too shallow. Volumetric accuracy; too shallow tends to create air suction
When putting in the solution, make the pipe dust vertically against the inner wall of the container, and let the solution in the tube flow down naturally along the wall of the container. After waiting for 10s to 15s, take out the pipette. When calibrating the pipette, the remaining liquid volume at the end has been considered, otherwise it will cause volume errors and affect the accuracy of the results.