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1. To understand the principle of electronic balance and master the use of electronic balance.
2. To learn to weigh the sample with direct method and decrement method.
Analytical balance is one of the most important and commonly used apparatus in chemical experiment.The commonly used is electronic balance.It uses electronic device to complete the adjustment of electromagnetic force compensation,so that the object can achieve the balance of force in the gravity field;or through the adjustment of electromagnetic torque,so that the object can achieve the balance of torque in the gravity field.
Electronic balance;Beaker;Weighing bottle;Weighing paper;Watch glass;Erlenmeyer flask;Medicine spoon.
Crude salt.
(1)Clean the balance and balance pan:Before using the balance,clean the balance pan and base plate with a soft brush.
(2)Level the balance:Check the level on the balance.If it is not level,adjust the level adjusting feet of the electronic balance to make the air bubble in the level in the center of the ring.
(3)Preheating:The electronic balance shall be preheated when it is powered on for the first time or powered off for a long time.The preheating time of each balance is different.The higher the accuracy level of the balance is,the longer the preheating time is required.Generally,the weighing can be carried out after the machine is started and preheated for 20 min.
(4)Calibration:If lying idle for a long time,position moves or environment changes,calibration shall be carried out generally.Press the calibration key CAL,the balance will display the required standard weight.Put on the standard weight until the g appears.The calibration is finished.
(5)Tare the balance:Place empty vessel on the pan.Be sure all the enclosure doors are shut.Press the“T”bar.Wait for the display to read“0.0000”.
(6)Measure the sample:Place sample on the pan.Close all the doors of the balance.Record the measurement.
(7)Turn power off and clean the chamber and pan:When weighing is finished,clean out the spills or residue in the pan and the balance.Close all doors of the balance.
(1)Direct weighing:Some stable,water-insensitive and noncorrosive reagents can be weighed out on balance directly.Weigh the mass of weighing bottle,bottle body and cap,and record the data in Table 1-1.
(2)Weighing by difference:This method assumes that thereceiver flask has a large enough opening that there is minimal risk for spillage.Weigh 0.4~0.5g NaCl,0.2~0.25g NaCl and 0.10~0.13g NaCl respectively.Record the data in Table 1-1.The specific steps are as follows:
Figure 1-1 Weighing by difference
①Get a weighing bottle which contains the material you want to weigh out.Weigh the weighing bottle on the analytical balance and record the weight,W 1 .
②Transfer material from the weighing bottle into a flask or other receiving container.When transferring,use one hand to hold the weighing bottle slightly tilted while tapping the weighing bottle to allow a small amount of the material to come out.Put the weighing bottle back on the balance pan and record the weight W 2 .Repeat the operation until the quality of the poured sample meets the requirements.
③To get the weight,W,of your transferred material,use the following formula:W =W 1 -W 2 .
See Table 1-1.
Table 1-1 Weighing data processing of electronic balance
1. The balance room should be kept clean and dry,and direct sunlight should be avoided.The balance should be installed on a stable and nonvibrating experimental platform.
2. Get and place the objects from the side doors.The mass should not outweigh the maximum weigh of the balance.Objects with corrosivity and hygroscopicity must be placed in an airtight container before weighing.
3. The temperature of the weighing substance should be the same as that of the room temperature.Hot and cold objects should not be directly put on the balance for weighing.The weighing should be carried out after the weighing object is placed at the room temperature.
4. Desiccant(commonly used discolored silica gel)should be placed in the balance,and the desiccant should be replaced regularly.
1. Is the higher the sensitivity of balance,the higher the accuracy of weighing?
2. Can small scoops be used for sampling in the weighing process of the declining weighing method?Give the reason.
3. How to ensure that the sample is not lost if using the weighing bottle?
