Ethanol distillation and boiling point determination experiment
Ethanol distillation and boiling point determination experiment
This experiment is from the official website of Southern Medical University
Operation method
Ethanol distillation and boiling point determination experiment
Principle
The boiling point is an important physical constant for organic compounds. A pure substance has a constant boiling point at a certain pressure. If there are impurities involved, the boiling point will be reduced or increased phenomenon, and in the process of distillation boiling point will gradually change. Therefore, the determination of the boiling point can be used to check the purity of a substance1. The determination of the boiling point is usually carried out incidentally during the distillation and purification of the substance (the constant method). The boiling point of purely liquid organic substances is usually determined by the trace method. Ethanol (C2H5OH) is a colorless, transparent liquid with a boiling point of 78.5°C. It is freely miscible with water. Dilute alcohol distillation, due to the volatility of ethanol is larger, the ethanol content in the vapor increases, and thus can be distilled with the help of the method to increase the concentration of alcohol.
Materials and Instruments
Heating jacket Alcohol lamp Thermometer General distillation apparatus Conical flask Long-necked glass funnel Measuring cylinder Zeolite Thiele tube Capillary tube Rubber ring Iron ring Move I. Atmospheric distillation of ethanol Caveat (1) Pure liquid organic compounds have a certain boiling point under a certain pressure and a very small boiling range (1 to 2°C). However, with a fixed boiling point of the liquid is not necessarily a pure compound. Because, certain organic compounds are often formed with other components such as binary or ternary azeotropic mixtures, they also have a certain boiling point. Therefore, the boiling point determination can not be used as the only standard for the purity of liquid organic compounds.(2) Zeolite must be added before heating. If you forget to add before heating; make-up must first stop heating, to be vaporized to the boiling point of cold below before adding. If you put zeolite into the liquid when it reaches the boiling point, it will cause violent boiling, and part of the liquid may rush out of the bottle causing burns or fire. If the boiling stops in the middle, new zeolite should be added before reheating.(3) distillation speed can not be too fast, otherwise easy in the neck of the distillation bottle caused by overheating phenomenon or condensation is incomplete, so that the boiling point read by the thermometer is high; at the same time the distillation can not be carried out too slowly, otherwise due to the thermometer can not be fully infiltrated by the thermometer's mercury ball for the evaporation of liquid vapors and make the thermometer read on the boiling point is low or irregular. For more product details, please visit Aladdin Scientific website.
1. Distillation unit installation
The most commonly used device for atmospheric distillation is shown in Figure 2-27. It consists of a distillation flask, thermometer, condenser tube, receiving tube and receiving flask.
Before installing the apparatus, first select the apparatus with appropriate specifications. The order of installation is the first from the heat source (gas lamp, alcohol lamp or electric stove) at the beginning, according to the "from top to top, from left to right (or from right to left)" order, in order to place the iron frame table, asbestos mesh, water bath and distillation flasks and so on. The distillation flask is clamped vertically with an iron clamp. When installing the condenser tube, you should first adjust the position so that it is coaxial with the distillation flask branch, and then loosen the condenser tube iron clamp, so that the condenser tube along the axis of rotation and the distillation flask is connected, so as not to break the distillation flask branch. Iron clamp should not be too tight or too loose, to clamp a little force can still turn appropriate. Iron clip should be padded with rubber and other soft materials, so as not to break the instrument. The entire device requires accurate and upright, whether from the front or side observation, the full set of devices in the axis of each instrument should be in the same plane. All iron clips and iron frame should be placed as neatly as possible on the back of the instrument.
2. Distillation
(1) add After the apparatus is assembled, remove the thermometer and pour 30mL of 75% ethanol into the distillation flask through the glass funnel or directly along the wall of the flask facing the mouth of the branch pipe (be careful not to let the liquid flow out of the branch pipe). Add a few grains of zeolite ②, plug the thermometer, and check that all parts of the apparatus are connected tightly and properly.
(2) Heating Turn on the condensate and heat it in a water bath. Observe the phenomena in the distillation flask and the rise in temperature. After heating for some time, the liquid boils and the vapor rises gradually. When it rises to the thermometer mercury ball, the thermometer mercury column rises sharply. At this point the flame should be controlled so that the vapors do not immediately rush out of the stub of the distillation flask, but condense back. After the temperature is stabilized, then slightly increase the flame and distill. Adjust the heating rate, control the distillation droplets to 1 to 2 drops per second is appropriate ③. Throughout the distillation process, droplets should always be kept on the mercury bulb.
(3) Boiling point observation and distillate collection Prepare two conical flasks as receivers before distillation, before the temperature stabilization of the fraction, often for the lower boiling point of the liquid. When the temperature tends to stabilize, the vaporized substance is a purer substance. At this time to replace another clean and dry receiver, note down the thermometer reading at this time. To be collected about 20mL ethanol (if it is a pure substance is steamed to the last drop), stop distillation, note down the thermometer reading. The two readings before and after are the boiling point range of ethanol.
(4) Removal of the apparatus Distillation is complete, first stop heating, a little cold, stop the water, dismantle the apparatus. The order of dismantling the apparatus is the reverse of assembling, first dismantle the receiver, and then dismantle the receiver tube, condenser tube and distillation flask in turn.
Determination of boiling point of ethanol by trace method
Micro-method for the determination of boiling point of the device as shown in Figure 2-26.
1. Preparation of boiling point tube
Boiling point tube consists of an inner and outer tube. The outer tube is a glass tube with an inner diameter of 3 to 4 mm and a length of about 5 cm closed at the bottom. Take a capillary tube with an inner diameter of about 1mm and a length of about 4cm as the inner tube, heat it on an alcohol lamp to close one end and leave it for use.
2. Sampling
Place 1 to 2 drops of anhydrous ethanol sample in the outer tube of the boiling point tube (liquid height of about 2 to 3 mm), into the inner tube (closed end upward, open end downward). Then attach the boiling point tube to the thermometer with a rubber band (the sample is positioned in the center of the mercury bulb) and place it in the oil bath of the Thiele tube (see Figure 2-25).
3. Boiling point determination
Heat the oil bath. As it heats, small bubbles will slowly escape from the inner tube as the gas expands, and when the boiling point of the liquid is reached or exceeded, a series of small bubbles will escape rapidly (while expelling the air from the inner tube). At this time to stop heating, so that the oil bath cools itself, the rate of bubble escape that slowly slow down. In the bubble is no longer emerging, and the liquid is just about to enter the inner tube of the moment (that is, the last bubble just want to shrink back to the inner tube - to pay attention to the careful observation), that the capillary vapor pressure and the external pressure is equal. Record the thermometer reading, the temperature at this point is the boiling point of the liquid.