Methods of checking the airtightness of chemical devices and their principles

Chemical experiment is an important way to learn chemistry, before the experiment, to ensure that the experimental device air tightness is good, is the basis for the success of the experiment. Airtightness check is mainly divided into: microthermal method (thermal expansion and contraction method), water injection method (liquid difference method), pumping and pumping method (syringe + long-necked funnel), etc. The principle of the check is to change the size of the internal pressure, the formation of the difference between the internal and external pressures, resulting in a column of water. Specific methods are as follows:

A, micro-thermal method (thermal expansion and contraction method)

Principle: through the micro-thermal so that the gas inside the device thermal expansion, overflow part of the gas, after cooling, the gas inside the device after the cold contraction of the pressure decreases, the outside air pressure is large, the pressure of the water into the conduit, the formation of water column.

Example 1, the heating of potassium permanganate to produce oxygen experimental device (test tube + catheter) airtightness check

assembled equipment, the catheter end into the water, constituting a closed system, with both hands to cover the test tube, observed that the mouth of the catheter in the water bubbles out, let go of the hand, the catheter into a section of the column of water, that is, the airtightness of good.

Example 2, hydrogen peroxide to produce oxygen experimental device (split funnel + conical flask) airtightness check

assembled equipment, catheter end into the water, close the split funnel in the stopper, constitute a closed system, with both hands over the conical flask (or with an alcohol lamp through the asbestos mesh to heat the conical flask, heating a few moments can be), observed the mouth of the catheter in the water bubbles, let go, the catheter into a section of the water column, indicating good airtightness. After letting go, a column of water enters the conduit, indicating a good airtight seal.

Second, the water method (liquid difference method)

Principle: the use of a split funnel to the sealed device in the water, water compressed gas inside the device so that the internal air pressure rises, greater than the external air pressure, and then water, the water will be left in the long-necked funnel, and the device to form the difference in the level of the liquid.

Example 1: Checking the airtightness of an experimental setup for the production of oxygen from hydrogen peroxide (long-necked funnel + conical flask)

Assemble the device, clamp the spring clip, and keep injecting water into the long-necked funnel, so that the water in the long-necked funnel will be higher than the surface of the water inside the device by a certain distance to form a liquid level difference, and then observe that there is no decrease in the stability of the liquid level difference, which indicates that the airtightness is good.

Example 2, qipu generator airtightness check

Close the qipu generator below the piston, from the upper mouth of the spherical funnel constantly injected into the water, until the bottom of the spherical funnel submerged in the water below. After closing the piston above the kip generator again, continue to inject water so that the liquid level in the spherical funnel is higher than the liquid level in the reactor, and after stopping the injection, observe for a period of time, and if the difference in the liquid level remains unchanged, it means that the airtightness is good.

Example 3, U-tube airtightness check

U-tube spring clamps at one end of the closed, at the other end of the injection of water until the formation of the liquid level difference between the left and right, and leave it for a period of time, the liquid level difference does not change, indicating that the airtightness is good.

Three, pumping and pumping method (syringe + long-necked funnel)

Principle: the use of syringe pumping and pumping to make the device inside the air pressure becomes larger and smaller. When pumping, the internal air pressure becomes smaller, the external air pressure is larger, the external air pressure is larger, and the external gas enters to form bubbles. When pumping, the air pressure inside the device is large and presses water into the long-necked funnel to form a section of the water column.

Example 1, carbon dioxide gas production device (long-necked funnel + conical flask) gas tightness check

Assembled the device, the conical flask of the two-hole stopper, one to put the long-necked funnel, the other hole is connected to the syringe. Fill the long-necked funnel with water until the bottom of the long-necked funnel is submerged below the surface of the water. Use the syringe to pump or draw air into the conical flask.

When pumping outward, bubbles come out of the lower end of the long-necked funnel, and when pumping inward, the level of the liquid in the filled long-necked funnel rises into a section of the water column, indicating a good airtight seal.