The atmospheric pressure measured in mmHg is the current atmospheric pressure in the laboratory. The normal boiling point is the temperature at which the vapor pressure of a liquid equal mmHg.
This is the atmospheric pressure at sea level. This is a reference value. Generally boiling points go down as the atmospheric pressure decreases, or the pressure above them in their container. The boiling point of a liquid is a physical property of that substance and is used to help identify it, or to determine its purity.
Measurement of boiling point can be done in a number of ways. One tactic that may be used to purify a liquid is to distill it. The process is common and one that is often used in the chemical laboratory. This exercise will distill a single liquid, and a mixture of liquids. The procedure that will be employed is called simple distillation.
Boiling points for a large number of liquids can be found in the Handbook of Chemistry and Physics.
This reference book is available in the laboratory and the library. Boiling points are often listed with a superscript that indicates pressure. If no reference is given, it is assumed to be at mm Hg.
Simple Distillation: A simple distillation apparatus is shown in Figure 1. This consists of a round-bottomed flask connected by means of a distillation adapter to a water-cooled condenser. A thermometer is held in place in the vertical arm of the distillation adapter by a special rubber connector at a height adjusted so that the top of the thermometer bulb is 5 — 10 mm below the opening of the side-arm.
A vacuum adapter is connected to the lower end of the condenser. The distilled liquid is collected in a clean, dry receiver, commonly an Erlenmeyer flaks or small-mouthed bottle. To reduce vapor losses and minimize fire hazards, it is desirable to insert the lower end of the adapter well into the mouth of the receiver.
A distilling assembly must have an opening to the atmosphere to avoid developing a dangerously high pressure within the system when heat is applied. Key Point: When conducting a distillation, the vapor should be richer in the lower boiling component than what you started with. Fractional Distillation: The vapors generated in the pot rise up the fractionating column and encounter cooler surfaces, upon which they condense see Figure 3. The inert material, such as copper sponge or glass beads, provides a large surface area, thus allowing many more evaporation-condensation cycles.
The column is attached to an adapter. In the top of this connector is the thermometer, which is used to read the temperature of the vapor, just as it condenses. The temperature reading is important, because, at normal conditions, the temperature of the vapor passing through is the same as the boiling point of the substance being collected.
Vapor that passes the thermometer condenses in the condenser, a double-walled tube that is cooled by water flowing through the outer layer, and drips into the receiver. Before moving on, let us look at benefits and drawbacks to a simple distillation versus a fractional distil- lation.
We have already said that the simple distillation is less efficient at separating liquids, because there is a smaller surface area inside the column, but it is usually much faster. For mixtures that contain only one volatile component, a simple distillation can be more than sufficient.
The fractional distillation is more efficient, and is suited for mixtures of volatile liquids. The closer the difference in the boiling points, the more demanding the distillation. The drawback is that fractional distillations typically take longer, because we want to achieve pseudo-equilibrium between vapor and liquid throughout this system. Slow boiling and ample time is important to achieve this goal.
Sounds easy, right? What can go wrong? Poor separation is the most likely problem you will face. This means that the fractions obtained are impure, and often contain traces of the other liquids found in the original mixture.
We will assume that we are performing a fractional distillation, and that we have several liquids that must be separated. Let us look at some of the technical aspects that are important for acceptable separation, and some key features for the assembly of the distillation apparatus.
Typical problems: Distillation too fast. The resulting explosion can cause dangerous flying glass, hot, corrosive chemicals to spatter and potentially hazardous fumes to be released. Additional heat must be supplied for a phase change to occur. The increase in heat and pressure inside the apparatus will increase and possibly explode the distillation equipment.
Water should always enter from the bottom of a condenser the end closest to the flask and exit from the top of the condenser. Doing it this way always ensures that your condenser will be full of cooling water. Ether peroxide is highly explosive.
Diethyl ether should not be kept for more than one year and should be stored in half-empty bottles.
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