Liquid mercury column

The comparatively inexpensive long-scale thermometer, widely used by students, is usually calibrated for complete immersion of the mercury column in the vapour or liquid. As generally employed for boiling point or melting point determinations, the entire column is neither surrounded by the vapour nor completely immersed in the liquid. The part of the mercury column exposed to the cooler air of the laboratory is obviously not expanded as much as the bulk of the mercury and hence the reading will be lower than the true temperature. The error thus introduced is not appreciable up to about 100°, but it may amount to 3-5° at 200° and 6-10° at 250°. The error due to the column of mercury exposed above the heating bath can be corrected by adding a stem correction, calculated by the formula ... 

The pressure of the atmosphere can be measured with a barometer, an instrument invented in the seventeenth century by Evangelista Torricelli, a student of Galileo. Torricelli (whose name coincidentally means little tower in Italian) formed a little tower of liquid mercury. He sealed a long glass tube at one end, filled it with mercury, and inverted it into a beaker (Fig. 4.4). The column of mercury fell until the pressure that it exerted at its base matched the pressure exerted by the atmosphere. To interpret measurements with a barometer, we need to find how the height of the column of mercury depends on the atmospheric pressure. 

Additionally, the pressure equivalent of the heights of the water above and below the initial point must be calculated and either added to or subtracted from the corrected pressure for each case. Because mercury is 13.6 times as dense as water, any column of water can be converted to mm Hg, or torr, by dividing the water column height, in mm, by 13.6. Water is used instead of mercury because liquid mercury and its vapors are highly toxic and cannot be safely used in the classroom laboratory. 

Figure 1.2 A barometer is a device for measuring pressures. A vacuum-filled glass tube (sealed at one end) is placed in a trough of mercury with its open end beneath the surface of the liquid metal. When the tube is erected, the pressure of the external air presses on the surface and forces mercury up the tube. The height of the mercury column li is directly proportional to the external pressure p...

There are problems to be considered and avoided when using liquid-in-glass thermometers. One type of these is pressure errors. The change in height of the mercury column is a function of the volume of the bulb compared to the volume of the capillary. An external pressure (positive or negative) which tends to alter the bulb volume causes an error of indication, which may be small for normal barometric pressure variations but large when, for example, using the thermometer in an autoclave or pressure vessel. 

I = length of mercury column in degrees above surface of liquid. 

The open end of the tube was placed in a bowl of mercury. The weight of the air on the surface of the mercury caused the liquid to travel up the tube until the pressure of the column of mercury equaled the pressure of the air on the bowl. The length of the mercury column gave a reading of air pressure. 

The reason for the use of a mercury column is that mercury is considerably heavier than liquids such as water, which appears to be a much more convenient (and less toxic) option. Water could certainly be used theoretically, but, since it is approximately 13 times less heavy than mercury, the sphygmomanometer column would need to be 13 times as high. The height of ceilings in typical clinical settings and the difficulty of reading the level of the liquid at the heights that would be needed effectively preclude such an option (Turner, 1994). 

It can be shown that r < h3, where h is the height of the mercury column, by considering the force of gravity and liquid flow in a capillary. So Ic h and l hx/2. This means that there is a minimum detection limit with current sampling this is around 10 7m (see Chapter 10 on pulse techniques). It is therefore important not to have too high a column of mercury. 

The earliest satisfactory, measurements on vapour pressure were made with water by Dalton, who passed the liquid into the vacuous part of a barometer tube surrounded by a water-jacket, and measured the depression of the mercury column. This simple method, also used by Gay-Lussac, Ure, Magnus, and Regnault, with improved apparatus, is subject to errors, e.g. for the change of shape of the mercury meniscus in contact with the liquid (with water, according to Regnault, this depresses the mercury column by 0T2 mm.) and the depression of the mercury column due to the weight of the liquid. It cannot, of course, be used with liquids (e.g. bromine) which attack mercury. 

Measurement Principle. The essential part of our measuring device consists of a precision capillary with a lumen of 2 to 4 mm. and a set-in platinum wire 0.1 mm. (f). Mercury served as a connection liquid to the reaction vessel. During the reaction, the mercury column travels down the capillary and gradually releases the platinum wire in the capillary. Wire and capillary must be carefully cleaned of grease otherwise the migrating mercury separates incompletely. The mercury itself was cleaned and distilled before use. 

For determination of the 100° point, 15 ml of pure distilled water is placed in an eight-inch tube with two clean boiling stones. The thermometer is suspended within the tube from a wire held by a clamp so that the bulb of the thermometer is 30 mm above the surface of the liquid. By means of a burner the lower end of the tube is heated until the liquid boils briskly. This is continued until steam issues from the mouth of the tube. When the temperature reading has become nearly constant, several readings are taken at short intervals. The barometric pressure and the length of the mercury column extending above the mouth of the tube are noted. By means of a second thermometer, the temperature at the... 

The mobile adsorption state seems to seldom occur in reality. De Boer [12] and other authors present the adsorption of krypton on the surface of liquid mercury as the only good example they do not mention any case of adsorption on solids. The conditions for mobile adsorption can hardly take place in the adsorption of heavy element halides on silica or metallic columns. Doubts can also be cast on the simplest picture of the ideal localized adsorption. An ideal crystal face does show ordered, equally deep potential wells on a map of the adsorption energy moreover, cutting of the crystal by certain planes (perpendicular to the surface) produces sections, which show one-dimensional adsorption wells separated by barriers reaching up to the zero adsorption potential. However, most of the possible sections show barriers, which do not reach the zero potential energy. As a consequence, a molecule can visit many neighboring sites before it is desorbed from the surface. 

Figure 13-12 A representation of the measurement of vapor pressure of a liquid at a given temperature. The container is evacuated before the liquid is added. At the instant the liquid is added to the container, there are no molecules in the gas phase so the pressure is zero. Some of the liquid then vaporizes until equilibrium is established. The difference in heights of the mercury column is a measure of the vapor pressure of the liquid at that temperature.

Pressure is defined as force per unit area. Fluids (liquids and gases) exert pressure in all directions. The pressure of a gas is equal to the pressure on the gas. A way of measuring pressure is by means of a barometer. The standard atmosphere (atm) is defined as the pressure that will support a column of mercury to a vertical height of 760 mm at a temperature of 0 °C. It is convenient to express the measured gas pressure in terms of the vertical height of a mercury column that the gas is capable of supporting. Thus, if the gas supports a column of mercury to a height of only 76 mm, the gas is exerting a pressure of 0.10 atm ... 

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