Mercury Column

Qualitatively, it is observed that the mercury surface initially is positively charged, and on reducing this charge by means of an applied potential, it is found that the height of the mercury column and hence Ae interfacial tension... 

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 ... 

Fig. 1. Gas thermometer A, helium gas B, mercury P, plunger for adjusting mercury column and P, pressure, where in (a), bulb is surrounded by water...

There are problems to be considered and avoided when using Hquid-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. 

Mercury thermometers are subject to separation of the mercury column or to inclusion of bubbles of the fiU gas. These may result from shipping and handling and cause a scale offset which can usually be seen upon visual examination, and they are always recogni2ed by a 0°C verification check. Manufacturers will suggest means by which these temporary defects may be cured. 

N = length of the mercury column m degrees from above the vessel to T. 

Q.S., Q.-S., abbret. (Quecksilbersaule) mercury column (Quecksilberstand) mercury level, quabbeln, jj.t. shake, quiver be flabby. Quacksalber. m. quack, charlatan. 

Atmospheric pressure can be measured by any of several methods. The common laboratory method uses a mercury column barometer. The height of the mercury column serves as an indicator of atmospheric pressure. At sea level and at a temperature of 0° Celsius (C), the height of the mercury column is approximately 30 inches, or 76 centimeters. This represents a pressure of approximately 14.7 psia. The 30-inch column is used as a reference standard. 

A device commonly used to measure atmospheric pressure is the mercury barometer (Figure 5.1), first constructed by Evangelista Torricelli in the seventeenth century. This consists of a closed gas tube filled with mercury inverted over a pool of mercury. The pressure exerted by the mercury column exactly equals that of the atmosphere. Hence the height of the column is a measure of the atmospheric pressure. At or near sea level, it typically varies from 740 to 760 mm, depending on weather conditions. 

Another unit commonly used to express gas pressure is the standard atmosphere, or simply atmosphere (atm). This is the pressure exerted by a column of mercury 760 mm high with the mercury at 0°C. If we say that a gas has a pressure of 0.98 atm, we mean that the pressure is 98% of that exerted by a mercury column 760 mm high. 

The apparatus shown in Figure 4-2C differs in that the right-hand tube is open. In this type of manometer, atmospheric pressure is exerted on the right-hand mercury column. Hence the pressure in the flask plus the height of the mercury column equals atmospheric pressure. In the example shown, the pressure is 755 — 650 = 105 mm, the same as pictured in the closed-end manometer, Figure 4-2B. 

Various tests can be applied, including the influence of the height of the mercury column of the dropping electrode on the diffusion current to decide if any of the above non-diffusion processes are operative.49... 

Self-Test 4.1A What is the atmospheric pressure when the height of the mercury column in a barometer is 756 mm ... 

A reaction is performed in a vessel attached to a closed-tube manometer. Before the reaction, the levels of mercury in the two sides of the manometer were at the same height. As the reaction proceeds, a gas is produced. At the end of the reaction, the height of the mercury column on the vacuum side of the manometer has risen 30.74 cm and the height on the side of the manometer connected to the flask has fallen by the same amount. What is the pressure in the apparatus at the end of the reaction expressed in (a) Torr (b) atm (c) Pa (d) bar ... 

FIGURE 8.1 The apparatus is a mercury barometer. The closeup on the left shows the vacuum above the mercury column. In the right-hand closeup, we see the effect of the addition of a small amount of water. At equilibrium, some of the water has evaporated and the vapor pressure exerted by the water on the mercury has lowered the height of the mercury column. The vapor pressure is the same however much water is present in the column. 

The mercury does fall, but the flow stops at a fixed height. The column of mercury stops falling because the atmosphere exerts pressure on the mercury in the dish, pushing the column up the tube. The column is in balance when the height of the mercury column generates a downward force on the inside of the tube that exactly balances the force exerted by the atmosphere on the outside of the tube. 

At sea level, atmospheric pressure supports a mercury column approximately 760 mm in height. Changes in altitude and weather cause fluctuations in atmospheric pressure. Nevertheless, at sea level the height of the mercury column seldom varies by more than 10 mm, except under extreme conditions, such as in the eye of a hurricane, when the mercury in a barometer may fall below 740 mm. 

The simplest device for measuring ECC at mercury is Gouy s capillary electrometer (Eig. 10.5). Under the effect of a mercury column of height h, mercury is forced into the slightly conical capillary K. In the capillary, the mercury meniscus is in contact with electrolyte solution E. The radius of the mercury meniscus is practically equal to the capillary radius at that point. The meniscus exerts a capillary pressure Pk = directed upward which is balanced by the pressure = ftpegg of... 

When potential is applied, the meniscus moves, owing to the resulting change in surface tension. By varying the height of the mercury column during the measurements... 

To reduce deviations in blood pressure measurement in the clinic, the patient and clinician should not talk during blood pressure readings. The measurement arm is supported and positioned at heart level with the blood pressure cuff encircling at least 80% of arm circumference. If a mercury or aneroid device is used, then the palpatory method must be used first to estimate the systolic blood pressure.18 If an automated device is used, this is not necessary. After the patient s cuff is inflated above the systolic pressure, the mercury column should drop at a rate of 2 to 3 mm per second. A stethoscope placed over the brachial artery in the antecubital fossa identifies the first and last audible Korotkoff sounds, which should be taken as systolic and diastolic pressure, respectively. A minimum of two readings at least 1 minute apart are then averaged. If measurements... 

Influence of mercury column height. In the above Ilkovic equations for the average limiting current m and [Pg.134]

G. Lippmann introduced the capillary electrometer to measure the surface tension of mercury (Fig. 4.10). A slightly conical capillary filled with mercury under pressure from a mercury column (or from a pressurized gas) is immersed in a vessel containing the test solution. The weight of the mercury column of height h is compensated by the surface tension according to the Laplace equation... 

Figure 2.1 (a) A schematic representation of the apparatus employed in an electrocapillarity experiment, (b) A schematic representation of the mercury /electrolyte interface in an electro-capillarity experiment. The height of the mercury column, of mass m and density p. is h, the radius of the capillary is r, and the contact angle between the mercury and the capillary wall is 0. (c) A simplified schematic representation of the potential distribution across the metal/ electrolyte interface and across the platinum/electrolyte interface of an NHE reference electrode, (d) A plot of the surface tension of a mercury drop electrode in contact with I M HCI as a function of potential. The surface charge density, pM, on the mercury at any potential can be obtained as the slope of the curve at that potential. After Modern Electrochemistry, J O M. 

---