Mercury meniscus

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... 

For example, the sudden drop in pressure indicates the mercury meniscus entering a wide region (pore body) from a narrow region (pore throat). One of the parameters obtained from such an experiment is the distribution of the pore body volumes, shown in Figure 3.7.4 with a pronounced peak at around 20 nL. The... 

Fig. 4.10 Capillary electrometer. The basic component is the cell consisting of an ideally polarized electrode (formed by the mercury meniscus M in a conical capillary) and the reference electrode R. This system is connected to a voltage source S. The change of interfacial tension is compensated by shifting the mercury reservoir H so that the meniscus always has a constant position. The distance between the upper level in the tube and the meniscus h is measured by means of a cathetometer C. (By courtesy of L. Novotny)...

Dilatometer. Reliable kinetic data on gamma-induced emulsion polymerization can be obtained only when the polymerization rate is measured continuously (7). The recording dilatometer used in our previous work had some disadvantages. A mercury meniscus traveled down a precision capillary, releasing a thin platinum wire within the capillary. The electrical resistance of this assembly was used as a measure for the... 

The mercury meniscus should not stand still for any long period of time. Therefore the motor of the syringe is reversed immediately after the meniscus has reached the platinum tip. Hence, even if there is no change in volume, the motor buret is always working to and fro, thus keeping the meniscus oscillating around the contact-no contact position. 

The beauty of the mercury manometer is that you can assess its accuracy by simply looking at it. If the mercury meniscus is at zero when there is no pressure in the cuff and the column moves smoothly with inflation and deflation it is accurate and can be used as the gold standard for pressure measurement. All other devices must be calibrated against a... 

If the relation between the e.m.f. applied to the terminals of the capillary electrometer, and the interfacial tension between mercury and the electrolyte, is plotted with tension as ordinate and the (negative) potential applied to the small mercury meniscus increasing as abscissae, the curve is called the electrocapillary curve.5... 

Pressure in the volumetric apparatus was measured by a McLeod gage and by a wide-bore (30-mm.) mercury manometer. Pressures measured with the McLeod gage were corrected for capillary depression of the mercury meniscus. Pressure in the gravimetric apparatus was controlled by regulation of the tempera-... 

Now consider a tapered tube, wider at the top. This represents a stable equilibrium. For a given pressure applied, the mercury is stable in only one position in the capillary. If the pressure is increased momentarily (or the surface tension is decreased, say, by a fluctuation in the applied potential), the mercury meniscus will move to a lower point, where the radius is a little smaller, to establish a new equilibrium, in accordance with Eqs. 51H-53H. The mechanical equivalent of this configuration is a sphere at the bottom of a concave surface, shown in Fig. 4H(b). Stability is attained by negative feedback. If the... 

Measurement of the electrocapillary curve consists of changing the potential stepwise and determining the pressure required to return the mercury meniscus to the same location in the fine capillary. A plot of this pressure as a function of potential is nothing but the electro-capillary curve, within a constant, as seen from Eq. 53H. The best way to determine the magnitude of this constant is by calibration with a known system. This requires, of course, one accurate determination of y by an independent method. Very careful experiments were performed by Gouy around the turn of the century, and these results are used even now as the primary standard for electrocapillary measurements. 

What is the effect of a small increase in pressure on the mercury in such a capillary The meniscus will descend slightly, but in its new position the radius is larger, so that the same pressure will tend to move it farther down, where the radius is even larger, and so on. Thus, while the general equations allow the existence of an equilibrium position in this situation, it will not be possible, in practice, to maintain a stable position of the mercury meniscus in the capillary. 

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. 

By careful manipulation of tap B and stopcock C the mercury level in the manometer is adjusted to the graduation mark above the smallest bulb. The difference in the height of the mercury menisci in the limbs of the manometer is measured with a cathetometer and noted. The volume of the gas sample is now adjusted by manipulating B and C, so that the mercury meniscus is precisely at the graduation mark between the bulbs. Again the difference in mercury levels is measured and noted. 

As ice contracts on melting by an amount which has been measured very accurately, we can deduce the mass m of ice, which has been melted, from the displacement of the mercury meniscus on the calibrated extension of the tube C. 

Evaluation of Results. At the beginning and end of a polymerization, the level of the mercury meniscus in the measuring capillary was read. The scannings of the Compensograph (conversion-time functions) were corrected using the calibration plot (pen deflection vs. meniscus level). The final conversion of the d spersions was determined. All values are based on the polymer content of the dispersions (milligrams of polymer per gram of dispersion) thus calculated, and by the assumption of a linear correlation between volume contraction and conversion. 

Figure 12.19 Shape of water or mercury meniscus in glass. A, Water displays a concave meniscus in a glass tube because the adhesive (H-bond) forces between the H2O molecules and the O—Si—O groups of the glass are stronger than the cohesive (H-bond) forces within the water. B, Mercury displays a convex meniscus in a glass tube because the cohesive (metallic bonding) forces within the mercury are stronger than the adhesive (dispersion) forces between the mercury and the glass.

In practice it is essential that the manometer and mercury be thoroughly clean to eliminate. stick-slip effects in the movement of the mercury meniscus. A tipping manometer is frequently used to transfer mercury from the reservoir into the arms of the manometer after evacuation. During the tipping operation it is essential that the mercury not reach the stopcock, because this is always greased and the resulting contamination will spread rapidly through the manometer [27],... 

If the inside surface of each tube were coated with wax, would the general shape of the water meniscus change Would the general shape of the mercury meniscus change ... 

Figure 11.19 Wax is a hydrocarbon that cannot form hydrogen bonds. Therefore, coating the inside of tube with wax will dramatically decrease the adhesive forces between water and the tube and change the shape of the water meniscus to an inverted U-shape. Neither wax nor glass can form metallic bonds with mercury so the shape of the mercury meniscus will be qualitatively the same, an inverted U-shape.

Here a = a (p, T) is the surface tension of mercury which on principle is a pressure and temperature dependent quantity , 0 = 140° is the contact angle between the mercury meniscus and the pore wall. Though the exact value of this parameter normally is unknown, a practical value of 14(T turned out to lead to physically reasonable results in many cases and hence is recommended for practical use [1.44],... 

---