Null manometer

Shchukarev et al. (5) measured the vapor pressure of WBr (cr, t) using a null manometer. Their results are given... 

The vapour pressures over CuSe03(cr) and Cu0 CuSe03(cr) were measured with a membrane null manometer and with a Knudsen effusion cell with mass spectro-metric detection by Makatun, Rat kovskii, and Pechkovskii [71MAK/RAT]. The dew point method was used by Bakeeva et al. [72BAK/BUK] for the same purpose. 

The vapour pressure of Se03(g) over Se03(cr) was measured in the temperature range 350 to 390 K by a membrane null manometer. A few measurements were also made with Se03(l). Several series of experiments were performed but the reproducibility was poor, which inevitably casts some doubts on the thermodynamic data obtained. The authors selected two series of experiments (Ci and C2) that showed concordant vapour pressure curves for the calculations. 

The processes of vaporization and dissociation in the vapour of the adducts AlBr3,NH3 and All3,NH3 have been studied using a membrane null-manometer the latter compound undergoes ammonolysis on heating. 

Figure 27 shows how to couple a null manometer to the system. In this case, a Bodenstein manometer connection is shown. 

Figure 9 Tomlin s static vapour-pressure apparatus with injection of liquids. A, vapour-pressure cell B, capacitance null manometer C, motorized valve D, variable leak E, F, metal bellows valve G, Teflon valves H, degassing apparatus M, nitrogen pressure regulator L, nitrogen ballast tank (35 1) V, to high vacuum line (Reproduced by permission from Royal Australian Institute of Chemistry, 5th National Convention, Papers of Physical Chemistry Division)...

With systems containing liquid columns, for example, a U-tube containing mercury used as a null manometer which may be only approximately balanced, or a piston gauge in which oil is used, the hydrostatic heads -evaluated according to equation (1) - of all the fluids must be added or subtracted appropriately. It may be most straightforward if all fluid heads are first converted into the corresponding heights of one fluid (most usually mercury) by multiplication by the inverse ratio of their densities at... 

For work above room temperature, precautions must be taken to ensure that the sample does not condense in cool parts of the apparatus with the result that the pressure is lowered, and one side of the manometer must therefore be kept at an elevated temperature - at least as high as the temperature of measurement. Measurements are now seldom made by means of a mercury manometer kept at the temperature of the sample, except at low pressures (say, up to 30 kPa), because it is preferable to have the precise measuring instrument at room temperature, and the sample is normally confined by a null manometer (either of the diaphragm type, or a short U-tube containing mercury) in the thermostat the pressure is transmitted by gas from the null manometer to the main measuring manometer. 

Figure 1 Apparatus for measurement of vapour pressure by the static method. A, sample vessel B, mercury cut-off C, null manometer D, hollow glass finger to reduce internal volume and limit movement of bellows) E, outer chamber of manometer F, metal plate G, connection to measuring manometer J, mercury-sealed joint K, mercury-sealed stopper and cap L, connection to vacuum (via a second mercury cut-off)...

If vapour pressure measurements are to be an essential part of the work to be undertaken, a cold cathode manometer is probably the best choice, despite the fact that it needs to be calibrated for each molecular species, and its use with mixtures of gases containing two or more species is correspondingly more difficult. If such mixtures are to be investigated, or if the chemicals concerned are corrosive, it is probably most efficient to use a mechanical gauge as a null-point instrument and to measure the pressure by means of a McLeod gauge. 

In the following procedures a standard glass vacuum line with high-vacuum stopcocks (lubricated with Kel-F-90 grease ) is used. Because of the reactivity of many of the compounds with mercury, it is convenient to use a null-point pressure device, such as a Booth-Cromer16 pressure gage or spiral gage. A mercury manometer covered with Kel-F-3 oil can be used. 

Spring balances are still used in certain research investigations when adsorption equilibration is very slow, e.g. for the study of hysteresis phenomena. For this purpose, it is advisable to replace the mercury manometer by a modem pressure gauge. However, in recent years spring balances have been largely superseded by electronic microbalances. The essential features of an electronic, null adsorption microbalance are indicated in Figure 3.11. 

The pressure was measured using a borosilicate glass Bourdon gage which was sensitive to 0.1 mm. of mercury as a null instrument to a mercury manometer. The final pressure was measured after the reaction vessel had been heated above 200° C. for 8 hours or longer. The maximum dead space of 14 cc. was efficiently flushed with oxygen at the start of each experiment and was duly corrected for in the calculation (2) of the ozone concentrations. The hollow-bore vacuum stopcocks used in the system were lubricated with Halocarbon (high temperature grade) stopcock lubricant. The usual precautions were taken to minimize the amount of mercury vapor in the system. 

Ishikawa and Abe [38ISH/ABE] used two different equipments, one with a null-point membrane manometer and the other with a directly attached manometer, and report ... 

The best device, however, is the quartz coil manometer, the coil of which can be heatedto500°C(in special cases to 600-700°C). In all cases the null point of the instrument must be checked after each measurement. Therefore the manometer should be provided with a heating coil, which doe snot need to be at the test temperature but must nevertheless be at a sufficiently high temperature to prevent condensation in the coil and in the capillary connections (which are likewise provided with a heating coil). With compensation to zero, the pressure is read off on the Hg manometer. In those cases where it cannot be ascertained by the usual method (with a thermometer and distillation flask) the boiling point is determined more accurately by extrapolation of the vapor pressure curve. 

A number of refinements have been suggested either to increase the accuracy or reduce the tedium of measurements. For example, Vance and Pattison [118] used a magic eye electrical zero point device for the manometer. Haikins and Jura [119] used a narrow bore mercury cutoff to serve as a null point instrument, the absolute pressure being measured on a wide bore manometer. Several authors have shown how the function of manometer and burette can be combined in a single device [120,121]. 

The determination of the volume of solvent vaporized in the unoccupied space of the apparams is difficult and can cause serious errors in the determination of the final solvent concentration. To circumvent the vapor phase correction, one can measure the concentration directly by means, for example, of a differential refractometer. The contact of solvent vapor with the Hg surface in older equipment may cause further errors. Complete thermostatting of the whole apparatus is necessary to avoid condensation of solvent vapor at colder spots. Since it is disadvantageous to thermostat Hg manometers at higher temperatures, null measurement instruments with pressure compensation were sometimes used. Modem electronic pressure sensors can be thermostatted within certain temperature ranges. If pressure measurement is made outside the thermostatted equilibrium cell, the connecting tubes must be heated slightly above the equilibrium temperature to avoid condensation. 

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