Reservoir, Mercury

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

The mercury reservoir is then lowered until the mercury flow just ceases (not further) and the electrode is allowed to stand in the air. It is good technique, at the beginning of each period of use, to immerse the capillary for ca 1 min. in 1 1-nitric acid while mercury is flowing, then wash it well with distilled water a further precaution is to allow the mercury drops to form in distilled water for about 15 minutes. 

When the experiment has been completed, clean the capillary as described above and then store it by inserting through a bored cork (or silicone rubber bung — normal rubber bungs which contain sulphur must be avoided) which is then placed in a test-tube containing a little pure mercury. Lower the mercury reservoir until drops no longer issue from the capillary, then push the end of the capillary into the mercury pool. 

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)...

A typical electrocapillarity system is shown in Figure 2.1(a). The mercury reservoir provides a source of clean mercury to feed a capillary tube the height of mercury in this tube can be varied such that the mass of the Hg column exactly balances the surface tension between the mercury and the capillary walls, see Figure 2.1(b). A voltage V is applied across the mercury in the capillary and a second electrode which is non-polarisable (i.e. the interface will not sustain a change in the potential dropped across it), such as the normal hydrogen electrode, NHE. The potential distribution across the two interfaces is shown in Figure 2.1(c). As can be seen ... 

These primers may also be tested in the "Gas Volume and Impulse Apparatus , briefly described as the "Test Set Mkl75-ModO in Ref 14, p 9-42. Its photographic view is given as Fig 9 23 (which is not shown here). The apparatus consists of a mercury reservoir, an upright capillary rube, and a firing chamber. When a primer is fired in this test set, the hot reaction products build up pressure in the firing chamber. This pressure is communicated to the upper surface of the mercury in the reservoir, and this, in turn, causes a column of mercury to rise in the capillary tube. 

Fig. 10. Typical dropping mercury electrode assembly. A, Mercury reservoir B, Tygon tubing C, Pt wire connection to mercury D, capillary E, reference electrode F, counter electrode.

Figure 14.1 Classical standtube and mercury reservoir assembly for use with dropping mercury electrodes.

The mercury reservoir usually has a volume of 100-200 cm3. The platinum wire used to make contact to the DME may be dipped in the mercury reservoir or sealed in the standtube. 

Once the capillary has been connected, the mercury reservoir should be filled with very pure mercury. For polarographic purposes, the triple-distilled mercury available commercially is usually satisfactory. If the mercury was previously used in voltammetric experiments, it should be chemically cleaned as indicated in Section I, and then twice or triply distilled. Due to the dangers associated with mercury vapors, it is normal practice to return dirty mercury to the supplier for commercial distillation. 

When filling the apparatus with mercury, one should be especially careful to avoid entrapped air bubbles. The classical DME requires a stand that facilitates changing the position of the mercury reservoir. A centimeter scale is often included, making it convenient to determine ht. Some early authors recommended an all-glass apparatus, but this seems inadvisable now that inert plastic... 

A capillary will work properly for a long time if special care is exercised both during experiments and when the electrode is not in use. An important rule of thumb is that solutions should never enter the capillary. The mercury reservoir should always be elevated and the flow of mercury observed before the capillary is placed in solution. When the polarographic work is finished, the DME should be withdrawn from the cell and transferred to a beaker containing distilled water (or a nonaqueous solvent, if used). Mercury should drop there for several minutes to wash the end of the capillary. Finally, the capillary should be carefully washed with distilled water. 

After washing and drying the capillary, many experimenters simply lower the mercury reservoir enough to stop the flow of mercury, but this procedure can occasionally cause problems. The inevitable metal impurities in mercury can be air-oxidized at the capillary tip, and the oxidation products may change the parameters of the electrode (namely, m and td). There is also a possibility that a small amount of the sample solution may remain in the capillary and then evaporate, leaving a solid residue this will also change the parameters of the electrode, and the residue may be very difficult to remove from the capillary. 

Many variations exist the model in Figure 14.3 has been used for many years at the Department of Chemistry at Warsaw University. The glass part of this electrode is shown in Figure 14.3a. It is composed of thick-walled glass tubing with an internal diameter of 4 mm, approximately 5 cm long. This forms a mercury reservoir, which is sealed to a thick wall capillary. The internal diameter of this capillary is commonly 0.18 mm but may vary somewhat. 

The mercury reservoir and the capillary should be completely filled with mercury. There must be no entrapped air A special vessel useful in filling the HMDE was described by Kemula [24]. This vessel should be equipped with a ground joint corresponding to that of the capillary, and the end of the capillary should extend to within several millimeters of the bottom. 

This type of electrode may be easily prepared in every laboratory. Modifications of this type of HMDE have been described by other authors [25]. The connection of the mercury reservoir to the plastic cap may be made in differ-... 

The potential of such streaming electrodes, placed in the sample solution, is dependent to some extent on the height of the mercury reservoir, ht, going... 

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