Thermostat, capillary

Figure 4-33. Block diagram of capillary electro- carried out close to the cathode in a region phoresis equipment. Capillary electrophoresis where the capillary is transparent, allowing equipment consists of a thermostatted capillary photometric or fluorimetric analysis of the whose ends are placed in the electrode buffer eluate. The detector system is linked either to a chambers these contain the electrodes attached recorder/integrator or to a PC. to a high-voltage power supply. Detection is...

A CE instrument comprises a high-voltage source and electrodes, containers for buffers, provision for holding and injecting samples, a thermostatted capillary housing and a detector. Most CE instruments use UV-vis absorbance detection, sometimes with spectral analysis capabilities. In instrumental separation sciences quantitative data is derived from the peak elution/mi-gration time and peak dimensions therefore any instrumental function which can affect these must be tested. 

The density determination may be carried out at the temperature of the laboratory. The liquid should stand for at least one hour and a thermometer placed either in the liquid (if practicable) or in its immediate vicinity. It is usually better to conduct the measurement at a temperature of 20° or 25° throughout this volume a standard temperature of 20° will be adopted. To determine the density of a liquid at 20°, a clean, corked test-tube containing about 5 ml. of toe liquid is immersed for about three-quarters of its length in a water thermostat at 20° for about 2 hours. An empty test-tube and a shallow beaker (e.g., a Baco beaker) are also supported in the thermostat so that only the rims protrude above the surface of the water the pycnometer is supported by its capillary arms on the rim of the test-tube, and the small crucible is placed in the beaker, which is covered with a clock glass. When the liquid has acquired the temperature of the thermostat, the small crucible is removed, charged with the liquid, the pycnometer rapidly filled and adjusted to the mark. With practice, the whole operation can be completed in about half a minute. The error introduced if the temperature of the laboratory differs by as much as 10° from that of the thermostat does not exceed 1 mg. if the temperature of the laboratory is adjusted so that it does not differ by more than 1-2° from 20°, the error is negligible. The weight of the empty pycnometer and also filled with distilled (preferably conductivity) water at 20° should also be determined. The density of the liquid can then be computed. 

Where fluid pressure is carried by a capillary tube, such as with the thermostatic expansion valve or pressure switches, the tube should be installed with due attention to the risk of it chafing against metal edges and wearing through. Tubes to manometers are usually in plastic, but may be copper. These must be carefully tested for leaks, as they are transmitting very low pressures. 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

For fast reactions (i.e., < 1 min.), open tubular reactors are commonly used. They simply consist of a mixing device and a coiled stainless steel or Teflon capillary tube of narrow bore enclosed in a thermostat. The length of the capillary tube and the flow rate through it control the reaction time. Reagents such as fluorescamine and o-phthalaldehyde are frequently used in this type of system to determine primary amines, amino acids, indoles, hydrazines, etc., in biological and environmental samples. 

Fig. 3.7 shows a comparison between a capillary thermostat in the right part of the figure and the ever more frequently used NTC temperature sensor whose analogue electrical signal can easily be processed by an electronic control system. The NTC sensor type is increasingly used, particularly in modern European machines that always have their own heater element and sometimes also an additional hot water connection. 

In cheap machines with simple capillary thermostats it is possible that due to the tolerances of this component the set temperature may vary by up to plus/ minus 10 °C. This is shown in the upper part of Fig. 3.8. If the cold water temperature is relatively high , the set lower temperature limit may be exceeded, and the heating may not be switched on at all. The washing process is then carried out with cold water with accordingly poor washing results. 

Fig. 17.8. Representation of a CE system. (1) Electrolyte compartments, (2) the capillary, (3) detector, (4) power supply, (5) sample carousel, (6) electrodes, (7) thermostatted areas and (8) data workstation.

As was already addressed in Section 17.2.4, despite the use of narrow bore capillaries, the temperature difference between the wall of the capillary and the surrounding air/hquid can rise up to several degrees (exceeding 70°C) [31,77]. This was shown in Fig. 17.6 and is known as the self-heating of the capillary due to the power production within the capillary. An efficient thermostatting procedure for the capillary is one of the primary prerequisites in order to perform reproducible CE. The uncontrolled temperature augmentation obviously has effects on sample stability, buffer pH and viscosity. Especially, certain biomolecules... 

In some experiments with the same apparatus isobutylene was not added to the solution of aluminium halide as a solution, but it was distilled in very slowly through a capillary from a thermostatted hanging burette. The unprecedented result of these experiments was that no polymerisation occurred and that the isobutylene could be distilled out of the mixture and was recovered almost quantitatively. A brief preliminary account of these experiments has been given [8] because of the novelty of this phenomenon, the experiments will be described in detail. The particulars are summarised in Table 3 and the conductivity changes during the experiments are shown in Figure 7. 

Basically, the calomel electrode consists of mercury, mercurous chloride (calomel), and chloride ion. The concentration of potassium chloride is 0.1 M in an aqueous-organic solvent (50 50) of the same nature as that contained in the solution to be investigated. The junction with the test solution is realized either with a capillary or a porous stone. When the capillary is used, a small hydrostatic pressure is maintained inside it in order to avoid any electrode contamination by the test solution. In the main part of our investigation, the porous stone junction was used. Moreover, the calomel electrode is thermostatted at 20°C, and temperature variations of this electrode giving appreciable emf variations involve uncertainty on the pon determination on the order of 0.2-0.3 poH unit/ 10°C. 

Fig.2.3. Dilatometer in a thermostat. Conversion-time curves are obtained through volume contraction in the capillary...

I.6.2. A variety of models If the most rigorous technique is not required, for example, if a monomer is to be polymerised at, say, 80 °C by a radical initiator, then solvent and monomer are run into the mixing chamber, the catalyst is added and left to dissolve, the assembly is then attached to a vacuum Une to allow the reaction mixture to be degassed by the conventional freeze-pump-thaw process and to facilitate the filling of the body and the capillary. When this has been done, the dilatometer is thermostatted and the height of the meniscus in the capillary is monitored by means of a catheto-meter. The simple dilatometer adequate for this can be modified for more... 

Methyl bromide was kept in an ampoule at 0 °C and was displaced by the introduction of 1.8517 p. s of mercury from the motorburette. The methyl bromide was passed through a 2-m stainless steel capillary heating coil which was placed in the thermostat water. In order to derive the enthalpy of reaction of liquid methyl bromide, the enthalpy of vaporization (23.0 kJmoR ) was subtracted from the value obtained for gaseous methyl bromide. 

The objective of this test was to assess the wax deposition tendency of Troll crude in a cool subsea flowline. The equipment consists of a thermostatted stirred vessel from which crude oil is circulated by a gear pump through a thermostatically controlled stainless steel capillary. Tne temperature in the vessel was maintained at 6S°C and the capillary tube was held at a constant temperature below the cloud point. 

The cell for rotating electrodes, Fig. 7, is usually cylindrical and surrounded by a water jacket for thermostatting purposes, but as long as the cell walls are more than 1 cm or so from the rotating assembly, there are usually no cell edge effects. The auxiliary electrode is very often contained in a separate compartment behind a glass frit in order to avoid contamination problems. A Luggin capillary, where required, can be positioned in various ways unless it is more than 0.5 cm from the electrode, it must be placed under the centre of the disc in order to avoid a non-equipotential surface this can cause some problems with disturbance of the fluid flow. 

Fig. 7. Typical rotating ring—disc electrode cell. A, rotating ring—disc electrode B, reference electrode with Luggin capillary C, counter electrode D, teflon lid E, porous frit F, thermostatted water jacket.

---