Apparatus and general technique

Originally, the number of coulombs passed was determined by including a coulometer in the circuit, e.g. a silver, an iodine or a hydrogen-oxygen coulometer. The amount of chemical change taking place in the coulometer can be ascertained, and from this result the number of coulombs passed can be calculated, but with modern equipment an electronic integrator is used to measure the quantity of electricity passed. 

Apparatus. The source of current is a potentiostat which is used in conjunction with a reference electrode (commonly a saturated calomel electrode) to control the potential of the working electrode. The circuit will be essentially that shown in Fig. 12.2(a) but with the addition of the integrator or of a coulometer. 

Hydrazine is used as depolariser at the platinum anode for metals which are not reduced by this compound  

The evolution of nitrogen aids in removing dissolved air. A salt bridge (4 mm tube) attached to the saturated calomel electrode is filled with 3 per cent agar gel saturated with potassium chloride and its tip is placed within 1 mm of the mercury cathode when the mercury is not being stirred this ensures that the tip trails in the mercury surface when the latter is stirred. It is essential that the mercury-solution interface (not merely the solution) be vigorously stirred, and for this purpose the propeller blades of the glass stirrer are partially immersed in the mercury. 

One of the outstanding advantages of the mercury cathode is that the optimum control potential for a given separation is easily determinable from polarograms recorded with the dropping mercury electrode. This potential 

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Column chromatography is an often used and routinely carried out technique which Is adaptable to all the major types of chromatography. Although such diverse operations as column adsorption, partition, ion exchange, exclusion and affinity chromatography are carried out in a column, the apparatus and general techniques used shaire a lot in common. The common features are discussed below. 

Stahl, E. (1969). Apparatus and general techniques. In Thin Layer Chromatography. A Laboratory Handbook, 2nd ed. E. Stahl (Ed.). Springer-Verlag, New York, pp. 86-105. 

Innumerable techniques, of almost infinite variety, have been proposed at one time or another for the measurement of the solubihty of soUds in liquids. No single method can be identified, however, as being generally applicable to all possible types of system. The choice of the most appropriate method for a given case has to be made in the light of the system properties, the availability of apparatus and analytical techniques, the skill and experience of the operators, the precision required, and so on. 

In general terss, the purge-and-trap technique is the nethod of choice Cor detemining organic volatiles in water because of its ease of operation. If greater sensitivity is required, the closed loop stripping apparatus should be used. 

Reports on microbiological assays usually dwell upon apparatus, general technique, measurement of growth, choice of assay organism, etc., but since there are many reviews of these subjects (B26, G3, G4, H19), they need not be discussed here. We shall deal here with the development and use of microbiological assays for clinical purposes. 

Radical Generation. The ESR spectrometer, flow system, and general procedure have been described (46). The apparatus was calibrated with freshly prepared diphenyl picrylhydrazyl (DPPH) solutions. The peroxy radical concentrations were determined by double integration of derivative spectra. A standard coal sample in the dual cavity allowed corrections to be made for changes in cavity Q. The rates of decay of the less reactive radicals were determined by stopped-flow techniques with manually or electrically operated valves. The decay was recorded... 

Much of what needs to be done about the techniques of removing water from glass apparatus and from solvents and reagents has been set out above in a more general context. It remains to point out a few pecularities which characterise the drying process rather than any other purification procedure and to discuss some drying agents. 

Hitherto, property measurements of BLM have been confined mainly to thickness, water permeability, electrical characteristics, and current-voltage. The bifacial tension (y6) of BLM is believed to be very small, and a value of about 1 dyne per cm. has been estimated (10). Since no detailed investigations of the bifacial tension of BLM have been reported, the immediate purpose of this work was to develop suitable techniques for y6 measurements. The results of measurements on BLM formed from various lipid solutions are given. The general applicability of the apparatus and method described here to studying other interfacial and bifacial phenomena is briefly discussed. 

As already mentioned in Chapter 1, there are mainly three geometries suitable for the measurement of flow birefringence, viz. those of the concentric cylinder apparatus, the adapted cone-and-plate apparatus and the slit-capillary with a rectangular cross-section. The general principles of the pertinent techniques have been described in the same chapter. The purpose of the present chapter is to give details of the design and construction in order to enable the reader to form a judgement as to the efficiency of the proposed methods, i.e. the relation between information and experimental effort. 

Caverly and Unwin [536] have described a rapid and sensitive technique for the determination of the residues of the fungicides furalaxyl and metalaxyl in soils. The soil sample is extracted with acetone in a Soxhlet apparatus and then the extract is analysed by gas chromatography using NP-selective detection. Recoveries are generally in excess of 80% with detection limits of 0.1 mg/kg. 

The purification of water has been treated exhaustively in the literature, and reviews have summarized general conclusions and recommended techniques for the ultrapurification and analysis of water.70,72 There is a general consensus among investigators that (1) water purified in quartz or plastic apparatus contains a lower level of cationic impurities than water purified in borosilicate or metal apparatus (2) storage of purified water for periods exceeding 30 days, even in plastic or Teflon containers, will result in an increase in cationic impurities and (3) resistivity measurements may be used as a survey technique but cannot be relied on for an unequivocal indication of water quality because they do not indicate nonionic impurities. 

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