Methods of visual observation

Methods of visual observation permit us to determine the phase constitution of a system and parameters of phase transformations, and to record an appearance (or disappearance) of phases and critical phenomena with changing temperature and pressure ( Vis.obs. in Table 1.1). These are the typical synthetic method. The composition of equilibrium phases can also sometimes be determined from the known composition of the initial charge. 

All the apparatus are equipped with special devices for stirring the contents of the capsules by rocking the furnace or shaking the capsule in the furnace or a Teflon-covered magnetic stirring bar in the case of a transparent cylinder. 

The error in the determination of phase transition temperatures in a capsule apparatus depends on the rates of phase reaction and variations in temperature as well as on thermal gradients along the capsule and between the capsule and die wall of furnace. The reproducibility of the temperature of such phase transitions as liquid immiscibility or critical phenomena is in the range of several tenflis of a Kelvin for the best measurements. However, an error can reach 1-2 K in most available publications on salt solubility measurements. 

Studies of aqueous systems using the sealed thick-walled capsules do not provide any information about the pressure. The influence of dissolution of the capsule wall on die composition and properties of the system under study is another complicated problem. 

Various visual cells used to study phase equilibria in non-aqueous systems at elevated temperatures and pressures have been described in Young (1978) and Aim and Fermeglia (2003). 

---

Ignition is determined on the basis of visual observation. This can be tricky when the material exhibits extensive flashing before sustained flaming. An alternative method based on the second time derivative of the mass of the specimen has been suggested to alleviate this problem.36-37... 

By means of method of visualization with the help of acoustic waves [1,2] we could get the microstructure images of steel samples on different depths from the surface. The analysis of acoustic images gave the possibility to calculate the dimensions of grains, to observe their transformation in the period of time or under external influences. In accordance with the theory of Hall - Peach there were defined the strength characteristics, for example flow limit ( Go,2) of the materials under study. The significance obtained o0 2 is in proper correspondence with values that are table one for the type of steel under consideration. 

In order to test the protective efficiency of the surface treatments, a number of 0100 mm and 060 mm cores were later on drilled out from the concrete decks. For Structure 1, the cores were removed from the deck slab after approximately four years, while for Structure 2, the cores were removed from the deck beams after approximately two years of exposure. Shortly after all surface treatments, a number of 060 mm cores had also been removed for control of penetration depth of the hydrophobic agent. This control was carried out by use of visual observations based on splitting of the removed cores and spraying with pure water [9]. The control of chloride penetration later on was based on successively grinding off small powder samples, from which the chloride content was analyzed by use of a spectrophotometric method [10]. 

A visual examination should be made to yield information on the general condition of the part, component or surface to be examined, including such conditions as the presence of scratches, wear, cracks, corrosion or erosion on the surface, or evidence of leaking. Optical aids such as television cameras, binoculars and mirrors may be used. Surface rephcation as a method of visual examination may be considered acceptable, provided that the resolution at the surface is at least equivalent to that obtainable by visual observation. Any visual examination that requires a clean surface or decontamination for the proper interpretation of results should be preceded by appropriate cleaning processes. 

Two methods of visualization will be used to observe the positions of the spots on the developed TLC plates. First, the plates will be observed while under illumination from a short-wavelength ultraviolet (UV) lamp. This is best done in a darkened room or in a fume hood that has been darkened by taping butcher paper or aluminum foil over the lowered glass cover. Under these conditions, some of the spots will appear as dark areas on the plate, while others will fluoresce brightly. This difference in appearance under UV illumination will help to distinguish the... 

If you use large plates (Section 20.3B), materials can be separated, and the separated components can be recovered individually from the plates. Plates used in this way are called preparative plates. For preparative plates, a thick layer of adsorbent is generally used. Instead of being applied as a spot or a series of spots, the mixture to be separated is applied as a line of material about 1 cm from the bottom of the plate. As the plate is developed, the separated materials form bands. After development, you can observe the separated bands, usually by UV light, and outline the zones in pencil. If the method of visualization is destructive, most of the plate is covered with paper to protect it, and the reagent is applied only at the extreme edge of the plate. 

Modern versions of the Winkler method improve the sensitivity and accuracy of the method by computer control of the titration procedure and the endpoint detection. Instead of visual observation of the decolouration of the blue starch-iodine complex, either the starch-iodine complex colour or the iodine colour itself is measured photometrically in the visible to ultraviolet (UV) spectral range. The spectral absorbance of an I3- solution (oxygen sample before titration) is depicted in Fig. 4-1. Grasshoff (1981) described a dead-stop titration of iodine with thiosulphate using amperometric endpoint detection. Bradburg and Hambly (1952) have compared various endpoint detections for iodine-thiosulphate titrations in low concentration ranges and stated relative sensitivities for visual-starch, colouri-metric-starch, amperometric, UV absorption as 1 0.2 0.002 0.0015. 

Differences between right and lefthanded subjects (hypothesis right-handed subjects show a slower visual cycle time at the left hemisphere, i.e., the right hand dvCTr > dvCTl). This hypothesis cannot be supported by the present method of direct observation of cycle time. 

Glassification of Phase Boundaries for Binary Systems. Six classes of binary diagrams have been identified. These are shown schematically in Figure 6. Classifications are typically based on pressure—temperature (P T) projections of mixture critical curves and three-phase equiHbria lines (1,5,22,23). Experimental data are usually obtained by a simple synthetic method in which the pressure and temperature of a homogeneous solution of known concentration are manipulated to precipitate a visually observed phase. 

The indirect methods discussed thus far have dealt with measurement of color only as it can be correlated with physical characteristics of materials and the effect of these materials on radiant energy. As has been pointed out, the reflectance spectro-photometric curve describes a property of the material. A change in the reflectance spectrophotometric properties may not always result in a change in visual color. The reason is that color of the object is not an unchangeable characteristic of the object itself, dependent only upon these reflectance properties, but is also dependent upon the quality of the illuminating light and the sensitivity of the observer s eye. Thus the measurement and description of visual color are psychophysical problems... 

Methods for the instrumental measurement of whiteness are well established but visual comparison remains important, even in well-equipped laboratories. Some degree of quantification is achieved by the method of paired comparisons, in which a panel of observers is presented with pairs of FBA-treated samples and asked to decide, without undue delay, which is the brighter. The total of positive scores can be used as a measure of whiteness and the results presented graphically as shown in Figure 11.7. Although time-... 

Equations (13) and (14) can be used to obtain quick estimates and to visualize the response of a polymer system under investigation. In any case, unless D t) and E(t) are varying very slowly on a log time scale, the distributions are valid only in the time range from the minimum time of observation, plus one decade, to the maximum timeArf observation, less one decade. Many more accurate and complex methods of estimating L(r) and //(T) have been proposed. These methods have been summarized by... 

Two methods were used to detect the onset of flocculation in the dispersions direct visual observation and turbidity/wavelength... 

In the light of these observations Olson and Chen [164] decided to use a correction factor for use in their visual endpoint calcium titration method involving titration with EGTA. They found that interferences by magnesium... 

The color of the quinonoid compounds that may be obtained by disproportionation can be sufficiently like that of the radicals to cause confusion if visual observation or broad-band spectrophotometry is used.11 For example, Preckel and Selwood, using paramagnetism as a measure of the amount of radical, reported that solutions of triphenyl-methyl derivatives more or less rapidly lost their paramagnetism. The decomposed solutions were still highly colored, but the color was no longer dependent on the temperature as it is in the case of a radical-dimer equilibrium mixture. What is more striking, and an even more subtle and dirtier trick on the part of nature, is the fact that Preckel and Selwood s non-paramagnetic solutions were still rapidly bleached by exposure to the air. It is clear that radical-like reactivity is not a safe criterion for the presence of radicals. It is also clear that the ebullioscopic method is particularly unsatisfactory in view of the excellent chance for decomposition. 

By this method the weighed dry product is dissolved in methanol and titrated with the Karl Fischer solution until the color changes from brown to yellow. The visual observation can be replaced by an ammeter, which shows an steep increase in current, when the titration is terminated (dead-stop-titration). The samples can be two to four times smaller than for the gravimetric method. To avoid the visual observation completely, iodine can be produced by electrolyzation and the water content is calculated by Coulomb s law. Such an apparatus (e. g. Fig. 1.97.1 and 1.97.2) is available commercially. The smallest amount of water to be detected by such instruments is 10 pg. Wekx and De Kleijn [1.84) showed, how the Karl Fischer method can be used directly in the vial with the dried product. The Karl... 

The usual method of detecting chromatographed tetracycline antibiotics involves fuming the paper with ammonia vapor and observing the yellow fluorescence under UV light. As little as 0.2-0.5 pg can be visualized by this technique. 

---