Spots form dark

Figure 1 Schematic drawing of PVC molecules with the chlorine atoms represented by circles. Dark areas represent the weak spots formed by clusters of partially negatively charged chlorine atoms.

In some experimental work (op. cit.) at the Bureau of Standards, Washington, to test out the Schwerin invention and to determine whether clays submitted to direct current separation would be improved by it, the conclusion was reached that most clays were not improved in quality by the separation and the success of the invention would lie in its being able to compete with the filter press. There seems some ground for believing that the process can remove such substances as pyrite from clays which tend to form dark spots or patches in the burned articles and it may have use in the clay and ceramic industries for removing substances not mentioned in the test work. 

FeS2, sometimes found as particles or nodules in clays, forming dark spots when the clay is fired. 

Salt glands of plants from Atriplex genus contain inclusions in the form of crystals of siliceous or sulphate salts of calcium and magnesium (Fahn, 1979). Usually the crystal particles also include phenols (see Chapter 7). The crystals are seen as dark dense spots within the structures on OCM images of the optical slices from the gland (Fig. 4). Profiles of signal intensity along... 

The consumption of the oxime can be checked by thin-layer chromatography on silica gel G with the solvent system chloro-form/methanol (95/5 v/v) and a spray reagent consisting of 5% potassium dichromate in 40% sulfuric acid. The oxime appears as an immediate dark spot and the aziridine as a yellow spot. The checkers observed identical mobilities (Rf 0.8) for both compounds. 

Cabon tetrachloride, n-hexane, chloroform, ACN, acetone, THF, pyridine, acetic acid, and their various mixtures were applied as mobile phases for adsorption TLC. Methanol, 1-propanol, ACN, acetone, THF, pyridine and dioxane served as organic modifiers for RP-TLC. Distilled water, buffers at various pH (solutions of and dipotassium hydrogen phosphate or potassium dihydrogen phosphate) and solutions of lithium chloride formed the aqueous phase. Carotenoids were extracted from a commercial paprika sample by acetone (lg paprika shaken with 3 ml of acetone for 30 min), the solution was spotted onto the plates. Development was carried out in a sandwich chamber in the dark and at ambient temperature. After development (15 cm for normal and 7cm for HPTLC plates) the plates were evaluated by a TLC scanner. The best separations were realized on impregnated diatomaceous earth stationary phases using water-acetone and water-THF-acetone mixtures as mobile phases. Some densitograms are shown in Fig.2.1. Calculations indicated that the selectivity of acetone and THF as organic modifiers in RP-TLC is different [14],... 

Fig. 13. On the left, representation of the ferritin molecule the subunits form a spherical shell containing a ferrihydrite crystal. On the right, a Transmission Electron Microscope picture showing the iron core contained inside the proteic shell, appearing as electron-dense dark spots.

Analyst 91(1082), 366-67(1966) CA 65, 3659(1966) (For detection of dinitro- and trinitro- aromatic bodies in industrial blasting expls, a 5—10 mg sample of expl to test is placed on a spot plate, and a drop of Me2CO-EtOH and a drop of 25% aq Me 4NOH. soln are added. The formation of a blue color indicates the presence of 2,4-DNT while 2,4,6-TNT gives a dark red color. A slight yellow color is formed initially with NG. If NG 2,4-DNT are both present, a green color is initially obtd. None of the other common components of blasting expls interfered. It is not indicated in CA which color is produced by m-MNT)... 

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