. Biological stains, a handbook on the nature and uses of the dyes employed in the biological laboratory. Stains and staining (Microscopy). The bond connecting one of the sulfonic groups with an amino group attached to a different benzene ring is assumed to exist in order to account for the fact that altho only two of the sulfonic groups are neutralized Avith sodium, the compound acts as tho it has no free acid. In other words, it is a case of intramolecular salt formation. Now when the tri-sodium salt is formed, this bond is broken down, whereupon the quinoid ring disappears and the fol- lowi
. Biological stains, a handbook on the nature and uses of the dyes employed in the biological laboratory. Stains and staining (Microscopy). The bond connecting one of the sulfonic groups with an amino group attached to a different benzene ring is assumed to exist in order to account for the fact that altho only two of the sulfonic groups are neutralized Avith sodium, the compound acts as tho it has no free acid. In other words, it is a case of intramolecular salt formation. Now when the tri-sodium salt is formed, this bond is broken down, whereupon the quinoid ring disappears and the fol- lowing compound is produced: SOrXa XH. XaS03 \_/ This compound, it will be seen, is a carbinol in structure, and as it lacks the quinoid ring it is colorless; but it is very readily converted into the di-sodium salt by the addition of acid, whereupon the color again appears. This property makes acid fuchsin of use as an indicator. The decolorized solution of acid fuchsin neutralized with sodium hydrate is called the Andrade indicator. It is used quite extensively in bacteriological work, because of the striking reaction when its color is restored by acid-forming bacteria. As an indicator to show hydrogen-ion concentration at all accurately, however, it is found to have much less value than the phthalein and sulphonphthalein dyes (see pp 83 to 86.) Acid fuchsin is a widely used plasma stain, which has also been recommended for a number of special uses. Among the best known are: the Van Gieson connective tissue stain, in which it is used with picric acid after haematoxylin to differentiate smooth muscle from connective tissue; the Ehrlich-Biondi stain, in which with methyl green and orange G it is employed in histology and for staining blood smears; and the Ehrlich tri-acid stain for blood, which is a "neutral" combination with orange G and methyl green. In plant histology it is used to stain the cortex, pith and cellulose walls; while the Pianese stain (with malachite
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