Violet, Acid Congo

Disazo indicators. In this group of indicators are included Congo red, benzopurpurin B, and benzopurpurin 4B, none of which are recommended for use as indicators for the determination of pH. Though the red alkaline form is soluble in water, the blue or violet acid form is insoluble. 

The decolorization potential of immobilized P. chrysosporium MTCC 787 for azo dyes Acid Orange, Acid Red 114, triphenylmethane dye Methyl Violet, diazoic dye Congo Red, vat dye Vat Magenta, thiazine dye Methylene Blue, and anthraqui-none Acid Green was demonstrated by Radha et al. [53]. Decolorization experiments were carried out with immobilized calcium alginate (Ca-ALG) beads of different sizes (2-6 mm). 

HC1 (Congo paper should be turned only faint violet), and the free acid crystallises out. It is filtered and washed with very little water. 

Congo Paper.—Used as an indicator for acids. Turned pure blue by mineral acids and violet by strong organic acids. 

Starch Variety Fuchsin Acidic Basic Methyl Violet Methylene Blue Congo Red Eosin... 

The carboxylic acids are considerably stronger acids than the phenols. They turn litmus red, and yield alkali metal salts which are neutral to litmus. They do not turn Congo red paper blue, however at best only a violet coloration is formed. In contrast to the phenols, the carboxylic acids dissolve even in bicarbonate solutions, and very easily in carbonate and ammonia solutions. The free acids are regenerated from their salts by strong mineral acids. 

Indicator papers frequently aid in the identification of chemicals. Strong mineral acids are acid towards methyl violet or thymol blue papers, moderately strong acids react acid towards Congo paper, and very weak acids are acid towards htmus and azolitmin paper. Strong bases show an alkaline reaction with turmeric or tropeolin 0 paper, medium strong bases with phenol-phthalein, and very weak bases with litmus or azolitmin paper. These papers are not recommended for use in quantitative analysis. ... 

Add a few grams of crushed-ice into the solution and acidify the contents by adding concentrated hydrochloric acid dropwise and carefully with constant stirring until the mixture is acid to Congo red paper (pH 5.0 Red pH 3.0 Blue-Violet). 

Alizarin Alizarin Red S Alizarin Yellow R Benzopurpurine 4B 4,4 -Bis(2-amino-l-naphthylazo)-2,2 -stilbenedisulfonic acid 4,4 -Bis(4-amino-l-naphthylazo)-2,2 -stilbenedisulfonic acid Brilliant Yellow Bromocresol Green Bromocresol Purple Bromophenol Blue Bromothymol Blue Chlorophenol Red Clayton Yellow Congo Red o-Cresolphthalein Cresol Red Crystal Violet Curcumin (Turmaric) p-(2,4-Dihydroxyphenylazo) benzenesulfonic acid, sodium salt... 

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