Adsorptive colorant purification methods

Sufficient color reduction is often achieved by recovery and purification methods. However, sometimes specific color removal is achieved by adsorption to, eg, activated carbon. 

The same resin was used for the purification via downstream processing of carminic acid, the natural colorant extracted from cochineal. By a direct adsorption method, a crude extract was applied on the polymeric bed gel and the adsorption kinetics studied using elution with hydrochloric acid and ethanol. The desorbed pure carminic acid concentrated under vacuum yielded a final product that complied with Codex Alimentarius requirements and FAO/OMS norms. 

A sequential analysis protocol includes three steps (1) extraction in water or other appropriate solvent for the colorant, (2) purification or concentration of the colorant, and (3) separation coupled with detection of the target molecule. Different methods of extracting synthetic colorants from foods have been developed using organic solvents followed by SPE protocols using as adsorption support RP-C18, amino materials, or Amberlite XAD-2. Eor qualitative evaluations, the easiest option for separating colorant molecules from unwanted ingredients found in an extract is SPE on polyamide or wool. 

The analysis of TNT in wastewaters is made simple and direct by LC using a UV detector at 220nm (Refs 81 158). An LC method suitable for the low level determination of Tetrvl in the presence of TNT, RDX and HMX is described (Ref 91). The adsorptive LC of TNT was demonstrated using poly(styrene-divinyl benzene) adsorbent and ethanol as the moving phase (Ref 112). HPLC was used for the separation of TNT from purification by-products of hexanitro-bibenzyl (Ref 69). Enzymatic action on TNT was supported by HPLC (Ref 155). HPLC chromatograms of TNT are included, together with data on TLC and color reactions of TNT in mixts (Ref 153). Pollutants in wastewater... 

Tetrahydrocarbazole (2.00 g, 11.68 mmol), chloranil (5.77 g, 23.68 mmol), and the minimum volume of boiling, sulfur-free xylene required to form a clear solution were refluxed until a few drops of the solution gave no red color when heated with sodium hydroxide (24 h). The solution was then cooled, separated from tetrachloroquinol, diluted with ether, shaken first with sodium hydroxide and then with water, and finally dried (sodium sulfate). On evaporation of the solvents, the impure crystalline compound separated and was purified by crystallization from methyl alcohol, benzene, or xylene when this method failed, purification was effected by chromatographic adsorption. 

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