Question

Alizarin, sudan III, and bromocresol green are types of dyes used for staining a variety of materials, ranging from clothing to triglycerides. A clever criminal has mixed all three of them together and used the mixture to make it difficult to analyze the clothing and DNA left behind at a crime scene. Find the structures and the acidity (pKa) of each dye and propose a method for separating them using the extraction technique learned in a lab experiment. You will need to identify three specific solvents, two aqueous and one organic. (Hint: the acid-base properties of the dyes and solvents is the key to solving this problem)

Answer 1

Alizarin or 1,2-dihydroxyanthraquinone (also known as Mordant Red 11 and Turkey Red) is an organic compound with formulaC14H8O4 that has been used throughout history as a prominent red dye, principally for dyeing textile fabrics.

PKa = 6.94

Alizarin is one of ten dihydroxyanthraquinone isomers. Its molecular structure can be viewed as being derived from anthraquinone by replacement of two neighboring hydrogen atoms (H) by hydroxyl groups (-OH).It is soluble in hexane and chloroform, and can be obtained from the latter as red-purple crystals, m.p. 277–278 °C.Alizarin changes color depending on the pH of the solution it is in, thereby making it a pH indicator.

Bromocresol green (BCG) is a dye of the triphenylmethane family (triarylmethane dyes). It is used as a pH indicator in applications such as growth mediums for microorganisms and titrations.

PKa = 4.90

In aqueous solution, bromocresol green will ionize to give the monoanionic form (yellow), that further deprotonates at higher pH to give the dianionic form (blue),which is stabilized by resonance.The acid and basic forms of this dye have an isosbestic point in their spectra, around 515 nm.An ethanol solution (0.04 wt%) of bromocresol green has been proposed for TLC staining and is suitable for visualisation of the compounds with functional groups whose pK_a is below 5.0 (carboxylic acids, sulfonic acids, etc.). These appear as yellow spots on a light or dark blue background; no heating is necessary. Bromophenol blue solution can be used for the same purpose.

Sudan III is a lysochrome (fat-soluble dye) diazo dye. It is structurally related to azobenzene.Sudan III is a red fat-soluble dye that is utilized in the identification of the presence of lipids, triglycerides and lipoproteins.Sudan III reacts with the lipids or triglycerides to stain red in colour.The oil will stain red with Sudan III dye since it is a lipid and contains triglycerides. However, since the oil is less dense than water and insoluble in water, the oil will form a layer or globules above the water and appear as a red layer above the water in the test tube.