Preperation Of Dyes

METHYL ORANGE INDICATOR:

Methyl orange is a pH indicator and due to its clear color change it is very often used in
titrations. Methyl orange changes color at the pH of a mid-strength acid and is usually
used in titrations for acids. Unlike a so called universal indicator, methyl orange does not
have a full spectrum of color change, but has a sharper end point.

Structure Of Methyl Orange

PREPERATION:

1. Although sulfanilic acid is insoluble in acid solutions, it is nevertheless necessary to carry out the diazotization reaction in an acid (HNO2, nitrous acid) solution. This problem can be circumvented by precipitating sulfanilic acid from a solution in which it is initially soluble. The precipitate which is formed is a fine suspension and reacts instantly with nitrous acid. The first step is to dissolve sulfanilic acid in basic solution.
2. In order to obtain the nitrosonium ion (NO+), sodium nitrite has to be treated with hydrochlorid acid. During the addition of the acid, the sulfanilic acid is precipitated out of solution as a finely divided solid, which is immediately diazotized
3. The finely divided diazonium salt is allowed to react immediately with dimethylaniline in the solution in which it was precipitated.

                                 

MALACHITE GREEN:      

                     

Malachite green is an organic compound that is used as a dyestuff and has emerged as a controversial agent in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper.

PREPARATION:

Grignard reagents are highly susceptible to water and much care will go into ‘drying’ the reaction before 

you begin. In order to insure water is kept out of this reaction you will flame dry your glassware, use 

anhydrous ether as the solvent and your reaction vessel will contain a drying tube to dry the 

atmosphere with in the reaction apparatus. 

Before you begin this lab you may want to recrystallize 4-bromo-N,N-dimethylaniline using methanol as 

solvent. Once recrystallized make sure that all methanol has evaporated from the crystals.