Microscale Preparation of Methyl Orange

Background and significance

The dyeing of fabric has been done for thousands of years but, it wasn’t until 1876 that Otto Witt made the first attempt to interpret color of dyes in terms of chemical structures. Witt proposed that dyes consist of conjugated P-bonded systems, chromophores, and salt forming groups, auxochromes, which are polar substituents that would modify color.  A chromophore is an unsaturated, electron withdrawing group attached to an aromatic ring system. Extended conjugation due to the chromophore decreasing dissociation energy of the pi and pi* transition so that the maximum intensity is in the visible range. The auxochrome is also an electron donating group attached to the organic ring system changing dissociation energy and maximum intensity.1

The dye made, methyl orange, is categorized as an azo dye,  a class of synthetic organic dyes that contain nitrogen as the azo group  (-N=N-) in the molecular structure which link two sp2 hybridized carbon atoms.  The general formula for making an azo dye requires two organic compounds one which acts as a coupling component and one that is a diazo compound. A methyl orange molecule is a water soluble anion that form ionic bonds between the dye and the fiber. This dye have a –SO3 group that form the ionic bond, or salt linkages with the NH3+ groups on the fiber.

For the making of the dye the “basic” steps were as follows:

1. Sodium carbonate reacts with sulfanilac acid, making the acid more soluble

[pic 1]

1. Sodium nitrite and hydrochloric acid react to produce water and the nitrosonium ion

2 H3O+ (aq) + NO2- (aq) → 3 H2O (l) + NO+ (aq)

1. Nitrosonium ion reacts with the amino of the sulfanilic acid, attaching a second N-atom on the sulfanilic acid (making a diazonium salt).

[pic 2]

1. Diazonium salt reacted with the N,N-dimethylaniline and the hydroxide ion to produce methyl orange

[pic 3]

Through the experiment dyeing of multi-fabric strip with both natural and synthetic fabrics is completed to test the fastness of the dye to these particular fabric threads.  Since the thread have different molecular structures the dye may not bond to it which is also what is being tested. The fabric strip that is used contain: Acetate, SEF (self-extinguishing fabric), Arnel, Bleached cotton, Creslan 61, Dacron 54, Dacron 64, Nylon 66, Orlan 75, spun silk, Polypropylene, viscose rayon, and wool.

Procedure

Started by dissolving 0.108 g of sodium bicarbonate (NaHCO3) in 5 mL deionized water in Erlenmeyer flask and adding .203 g sulfanic acid monohydrate (C6H7NO3S) and heating until fully dissolved. Solution was cooled to room temperature and .082 g sodium nitrite (NaNO2) was added and solution stirred until dissolved.  Solution was cooled in an ice water bath for 9 minutes, swirling frequently, until temperature was below 10°C.  Once cooled, 0.25 mL of concentrated HCL was added where a diazonium salt of anthranilic acid (C7H7NO2 ) separated as participate. (If needed, keep suspension in ice bath until ready for further use).

The next step was completed by combining approximately 0.14 mL of N, N-dimethylamline (C6H5N(CH3)2) and 0.14 mL of glacial acid in a test tube. This solution was added to a cooled suspension of diazotized sulfanilac acid in flask using pipet, color of solution started as yellow then turned to a dark red color. Mixture was then cooled for 10 minutes in ice bath. After 10 minutes, 1.5 mL of 10% aqueous sodium hydroxide (NaOH) was added slowly, while stirring, continuing to cool in ice bath.

The pH was then tested to determine basicity (if not basic, more sodium hydroxide should be added and rechecked). The basic solution was of a thick, red color and consistency. The now basic solution was heated to boiling for 15 minutes to dissolve newly formed methyl orange, though not all dissolved. Sodium Chloride (NaCl), 0.503 g, was added, allowed to cool and placed in ice bath to complete crystallization. Mixture was transferred to tube, centrifuged, and liquid decanted. Flask was rinsed with a portion of saturated aqueous sodium chloride, added to solid in tube, and again centrifuged.