Alkaline fading reactions

One of the most typical examples demonstrating the importance of the hydro-phobic contribution may be alkaline-fading reactions of triphenylmethane dyes, which take place between dye cations and hydroxyl ions to form carbinols167. ... 

A flow injection method based on the catalytic action of iodide on the colour-fading reaction of the FeSCN2+ complex was proposed and applied in order to determine iodine in milk. At pH 5.0, temperature 32°C and measurements at 460 nm, the decrease in absorbancy of Fe -SCN (0.10 and 0.0020 mol /I) in the presence of N02" (0.3 mol/ 1) is proportional to the concentration of iodide, with a linear response up to 100.0 pg/1. The detection limit was determined as 0.99 pg/1 and the system handles 48 samples per hour. Organic matter was destroyed by means of a dry procedure carried out under alkaline conditions. Alternatively, the use of a Schoninger combustion after the milk dehydration was evaluated. The residue was taken up in 0.12 mol/1 KOH solubilization. For typical samples, recoveries varied from 94.5 to 105%, based on the amounts of both organic matter destroyed. The accuracy of the method was established by using a certified reference material (IAEA A-11, milk powder) and a manual method. The proposed flow injection method is now applied as an indicator of milk quality on the Brazilian market (de Araujo Nogueira et al., 1998). 

The alkaline fading of crystal violet (4-Me2NC6H4)3C, in micelles of CTABr modified by 1-hexanol or by 1-octanol has been treated quantitatively by estimating the local concentration of OH in the interfacial region by solving the Poisson-Boltzmann equation. The local second-order rate constants calculated by this method are very similar to those calculated for normal aqueous micelles of CTABr [107], but these rate constants are uncertain because the authors did not correct for the competing reaction with alkoxide ion formed by deprotonation of the alcohol by OH (cf Refs. 79 and 80). 

The alkaline fading of crystal violet (4-Me2NC6H4)3C+ in cationic W/O microemulsions of CTABr-alcohol-cyclohexane occurs in the water pools [113]. The corresponding reactions of crystal violet and malachite green, (4-Me2NC6H4)2C Ph, in anionic W/O AOT-isooctane microemulsions also occur in the water pools, and rate data were analyzed in terms of their size and the competing base-promoted hydrolysis of AOT [114]. This reaction with crystal violet is strongly inhibited by ClOj, which pairs with the carbocation and blocks nucleophilic attack,... 

The alkaline fading of crystal violet in AOT micelles formed in supercritical ethane as a continuous phase has been studied by Ikushima et al. [20,70,71] (Scheme 12.5). The reaction rate of fading in this system was reported to increase compared with that in H20/AOT/heptane at high pressures under similar conditions. 

The transformation from colorless to red-brown takes place between pH 11.8-14. The alkaline form is unstable, and the red-brown color fades on standing. The speed of this reaction increases with alkalinity. 

The nitroprusside reaction is noteworthy for being highly specific for —SH groups (if carried out in faintly alkaline solutions) but notoriously difficult for defining detail of distribution. The color developed fades quickly also fairly thick sections are required to give adequately robust... 

Nitrous oxides in combination with stabilizers, such as phenols, can lead to discoloration. This discoloration is known as gas fading [534]. The reaction of nitrous gases with 2,6-di-tert-butyl-p-cresol (BHT) leads to the formation of a nitro-sated BHT (2,6-di-tert-butyl-p-nitrophenol), minimal amounts of which in an alkaline medium will lead to intensive yellowing [534]. Other phenolic stabilizers can also cause yellowing or reddening in the presence of nitrous gases [534]. 

---