Abstract: Process for the preparation of 1-amino-2-naphthol-4-sulfonic acid by treating 1-hydroxylamino-2-tetralone-4-sulfoniuc acid with an alkali metal pyrosulfite in mineral acid medium in the presence of catalytic amounts of a copper compound.1-Amino-2-naphthol-4-sulfonic acid is an intermediate for the manufacture of dyes.

Abstract: The invention relates to an automatically controlled continuous method of eliminating excess nitrite in diazotization solutions of aromatic amines, which comprises controlling the addition of the agent employed for eliminating the nitrite ions by means of an electrochemically controlled variable.The process permits a smooth monitoring of the nitrite ion concentration in diazotization solutions. It is advantageous that the determination of the electrochemical controlled variable is almost instantaneous and that the addition of agent for removing nitrite ions can be controlled in simple manner via the fluctuations in the potential.

Abstract: The invention relates to a process for the preparation of 1-diazo-2-naphthol-4-sulfonic acid by diazotizing 1-amino-2-naphthol-4-sulfonic acid in weakly acid medium and in the presence of iron ions, using only 1 to 30 millimoles of an iron compound per mole of 1-amino-2-naphthol-4-sulfonic acid. Such an insignificant amount is sufficient to protect the 1-amino-2-naphthol-4-sulfonic acid effectively from being oxidized to 1,2-naphthoquinonesulfonic acid and to afford the desired reaction product in good yield.

Abstract: A process for the preparation of formulations of water-soluble organic dyes starting from an aqueous salt-containing suspension of the untreated dye and using membrane separation methods, which process comprises substantially freeing the aqueous suspension of the untreated dye from by-products and reducing the salt concentration by more than half in a first membrane separation treatment, then diluting the dye suspension in a further optional step and effecting deionization and concentration in a second membrane separation treatment and converting the concentrate so obtained into a liquid or solid commercial formulation. This multi-step membrane separation process, e.g. ultrafiltration combined with hyperfiltration, results in a high rate of permeation through the membrane and avoids operating problems caused by membrane clogging.