Acid Ammonium o-Sulfobenzoate

Acid ammonium o-sulfobenzoate has been prepared by the hydrolysis of saccharin with concentrated hydrochloric add.1 The above method is a modification of that of White and Acree.2... 

In many cases, particularly for aryl acid anhydrides, the sodium, ammonium, or potassium salt of the acid is heated with sufficient phosphorus oxychloride or thionyl chloride to convert half of the salt into the acid chloride. The latter reacts with the other half of the salt to give the anhydride. This method is used for the preparation of the anhydride of o-sulfobenzoic acid (see page 339). Finally, a few dicarboxylic acids, such as phthalic acid, which contain two carboxyl groups attached to adjacent carbon atoms, when heated above their melting point lose water and form the anhydride. 

F) Preparation of Sulfonphthaleins. The preparations described involve (1) hydrolysis of o-sulfobenzoic imide (saccharin) to the ammonium salt of o-sulfobenzoic acid (2) preparation of o-sulfo-benzoic anhydride (3) preparation of phenolsulfonphthalein (phenol red) (4) bromination of phenolsulfonphthalein to tetrabromo derivative (bromophenol blue) (5) preparation of o-cresolsulfon-phthalein (cresol red) (6) bromination of o-cresolsulfonphthalein to the dibromo derivative (bromocresol purple)The equations for the steps are ... 

Amino-2-suIfobenzoic Acid. p-Amino-o-sulfo-benzoic acid. C7H,NOsS mol wt 217.20. C 38.71%, H 3.25%, N 6.45%, O 36.83%, S 14.76%. Prepd by treating the potassium salt of 4-nitro-2-sulfobenzoic acid with ammonium sulfide Hart, Am. Chem. J. 1, 351 (1879/80) Kastle. ibid. 44, 490 (1910). 

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