Preparation of naphthalene-1,4-disulfonic acid
50 g of sodium naphthionate are diazotized in the usual way. When diazo compound separates the sulfur dioxide gas is then passed in until the solution is saturated, the temperature being kept below 0° C. Copper powder is then added very gradually until the evolution of nitrogen ceases, a slow stream of being passed through during the addition. The obtained reaction mixture is then filtered, and common salt is added to saturate the filtrate, when the sodium salt of naphthalene-1,4-disulfonic acid (1 : 4-sulfonicsulfinic acid) separates, and after filtration is recrystallized from water. The free naphthalene-1,4-disulfonic acid may be isolated by passing hydrochloric acid gas into the solution in water of the sodium salt.
Systematic organic chemistry, by W. M. Cumming, 327, 1937.