In a 4-l. beaker, 290 cc. of water is heated to boiling, and 260 g. (1.1 moles) of crystallized sodium sulfide (Na₂S·9H₂O) and 34 g. of powdered sulfur are dissolved by heating and stirring. A solution of 40 g. of sodium hydroxide in 100 cc. of water is then added and the mixture cooled, first in cold water, and finally by a freezing mixture of ice and salt.

In a 2-l. beaker, set in a freezing mixture and provided with a stirrer and a thermometer for reading temperatures to 0°, are placed 500 cc. of water, 137 g. (1 mole) of anthranilic acid, and 200 cc. of concentrated hydrochloric acid; the stirrer is started and the mixture cooled to about 6°. Meanwhile 69 g. (1 mole) of sodium nitrite is dissolved in 280 cc. of hot water and the solution cooled in ice; portions are then placed in a separatory funnel of convenient size, supported in such a way that the lower end of the stem extends beneath the surface of the anthranilic acid solution. When the temperature has fallen to 5°, the nitrite solution is run in; about 500 g. of cracked ice is added at such a rate as to keep the temperature below 5°. This takes about ten minutes (Note 1). A drop of the solution should give an immediate blue color with starch-iodide paper.

The stirrer and thermometer are now transferred to the alkaline sulfide solution, the temperature of which must be below 5°. The diazo solution is added over a period of twenty to thirty minutes along with 950 g. of ice to prevent the temperature from rising above 5°. When addition is complete, the water bath is removed and the mixture allowed to warm up to room temperature; after two hours the evolution of nitrogen ceases (Note 2). About 180 cc. of concentrated hydrochloric acid is added until the solution is acid to Congo red paper, and the precipitate of dithiosalicylic acid is filtered and washed with water.

To remove the excess sulfur, the precipitate is dissolved by boiling with a solution of 60 g. of anhydrous sodium carbonate (soda ash) in 2 l. of water, and the mixture is filtered while hot. It is divided into five equal parts (Note 3), and the dithiosalicylic acid is reprecipitated as before with concentrated hydrochloric acid. The solid is filtered, the cake being sucked as dry as possible.

The moist cake is mixed with 27 g. of zinc dust and 300 cc. of glacial acetic acid in a 1-l. round-bottomed flask, and the mixture is refluxed vigorously for about four hours (Note 4). When the reduction is complete, the mixture is cooled and filtered with suction. The filter cake is washed once with water and then transferred to a 1-l. beaker. The cake is suspended in 200 cc. of water, and the suspension is heated to boiling. The hot solution is made strongly alkaline by the addition of about 40 cc. of 33 per cent aqueous sodium hydroxide solution. The alkaline solution is boiled for about twenty minutes to ensure complete extraction of the product from the filter cake, filtered from the insoluble material (Note 5), and the thiosalicylic acid is then precipitated by the addition of sufficient concentrated hydrochloric acid to make the solution acid to Congo red paper. The product is filtered with suction, washed once with water, and dried in an oven at 100–110°. The yield of a product which melts at 162–163° is 110–130 g. (71–84 per cent of the theoretical amount based on the anthranilic acid).

This product is sufficiently pure for most purposes (Note 6).

For recrystallization 5 g. of this material is dissolved in 20 cc. of hot 95 per cent alcohol, and 40 cc. of water is added. The solution is boiled with a little decolorizing carbon, filtered hot, and then allowed to cool. The product crystallizes in yellow flakes. The yield of recrystallized material is 4.7 g.; the melting point of the material is 163–164°.