The potassium salt of toluene p-nitro-o-sulphonic acid is easily obtained, as already stated, by heating p-nitro toluene on the water bath with three times its weight of fuming H_{2}SO_{4}, neutralizing with chalk and to the solution of calcium salt thus obtained adding a slight excess of K_{2}CO_{3}. On filtering from. the precipitated CaCO_{3} and evaporating slightly, the salt is obtained in long needle shaped crystals of a pale straw yellow color. This is ┌─ │CH_{3} C_{6}H_{3}┤SO_{2}OH (o) │NO_{2} (p). └─ The reduction of the nitro group is best effected by means of tin and HCl, in the proportion, salt 5 parts, tin 6 parts and concentrated HCl 30 parts. The amido acid forms a compound with tin which crystallizes from the HCl together with stannous chloride. This compound may be broken up and the tin removed by continued boiling with water. A better method of removing the tin is by dissolving the compound in Na_{2}CO_{3}. This forms a salt with the amido acid and throws down the tin as Sn(OH)_{2}, a white flocculent precipitate. On filtering and adding to the solution conc. HCl, the free amido acid is deposited in characteristic colorless, rhombic crystals, having the formula ┌─ │CH_{3} C_{6}H_{3}┤SO_{2}OH (o) │NH_{2} (p) └─ The method at first employed for preparing the hydrazine compound consisted in treating the amido acid, suspended in HCl, with potassium nitrite and then with stannous chloride. The tin was then removed from the solution by the addition of sodium carbonate and the hydrazine compound thrown down with HCl. This method however gave poor results the yield being only about 50% of the theoretical. Another method was accordingly substituted for the above, namely that of Strecker and Römer (Ber. IV. s 784.) By this the diazo compound is made first and isolated. This is done by suspending the finely powdered acid in absolute alcohol, cooling and passing a current of the oxides of nitrogen through in the ordinary way. The acid changes in appearance, becoming more crystalline and slightly darker and settles quickly on being shaken. The reaction here may be expressed thus-- ┌─ ┌─ │ CH_{3} │ CH_{3} C_{6}H_{3} ┤ SO_{2}OH + HNO_{2} = C_{6}H_{3} ┤ SO_{3} + 2H_{2}O │ NH_{2} │ \ └─ │ N=N └─ When the reaction is completed as shown by the appearance of the suspended powder it is filtered and while still fresh is added to a solution of acid sodium sulphite as long as it continues to dissolve readily.