Abstract
A sample containing less than 30γ of sulfur is placed in a flask in an apparatus as shown in Fig. 1. Treat with acid to convert all forms of sulfur into sulfate. Then heat the solution with a reducing agent (hydrogen iodide+formic acid+red phosphorus), while nitrogen is being passed in to drive off the hydrogen sulfide. Hydrogen sulfide is absorbed in a solution of zinc acetate; and methylene blue coloration is developed by reaction with p-aminodimethylaniline in the presence of iron chloride. Quantitative determination of a small amount of sulfur is made by estimating the light absorbency of the color at 665 mμ.
The absorption curve of methylene blue formed by this method is indicated in Fig. 2. The forms of absorption curve, as indicated in the graphs, are different depending on the conditions of development of coloration. In particular, the effect of the concentration of acid is characteristic and strong (Fig. 3). The effects of the amount of p-aminodimethylaniline and of the other substances are shown in Tab. 1. The effects caused by changing the conditions of reduction of sulfate to hydrogen sulfide are indicated in Tab. 2. The relationship between the amount of sulfur and the light absorbency value estimated is linear as indicated in Fig. 5 The presence of 1 mg each of Cu, Zn, Co, Fe, Al, As, Sb, Bi, Sn, Pb, and Mn causes no interference. But S and Te should not be present. However, the presence of up to several g of Ni, etc. is allowable (Fig. 4). The results of application of this method to the determination of a small amount of sulfur in nickel metal and in pure selenium are given in Tab. 3; it was possible to determine as little as 0.0001% sulfur in 6 hours.
