Spectrophotometric determination of silicon in thorium oxide

A method is described for the spcctrophotometric determination of microgram quantities of silicon in the presence of thorium. Thorium oxide is dissolved by heating it at 70 to 8o°C in 4M HNO₃ that contains 2 drops of HF. Under these conditions, no silicon is lost through volatilization. The silicon is estimated as the blue ailicomolybdate complex. Under the conditions selected for development of color, a precipitate of thorium molybdate is obtained ; the thorium molybdate is, however, dissolved without affecting the color of the complex by the addition of tartrate and adjusting the pH of the solution to 3.0. The lower limit of detection is about 5 p.p.m. of silicon.