Abstract
As a series of studies on automatic chemical analysis of steel, an automatic spectrophotometric method has been developed for determination of silicon using the automatic analyzer that the present authors have previously reported for determination of acid-soluble aluminum, manganese and phosphorus in steel.
The analyzer (Fig. 1) was automated and mainly composed of a sample changer, solution adding devices, a coloring reaction vessel, a spectrophotometer, a controller and recorder.
The spectrophotometric method based on the principle of forming silico-molybdenum blue was found to be suitable to automatic determination of silicon in steel by the analyzer.
The optimum conditions in case of 50.00 ml of sample solution and 25.00 ml of beaker washings were obtained as follows from the fundamental studies (Fig. 2 to Fig. 7). The concentration of hydrochloric acid; 0.64 N, the amount of ammonium molybdate solution (20%); 8.0 ml, hydrofluoric acid (15%); 8.0 ml and ferrous ammonium sulfate solution (30%); 9.0 ml. The reaction time at room temperature for forming silico-molybdenum yellow was about 2.5 minutes.
The effects of less than 50 mg of nickel, chromium, cobalt, molybdenum, 1 mg of phosphorus and 0.5 mg of arsenic in the final solution were negligible in the present automatic analytical procedure in which the absorbance of silico-molybdenum blue was always corrected by the blank absorbance.
The automatic analysis of silicon is performed according to the predetermined program (Fig. 8). The sample solution (50.00 ml) with beaker washings (25.00 ml) are transferred quantitatively to the reaction vessel from the sample changer. The sample solution is manually prepared by dissolving 0.1 gram of steel sample with hydrochloric acid (35%) and hydrogen peroxide (30%). The solution is diluted to 250 ml and one fifth aliquot is presented to the apparatus.
The solution in the reaction vessel are circulated through the circulating pipe which is composed of a bubble remover, an absorbance measuring flow-cell, a gas blowing tube and a magnetic drain cock. After measuring the blank absorbance, ammonium molybdate solution is added. The formation of silico-molybdenum yellow in the vessel is accomplished by circulating the solution at room temperature for about 2.5 minutes. Hydrofluoric acid (15%) and then ferrous ammonium sulfate solution (30%) are added for decomposition of phosphorus- and arsenic-molybdenum yellow and reduction of silico-molybdenum yellow. After the reduction, the absorbance of colored silico-molybdenum blue solution can be measured. The solution is drained off and the vessel is completely washed by 0.14 N nitric acid and water.
This automatic analysis had a merit of high accuracy and simplicity. The time required for analysis of one sample was 6 minutes. The analytical results of steel samples were well agreed with the standard values as shown Table I.
