The most widely used method for the determination of silicon and phosphorus is the spectrophotometric method. This involves the formation of the yellow silicomolybdate and phosphomolybdate, followed by reduction of these complexes to molybdenum blue.
During the investigation leading to the colorimetric determination of silicon in the presence of phosphorus, PAUL AND POYER 1 reported the separation of silicomolybdate from phosphomolybdate by selective extraction of the latter with ethyl acetate. Reduction of the silicomolybdate in the aqueous phase provided an accurate and reliable method for determination of silicon in the presence of phosphorus.
The possibility of simultaneously determining phosphorus in the solvent phase was considered. De SESA AND ROGERS 2 have reported the simultaneous determination of silicon and phosphorus by measurement of the total absorbance of the yellow molybdate at 332 m/z, extraction of the phosphomolybdate with isoamyl acetate and measuring the absorbance of the organic phase at 330 m#. Correction of the total absorbance in the aqueous phase for phosphorus, using the previously determined absorbance index of phosphomolybdate, gave the silicon content. This technique though feasible was time-consuming. However, while investigating the solvent extraction technique, a spectrophotometric method was developed for the simultaneous determination of silicon and phosphorus as molybdenum blue.
This paper reports the investigations leading to this method and the tests conducted on the procedure.
EXPERIMENTAL
Reagents
All reagents were of AnalaR grade. Distilled water was used throughout. Silicate solutions : Solutions of soluble silica were prepared by fusing io.o mg of finely divided quartz with o.5 g of powdered sucrose and i-5 g of sodium peroxide in a Parr-bomb, and dissolving the melt in IOO ml water in a polythene beaker. Further solutions of silicate were prepared by dilution.
Phosphate solutions : A stock solution of phosphorus was prepared by dissolving 0.5o g of sodium dihydrogen phosphate (dihydrate) and diluting to ioo ml. Further solutions of phosphate were prepared from it by dilution.