A NOTE ON MIXED INDICATORS IN ALKALIMETRIC TITRATION

Simpson (Ind. Eng. Chem., 1924, 16, 709) introduced a great simplification in the titration of carbonate to the bicarbonate stage with his new mixed indicator consisting of 6 parts of Thymol blue and I part of Cresol red. It seemed of interest to investigate if other indicators of this type may be found. Amongst many combinations tried, the following have yielded useful results. In a few cases the end-point is much more sharp than with Simpson's indicator. Aliquot parts of a N/ ro solution of pure sodium carbo­nate, obtained by heating Analar sodium carbonate to constant weight at 300° for 2 to 4 hours, were titrated to the bicarbonate stage; then varying quantities of solid bicarbo­nate were added to the carbonate solution and the carbonate was determined by titration to the bicarbonate stage. The results are tabulated below. In all cases two or three drops of the indicator were added and the liquid diluted to zoo ml.