A SCHIFF REAGENT OF CALIBRATED SENSITIVITY

Abstract

The sensitivity of Schiff reagent for the detection of low concentrations of aldehyde increases progressively as the concentration of sulfurous acid in the reagent is lowered. Calibration of sensitivity is readily accomplished by titrating the Schiff reagent against the iodine of Lugol's solution. Stability of Schiff's solution depends on decreasing the evaporation of sulfur dioxide and of retarding the change of sulfite to sulfate. For practical purposes these ends can be achieved by storage in the refrigerator of a solution with greater sulfurous acid content and acidity than the solution used for staining. Evacuation of the excess sulfur dioxide or mixing in proper proportions with used reagent allows the rapid production of a solution of the desired sensitivity. If chemical preservatives are required, 0.5% hydroquinone gives very good results. Spectrophotometric measurement of the colored solutions produced by adding aldehyde to Schiff reagent indicates that at least two distinct compounds may result. The one that is usually mentioned as characteristic of a positive response of aldehyde to the Schiff reagent has a violet color. When the concentration of aldehyde is so low that only a Schiff solution of low sulfurous acid titer will react with it, a red color is produced. Spectrophotometric analysis shows this compound to be different from the original basic fuchsin and validates its use as an endpoint in the reaction of aldehyde with Schiff's reagent.