A Semi-micro Method for the Determination of Reducing Sugars.

The MUNSON-WALKER method (3) with the modification of BERTRAND (1) has long been used for the determination of reducing sugars in plant extracts. The large amiiounts of solutions required by this method, however, make necessary the frequent preparation and standardization of solutions, which is inconvenient when large numbers of determinations are being made over an extended period of time. For plant materials containing not more than 10 to 12 per cent. sugar, it seems unnecessary to use the amounts of Fehling 's solutions originally specified. ILJIN (2) in adapting the MUNSONWALKER method to a micro scale, used 3-nml. portions of Fehling's solutions A and B. Where more than one per cent. is encountered, however, the dilutions made necessarv by this modification in order to keep within his range of 1 to 25 mg. of sugar, result in too great a dilution factor in the final value. An additional advantage of reducing the amounts of reagents used would be in iuaking possible carrying out the reduction and subsequent washing of the copper oxide precipitate directly in centrifuge tubes, thus eliminating the tedious and inconvenient procedure of filtering the precipitate on a Gooch crucible. The use of the centrifuge for micro-sugar analysis has beeni reported by ILJIN (2) and PHILLIPS (4). Another objection to the MUNSON-WALKER method is the practice of boiling the sugar solutions over an openi flamiie. QUISUMBING and THOMAS (5) have shown that this practice is sutbject to error, since at 1000 C. the endpoinlt of the copper reduction is taken on a continuously ascending curve, so that smiiall changes in time or temperature will result in appreciable error. They pointed out the desirability of using a temperature at which the reduction curve would eventually becomie level, at which poinit the end-point could be taken. A temperature of 800 C. was found best for this purpose. SHAFFER and SOMOGYI (8) have also stressed the importance of taking the end-point of reduction on the level portion of the curve. The use of standard potassium permanganate solution for titrating the reduced cuprous oxide according to the BERTRAND modification, has the disadvantages of instability of solutioni, sensitiveness to foreign organic matter and poor end-point, especially when the solutions cannot be thoroughly cleared. In view of the recent development of an inside indicator for potassium dichromate (7), it would appear desirable to use this solution instead of potassium permanganate for titration of the reduced copper oxide. 719)