Abstract
Sucrose is completely inverted by heating in a solution of picric acid. This fact has been made use of in the estimation of cane sugar in solutions and extracts. After the cane sugar is hydrolyzed by heating with picric acid under properly controlled conditions, the invert sugar formed is determined by a modified Lewis-Benedict colorometric method. Glucose and fructose are determined at the same time.
The technic of the method is as follows: 1 c.c. of the clear liquid containing the sugars is transferred to each of two graduated narrow test tubes containing 2 c.c. saturated solution of picric acid. One of the tubes also contains 1 c.c. 20 per cent. sodium carbonate. The two tubes are then immersed into a bath of boiling water. After ten minutes 1 c.c. of 20 per cent. sodium carbonate is also added to the tube containing the acid mixture. After heating for 30 minutes the color in each tube has reached its maximum and they are allowed to cool. The contents of the tubes are diluted to a suitable volume and matched against a standard solution in a colorimeter. The readings for the tube which was alkaline at the beginning of the heating represents the glucose and fructose and the other tube represents these sugars plus the invert sugar from the sucrose.
When this method is applied to solids or semisolids, such as mashed fruit pulps, 1-10 grams are taken and triturated in a mortar with 1OO C.C. water including the moisture of the sample and a clear liquid obtained by filtering or centrifuging.
Picric acid may also be used as the inverting agent in the determination of cane sugar by polarizing. The picric acid has no influence on the rotation of the polarized light and in many instances it acts as a clarifier and as a remover of soluble proteins.
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