It will be of interest to add the observation that the gyrodynat of maltose has lately been redetermined by Ost, who finds it to be [a]D²⁰ ° = 137°.04 ± 0.19.[155]
189. Solutions of Starch at High Pressure.—Starch may also be brought into a condition suited to polarization by dissolving in water at a high temperature and pressure. The solution is accomplished in an autoclave as described in [181].
From two to three grams of starch are used and from eighty to ninety cubic centimeters of water. The starch is first reduced to a pasty state by heating with the water and, when evenly distributed throughout the flask, is rendered soluble by heating from three to five hours in an autoclave at from two to three atmospheres. The material is entirely without action on an alkaline copper solution. After heating, the volume of the solution is completed to 100 cubic centimeters and it is then polarized. The gyrodynat of starch dissolved in this way varies from [a]D = 196°.5 to 197°.[156]
Starch is prepared by Baudry for polarization by boiling with salicylic acid.[157] The gyrodynat of starch dissolved in this way is [a]D = 200°.25.
190. Polarization after Solution in Dilute Nitric Acid.—Guichard recommends saccharification with ten per cent nitric acid (ten cubic centimeters strong acid, ninety cubic centimeters water).[158] This treatment, even after prolonged boiling, gives only a light straw color to the solution which does not interfere with its polarization with a laurent instrument.
In working on cereals four grams of the finely ground material, in which the bran and flour are intimately mixed, are used.
The material is placed in a flask of about 500 cubic centimeters capacity, with 100 cubic centimeters of the dilute acid. The flask is closed with a stopper carrying a reflux condenser. After boiling for an hour the contents of the flask are filtered and examined in the saccharimeter. The dextrose formed is determined by the polarimetric data and the quantity of starch transformed calculated from the dextrose. The following formula is used:
| A = | av × 25 × 0.016 |
| 2 × 52.8 |
In this formula a = the rotation in angular degrees, v = the volume of the liquid and A = the starch transformed.
In this method no account is taken of the sucrose and other sugars which are present in cereals. In the case of sucrose the left-handed sugar produced by treatment with nitric acid would diminish the rotation to the right and thus introduce an error. On the other hand the dextrose formed from the fiber of the bran would be calculated as starch. If these two errors should be compensating the method might prove practical.