Iodine was discovered by Barnard Courtois in 1811 in France. It has been successfully researched and used in biochemistry and clinical research since 1825. In analytical work, however, it is necessary to draw attention to the fact that iodine can occur in biological material as free iodine, or in some other forms such as iodoaminoacids and iodoproteins. Three chemical methods for the quick determination of quinolizidine alkaloids were developed for use in practical breeding in 1927277. All of these methods were based on the use of iodine. In the first of Sengbusch's methods, the alkaloids were extracted from whole seeds or leaves by means of hot water, and precipitated with iodine-quicksilverpotash of iodine. By this method of alkaloid analysis, the first "sweet" plants (without indication of alkaloid content) of L. luteus were found277. With material of low alkaloid content, the second method could be evolved in which the alkaloids were extracted with hydrochloric acid instead of hot water. According to Sengbusch277, this method is mainly suitable for the investigation of leaf material, whereas in the testing of seeds, precipitation of non-alkaloid substances apparently also occurs. The hydrochloric acid method with cold water extraction was then developed. In this method, the alkaloids are extracted from the seed with cold water and are then precipitated with iodine-potash. According to Sengbusch277, this method permits the testing of seed-material without damage to germination so that the tested seeds can be sown. This is very important, for example, in plant breeding work.
The use of iodine in determining total alkaloid content has been developed by plant breeders in field conditions up to the present. The basic iodine solution contains 100 g J and 140 gKJ as well as 1000 ml water. Before application, the basic iodine solution is diluted by 1:3 or 1:5. The colour of leaves without alkaloids is not changed after application of the iodine solution. Following this method, the leaf colour changes to red-brown277 279.
Very similar to the method of Sengbusch was that developed by Schwarze, during which the juice of leaves was transferred onto blotting paper. After that, the blotting paper was put into the iodine-potash-iodine solution. The brown colour which appeared on the blotting paper was caused by alkaloids, and the colour of the blotting paper in which alkaloids were absent was slightly yellow or green4. A similar solution is also known as the old Dragendorff reagent, and as the KI/I2 test. The use of KI/I2 needs 2-3 hours for sample determination.
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