CH. viii.] WOHLGEMUTH'S METHOD. 193 



of i per cent, soluble starch to erythro-dextrin in 30 minutes, then 

 i cc. would convert 16 x 5 = 80 cc. of starch. If the temperature 



of the bath is 40 C, then D ^ = 80. 



30 



NOTES. i. In the experiment as conducted above, the possible error 

 is nearly 100 per cent. A nearer approximation can be made by repeating the 

 experiment with more gradual dilutions that will depend on the result obtained. 

 Thus, if T \ cc. gives a red and ^ cc. gives a violet, D may be anything between 

 So and 160. To i cc. of the enyzme add 15 cc. of distilled water, mix well, and 

 measure i, 0-9, 0-8, 0-7, 0-6 and 0-5 cc. into a series of 6 tubes. Make up 

 the volume to i cc. in each case by the addition of distilled water. Add 5 cc. 

 of the soluble starch and repeat the experiment. One of the following values 

 for D will then be obtained : 80, 88, 100, 114, 133, or 160. 



2. Though in Wohlgemuth's original method the instructions are to 

 use pure i per cent, soluble starch, the author finds that the results obtained 

 are much more reliable if the hydrogen-ion concentration and the salt content 

 be maintained at the optimum. The starch solution is prepared by treating 

 100 cc. of 2 per cent, soluble starch (see p. 391) with 25 cc. of a buffer solution 

 of P H = 6-7 (see Note i, Ex. 237), 25 cc. of i per cent, sodium chloride and 

 50 cc. of distilled water. The starch solution should be freshly prepared. 



3. It is important to add exactly the same amount of iodine to each 

 tube in all the experiments. The iodine should be measured by means of a 

 dropping pipette (see fig. 5). 



242. The Method of " the first change." Measure 10 cc. of 2 

 per cent, soluble starch (see p. 391) into a test-tube. Add 2 cc. of 

 a buffer solution of P H = 6-7 (seeNote i, Ex. 237) and 2 cc. of 0-5 per 

 cent, sodium chloride. Place the tube in a water bath at 40 C. for 

 some minutes until it has attained the bath temperature. Have 

 ready a series of tubes containing 3 cc. of distilled water, to each of 

 which has been added a single- drop of o-oi N. iodine by means of a 

 dropping pipette (fig. 5). Now add 2 cc. of the enzyme to the 

 starch tube by means of a 2 cc. pipette, discharging this by blowing 

 through it for a second into the solution. Note the time of the 

 addition of the enzyme. Seal the tube by the thumb and mix by 

 shaking ; immediately replace the tube in the water bath. Insert a 

 quill tube. At the end of i minute allow a single drop of iodine to 

 fall into one of the iodine tubes, shake by mixing, and place this tube 

 in the first hole of a test-tube rack. At the end of 2 minutes, allow 

 another drop to fall into another iodine tube, shake and place this 

 in the second hole of the rack. Continue in this way until the 

 colour given by the digestion mixture is violet. Now examine the 

 tubes carefully in diffuse daylight, and select the tube in which the 



