130 



THE CARBOHYDRATES. 



[CH. V. 



3. Special Burette. Folin and McEllroy describe an ingenious method of 

 measuring small quantities of fluids by means of an accessory fine tip fitted to an 

 ordinary burette. When the fluid is dropping from the burette very slowly, 

 the size of the drop is constant for a particular fluid. So if 

 the number of drops emitted by a given tip for a delivery of 

 i cc. be known, the volume of the number of drops required 

 for the titration can be readily calculated. The author pre- 

 fers to use the special 5 cc. burette shewn in fig. 14. It is 

 provided with an accessory tip, which is drawn out to a fine 

 point. As recommended by Folin and McEllroy, the burette 

 must be filled by suction. Elaborate washing of the burette 

 in the intervals between successive estimations is thereby 

 rendered unnecessary. 



Method. Weigh out approximately 5 grams, of 

 the phosphate mixture and transfer it to a clean, dry 

 tube, conveniently 7 inches by f ths inch. Add 5 cc. 

 of the 6 per cent, solution of copper sulphate, shake 

 and heat to boiling. A deep blue, nearly clear, 

 solution is obtained. It is advisable to improvise 

 a test-tube holder by folding a piece of paper. Run 

 in about 0-5 cc. of the sugar solution from the burette 

 and boil very gently for 2 minutes by the watch. The 

 tube should be held at an angle and moved about in 

 a small flame : excessive concentration and loss by 

 spurting can be thus avoided. If there is an appre- 

 ciable amount of sugar present a chalky white 

 precipitate appears. If the blue colour entirely 

 disappears there is more than 5 per cent, of sugar 

 present, and the estimation must be repeated with 

 a diluted solution. If the copper is only slightly 

 reduced, yielding only a small amount of cuprous 

 thiocyanate, add a further amount of the sugar 

 solution and boil gently for another minute. If now 

 the greater part of the copper has been reduced, 

 complete the titration by adding a drop at a time, 

 boiling for i minute after each addition. The total 

 period of boiling must not [be less than 4 minutes, 

 and should not exceed 8. The copper value has been 

 adjusted to a period of 5 to 6 minutes. 



Fig. 14- 



Burette with 

 accessory tip. 



A second estimation is often necessary. With a little experi- 

 ence it is easy to judge of the amount that should be added, so that 



