346 ANALYSIS OF URINE. [CH. XIII. 



flask. The filtrate should be quite clear and colourless. Add a 

 pinch of zinc dust and 2 drops of concentrated hydrochloric acid, 

 shake and allow it to stand for 5 to 10 minutes. Filter through a 

 small dry filter into a dry test-tube. 



Measure I to 4 cc. of this filtrate (so that 0-5 to 2 mg. sugar are 

 taken) into one of the graduated tubes. If less than 4 cc. are taken 

 make the volume up to exactly 4 cc. with distilled water. Add I cc. 

 of the 20 per cent, sodium carbonate and 4 cc. of the picric-picrate 

 mixture and plug the tube with cotton wool. If the standard is pre- 

 pared from glucose, measure I cc. (i.e. i mg.) into another graduated 

 tube (marked " S "). Add 3 cc. of water, i cc. of the sodium 

 carbonate, 4 cc. of the picric-picrate mixture and plug with cotton 

 wool. Immerse both tubes in a boiling water bath and note the 

 time. After exactly 10 minutes remove the tubes and cool thoroughly 

 under the tap. Dilute the " S " tube to 25 cc. If picramic acid is 

 used as a standard, fill a tube with some of the dilute solution. 

 Dilute the other tube to 12*5 cc. or to 25 cc. depending on the colour 

 obtained. If on diluting to 25 c.c. the colour is still much darker 

 than the standard, the experiment must be repeated, using less of 

 the final filtrate from the zinc or diluting the urine and starting again 

 from the beginning. 



When the two colours are roughly^the same, compare with 

 the standard in a colorimeter (see p. 384), setting the standard at a 

 height of 15 mm. It may be necessary to filter the solution contain - 

 ing the urine from a slight precipitate that appears on heating with 

 the alkali and picrate. 



Calculation. This depends on the amount of final filtrate taken and on 

 the dilution. Suppose that 2 cc. of final filtrate were taken and it was diluted 

 to 1 2 -5 cc., and that the reading was 17-4 mm., against the standard at 15. 

 Now 2 cc. of the filtrate contain i cc. of urine, and the standard contains i mg. 

 of glucose (or corresponds to this if picramic acid be used) . Since the urinary 

 solution was only diluted to 12-5 cc., whilst the standard was made up to 

 25 cc., the result must be halved. 



mg. of glucose in i cc. 15 i 

 ~T~ = iT^T x 2- 



In general 



Reading of " S " Volume after heating ^ 

 mg. of glucose in i cc. = Readingof Tj * vtfS^litaSteli^d x 25' 



15 12-5 2 



= ^T 4 * -T X 2-5' 



