THE COLORIMETER 37 



of N/10 hydrochloric acid. The air current should be only 

 moderately rapid for the first two minutes but at the end of this 

 two-minute period the current should be run at its maximum 

 speed for an interval of eight minutes. 



Disconnect the flask, dilute the contents to about 60 c.c. with 

 ammonia-free water and dilute similarly 1 mg. of nitrogen in the 

 form of ammonium sulphate in a second volumetric flask. Nessler- 

 ize both solutions as nearly as possible at the same tune with 

 5 c.c. of Nessler-Winkler solution diluted, immediately before 

 using, with about 25 c.c. of ammonia-free water to avoid turbidity. 

 Immediately fill the two flasks to the mark with ammonia-free 

 water, mix well and determine the relative intensity of the two 

 colors by means of a Duboscq colorimeter. 



The color of the unknown should be adjusted to that of the 

 standard both from above and below the level of the latter. The 

 matching of the colors is ordinarily very easy. It is desirable 

 to make the readings, by diffused daylight if possible. If electric 

 light must be used, a sheet of smooth white paper should be inter- 

 posed between the colorimeter and the source of light. 



Calculation. The reading of the standard divided by the 

 unknown gives the nitrogen in milligrams in the volume of the 

 urine taken. Calculate the total nitrogen output for the twenty- 

 four-hour period, 



THE COLORIMETER l 



For this method as well as for a number of other methods 

 commonly used in urinary and blood analysis an instrument 

 known as a colorimeter is required. Through its aid we are able 

 accurately to measure the respective depths of color in two solu- 

 tions and hence to calculate the comparative amounts of sub- 

 stances which form colored compounds in a quantitative manner. 

 The most satisfactory instrument for this purpose is the Duboscq 

 colorimeter. This enables the two colored solutions to be com- 

 pared in the same optical field and with a degree of accuracy 

 of about 1 per cent. The later type of the Duboscq colorimeter 

 with cylinders instead of prisms movable is to be preferred, partic- 

 ularly as this type may be readily adapted to the comparison of 



1 Hawk: Practical Physiological Chemistry, Sixth Edition, 1918. 



