342 PHYSIOLOGICAL CHEMISTRY 



an essential point, as the method is accurate only within these 

 limits. 



By experiment it has been determined that 10 mg. of pure 

 creatinine gives a color of such strength that 8.1 mm. of such a 

 solution exactly matches 8.0 mm. of bichromate under the 

 above conditions. From the reading obtained with the unknown 

 solution it thus is possible to calculate the amount of creatinine 

 present in the volume of urine used. Remember that the larger 

 the amount of creatinine, the deeper will be the color, and the 

 shorter the column of liquid necessary to match the color of 

 the standard. If a reading of 8.1 mm. corresponds to a creat- 

 inine content of 10 mg., calculate the amount of creatinine cor- 

 responding to the reading of your solution, not forgetting that 

 the proportion will be inverse. Calculate the amount of creat- 

 inine in the volume of urine used and in the 24-hour specimen. 



From the formula of creatinin calculate what per cent of 

 nitrogen it contains, and then the amount of creatinine nitro- 

 gen present in the 24-hour sample. Calculate the per cent of 

 the total nitrogen which this represents. 



41. Microchemical Determination of Creatinin in Urine. 



(Folin). 



A suitable and convenient "creatinin reagent" is prepared 

 by adding 75 c.c. of 10^ sodic hydrate to a liter of saturated 

 picric acid solution. If the picric acid is pure and the alkaline 

 solution is kept away from the light and from dust it keeps well 

 for several days. It is usually more safe, however, to prepare 

 only so much of the solution as is used up the same day. For a 

 single determination it is not worth while to prepare the re- 

 agent; employ instead the picric acid solution and the alkali, 

 using 20 c.c. of the former and 15 c.c. of the latter. 



By means of an Ostwald pipette transfer 1 c.c. or 2 c.c. urine 

 to a 100 volumetric flask. To another similar flask transfer 1 

 c.c. of a standard creatinin solution (1.61 g. of creatinin zinc 

 chlorid dissolved in one liter of tenth normal hydrochloric acid), 

 1 c.c. of which contains 1 mg. of creatinin. To each flask add 

 20 c.c. of picric acid solution, then add from a buret 1.5 c.c. 



