260 THE URINE. 



tassium bichromate solution, 10 per cent, sodic hydrate, and a satu- 

 rated (1.2 per cent.) picric acid solution. 



If to 10 mgrms. of chemically pure kreatinin dissolved in 10 c.c. 

 of water in a 500 c.c. volumetric flask are added 15 c.c. of picric 

 acid solution and 5 c.c. of sodic hydrate, the maximum color is ob- 

 tained at the end of five minutes. If at the end of this time the 

 solution be diluted to the 500 c.c. mark and at once compared with 

 the standard bichromate solution, it will be found that 8.1 mm. of 

 the kreatinin-picrate solution have in the colorimeter exactly the 

 same shade and depth of color as 8 mm. of the bichromate 

 solution. 



The actual determination in urine is carried out in exactly the 

 same way, substituting 10 c.c. of urine for the kreatinin solution. The 

 more kreatinin that is present in the 10 c.c. of urine the deeper will, 

 of course, be the color of the solution obtained. Supposing the 

 colorimetric observation shows that 7.1 mm. of the urine picrate 

 solution are equal in color to 8 mm. of the standard. The 10 c.c. 



8 1 

 of urine would then contain 10 X ^r 11.4 mgrms. of kreatinin. 



The following precautions are to be observed in the determination : 



1. Make first a preliminary colorimetric observation, using half- 

 normal potassium bichromate solution in both cylinders of the col- 

 orimeter, adjusting first one to the 8 mm. mark. The average of 

 three or four readings of the other cylinder should also be 8 mm., 

 and after the first observation no two should differ by more than 0.2 

 mm. This preliminary observation takes only two or three minutes, 

 and is exceedingly useful in making the eye sure of the correct point 

 to be ascertained. 



2. Exactly 8 mm. of the half-normal potassium bichromate solu- 

 tion must be used as the standard for comparison ; 16 or 24 mm., 

 for example, cannot be substituted on the basis of the calculation 

 given above because the kreatinin picrate solution absorbs light at 

 an entirely different rate from that of the bichromate solution. 



3. For the reason given in the preceding paragraph it is necessary 

 to make each determination with a quantity of urine containing not 

 less than 5 or more than 15 mgrms. of kreatinin. Within these 

 limits the determination as described is correct within 0.2 rngrm. 



4. Sugar and albumin do not interfere with the determination. 

 Acetone, diacetic acid, and hydrogen sulphide do interfere. Where 

 these are present the urine should be measured into a porcelain evap- 

 orating-dish and heated on a water-bath with 10 c.c. of 10 per cent, 

 hydrochloric acid for about half an hour. When the dish is again 

 cooled, the reagents are added directly into the dish, and finally 

 rinsed into the volumetric flask after five minutes. 



5. The color due to the urine is ordinarily of no appreciable con- 

 sequence because of the great dilution. Urines containing bile-pig- 

 ments can, however, first be cleared by the addition of egg-albumin 

 and then removing this by coagulation (heat). 



