2 12 CAPILLARY TUBE COLORlMETftY 



Saturated Picric Acid Solution. Prepare from pure picric acid. 

 10% Sodium Hydroxide. Prepare from Merck's reagent "from 



sodium." 

 Folin Reagent. Freshly prepare before use by mixing 5 vol. 



saturated picric acid with 1 vol. of the 10% sodium hydroxide. 



PROCEDURE 



1. Introduce separate columns of saturated picric acid (25 mi- 

 crometer divisions), tungstic acid supernatant (page 198) or other 

 unknown soln. (60 divisions), and 10% sodium hydroxide (5 divi- 

 sions) in that order into a capillary tube (0.35 mm. inside diameter) . 



2. Seal off the ends of the tube and place in a closed box until 

 the other tubes are prepared. 



3. Darken the room for the following operations. 



4. Prepare as rapidly as possible the standard color solns. in 

 test tubes by adding 1 ml. Folin reagent to 2 ml. standard soln. 



5. With no loss of time mix the liquids in the capillary tubes 

 by repeated centrifugations and plan to begin color comparisons 

 10 min. after the first centrifugation. 



6. In this 10 min. interval transfer the contents of each capillary 

 tube and a portion of each standard mixture to pieces of capillary 

 tubing of uniform bore, and seal the ends of each piece with 

 plasticine. 



7. Place each sealed piece of tubing in a labeled space on a 

 milk glass plate for color comparison. 



8. Compare the colors in a dark room under a 200 watt bulb 

 equipped with a straw-colored light filter, or illuminate the milk 

 glass plate from underneath using a straw-colored filter between the 

 plate and the light source. 



note: The particular order of procedure given must be followed since 

 Folin reagent darkens at a faster rate and more extensively in capillary 

 tubes than it does in larger volumes in test tubes. Furthermore, this change 

 is mtensified and accelerated by daylight, which makes it imperative to 

 protect the solutions from light. The yellow picric acid color interferes 

 with the comparisons of the red colors developed, and hence it is necessary 

 to use a straw-colored light filter. The color produced by 2.0 milligram 

 per cent creatinine is about the palest which can be reliably estimated in 

 the tubes used. The most advantageous colors are those produced in the 

 range 2.5 to 5.0 milligram per cent. 



