198 CAPILLARY TUBE COLORIMETRY 



30 cm. length of unifonii tubing into 2 cm. pieces. Transfer the 

 colored solutions to these pieces by breaking off the sealed ends of 

 the tubes, inserting one end of a tube into small rubber tubing held 

 in the mouth, and placing the other end in contact with the piece 

 into which the solution is to be transferred. Apply gentle pressure to 

 effect the transfer and quickly seal the ends of the tube with plasti- 

 cine, taking care to avoid contact between the plasticine and the 

 solution in the tube. In order to prevent blowing the liquid from one 

 tube right through the other, the two tubes should be held in the 

 position of a wide-angled V during the transfer. 



8. For the comparison of blue colors, place two desk lamps fitted 

 with 100 watt bulbs side by side about 6 in. over the milk glass plate. 

 The use of two lamps prevents shadows. For colors at the red end 

 of the spectrum suspend a 150 watt lamp provided with color filters 

 over the plate. Place the standard tubes, one at a time, beside the un- 

 known on the plate for comparison. It is sometimes helpful to cover 

 the tubes with a piece of white paper in which a rectangular window 

 has been cut so that the visible columns are of the same length. 



note: In some cases it has been found that the intensity of the color 

 developed when minute quantities of test solution and reagent are mixed 

 in capillar}' tubes is not the same as when the liquids are mixed in the 

 same proportions in macro quantities. However, when this difference does 

 not exist, it is obviously less laborious to prepare the series of standard 

 color mixtures in macro volumes and transfer them to capillary tubes. 



3. Methods 



PREPARATION OF PROTEIN-FREE SUPERNATANTS 



The following procedures were employed by Richards' group 

 ( 1933) for frog plasma. The proportion of plasma to precipitating 

 reagent and the final dilution may be varied to suit the particular 

 kind of blood used. In general it is best to keep the dilution of plasma 

 as low as possible for capillary tube colorimetry. 



Tungslic Acid Supernalanls 



1. Collect the blood directly in one of the micropipettes. A few 

 grains of dry sodium oxalate may be placed in the pipette in advance. 



