BLOOD ANALYSIS 279 



cylinder for the standard and replacing the other cylinder and prism by the polari- 

 scope tube, containing the unknown, which usually fits properly in place in the 

 colorimeter. 



The unknown solution remains stationary (100 mm.) and the standard is 

 adjusted until the colors match. An iris diaphragm, such as is used in microscopical 

 work, must be attached to the side of the colorimeter holding the standard to reduce 

 the light passing through the standard solution. This is necessary in order to 

 obtain two fields of the same tint. The calculation is made in the usual manner for 

 colorimetric determinations, the amount of ammonia in the unknown being directly 

 proportional to the reading of the standard and its concentration. 



7. Total Nitrogen. The total nitrogen of the blood may be readily determined 

 by the regular Kjeldahl method (see Chapter XXVI). One c.c. of the blood ac- 

 curately measured is used in this method. The microchemical method of Folin 

 and Farmer, as outlined in the same chapter, may also be employed. In this case 

 transfer i c.c. of the well-mixed blood to a 25 c.c. flask, make to the mark with 

 distilled water, mix thoroughly and take i c.c. of this diluted blood for the digestion 

 and determination as there given. 



8. Sugar, (a) Method of Lewis and Benedict. 1 Principle. The red 



color obtained by heating a glucose solution with picric acid and sodium 

 carbonate is employed as the basis of the colorimetric determination. 

 The blood protein is removed by precipitation with picric acid. 



Procedure. Two c.c. of blood are aspirated through a hypodermic needle 2 

 and a piece of rubber tubing into an Ostwald pipette, a little powdered potassium 

 oxalate in the tip of the pipette preventing clotting. The blood is drawn up a 

 little above the mark and the end of the pipette is closed with the finger. After 

 the rubber tubing and needle are disconnected, the blood is allowed to flow back 

 to the mark and is discharged at once into a 25 c.c. volumetric flask containing 



5 c.c. of water. The contents of the flask are shaken to insure thorough mixing 

 and the consequent hemolysis of the blood. Then 15 c.c. of saturated aqueous 

 solution of picric acid are added, as well as a drop or two of alcohol to dispel any 

 foam, and the contents of the flask are made up to the mark with water and then 

 shaken. After filtration 8 c.c. aliquots are measured out into large Jena test- 

 tubes for duplicate determinations. Two c.c. of saturated picric acid solution and 

 exactly i c.c. of 10 per cent sodium carbonate are added (as well as two glass 

 beads and 2 or 3 drops of mineral oil), and the contents of the tube are evaporated 

 rapidly over a direct flame until precipitation occurs. About 3 c.c. of water are 

 added, the tube is again heated to boiling to dissolve the precipitate, the contents 

 of the tube are transferred quantitatively to a 10 c.c. volumetric flask, 3 cooled, 

 made up to the mark, shaken, and then filtered through cotton into the chamber 

 of a Duboscq colorimeter (see Fig. 153, p. 486). The color is compared at once 

 with that obtained from 0.64 mg. of glucose, 5 c.c. of saturated picric acid, and i 

 c.c. of 19 per cent sodium carbonate, when evaporated to precipitation over a free 



1 Lewis and Benedict: Jour. Biol. Chem., 20, 61, 1915. For modification see Myers 



6 Bailey: Jour. Biol. Chem., 24, 147, 1916. 



2 It may be more convenient to draw about 5 c.c. of blood directly into a test-tube 

 containing a little finely powdered potassium oxalate and removing 2 c.c. portions of this 

 with the Ostwald pipette. 



3 In case of hyperglycemia the final volume of the reaction fluid is made 25*c.c. or 50 

 c.c., and the results are accordingly multiplied by 2.5 or 5.0. 



