PHOSPHORIC ACID IN BLOOD 179 



3. Standard phosphate, (a) Stock standard, containing in 

 100 c.c. 0.0834 gm. of pure acid potassium phosphate. (6) Stand- 

 ard for use: dilute 25 c.c. of the above to 500 c.c. Each 5 c.c. of 

 this standard contains 0.15 mg. of HsPCU. The dilute standard 

 has been found to deteriorate in hot weather and should be made 

 up at least once a month. 



4. Sodium hydroxide (from sodium) 10 to 20 per cent. 



5. Concentrated sulfuric and nitric acids free from all but traces 

 of phosphates (see below). 



6. Dilute sulfuric acid. One part of the concentrated acid 

 and 3 parts water. 



Blanks run with the acids, alkali, and ammonium sulfate 

 should show only a slight cloudiness on standing for one hour 

 with the reagent. The ordinary highest grade reagents obtainable 

 up to the present time have been found satisfactory with the 

 exception of the sodium hydroxide which should be made from 

 the metal. If the distilled water is stored in metal tanks it may 

 contain lead which is itself a fairly good precipitant for phos- 

 phates. The water should therefore be stored in glass. 



Because of the minute amounts of phosphoric acid determined 

 the greatest care should be used to avoid contamination, and free 

 use should be made of good distilled water. For the same reason 

 glass-stoppered measuring flasks should be used throughout. 



Reagents. With impure samples of molybdic acid a highly 

 colored solution is often obtained which is difficult to use. It 

 has been found that if the colored solution is exposed to sunlight 

 for a time the color fades and, if the reagent is then taken 'out of 

 the direct light, does not reappear. (If placed in the dark, the 

 color reappears to some extent, and, if exposed to sunlight too 

 long, the reddish purple color changes to a deep yellow which is 

 almost as difficult to work with as the original color.) If pure 

 molybdic acid is used the reagent is colorless but with the above 

 precautions it has been found possible to use acid of varying 

 degrees of purity. 



Methods. The quantities of blood taken are based on the 

 average values for normal human blood and variations in amount 

 of blood or strength of standard may be necessary in special 

 cases. 



The tubes with the beads should be given a preliminary 

 ' steaming out " by boiling concentrated sulfuric acid in them 



