554 PHYSIOLOGICAL CHEMISTRY 



of urine used. Since 100 c.c. of urine was taken this value also expresses the 

 percentage of P 2 O 5 present. 



Calculate the quantity of earthy phosphates, in terms of P 2 O 5 , present in the 

 24-hour urine specimen. 



The quantity of phosphoric acid present in combination with the alkali 

 metals may be determined by subtracting the content of earthy phosphates 

 from the total phosphates. 



Interpretation. Ordinarily the earthy phosphates make up from 

 30-40 per cent of the total phosphate excretion. The amount varies 

 with the -excretion of calcium and magnesium (which see). 



3. Total Phosphorus. (a) Volumetric Procedure. Principle. The 

 organic matter is destroyed by digestion with a mixture of sulphuric 

 and nitric acids or some other oxidizing agent. The phosphorus is then 

 precipitated as the phosphomolybdate and determined gravimetrically 

 or volumetrically. 



Preparation of the Solution. Pipette 10 c.c. of urine (or an amount of sub- 

 stance containing about 20 mg. of PzOs) into a Kjeldahl flask. Add 10 c.c. of a 

 mixture of equal parts of concentrated H 2 SO 4 and concentrated HNO 3 . Digest 

 over a low flame until red fumes cease to come off. If the mixture darkens due 

 to the charring action of the sulphuric acid, add nitric acid from a separatory 

 funnel a few drops at a time and continue the digestion. When the mixture 

 remains clear on evaporation to the point where white sulphuric fumes come off 

 the digestion is completed by heating for 10-15 minutes longer. Cool and transfer 

 the solution to a 400 c.c. Erlenmeyer flask with the aid of enough water to 

 make a total volume of about 75 c.c. 1 



Instead of oxidizing the material as described above it may be 

 ignited with magnesia to destroy organic matter. About 2 grams of 

 the solid substance or 25 c.c. of urine (previously evaporated nearly 

 to dryness) are mixed with a little more than an equal bulk of mag- 

 nesium oxide in a porcelain dish of about 30 c.c. capacity. Five c.c. 

 of magnesium nitrate solution (see Reagents and Solutions, page 602) 

 are added and the mixture heated very gently at first, then gradually 

 to bright redness. The mixture is cooled and transferred with water 

 to a 250 c.c. flask. An excess (20-30 c.c.) of HC1 are added and the 

 mixture boiled a few minutes. Remove from the flame and add at 

 once enough barium chloride solution to precipitate any sulphate 

 present. Cool, make to mark, filter and take an aliquot for analysis. 



Precipitation of the Phosphomolybdate. Neutralize the solution with 

 ammonia, make slightly acid with nitric acid, and add 15 grams of ammonium 

 nitrate in substance (or 25 c.c. of a 60 per cent solution). Heat on a water-bath 

 to 6o-65C. (not higher) and add 30-40 c.c. of molybdate solution, 2 stir and let 



1 In the case of urine it is possible to neutralize this acid solution with ammonia, make 

 it acid with acetic acid and titrate with uranium acetate as in the preceding method. 



2 Made by adding 5 c.c. of concentrated HNOs to 100 c.c. of the ordinary molybdate 

 solution (see Reagents and Solutions, page 602). 



