URINE 571 



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-6sC. (not higher) and add 30-40 c.c. of molybdate solution, 1 stir and let 

 stand for about 15 minutes at 60-65. Filter at once through a small paper, 2 

 washing the precipitate twice by decantation with i per cent potassium nitrate 

 solution using about 25 c.c. each time, stirring up the precipitate well in each 

 case, and allowing to settle. Transfer the precipitate to the filter and wash with 

 i per cent potassium nitrate solution until two fillings of the filter (collected 

 separately) do not greatly diminish the color produced with phenolphthalein 

 by i drop of the standard alkali. 



Titration of the Phosphomolybdate. Transfer the precipitate and filter 

 back to the original beaker and dissolve in a small excess of N/5 NaOH (about 

 2-3 c.c. more than required to completely dissolve the yellow precipitate). Add 

 about 100 c.c. of boiled and cooled water and a few drops of phenolphthalein 

 as an indicator (a red color should be observed indicating excess of NaOH) 

 and titrate the excess of NaOH with N/io acid. 



Calculation. Divide the number of cubic centimeters of N/io acid required 

 by 2 and subtract from the number of cubic centimeters of N/5 NaOH used. 

 This gives the number of cubic centimeters of N/5 NaOH required. Multiply 

 by 0.618 (the equivalent of i c.c. of N/5 NaOH in P 2 O 5 ) and obtain the number 

 of milligrams of P 2 O 6 hi 10 c.c. of the urine analyzed. Calculate the daily output 

 of P 2 O 5 in this case from the 24-hour volume. 



Interpretation. Nearly all of the phosphorus of the urine exists as 

 alkali and earthy phosphates. Consequently the total phosphorus 

 varies in the same way as the total phosphates (which see). A small 

 portion of the phosphorus of the urine may exist in organic com- 

 bination though never in a reduced form. This organically bound 

 phosphate may amount to from 1-4 per cent of the total phosphorus 

 excretion. Little is known with regard to the compounds in which it 

 occurs. Possibly some glycerophosphoric acid may occur either free 

 or as lecithin. 



Gravimetric Modification. The phosphorus may be determined somewhat 

 more accurately by substituting a gravimetric procedure for the above titration. 

 In this case the washed phosphomolybdate precipitate is dissolved on the filter 

 paper with ammonium hydroxide and hot water to make a volume of not more than 

 100 c.c. Nearly neutralize with HC1, cool, and add about 10 c.c. of magnesia 

 mixture (see Appendix) from a burette. Add slowly (about i drop per second) 

 stirring vigorously. After 15 minutes add 12 c.c. of ammonium hydroxide 



1 Made by adding 5 c.c. of concentrated HNO 8 to 100 c.c. of the ordinary molybdate 

 solution (see Reagents and Solutions, page 638). 



2 It is better to use a special filter tube of about i^ inches diameter (similar to a Gooch 

 filtering tube) in which is placed a perforated porcelain plate i3^ inches in diameter, covered 

 with a layer of asbestos y% inch thick. Filtration is carried out with suction and is very 

 rapid. Ordinary Gooch crucibles lined with asbestos may also be used but are not so satis- 

 factory. The asbestos used should be specially prepared (see Appendix). For a good dis- 

 cussion of the details of procedure and sources of error of this volumetric method see 



Hibbard: /. Ind. Eng. Chem., 5, 998, 1913. 



