URINE 555 



stand for about 15 minutes at 60-65. Filter at once through a small paper, 1 

 washing the precipitate twice by decantation with 2 per cent ammonium 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 

 2 per cent ammonium nitrate solution until two fillings of the filter (collected 

 separately) do not greatly dimmish 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 1.268 (the equivalent of i c.c. of N/5 NaOH in PoO s ) and obtain the number 

 of milligrams of P^O 5 in 10 c.c. of the urine analyzed. Calculate the daily output 

 of PsO 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 

 solution (sp. gr. 0.90). Let stand for some time (two hours is usually enough) 

 then filter and wash the precipitate with 2.5 per cent ammonia until practically 

 free from chlorides. Ignite to whiteness or to a grayish-white ash and weigh. 

 Multiply the weight of magnesium pyrophosphate thus obtained by 0.637 to get 

 the weight of P 2 O 5 . 



Calculation. Calculate as explained above. 



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

 filtering tube) in which is placed a perforated porcelain plate i Jjf inches in diameter, covered 

 with a layer of asbestos 1/8 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. 



