Acidify the solution of soluble ash, obtained in 9, with dilute nitric
acid and determine phosphoric acid (P_2_O_5). For percentages up to 5
use an aliquot corresponding to 0.4 gram of substance, for percentages
between 5 and 20 use an aliquot corresponding to 0.2 gram of substance,
and for percentages above 20 use an aliquot corresponding to 0.1 gram of
substance. Dilute to 75-100 cc., heat in a water-bath to 60°-65° C., and
for percentages below 5 add 20-25 cc. of freshly filtered molybdate
solution. For percentages between 5 and 20 add 30-35 cc. of molybdate
solution. For percentages greater than 20 add sufficient molybdate
solution to insure complete precipitation. Stir, let stand in the bath
for about 15 minutes, filter _at once_, wash once or twice with water by
decantation, using 25-30 cc. each time, agitate the precipitate
thoroughly and allow to settle; transfer to the filter and wash with
cold water until the filtrate from two fillings of the filter yields a
pink color upon the addition of phenolphthalein and one drop of the
standard alkali. Transfer the precipitate and filter to the beaker, or
precipitating vessel, dissolve the precipitate in a small excess of the
standard alkali, add a few drops of phenolphthalein solution, and
titrate with the standard acid.