16 METHODS OF ANALYSIS. 



of hot distilled water on the precipitate, and again decant the clear solution 

 onto the filter. Dissolve the precipitate in the beaker with a few drops of 

 hydrochloric acid, add a little water, and reprecipitate, boiling hot, by adding 

 ammonium hydroxid, to a slight alkalinity, and a little ammonium oxalate 

 solution ; allow to stand as before and filter through the same filter ; transfer 

 the precipitate to the filter and wash it free from chlorids with hot water ; dry, 

 ignite the precipitate over the blast lamp until it ceases to lose weight, and 

 weigh as calcium oxid. The filtrate and washings become Solution D. 



(e) MAGNESIUM. OFFICIAL. 



Evaporate Solution D on the water bath to dryness and carefully heat to 

 expel ammonium salts. Take up the residue, with 20 or 25 cc hot water and 

 about 5 cc hydrochloric acid, filter, and wash. Concentrate to about 50 cc, cool 

 and add sufficient acid sodium phosphate to precipitate the magnesium ; then 

 add gradually ammonium hydroxid, with constant stirring, until the solution 

 is distinctly alkaline. Test with acid sodium phosphate to be sure that suffi- 

 cient has been added. Allow to stand one-half hour, then add gradually 30 cc 

 of strong ammonium hydroxid, cover closely to prevent escape of ammonia, 

 and let stand in the cold. Filter after 12 hours, wash the precipitate free from 

 chlorids, using 2.5 per cent ammonia water, dry, burn at first at a moderate 

 heat, then ignite intensely, and weigh as magnesium-pyro-phosphate (Mg 2 P 2 O T ). 



(f) PHOSPHORIC ACID. (See also Appendix, p. 234.) 



(1) GRAVIMETRIC METHOD. OFFICIAL. 



Evaporate 100 or 200 cc of Solution A to about 25 or 30 cc, neutralize with 

 ammonium hydroxid and add about 10 cc additional, neutralize the excess of 

 ammonium hydroxid with nitric acid, gradually add at once about 20 cc of 

 molybdate solution o ("I. Fertilizers," (1) (c), p. 2) ami place ilie beaker In 

 a water bath at a temperature of 40 to GO C. When the precipitate has settled 

 sufficiently, draw out with a pipette about 5 cc of the clear liquid and test by 

 allowing it to run into 5 cc of warm molybdate solution. If any precipitate be 

 produced, return the test liquid to the main portion, add more molybdate solu- 

 tion, and repeat the operation until all the phosphoric acid is precipitated. After 

 standing several hours at a temperature not above GO C., filter off the ammo- 

 nium phosphomolybdate. Wash the precipitate thoroughly with cold water, 

 dissolve with ammonium hydroxid, and determine the phosphoric acid as mag- 

 nesium pyrophosphate, as directed under total phosphoric acid in fertilizers, 

 page 3. 



(2) VOLUMETRIC METHOD. PROVISIONAL, 



Proceed as in the gravimetric method (1) until all the phosphoric acid is 

 precipitated and then finish the determination as follows: 



After standing for 3 hours at a temperature not above 60 C., filter on a 

 small filter paper or on a gooch crucible and wash with cold water until two 

 fillings of the filter do not greatly diminish the color produced with phenol- 

 phthalein by 1 drop of standard alkali. Return the filter and precipitate to 

 the same beaker used for precipitating the phosphomolybdate, dissolve the 

 yellow precipitate in standard sodium or potassium hydroxid, add a few drops 

 of phenolphthalein solution and titrate excess of alkali with standard acid; 

 1 cc of the standard alkali should be made to equal 0.0005 gram of phosphoric 

 acid (P 2 O B ). 



It is better to pour the solution 6f phosphate Into the molybdate solution. 



