DETERMINATION OF BASES 93 



gether with the precipitate, is then poured onto the same 

 niter paper as before and thoroughly washed with hot 

 water. Transfer paper to original beaker containing hot 

 1:10 sulphuric acid. After the paper has been immersed 

 in the liquid it is brought up on the side of the beaker by 

 means of a glass rod. Then the solution is titrated to the 

 end point with potassium permanganate. The paper is 

 now put back into the liquid and the titration finished. 

 Some prefer to ignite to constant weight the precipitate 

 left on the filter paper and calculate it as calcium oxide. 



Magnesium. Usually magnesium is determined by the 

 method adopted by the Association of Official Agricultural 

 Chemists which is as follows: "Evaporate the filtrate 

 from the above determination on water bath to dryness 

 and carefully heat to expel ammonium salts. Take up 

 the residue, with 20 to 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 am- 

 monium hydroxide, with constant stirring, until the 

 solution is distinctly alkaline. Test with acid-sodium- 

 phosphate to be sure that sufficient has been added. Al- 

 low to stand one-half hour, then add gradually 10 cc. of 

 strong ammonium hydroxide, cover closely to prevent 

 escape of ammonia, and let stand in the cold. Filter 

 after 12 hours, wash the precipitate free from chlorides, 

 using 2.5 per cent ammonia water, dry, burn at first at 

 moderate heat, then ignite intensely, and weigh as mag- 

 nesium-pyro-phosphate (Mg 2 P 2 O 7 )." If this precipita- 

 tion is done from a hot solution there is less danger of 

 tertiary magnesium phosphate being formed. Colori- 

 metric and titration methods are used occasionally. 



Sodium. Sodium determinations are seldom made in 



