30 METHODS OF ANALYSIS [Chap. 



in the usual way; or, (2) Treat a weighed portion of the ash, as prepared under 2, 

 with dilute hydrochloric acid. Evaporate to dryness; pulverize and dry thoroughly 

 to render the silica insoluble. Moisten the dry residue with 5-10 cc. of hydrochloric 

 acid, add about 50 cc. of water, allow to stand on the water bath for a few minutes, 

 filter on an ashless filter, wash, dry, ignite and weigh to determine the combined 

 weight of the silica and sand. Deduct the weight of the sand found above to obtain 

 that of the silica. The soluble silica cannot be separated from the residue after 

 ignition. 



5 CARBON-FREE ASH.-OFFICIAL. 



Subtract the weights of the carbon found in 4 and the carbon dioxid found in 3 

 from that of the total ash used in 3. 



6 FERRIC AND ALUMINIUM OXIDS.— OFFICIAL. 



(Applicable for plant materials other than seeds.) 



Pipette an aliquot of A, under 4, corresponding to 0.5 gram of ash, into a 250 cc. 

 beaker. If ferrous iron is present, oxidize it by boiling with a few cc. of hydrogen 

 peroxid or of concentrated nitric acid. Cool, add ammonium hydroxid until a 

 precipitate begins to form, then nitric acid until just clear, and finally add 2-3 cc. 

 of concentrated nitric acid in excess. Add 25 cc. of 50% ammonium nitrate solution, 

 phosphate free, heat to 40°C., and add slowly, with constant shaking, a moderate 

 excess of molybdate solution [1, 4 (a)], and allow to stand for 1 or 2 hours at a tem- 

 perature not exceeding 40°C. After standing for an hour pipette 5 cc. of the clear 

 solution into an equal volume of warm molybdate solution. If a precipitate forms 

 in the test portion return it to the original solution and add more molybdate solu- 

 tion. Allow to stand at room temperature for several hours, preferably overnight. 

 Filter, wash with about 75 cc. of ammonium nitrate solution (2.5%, phosphate free, 

 and slightly acidified with nitric acid) and combine the filtrate and washings. Des- 

 ignate as B. Reserve the precipitate for the determination of phosphoric acid as 

 described under 11. i 



Without concentrating solution 5, cai't-'ouply^eatralize with ammci,\ roxid, 



add a very slight excess of the alkali, avoiding a temperature higher than 40''C., and 

 allow to stand at this temperature until the precipitate completely settles. Filter 

 the clear supernatant liquid, wash the precipitate a few times by decantation with 

 hot water before transferring to the filter, then wash 4 or 5 times on the filter. Dis- 

 solve the precipitate on the filter with hot nitric acid (1 to 5), wash and reprecipitate 

 as before. The combined filtrates and washings from the first and second precipi- 

 tations should not exceed 500 cc. and should not be concentrated by evaporation. 

 Designate as C and reserve for the determination of calcium and magnesium as 

 described under 8. The same filter may be used for the second filtration, and the 

 volume of the solution for the reprecipitation need not exceed 100 cc. Before the 

 second filtration is made, a small quantity of ashless filter paper pulp should be 

 added in order to facilitate the washing and leave the precipitate finely divided 

 after the ignition, so that it can be easily fused with potassium hydrogen sulphate for 

 the iron determination. Dry and ignite the precipitate and weigh as ferric and 

 aluminium oxids. 



The iron oxid is determined in the following manner : Fuse, in a platinum crucible, 

 the ignited precipitate with about 4 grams of fused potassium hydrogen sulphate. 

 This fusion takes but a few minutes and must not be continued beyond the time 

 actua ly needed. When completed the crucible is set aside and allowed to cool. 

 Add 5 cc. of concentrated sulphuric acid and heat until copious fumes of sulphuric 



