ON THE ESTIMATION OF POTASH AND PHOSPHORIC ACID. 31 



aluminium remain chiefly undissolved. As, however, a small and not very 

 constant amount of iron undergoes solution, the advantage of this method 

 is considerably diminished. 



Separation of the Iron and Aluminium. 



In all cases in which more than traces of iron or aluminium are present, 

 the Committee is strongly of opinion that they should he separated. In the 

 first place several of the most satisfactory methods of determining phosphoric 

 acid are vitiated by the presence of these metals ; and secondly, the manorial 

 value of the sample is affected by their presence. It is therefore doubly 

 important that they should not be ignored. 



The removal of the aluminium and iron from the solution is readily 

 effected by neutralizing any excess of acid witli ammonia and adding 

 ammonium acetate, when iron and aluminium are thrown down as phos- 

 phates, which may be filtered off and weighed. The operation should be 

 conducted in a cold or but slightly warm solution. If the liquid be heated, 

 a calcium phosphate is thrown down. 



The precipitate can be conveniently analyzed by the following method, 

 contributed by Mr. 11. Warington : — " The precipitated phosphates of iron 

 and aluminium are washed, ignited, and weighed, rcdissolvcd in strong hy- 

 drochloric acid, and the iron determined volumetrically. From the iron 

 the quantity of ferric phosphate in the precipitate is calculated, the phosphate 

 of aluminium found by difference, and thus the iron, aluminium, and phos- 

 phoric acid in the precipitate are obtained. A little phosphoric acid is liable 

 to be removed from the precipitate during washing, and basic salts are thus 

 reckoned in the calculation as of normal composition." 



The Oxalic-Acid Method. 



In employing this method it is very desirable to previously separate iron 

 and aluminium acetate. Besides the advantages already mentioned, this 

 precaution renders it unnecessary to add an organic acid beforo precipitating 

 the phosphate with magnesia. The use of an organic acid prevents the 

 complete separation of the lime (oxalate of calcium being soluble in citrate 

 of ammonium), and tends to falsify the subsequent precipitation with 

 " magnesia mixture." 



The presence of ammonium acetate facilitates rather than prevents tho 

 precipitation of the calcium as oxalate. 



On rendering the filtrate from the oxalate of calcium precipitate alkaline 

 with ammonia, a small additional precipitation of oxalate of calcium may 

 occur. If the solution of the manure has been made with acid, and sub- 

 sequent evaporation of the acid liquid to dryness has been neglected, the 

 precipitate here formed may contain silica or fluoride of calcium. If the 

 separation of the iron and aluminium has been omitted, citric acid must be 

 added before making the solution alkaline with ammonia. Of course it' a 

 precipitate is formed at this stage, from whatever cause, it must be separated 

 beforo adding " magnesia mixture.'' 



Direct Citric-Acid Method. 

 In this method the iron, aluminium, and calcium are all retained in solu- 

 tion by means of citrate of ammonium, and no attempt is made to separate 

 the calcium as oxalate ; but the phosphate is at once precipitated from the 

 ammoniacal solution by " magnesia mixture." Although in the hands of 

 several chemists of high repute this convenient method gives very good re- 



