ON THE ESTIMATION OF POTASH AND PHOSPHORIC ACID. 29 



Oxalic-Acid Metliod. 



Of the chemists whose processes of analyzing phosphates have been com- 

 municated to the Committee, a decided majority precipitate the phosphoric 

 acid as the double phosphate of magnesium and ammonium, after previously 

 separating the calcium as oxalate. 



Although there is no great diiference in the general outline of the method 

 followed, the most extraordinary variations occur in the details of the 

 instructions, and in the precautious recommended to insure accuracy. 



By far the greater number of chemists precipitate the calcium as oxalate" 

 after neutralization of the excess of acid. Some add citric acid prc-siously to 

 employing oxalate of ammonium. According to Mr. K. Wariugton this 

 modification " occasions a deficiency of lime, oxalate of calcium being soluble 

 in citrate of ammonium." Those chemists who add citric acid before pre- 

 cipitating the calcium usually employ an acetate to get rid of free mineral 

 acid. Of course, the addition of citric acid then becomes a necessity when 

 iron and aluminium are present, unless the precipitated phosphates of these 

 metals are filtered ofl;' and estimated separately. This plan appears to have 

 several advantages, and is recommended by some chemists of wide experience. 

 By employing it, the phosphates of iron and aluminium (by most chemists 

 believed to have a very limited manurial value) are separately estimated, and 

 the subsequent addition of citric acid is rendered unnecessary and the iron 

 introduced by its use avoided. 



If the phosphates of iron and aluminium are not previousl}? separated, the 

 general plan is to neutralize the solution with ammonia till a slight turbidity 

 ensues, to clarify the liquid by the addition of a few drops of oxalic acid, to 

 add a moderate excess of ammonium oxalate, to heat the liquid nearly to 

 boiling, and to filter off the precipitated oxalate of calcium. Some chemists 

 filter again after cooling. 



After careful washing of the precipitate, the filtrate is usually concentrated, 

 a moderate quantity of citric acid added and then excess of ammonia. A 

 precipitate may here occur of silica, fluoride of calcium, or oxalate of calcium. 

 The more careful analysts leave the solution for a time to make sure that 

 the liquid remains clear, or to filter from any precipitate. 



Precipitation hy Magnesia. 



The clear solution is next precipitated by " magnesia mixture," which is 

 universally admitted to be better made with chloride than with sulphate of 

 magnesium. It is also clearly proved and generally recognized that a large 

 excess of the precipitant should be avoided, some chemists recommending a 

 preliminary analysis of the sample with the view of adding the approximately 

 theoretical quantity of solution. On the other hand, it has been proved that 

 complete precipitation is very slow except in presence of a considerable excess 

 of " magnesia mixture." 



Very great variation occurs with respect to the concentration and tem- 

 perature of the solution at the time of precipitation. A few chemists recom- 

 mend precipitation in a hot solution, but the majority direct precipitation in 

 the cold ; one or two recommend the use of a dilute solution, while others 

 concentrate, if necessary, to a certain bulk ; and some make a correction for 

 the solubility of the double phosjjhate in the liqiiid. 



The amount of free ammonia present during the precipitation varies 

 from a moderate excess to one fifth of its bulk of the strongest ammoniat 

 (•880). 



