124 Analysis and Valuation of Manures [198, 199 



gently round the beaker until all the substance has been 

 moistened with it. Warm for a few minutes on the water bath, 

 then add about 20 c.c. of dilute HC1 and allow to digest for ten 

 minutes. Filter off, and wash with hot water until the washings 

 are no longer acid to litmus paper. Transfer the filter paper, 

 without drying, to a weighed platinum dish. Heat over an 

 Argand turned down very low, keeping the dish covered with 

 a square of platinum foil. When the paper is dry, turn up the 

 flame so as just to char the paper. As soon as fumes cease to 

 come off, turn up the Argand to its full power until the paper 

 is completely burned. Cool in a desiccator, and weigh. Sub- 

 tract the weight of the dish and filter ash. Calculate the 

 percentage of the residue, and enter it as sand and insoluble 

 silicates. 



198. Lime. — To the filtrate from the sand add ammonia 

 until a precipitate is formed. Dissolve this by adding 2 grams 

 of citric acid. 1 If the precipitate all dissolves, add ammonia 

 until it reappears, and strong acetic acid until it goes again. 

 If it does not dissolve, the ammonia may be omitted. By going 

 through these operations it will be ensured that sufficient 

 ammonia is present to convert the whole of the citric and a 

 portion of the acetic acid into ammonium salts. Raise to a 

 boil, add 2 grams of solid ammonium oxalate, and estimate the 

 lime exactly as described in paragraphs 42-45. The reason 

 for adding citric acid is that ammonium citrate prevents the 

 precipitation of phosphates of iron and aluminium. 



199. Phosphoric Acid. — Allow the filtrate from the lime, 

 which should not exceed 200 c.c, to cool, or cool it by placing 

 the beaker in a vessel through which passes a stream of cold 

 water. Then add 50 c.c. of strong ammonia and 40 c.c. of 



1 Citric acid sometimes contains insoluble matter. It is, therefore, 

 better to dissolve the 2 grams of acid in water, and filter, if necessary, 

 before adding to the liquid. 



