TOTAL PHOSPHORIC ACID IN BASIC SLAGS 



In all cases the complete precipitation of phosphoric acid is 

 rendered certain by an addition of molybdate solution to the ni- 

 trate. 



Schenke calls attention to the important point that if the method 

 of the German stations is strictly followed with the citrate method 

 of determination no precipitate at all of phosphoric acid is ob- 

 tained where only very small quantities are present. This is the 

 case both with certain soils which contain only a trace of phos- 

 phoric acid and certain cattle foods. In such cases the above 

 stated molybdate method gave agreeing results even in these small 

 quantities. The reason for this is said to be the increased solu- 

 bility of the precipitate of magnesium ammonium phosphate in 

 the double quantity of ammonium citrate solution which is used. 



188. Estimation of Total Phosphoric Acid in Basic Slags Solu- 

 ble in Citric Acid Solutions. Since the value of basic slag is no 

 longer determined solely by the total content of phosphoric acid 

 therein, the agricultural analysts have never been able to agree 

 upon a satisfactory plan for the valuation of phosphoric acid 

 available for plant growth. The use of citric acid solutions for 

 dissolving the supposed available phosphoric acid is the one which 

 is most commonly employed. It is easily seen, however, that 

 the activity of a solution of this kind depends upon the amount of 

 free lime in the sample, its state of subdivision, the temperature 

 at which the solution takes place and the agitation to which the 

 mixture is subjected. The disturbing influence of silicic acid is 

 also to be taken into consideration and this has been fully dis- 

 cussed. Mach has found in a large number of determinations 

 that the Wagner method in most cases gives satisfactory results 

 while the direct precipitation by the Bottcher method sometimes 

 fails, and in its present form is not to be regarded as reliable as 

 the German experiment station method with previous separa- 

 tion of silica. 67 It is concluded, therefore, that as a result of the 

 study of all the various methods, the one which is based upon the 

 previous separation of the silicic out of the citric acid extract is 

 the most reliable. 



In a study of the different forms of precipitation of total phos- 



67 Die landwirtschaftlichen Versuchs-Stationen, 1905-6, 63 : 81. 



