26 Estimations occurring in Agricultural Analysis [36. 37 



then transfer as much as possible of the precipitate to a weighed 

 platinum dish. Replace the filter in the funnel, and wash it 

 two or three times with small quantities of boiling distilled 

 water, allowing the washings to run into the platinum dish. 

 When the adhering precipitate has entirely disappeared from 

 the filter, place the dish on the water bath until the water has 

 evaporated. Remove it to the steam oven for a few minutes. 

 Cool in a desiccator, and weigh. 



36. Calculation. — From the weight of K 2 PtCl 6 the weight 

 of K may be obtained, or, as is more usual in agricultural 

 chemistry, an equivalent quantity of K 2 may be calculated. 

 (See paragraph 251.) 



When a large number of analyses have to be made in a 

 given time, it is of importance to make the calculation as short 

 as possible. To this end ' factors ' are usually employed'. 

 The factor for calculating how much K 2 is represented by 

 a given quantity of K 2 PtCl 6 is -19308, or, as is more frequently 

 used, "193. Thus in calculating out the result of this estima- 

 tion we use the following formula : 



Wt. of K 2 PtCl 6 x 100 



Wt. of substance taken X ' I93 ~ /o ot KaU " 



ESTIMATION OF PHOSPHORIC ACID (P 2 O s )  



37. Substance used.— Sodium hydrogen phosphate, 

 Na : HP0 4 .i2H 2 0. 



Method employed. — The phosphoric acid is precipi- 

 tated as Mg.NH 4 .P0 4 .6H 2 0, which is ignited and weighed as 

 Mg 2 P 2 7 . 



1 When phosphatic manures are guaranteed to contain a certain per- 

 centage of phosphoric acid, P. 2 3 is usually meant. The one exception to 

 this rule is liquid phosphoric acid, which is generally guaranteed to con- 

 tain a percentage of H 3 P0 4 . 



