PRECIPITATION OF THE PHOSPHATE 135 



centimeters of the real solution and the error at the most would 

 be less than o.oi. This error could be reduced to one-half by dis- 

 solving only two grams of the material in place of four, or by mak- 

 ing the volume up to 200 instead of 100 cubic centimeters. 



In general it may be said that the errors which do not exceed 

 o.oi of the total matter under treatment, are negligible for all 

 industrial products. The method of volumetric solution does 

 not present any further inconvenience. It deserves to be and 

 has been generally adopted by reason of its rapidity in all the 

 laboratories where many analyses are to be made. In the volu- 

 metric method great care should be taken not to make up to the 

 volume until after the cooling to room temperature, which may 

 be speedily secured by immersing the flask in cold water. Care 

 should also be exercised in removing the sample for analysis by 

 means of the pipette immediately after filtration, and filtration 

 should take place as soon as the volume is made up to the stand- 

 ard. By operating in this way the possible variations from 

 changes of volume due to changes of temperature are avoided. 



(8) Examination for Arsenic Acid. When the sample exam- 

 ined contains pyrites, arsenic is often present. When the decom- 

 position has been effected by means of nitric acid, arsenic acid 

 may be produced. This deports itself in all circumstances like 

 phosphoric acid, and if it is present in the matter under exami- 

 nation, it will be found united with the phosphoric acid and de- 

 termined therewith afterwards. It is easy to avoid this cause of 

 error by passing first a current of sulfurous acid through the 

 solution, carrying it to the boiling point in order to drive out the 

 excess of sulfurous acid, and afterwards precipitating the arsenic 

 by a current of hydrogen sulfid. After filtration, the rest of the 

 operation can be carried on as already described. 



1 20. Precipitation of the Phosphate in Presence of Citrate. 

 By means of an accurate pipette a quantity of the solution repre- 

 senting from 0.125 to 0.250 gram or more is measured, according 

 to the presumed richness of the product to be examined. In order 

 that the following operations may go on well, it is advisable that 

 the quantity of phosphoric acid contained in the sample should be 

 about 50 milligrams. The sample being measured is run into a 



