Phosphorous, Hypophosphoric and Phosphoric Acids. 97 



shown by the experiments of Table IV, in which the titra- 

 tion of a N/50 iodine solution with thiosulphate gave 

 practically the same result whether the titration was 

 performed in neutral solution in the ordinary way, or in 

 acid solution, after the iodine had previously stood for 

 2y 2 hours with the amount of disodium phosphate recom- 

 mended above. 



The details of the procedure finally adopted for the 

 estimation of phosphorous acid in the presence of hypo- 







Table IV. 









Iodine solution 



approximately N/50. 





Iodine 



Na 2 HP0 4 







H 3 P0 4 



Thiosulphate 



solution 



110 grams 





Time 



85% 



solution 



taken 



per liter 





hours 



solution 



required 



cm 3 



cm 3 







em 3 



cm 3 



50-00 









. . 



49-21 



5000 









, . 



49-20 



5000 



50-66 





2% 



5 



49-16 



5000 



5000 





2y 2 



5 



49-18 



Note: — One gram of potassium iodide was added in the final titration 

 to increase the sensitiveness of the starch reaction. 



phosphoric acid are as follows: A suitable quantity of 

 the solution to be analyzed is measured into a glass- 

 stoppered conical flask of 300 cm 3 capacity. If strongly 

 acid or strongly alkaline the solution is brought approxi- 

 mately to the neutral point toward litmus by cautious 

 addition of sodium hydroxide or hydrochloric acid. This 

 adjustment need only be very rough, but the litmus paper 

 must be used very sparingly, and in such a way as to 

 avoid the introduction of paper fiber, or of appreciable 

 quantities of the indicator, into the solution. When 

 phosphorous acid is the only acid present, this prelimi- 

 nary neutralization may be omitted if the quantity of 

 disodium hydrogen phosphate employed is not less than 

 that recommended above. This salt is now added, either 

 as crystals or in solution, and an excess of a standard 

 solution of iodine is measured into the flask, 14 which is 

 then stoppered and set aside in the dark for at least two 

 hours. Finally the solution is acidified by adding a 

 measured quantity of phosphoric acid 15 (which must be 



14 If a number of determinations are to be made much time can be saved 

 by adding the iodine solution with an accurately calibrated pipette. When 

 this is done, it is best to use the same pipette in determining the ratio 

 between the thiosulphate and iodine solutions. 



15 Though not tested, it is probable that hydrochloric or sulphuric acid 

 could be used for this purpose. 



Am. Jour. Sci. — Fourth Series. Vol. XLV, No. 26(>.— February. 1918. 



