Conductivity of Ilypophosphoric Acid Solutions. Ill 



prepared by dissolving the crystalline hydrate of hypo- 

 phosphoric acid. Both of these methods of preparation 

 have disadvantages. In the former, impurities may be 

 introduced by failure to wash the lead hypophosphate 

 until free from adhering salts, or by incomplete removal 

 of the hydrogen sulphide or its possible oxidation prod- 

 ucts. In the latter, the high solubility and deliques- 

 cence of the hydrate make it very difficult to free the 

 crystals from traces of the syrupy mother liquor, the 

 presence of which not only contaminates the preparation 

 but induces hydrolysis of the crystals at such a rapid 

 rate that solutions prepared from them almost inevitably 

 contain more or less phosphorous and phosphoric acid. 

 The conductivities found by Parravano and Marini at 

 25° are on the average about 15% higher than those of 

 Rosenheim and Pinsker, measured at 25-6°. 



Knowing from practical experience the difficulty in 

 preparing a pure solution of hypophosphoric acid by way 

 of the hydrate, we have preferred to adopt the principle 

 used by Parravano and Marini. Two solutions, A and 

 B, have been employed in our conductivity measure- 

 ments, the former prepared by the action of hydrogen 

 sulphide on lead hypophosphate, the latter in a similar 

 manner from copper hypophosphate. 



In preparing solution A, acid sodium hypophosphate, 

 four times recrystallized, was treated in hot solution 

 (80°-90°) with an excess of lead acetate, and the mixture 

 heated on the steam bath for several hours to bring the 

 precipitate into a dense and easily washable condition. 

 The precipitate was then collected on a suction filter, 

 transferred to a porcelain dish, and digested for some 

 hours with distilled water over a steam bath, again fil- 

 tered, and the alternate digestion and filtering continued 

 until the filtrate gave no test for lead salt, and a portion 

 evaporated in platinum showed no residue. 



To decompose the lead hypophosphate it was placed, 

 together with enough conductivity water to make the 

 resulting solution of H 4 P 2 O f) about 0-08 molar, in a large 

 bottle surrounded by a cooling bath of ice and water, and 

 closed by a stopper carrying a rotary glass stirrer, as 

 well as the inlet and outlet tubes for the gas. "With the 

 stirrer in operation, driven by an electric motor, hydro- 

 gen sulphide was passed in until tests of the liquid 

 showed that the decomposition was complete and the 

 hydrogen sulphide permanently in excess. 



