Section 48. Preparation of Substances and Solutions. up 



METHOD OF PROCEDURE. 



The procedure was substantially the same as that described by Noyes 

 and Coolidge.* In the case of solutions which were liable to adsorption 

 by the platinum, this effect was largely avoided by allowing the solution 

 to remain in the bomb between the experiments (usually over night) and 

 rinsing the bomb with solution only before introducing a fresh portion 

 for a new experiment. In passing to a diluter solution, the bomb was first 

 steamed out at 218 with the diluter solution instead of with water. 



In the experiments with a platinized lower electrode, the air pressure 

 within the bomb was reduced at the start to 2 cm. of mercury. In sub- 

 sequent experiments with unplatinized electrodes, the measurement at 18 

 was made under atmospheric pressure and then the pressure was reduced 

 to about 10 cm. of mercury before going on to the higher temperatures. 



For each measurement of resistance, three different box resistances 

 were used, generally in the order 100, 110, 111. These series of read- 

 ings were made at 5-minute intervals, and the bomb was kept in the bath 

 until three or more successive series of readings showed no progressive 

 change. 



In the case of all the solutions which showed any variation in the read- 

 ing, the bomb was removed from the bath, shaken, and returned as quickly 

 as possible, in order to determine whether there was any change in the 

 solution around the electrode. 



48. PREPARATION OF THE SUBSTANCES AND SOLUTIONS. 



For the preparation of sodium chloride, pure commercial salt was twice 

 reprecipitated from saturated solution with hydrochloric acid, filtered, 

 washed, and ignited. Tests for potassium and for sulphate gave negative 

 results. For potassium chloride, a Kahlbaum preparation was purified by 

 reprecipitation from saturated solution in best water by pure hydrochloric 

 acid and subsequent washing and ignition. A flame test showed no so- 

 dium. 



The potassium chloride and sodium chloride solutions, except the 0.0005 

 normal sodium chloride solution, were prepared by weighing out a proper 

 quantity of freshly ignited salt (corrected for the buoyancy of the air) 

 and dissolving it in water in a graduated 2,000 or 500 c.cm. flask at 21, 

 the flask being so calibrated as to contain 2,000 or 500 grams of water at 

 that temperature. The 0.0005 normal sodium chloride solution was made 

 by diluting a 0.002 normal solution by means of a 500 c.cm. and a 2,000 

 c.cm. flask. 



Hydrochloric acid was prepared by heating sodium chloride of the same 



*Section 6, Part II. 



