82 CONDUCTIVITY AND VISCOSITY OF SOLUTIONS 



where md f (1 p'} is the amount of water in the alcohol and yd the 



k 

 amount of water to be added to make the amount - (100 x). 



JU 



Substituting for k, md'p', the expression becomes 



md'p'(lW-x) wd'(100 p'-x) 



-^- '- = md'(l-p')+ydoT -~ =y 



If p' is in percentage this becomes 



md'(px) _ 

 ~~ 



In this connection a question arose as to whether or not the volume 

 method of making up the solvent was experimentally accurate. On 

 the face of it the gravimetric method appears to be a safer one, but it 

 is also much longer and more tedious. A test was therefore made of 

 both methods. The calculated amounts of water and alcohol, that is, 

 the apparent weights of the two necessary to give a 50 per cent mixture 

 by weight (in vacuo), were weighed into a glass-stoppered flask and 

 thoroughly mixed. A density determination of the mixture was made 

 at 25 and found to be 0.909826, or 50.01 per cent alcohol. 



In like manner a mixture was made up by volume, using the quan- 

 tities of alcohol and water calculated from the above formula necessary 

 to make a 50 per cent mixture by w r eight ; the specific gravity in this 

 case being 0.90980, corresponding to 50.02 per cent alcohol. 



DISSOLVED SALTS. 



Sodium iodide and potassium iodide were used in this investigation. 

 They were obtained from Kahlbaum and were extra pure material. 

 In fact, it was not even necessary for us to recrystallize them. We 

 analyzed them, ground them fine, and placed them in a desiccator to 

 dry them thoroughly before weighing. 



Potassium iodide is not very soluble in pure alcohol. It was with 

 great difficulty that we were able to make a N/8 solution of it in the 95 

 per cent alcohol. 



PIPETTES. 



25 c.c., 50 c.c., 100 c.c., 150 c.c., and 200 c.c. pipettes, carefully 

 calibrated by weighing the water they would deliver, were employed, 

 together with a 10 c.c. graduated pipette, for making up the mixed 

 solvents. 



CONDUCTIVITY CELLS. 



The conductivity cells were of the same type as those used here 

 for such work. The general method of determining conductivity 

 previously described 1 was also employed. Since a number of changes 

 in the temperature regulation, which will be spoken of later, were made 

 at the beginning of this work, and since these changes necessitated 

 rewiring of the system for the determining of conductivity, we tested 



'Amer. Chem. Journ., 46, 56 (1911). 



