TOUIAN. — ELECTROMOTIVE FORCE PRODUCED IN SOLUTIONS. 1 37 



^m ; in other words, the farther apart the concentrations /-'and P' are 

 taken. On the other hand, if we are interested in the partial volume 

 of the solute at the concentration P, we must determine the limit of 

 Av/Ajn as P' is brought nearer and nearer to P. A satisfactory solu- 

 tion of the problem may often be obtained by determining Ar/Am for 

 several different values of F', if possible making it in some cases larger 

 and in others smaller than /-'. These values will then permit a close 

 estimate of the limit of A^/A?^^ at the concentration P. 



If a calculation of the (|uantity Ar/Am from specific gravity data is 

 desired, equation (3) has to be changed merely by the substitution of 

 the densities d and d' for the weights 31 and M' and by placing the 

 volume V etjual to unity, giving the formula 



^0 ^ (i-p)d-(i-pyr 



Am (1 - P) dP'd' - (1 - P') d'Pd ^^ 



For the purpose of determining the partial volume of iodine in iodide 

 solutions picnometer weighings were made of a solution of approxi- 

 mately quarter normal potassium iodide containing varying quantities 

 of iodine. The partial volume was desired in solutions very dilute in 

 iodine (y^o I2), and hence the solutions were made as dilute in iodine 

 as was consistent with a reasonable degree of accuracy in the calcula- 

 tion of Av/Am between pure potassium iodide solution and the iodine 

 solution in question. It is especially desirable to make the determi- 

 nations with solutions as dilute as the accuracy of the method will 

 permit, since the exact measurements of Kohlrausch and Hallwachs ^"^ 

 have shown that for several substances there is a rapid change in 

 partial volumes when considerable dilutions are reached. 



For the measurements, two separate solutions were made up from the 

 same KI solution each containing about 1.5% I2. The concentration 

 of the solutions was determined by weighing the iodine used and 

 transferring it directly from a glass-stoppered weighing tube into the 

 solution. The amount of solution used was also determined by weight. 

 From each of these original solutions two more were prepared by dilu- 

 tion with weighed quantities of the pure potassium iodide solution, 

 giving in all six iodine solutions wliose densities were determined. 

 The density of the original KI solution was also determined. 



The volume of the picnometer used was about 21 cc, and was exactly 

 determined by w^eighing the picnometer filled with pure water. The 



" Wied. Ann. 53, 14 (1894). 



