400 J. R. Reedy — Anodic Potentials of Silver. 



Art. XXIX. — Anodic Potentials of Silver : II. Their Role 

 in the Electrolytic Estimation of the Halogens • by John 

 Henry Keedy. 



(Contributions from the Kent Chemical Laboratory of Yale Univ. — cclxxi.) 



The first attempt at the estimation of the halogens by- 

 electrolytic fixation on silver electrodes was made by Vort- 

 mann.* The halides in alkaline solution were electrolyzed 

 between a copper cathode and a silver anode, using a potential 

 of about two volts. At the end of the electrolysis the anode 

 was ignited to decompose the silver oxide formed during the 

 process, and the deposited halogen was determined by gain in 

 weight. Even in alkaline solution small amounts of silver 

 were transported to the cathode. The deposit was not very 

 adherent, and Vortmann sought to overcome this by adding 

 sodium-potassium tartrate to the solution. The problem of 

 separating the halogens was not considered. 



The quantitative separation and estimation of the halogens 

 was first attempted by Specketer. f The halides were dissolved 

 in - 5 molar sulphuric acid, and during the electrolysis a stream 

 of hydrogen was constantly passed through the solution to 

 provide stirring, and at the same time to make the platinum 

 cathode function as a hydrogen electrode. Considering the 

 anode reversible with respect to the Ag'-ion, Specketer 

 assumed that the maximum anode potential required in any 

 determination would be the potential of a silver electrode in a 

 saturated solution of the silver halide in question, and that this 

 value could be calculated from solubility data, by means of the 

 Nernst formula for electromotive force. In this way he 

 calculated the potentials necessary for the precipitation of the 

 halogens to be — "06 volts;}; for iodine, +'15 volts for bromine 

 and +*50 volts for chlorine ; that is, a potential of — - 06 volts 

 should deposit all the iodine free from chlorine and bromine, 

 and +*15 volts all the bromine free from chlorine. In practice 

 somewhat larger potentials were found necessary. In order to 

 make this separation satisfactorily, Specketer found that the 

 following precautions must be observed : (1) A constant volt- 

 age must be employed ; (2) a definite acidity of solution must 

 be maintained, — for example, 0*5 molar sulphuric acid : (3) 

 atmospheric oxygen must be excluded, since it has a marked 

 effect on the potential of the hydrogen electrode. Specketer 

 found it impossible to determine chlorine on a silver anode, 



*Monatsh. Chem., xv, 280 ; xvi, 674. 



fZeitsehr. Elektroch., iv, 542. 



\ Referred to the hydrogen electrode in - 5 molar sulphuric acids as 



volts. 



