eutech-instruments iodide epoxy User Manual

Instruction Manual 

 

Iodide Electrode

 

 

12

 
The iodide ion electrode may be stored for short periods of time in 1.0x10

-2

M iodide solution. For 

longer storage (longer than two weeks), rinse and dry the sensing pellet and cover the membrane tip 
with any protective cap shipped with the electrode. The reference portion of the combination 
electrode (or the outer chamber of the reference electrode) should be drained of filling solution, if 
refillable, and the rubber insert placed over the filling hole. 
 
 
ELECTRODE THEORY 
 

 
An iodide ion electrode consists of a sensing membrane bonded into a glass or an epoxy body. The 
membrane is composed of silver iodide/silver sulfide. When an electrode potential develops across 
the membrane, the electrode is in contact with solution containing iodide ions and is capable of 
measuring free iodide ions. This electrode potential is measured against a constant reference 
potential, using a standard pH/mV meter or an ion meter. The level of the iodide ions, 
corresponding to the measured potential, is described by the Nernst equation: 
 
 

 

E = Eo - S log X 

 where: 
 

 

E = measured electrode potential 

 

 

Eo= measured potential (a constant) 

 

 

S = electrode slope (

∼57 mV/decade) 

 

 

X = level of iodide ions in solution 

 
The activity, X, represents the effective concentration of the ions in solution. The activity is related 
to the free ion concentration, Cf, by the activity coefficient, 

γ, by: 

 
 

 

X = 

γ Cf 

 
Activity coefficients vary, depending on total ionic strength, I, defined as: 
                              
 

 

I = ½ 

Σ CxZx

2 

 

 where: 
 

 

Cx =  concentration of ion x 

 

 

Zx =  charge of ion x 

 

 

 

Σ  =  sum if all the types of ions in the solution 

 
In the case of high and constant ionic strength relative to the sensed ion concentration the activity 
coefficient, 

γ , is constant and the activity, X, is directly proportional to the concentration. 

 
To adjust the background ionic strength to a high and constant value, ionic strength adjuster is 
added to samples and standards. The recommended ISA for iodide ions is NaNO

3

. Solutions other 

than this may be used as ionic strength adjusters as long as ions that they contain do not interfere 
with the electrode's response to the iodide ions. Samples with high ionic strength (greater than 
0.1M) do not need ISA added and standards for these solutions should be prepared with a 
composition similar to the samples.