eutech-instruments cadmium epoxy User Manual

Cadmium Electrode 

 

Instruction Manual

 

 

14

interference from hydroxide is given by a dashed line to the left of the dotted area on Figure 4. 
Within this dotted area, cadmium combines with hydroxide to form Cd(OH)

2

. Since only free 

cadmium concentration can be measured with the cadmium ion electrodes, a false reading results. 
 

 
A cadmium electrode will last six months in normal laboratory use. On-line measurements might 
shorten operational lifetime to several months. In time, the response time will increase and the 
calibration slope decreases to the point calibration is difficult and electrode replacement is required. 
 

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

-2

M cadmium 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 
 

 
The electrode consists of sulfides of cadmium and other metals bonded into an epoxy or glass body. 
When an electrode potential develops across the membrane, the membrane is in contact with a 
solution containing cadmium ions. The electrode potential is measured against a constant reference 
potential, using a standard pH/mV meter. The level of cadmium ions, corresponding to the 
measured potential, is described by the Nernst equation: 
 
                         E = Eo + S log X 
 
           where:    E   = measured electrode potential 
       

              Eo = reference potential (a constant) 

       

              S   = electrode slope (

∼27 mV/decade) 

                         X  = level of cadmium ions in solution 
 
The activity, X, represents the effective concentration of free cadmium ions in solution. Both 
bound, Cb, and free, Cf, cadmium ions are included in the total cadmium ion concentration, Ct. The 
cadmium ion electrode will only respond to free cadmium ions, the concentration of which is: 
 
                      Cf = Ct + Cb 
 
The activity, X, represents the effective concentration of the ions in solution. The activity is related 
to the free cadmium ion 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