eutech-instruments cadmium epoxy User Manual

Instruction Manual 

 

Cadmium Electrode

 

 

11

                            MCd

+2

 =  ———— 

                                     

VCd

+2

 

       

where: 

                    MCd

+2

 = concentration of cadmium ion in the sample (moles/liter) 

                           Vt = volume of EDTA added at endpoint 
                          Mt = EDTA concentration (moles/liter) 
                      Cd

+2

 = volume of unknown sample (100 ml) 

 
 
ELECTRODE CHARACTERISTICS 
 

 
Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every 
hour. Factors like temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is 
independent of concentration within the electrode's operating range. 
 

 
A surface layer of silver metal may be formed by strongly reducing solutions. A layer of silver salt 
may be deposited on the membrane if high levels of ions forming very insoluble salts are present in 
the sample. Proper performance can be restored by polishing. See the section entitled 

 for proper polishing procedure. 

 
The cadmium ion electrodes do not respond to anions or most cations. The electrode membrane is 
poisoned by solutions containing copper, mercury, and silver. These ions must be absent from the 
solution. 
 
If the level of ferric ion or lead ion exceeds the level of cadmium ion in the sample, the 
measurement will be affected. Sodium fluoride added to the sample will eliminate ferric ion. 
 

 
Sulfide, carbonate, oxalate, phosphate, hydroxide, and other ions precipitate insoluble cadmium 
salts. The level of cadmium ion, the level of the precipitating ion, and the pH of the sample 
determine formation of a precipitate. 
 
A wide variety of species, including acetate, ammonia, bromide, chloride, citrate, cyanide, and 
EDTA, form complexes with cadmium. The total cadmium concentration, the concentration of the 
complexing species, the solution pH, and the ionic strength all determine the extent of 
complexation. Complexation reduces the free cadmium ion concentration and, since the electrode 
responds only to free cadmium ions, a false reading results.