eutech-instruments silversulfide epoxy Manual Do Utilizador

Silver/Silver Sulfide Electrode 

 

Instruction Manual 

 

18

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. Performance may be restored by polishing. See section 

 for proper 

polishing procedure. 
 
All silver samples must be free of mercury. Sulfide samples will not have mercury present due to 
the extreme insolubility of HgS and Hg

2

S. Biological samples and protein in food interferes with 

silver measurements, but the protein interference can be removed by acidifying to pH 2-3 with 1M 
HNO

3

. 

 

 
Total concentration (Ct) whether sulfide or silver ions, consists of free ions (Cf) and complexed or 
bound ions (Cc) in solution: 

Ct = Cf + Cc 

 

Since the electrode only responds to free ions, any complexing agent in the solution reduces the 
measured concentration of ions. Silver ions complex with many species, notably cyanide, 
thiosulfate, ammonia and chelants such as EDTA. 
 
Sulfide ions form complexes with hydrogen ions (HS

-1

 and H

2

S). Sulfide ions also form soluble 

complexes with elemental sulfur (S

2

-2

, S

3

-2

, S

4

-2

, etc.) and tin, arsenic, and antimony ions. 

 

 
Samples and standards should be within ±1oC of each other, since electrode potentials are 
influenced by changes in temperature. Because of the solubility equilibrium on which the electrode 
depends, the absolute potential of the reference electrode changes slowly with temperature. The 
slope of the electrode, as indicated by the factor "S" in the Nernst equation, also varies with 
temperature. Table 3 gives values for the "S" factor in the Nernst equation for each ion. 
 

   TABLE 3: Temperature vs. Values for the Electrode Slope 

 
  

Temp 

(oC)  

     

 S

-2

  

 

Ag

+1

 

 

 

 

0 

  

27.1 

 54.2 

 

 

10 

  

28.1 

 56.2 

 

 

20 

  

29.1 

 58.2 

 

 

25 

  

29.6 

 59.2 

 

 

30 

  

30.1 

 60.1 

 

 

40 

  

31.1 

 62.1