eutech-instruments lead epoxy ユーザーズマニュアル

Instruction Manual 

 

Lead Electrode

 

 

13

ELECTRODE CHARACTERISTICS 
 

 
Electrode measurements reproducible to 

±2% can be obtained if the electrode is calibrated 

frequently. Factors such as 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 lead ion electrodes do not respond to anions or to 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 or cadmium ion is less than the level of lead ion, no interference occurs. If the 
level of ferric or cadmium ion is more than the level of lead ion, interferences will be present, 
resulting in false readings. The ferric ion interference is eliminated by pH adjustment to above pH 4 
by the addition of NaOH. 
 

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

 
Samples and standards should be within ±1

o

C of each other, since electrode potentials are 

influenced by changes in temperature. A 1

o

C difference in temperature results in a 4% error at 

1.0X10

-3

M lead ion concentration. 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 the lead ion.   
 
       TABLE 3:  Temperature vs. Values for the Electrode Slope 
 
 

   

      Temp(

o

C)     

   S   

 

 

   

0 

 

27.10 

  

 

10 

  28.09 

  

 

20 

  29.08 

  

 

25 

  29.58