eutech-instruments iodide epoxy User Manual

Instruction Manual 

 

Iodide Electrode

 

 

9

   TABLE 2: Maximum Allowable Ratio of Interfering Ion to Iodide Ion 
 
       

 

Interference       

       Maximum Ratio 

            

Cl

-1

 

(1) 

  1.0X10

6

    

 

 

S

2

O

3

-2

(2) 

  1.0X10

5

 

 

 

Br

-1

 

(1) 

  5.0X10

3

 

 

 

 

CN

-1

 

(3) 

  4.0X10

-1

 

 

 

S

-2

 

 

(3) 

  1.0X10

-6

 

 

(1)  Gran's plot titration can be used to measure mixed halides in solution. 
 
(2)  These substances represent complexing species whose maximum level can be exceeded 

without electrode damage. Values shown represents a 1% error. 

 
(3) Add Ni

+2

 to remove sulfide or cyanide interferences. 

 

As an example of Table 2's use when using the iodide ion electrode: 
 
What is the maximum level of iodide tolerable in a sample whose iodide concentration is 10-4M? 
 
Using Table 2, the maximum ratio is: 
 
  

[Br

-1

] 

           

⎯⎯⎯⎯   = 5X10

3 

      

  [I

-1

]   

 
 

or    [Br

-1

] = 5X10

3

 [I 

-1

] 

             

     = 5X10

3

 (1X10

-4

) 

 
      [Br

-1

]  = 5X10

-1

M maximum bromide concentration for no interference 

 

 

Total concentration (C

t

) consists of free ions (C

f

) and complexed or bound ions (C

c

) in solutions: 

                        

 

 

 

 

C

t

 = C

f

 + C

c

 

 
Since the electrode only responds to free ions, any complexing agent in the solution reduces the 
measured concentration of ions. 
 
Iodide ions complex with some metal ions. Table 3 lists the levels of complexing metals causing a 
20% error. 
 
   TABLE 3:  Levels of Complexing Agents Causing a 20% Error at 1.0X10

-4

M Iodide 

 
 

 

 

Ion                Concentration 

 

 

 

Bi

+3

  

 

2.0X10

-5

M ( 4 ppm)   

 

 

 

Cd

+2

  

5.0X10

-4

M ( 50 ppm)   

 

 

 

Pb

+2

  

5.0X10

-3

M (1,000 ppm)