Measuring principle
An ion selective electrode is an electrochemical sensitive element whose potential is linear with the logarithm of the ion activity in a given solution. It is a type of electrochemical sensor that uses the membrane potential to measure the ion activity or concentration in a solution. The component is a sensing film at the tip of the electrode. Ion-selective electrodes are also called membrane electrodes. These electrodes have a special electrode film. The electrode film has a selective response to specific ions. The relationship between the potential of the electrode film and the ion content to be measured conforms to the Nernst formula. Such electrodes have the characteristics of good selectivity and short balance time, and are the most used indicator electrodes for potential analysis.
The ion selective electrode method is a branch of potential analysis and is generally used for direct potentiometry or potentiometric titration. Its characteristics are: the concentration of specific ions in the solution can be determined; the application range is wide; it is not affected by the color and turbidity of the reagent.
Measuring structure
The sensitive membrane of the electrode is fixed on the top end of the electrode tube, and the tube is filled with an internal filling solution, wherein the internal reference electrode is inserted
(
Usually
Ag
│
AgCl
electrode
)
The effect of the internal filling solution is to maintain the stability of the inner surface of the membrane and the internal reference electrode potential.
The ion selective electrode is a half battery
(
Gas sensor exception
)
, its potential cannot be measured separately, but must be combined with a timely external reference electrode to form a complete electrochemical cell, and then measure the electromotive force of the battery, which includes the following
:
E=Em+En+E1-Ew
In the middle
En
Is the potential of the internal reference electrode
;
Ew
versus
E1
It is the potential of the external electrode and the liquid junction potential of the liquid junction.
In the general measurement, all three of the above requirements are required to remain unchanged, so the electromotive force
E
versus
Em
There is only one constant term between them, and its change can fully reflect
Em
The change.
Application of ion selective electrodes
Ion-selective electrodes are widely available for analysis. The correct ion measurement method, ion meter, and electrode can be used to measure the concentration of the corresponding ion. It has been successfully applied in the fields of environmental monitoring, water quality monitoring, soil analysis, food testing, and drug analysis.
Ion electrode applications in various industries
Introduction to actual product application:
(
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1.
Determination of fluoride ion content in water by fluoride ion selective electrode
equipment
(
Shanghai Lei Magnetic
)
laboratory apparatus:
PXSJ-216F
Or other models of ion meters, Dalong
MS-H280-Pro LED
Digital heating type magnetic stirrer
Experimental electrode:
PF-1-01
Fluoride ion selective electrode,
232-01
Reference electrode (cabbage electrode)
2.
Determination of choline chloride content
First, the experimental principle
The choline chloride contained in the sample was dissolved in water, and the filtrate was filtered. Chloride ions are dissolved in the filtrate.
The content of choline chloride is determined by the silver amount method.
The silver electrode can be judged at the end point depending on the potential.
Second, the instrument
ZDJ-5
Automatic potentiometric titrator
216-01
Silver ion selective electrode
217-01
Type reference electrode
Third, the reagent
(1) 0.1 mol/l
Silver nitrate standard titration solution
(2)
Because the ultrapure water is used, the blank is
0
(3)
sample
1
(
50%
Silicon powder), sample
2
(
50%
Corn cob type powder)
Fourth, the experimental steps
Accurate weighing
0.7g
Sample (accurate to
0.0001g
)
,
Placed
100ml
In the volumetric flask, dilute to the mark with water and place
20min
After filtering. Accurate removal
25ml
in
100ml
In the beaker, titrate. Each group is paralleled twice.
The content of choline chloride expressed by mass fraction
Calculation formula:
C
(
VV 0
)×
0.13963
×
4
X=
————————————
×
100
(
1
)
M
C
- the actual concentration of silver nitrate standard solution (
0.1mol/L
)
V
——The volume of the silver nitrate standard solution consumed by the sample
Ml
V 0
- blank test consumes the volume of silver nitrate standard solution
Ml
Because the ultrapure water is used, the blank is
V 0 = 0
M
- the quality of the sample (
g
)
0.13963
--versus
1.00ml
The silver nitrate standard titration solution is equivalent to the mass of choline chloride expressed in grams. ,
4
- dilution factor
V. Results calculation
sample
1
(
50%
Silicon powder)
Sample quality:
0.7045 g
(
1
End point potential:
282.2
mV
Consumption volume:
6.384
Ml
X=50.61%
sample
2
(
50%
Corn cob type powder)
Sample quality:
0.7250 g
(
1
End point potential:
287.1
mV
Consumption volume:
6.573
Ml
X=50.64%
Six, discussion
1
It can be seen from the titration curve that the choline chloride is determined by the pre-titration mode, the titration curve potential is large, and the end potential corresponding to the volume of the titrant added is obvious.
2
The titration data showed that the content of choline chloride was determined by automatic potentiometric titration, and the reproducibility of the results was good. Mainly this method avoids the error of the human eye in determining the color change of the indicator.
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