Transcript Residual current

Voltammetry
伏安分析法
Read: pp. 716 –753
Problems: 25-1,2,3,6,13
Basic principle of voltammetry
Voltammetry:
A group of electrochemical methods based on
measuring current (i)- applied potential curve during
electrolysis
- only a small amount of sample (analyte) is used
Polarography:
Invented by J. Heyrovsky (Nobel Prize 1959). Differs
from voltammetry in that it employs a dropping mercury
electrode (DME) as Working electrode to continuously
renew the electrode surface.
1. Polarographic analysis process and the
conditions for polarographic wave formation
Polarographic analysis ：
Electrolytic analysis carried out
under special conditions.
specific characteristics：
A、a polarized electrode and a dep
olarized electrode are used as work
ing electrode
B、No stirring
Incomplete electrolysis (only a s
mall amount of analyte is consumed)
Polarized electrode and depolarized electrode
If the electrode potential has great changes
when infinite small current flow through the
electrode, such electrode is referred to as
polarized electrode. eg. DME ;
If the electrode potential does not change with
current , such electrode is called ideal
depolarized electrode. eg. SCE
Three electrode cell:
Working
Reference
Counter/auxilliary
current flows between
working and counter
electrodes.
Potential controlled by
potentiostat between
working and reference
electrodes.
Two special electrodes
Supporting electrolyte : Usually relatively higher conc
entration of strong electrolytes (alkali metal salts) serv
es as supporting electrolyte
Dissolved oxygen is usually removed by bubbling
nitrogen through the solution
Voltage scanning Under unstirred state, recording
voltage - current curve
①～ ②residual current
③ electrolytic current
Cd
2+
+2e + Hg = Cd(Hg)
2Hg + 2Cl- -2e = Hg2Cl2
④～⑤limiting diffusion current
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+
i (A)
+
2e-
Cd
Current starts to be registered at the
electrode
E½
Working electrode is
no yet capable of
reducing Cd2+ 
only small residual
current flow through
the electrode
-0.2
-0.4
All Cd2+ around the electrode has
already been reduced. Current at
the electrode becomes limited by
the diffusion rate of Cd2+ from the
bulk solution to the electrode.
Thus, current stops rising and
levels off at a plateauid
Current at the working
electrode continue to rise as
the electrode become more
Base line
reducing and more Cd2+
of residual
around the electrode are being
current
reduced. Diffusion of Cd2+
does not limit the current yet
-0.6
V vs SCE
-0.8
-1.0
-1.2
-1.4
Limiting current
Related to concentration
E½ at ½ i
0.5mmol镉离子极谱图
Limiting diffusion current -A basis of
polarographically quantitative analysis
When the applied voltage exceeds the decomposition
voltage, diffusion-controlled current is expressed as:
i = K(C-C0)
When the applied voltage gets more negative, C0 →0,
current becomes only diffusion limited, then
id = KC
Id reaches a limiting value proportional to ion concentration
C in bulk solution, and do not changes with applied voltage
longer
Half-wave potential —polarographic qualitative analysis
The potential at which the current is equal to o
ne half the limiting current is called the half-wa
ve potential and given the symbol E1/2.
How it works?
▲ The applied voltage is gradually increased,
typically by going to a more positive( more negative
decomposing potential)
▲ A small residual current is observed.
▲ When the voltage becomes great enough,
reduction occurs at the analytical electrode causing a
current.
▲ The electrode is rapidly saturated so current
production is limited – based on diffusion of the
analyte to the small electrode.
How it works ?
The reduced species alters the surface of the
mercury electrode.
To prevent problems, the mercury surface is
renewed by “ knocking off ” a drop –providing a
fresh surface.
This results in an oscillation of the data as it is
collected.
2. The diffusion current theory and
polarographic wave equation
We have already known:
id = KC
In above equations, K is called Ilkovic constant, it is expressed
as follows:
K = 607 n D1/2m2/3t1/6
Thus,
id = 607nD1/2m2/3t1/6C
id = 607nD1/2m2/3t1/6C
Concentration of
electro-active
analyte(mmol.L-1)
Drop time
(sec)
Mercury
mass
flow
Diffusion
coefficient
-1)
rate(mg.sec
of electroactive Number of transferring
Average limiting diffusion
electrons in electrode
current denoting averageanalyte in
2
-1
solution(cm
.sec ) reaction(e/mol)
current on mercury drop
from
drop forming to falling (A)
From above equation, we can find that when
temperature, matrix solution and capillary
characteristic are kept constant, id is proportional to C
polarographic wave equation：
RT
i
E  E1 / 2 
ln
nF id  i
When i = ½ id , log term in above equation is equal to zero,
corresponding potential is called halfwave potential E1/2
●E1/2 independent on the concentration
●basis of qualitative analysis
3. Interference current
in classical DC polarography
● Residual current
(1) redox reactions of impurities in solution
(2) charging of Hg drop
(non-faradaic current / non-redox current)
● Migration current
The current produced by static attraction of the
electrode to sought-for ions
● Polarographic Maximum (or malformed peak )
Complex artifactual
phenomenon
Less likely at low drop
rates, in concentrated
electrolyte, or low
concentration of
electroactive species
Lessened by inclusion of
surfactants in medium
● Oxygen wave
Dissolved oxygen is easily reduced at many working
electrodes. Thus an aqueous solution saturated with air
exhibits two distinct oxygen waves.
The first results from the reduction of oxygen to
hydrogen peroxide:
O2 + 2H+ + 2e-  H2O2
The second wave corresponds to the further reduction of
hydrogen peroxide:
H2O2 + 2H+ + 2e-  2H2O
Sparge solutions with high purity N2 or Ar for 5-20 min
Factors that affect limiting diffusion current
Characteristics of capillary
– hight of Hg
Potential of dropping Hg el
ectrode
Factors that affect
half-wave potential
Composition of solution
Temperature
Type and concentration of
supporting electrolyte
Temperature
Forming complex
Acidic of solution
Question
Why a reference electrode with large area and a
dropping mercury electrode with very small area are
used to electrolyze in polarographic analysis ?
Why large amount of supporting electrolyte is
added to sample solution?
Why does nitrogen gas pass through the solution
before electrolysis ?
In the process of polarographic analysis whether
or not to carry out stirring the solution? Why?
4. Polarographically quantitative analytical
methods
（id）avg = K·c
●Direct comparison method
●Calibration curve method
●Standard addition method
5. Applications
Fundamental studies
Inorganic applications
Organic applications
Applications in pharmaceutical and biochem fields