COD的测定---快速消解法(英文版)

ExperimentⅩthe determination of chemical
oxygen demand
——the Rapid Digested Spectrophotometry method
The purpose and requirements of the experiment
1.
Learn the Rapid Digested Spectrophotometryto determinateMethodCOD.
2.
Compare the advantages and disadvantage of Potassium Dichromate Method
withSpectrophotometry.
3.
Learn why and how to eliminate the influence of Chloridion.
4.
Know whether shake up the solution in different circumstances when we use
spectrophotometry to determinate samples.
Experimental program
1.
Experimental instruments and drugs
Digester device
12 digester tubes, pipette
Spectrophotometer
Digestive juice II
Screening agent
Catalyst
2.
Experimental steps
The preparation of sample wastewater: take 3 parts 3mL sewage, 2 parts vacant
sample water and 2 parts 3mL, 2 parts 5mL, 10mL 20mL 40mL respectively 1 part
standard solution into 12 digester tubes, then add 1mL screening agent3mL
digestive juice II and 5mL catalyst into it, finally screw their lids andshake up.
Digester and Determining: put tubes into digester device and setDigester
device’s temperature in 160 and time in 25 min. when time is up, let tubes in
digester for cooling 15 minutes, the next let it cool in air for 10min. after add
respectively 3mL clean water into 12 digester the wavelength of
Spectrophotometer in 600nm. The first determine 4 dishes difference and mark

which tube is put into which dish to determine, then determine each tubes and
writ every data down table. Let the Abs be the ordinate and the concentration of
standard solution be the abscissa and draw up the graph. Through the graph,
calculate the COD.
Experimental result and data handling
The Abs and the standard solution data

A
1


volume
standard
solution（ml）
Concentration
mgL
Determination
Abs
Actual Abs
A
2
A
3
A
4
A
5
A
6
A
7

3ml
30
3ml 5ml
50
5ml 10ml
100
20ml
200
0.161
0.151
0.151
40ml
400
0.173
0.174
0.174
0.113 0.114 0.155 0.157 0.127
0.114 0.109 0.145 0.158 0.122
0.1115 0.1515 0.122

From the table we can draw relation between Abs and the solution.

0. 2
0.18
0.16
0.14
A
b
s

1< br>0.12
0.1
0.08
0.06
0.04
0.02
0
250
y = 0.000x + 0.121
R? = 0.638
the concentration standard solution(mgL)

Fig. 1 Abs-the concentration of standard solution
According to fig. 1 and the following formula we can calculate the COD in the
sewage.
COD
(
O
2
)?
A< br>?
F
?
K
(
mg

L
)

Notes: A refers to the absorbance of sample sewage.
F refers todilution multiple.
K refers to reciprocal of curve slope
The data of detainment and calculate about sewage
Sample sewage
Determination
Abs
Actual Abs
COD

S1
0.183
0.178
567
S2
0.165
0.155
337
S3
0.16
0.161
397
S01
0.094
0.089
-323
S02
0.237
0.227
1057
Conclusion
1. Data reliability analysis
2. Data reliability analysis
From the formula we can see R
2
which mean the requirement quite well
R
2
?0.6383?0.995

The average of the concentration
x?
1
n
i
?1
?
x
i
?
n
n
1
(567?337?397)?43 4

3
The standard deviation of the concentration
σ?
?
(
x
i
i?1
?
x
)
2
?
n
?1
s
17689?94 09?1369
?119

2
The relative standard deviation of data
CV
1
?
x
?100%?< br>119
?100%?27.51%

434
From the data, we can see that our R
2
is severely lower standard data which
mean that the standard solution data have big error when we digested and
determinated. Similarlythe relative standard deviation (CV
1
=27.51%) is very bad
which mean the data have a lot of fluctuation. It is obvious that the first data is
very higher than other data. When we remove it, CV
1
become to 11.56%, but it is
still big than credible my opinion, the experimental error mainly comes
from the phase of digested, when we opened the lids, we found the solution in

tube have been reduced and the reduced value is different. Hereby, after we
added 3mL clean water, every tube’s concentrationis also different. I think that
we have mistaken the meaning of adding 3mLwater which should be adding
clean water to 15ml. We should pay attention to the attitude about experiment
steps.
3. Some standard specifications of total chromium and
hexavalent chromium in GB
COD emission limit in
(GB 8978-1996)
scope of application
beet sugar production, synthetic
fatty acid, wet-progress
fiberboard, dye, scouring,
organophosphorus pesticide
industries
monosodium glutamate, alcohol,
Chemical
Raw Medicine,
oxygen
Biopharmaceutical ramie
demand(COD)
degumming, leather, Purified
Pulp industries
petrochemical industry
Urban secondary wastewater
treatment plants
Units; mgL Ⅰ Ⅱ Ⅲ
100 200 1000
100
60
60
300
120
120
150
1000
500
other pollutant discharging unit 100
Notes: COD has been classified for the second pollutants
500
The standard limited value of chemical oxygen demand (COD) in
(GB3838-2002)

Units; mgL
COD ≤

From the standard, we can the sample water have exceeded standard of surface
water more than 10 times, if compare with integrated wastewater discharge
I
15
II
15
III
20
IV
30
V
40

standard it is accord Ⅲ standard. But compare withthe last report in Potassium
dichromate method, the value have exceeded nearly 3 times. From the words of
teacher, we know the method is hardly to determinate accurately. Although the
method is quickly, errors are easy appearing.

4. Treatment technologies
Physical method:the methods mainly wipe off the big organic that will cost
oxygen in physical methodincluding filtering, precipitation, centrifugal
separation.
Chemicalmethod: we can add some Chemical substances in wastewater to react
with the organic pollutants substance that cost oxygen, such as Oxidation
reduction, Chemical Precipitation
Biological treatment: by microbial metabolism, we transform the organic
pollutants and microbial inorganic nutrients into stable, harmless substance, like
Activated Sludge Process, Biological Membrane method, Wetland Treatment.
Questions and discussion
1. Look up some material to explain the measuring methods of COD and their
advantages and disadvantages of.
Answer:from materials we can found these methods: Potassium Dichromate
Standard Method, Potassium Permanganate Standard Solution, and Microwave
Digester Method.
Potassium Dichromate Standard Method and Potassium Permanganate
Standard Solution are both accurate highly and smart applicability. But both methods
have to cost much time to heat reflux and they also produce new pollution, such as
mercury and silver.
Microwave Digester Method is faster than Potassium Dichromate Standard
Method and Potassium Permanganate Standard Solution, and it cost less time and
drugs, its operation is also easy. But it is hard to determinate with high accuracy.
2. Look up some material to explain how Chloride affects the determination of
COD, and how to overcome it.

Answer: Chloride in solution can consume catalyst and oxidant which is used to
react with sample water. In other word, if there are many Chlorides, it will pull up
the value of COD. We can add screening agent (mercury sulfate solution) into
solution to react with Chlorides before adding catalyst.
3. How to determinate COD in the water with low COD (20mgL) and high
concentration of Chloride (more than 60000mgL).
Because the concentration of Chloride is very high, it can’t be added screening
agent to cost so many Chlorides. But we can put silver pillar into sample water,
when Chloride get through the resin of silver pillar, it can react with the silver ion
in resin pillar, after reacted they will become Silver chloride precipitation. And
silver pillar can’t react with other ion except Chloride, so it will not affect the
constitution of sample water. By the method, we can only reduce concentration
of Chloride, we can determinate COD by ordinary methods.