1 Application range
This standard specifies the method of measuring chemical oxygen demand in water.
This standard applies to all types of water samples with a COD value greater than 30 mg/L. The upper limit for the determination of undiluted water samples is 700 mg/L. Exceeds water dilution determination.
This standard does not apply to brines containing chloride concentrations greater than 1000 mg/L (after dilution).
This standard applies to all types of water samples with a COD value greater than 30 mg/L. The upper limit for the determination of undiluted water samples is 700 mg/L. Exceeds water dilution determination.
This standard does not apply to brines containing chloride concentrations greater than 1000 mg/L (after dilution).
2 Definitions
Under certain conditions, the concentration of oxygen corresponding to the potassium dichromate consumed by the dissolved substances in the water sample and the suspended solids during the oxidation treatment with potassium dichromate.
3 Principle
A known amount of potassium dichromate solution was added to the water sample and silver salt was used as a catalyst in a strong acid medium. After boiling back, the test sample was tested with ferrous ammonium sulfate to titrate the water sample. The reduced potassium dichromate has the concentration of ferrous ammonium sulfate that is a favorite of Western Europe and is converted into a consumed oxygen concentration.
In acidic potassium dichromate, aromatics and pyridine are difficult to be oxidized, and the oxidation rate is low. Under the catalytic action of sulfuric acid, linear aliphatic compounds can be effectively oxidized.
In acidic potassium dichromate, aromatics and pyridine are difficult to be oxidized, and the oxidation rate is low. Under the catalytic action of sulfuric acid, linear aliphatic compounds can be effectively oxidized.
4 Reagents
Unless otherwise specified, the reagents used in the experiment were all analytically pure reagents that met the national standards. The test water was distilled water or water of equivalent purity.
4.1 Silver sulfate (Ag2SO4), chemically pure.
4.2 Mercury sulfate (Hg SO4), chemically pure.
4.3 Sulfuric acid (H2SO4), p=1.84 g/Ml.
4.4 Silver Sulfate-Sulfuric Acid Reagent: Add 10 g of silver sulfate (4.1) to 1 L of sulfuric acid (4.3), place and dissolve 1-2 angels, and mix well. Shake carefully before use.
4.5 Potassium dichromate standard solution:
4.5.1 Potassium dichromate standard solution with a concentration of C(1/6K2Cr2O7)=0.250mol/L: Dissolve 12.258g potassium dichromate after drying at 105°C for 2h in water and dilute to 1000mL.
4.5.2 Potassium dichromate standard solution with a concentration of C (1/6K2Cr2O7) = 0.0250 mol/L: Dilute the 4.5.1 solution 10 times.
4.6 Ammonium ferrous sulfate standard titration solution
4.6.1 Standard titration solution of ammonium ferrous sulfate with concentration of C[(NH4)2Fe(SO4)2·6H2O]≈0.10mol/L: dissolve 39g of ammonium ferrous sulfate [(NH4)2Fe(SO4)2·6H2O] In water, 20 ml of sulfuric acid (4.3) was added, and after the solution was cooled, it was diluted to 1000 ml.
4.6.2 Prior to daily use, the concentration of this solution (4.6.1) must be accurately calibrated with a potassium dichromate standard solution (4.5.1).
Take 10.00 mL potassium dichromate standard solution (4.5.1) in a conical flask, dilute it with water to about 100 mL, add 30 mL of sulfuric acid (4.3), mix, cool, add 3 drops (about 0.15m L) The tester Tie Ling Ling indicator (4.7) uses ferrous ammonium sulfate (4.6.1) to titrate the color of the solution from yellow to blue-green to red-brown, which is the end point. Record the consumption of ammonium ferrous sulfate (mL).
4.6.3 Calculation of concentration of ferrous ammonium sulfate standard titration solution:
10.00*0.250 2.50
C[(NH4)2Fe(SO4)2·6H2O]=
V V
In the formula: V—milliliters of ferrous ammonium sulfate solution consumed for titration.
4.6.4 Standard titration solution of ammonium ferrous sulfate with concentration of C[(NH4)2Fe(SO4)2·6H2O]≈0.010 mol/L: Dilute the 4.6.1 solution 10 times with potassium dichromate standard solution. (4.5.2) Calibration. The titration procedures and concentration calculations are the same as 4.6.2 and 4.6.3, respectively.
4.7 Potassium hydrogen phthalate standard solution, C (KCr6H5O4) = 2.0824m mol/L: Weigh 0.4251 g of potassium hydrogen phthalate (HOOCC6H4COOK) dried at 105°C for 2 hours, dissolve in water, and dilute to 1000Ml. uniform. With potassium dichromate as the oxidizing agent, the COD value of the complete oxidation of potassium hydrogen phthalate is 1.176g oxygen/g (referring to 1g oxygen consumed by 1g potassium hydrogen phthalate 1.176g), so the theoretical COD value of the standard solution is 500mg /L.
4.8 1,10-phenathroline monohy drate indicator solution: dissolve 0.7g of ferrous sulfate heptahydrate (FeSO4.7H2O) in 50mL of water, add 1.5g of 1,10-phenanthroline, stirring To dissolve, add water to dilute to 100mL.
4.9 Explosion-proof boiling glass beads.
5 Instruments
Common laboratory instruments and the following instruments.
5.1 Return Device: A full glass backflow device with a 250 mL Erlenmeyer flask with a standard 24 gauge orifice. The length of the reflux condenser is 300-500mm. If the sample volume is more than 30mL, use a full glass reflux device with a 500mL conical flask. (3 bubble glass burr return tube, plus the double role of the cooling water in the partial ball return tube and the fan in the machine, to ensure the sample's reflux cooling)
5.2 heating device. (YHCOD-100 type COD automatic digestion reflux meter)
5.3 25mL or 50mL acid burette.
6 Sampling and samples
6.1 Sampling
Water samples should be collected in glass bottles and should be analyzed as soon as possible. If it cannot be analyzed immediately, add sulfuric acid (4.3) to pH<2 and store at 4°C. However, the preservation time is not more than 5 days. The volume of water sample collected must not be less than 100 mL.
6.2 Preparation of samples
The sample was shaken well and 20.0 mL was taken as a sample.
7 steps
7.1 For water samples with a COD value of less than 50 mg/L, oxidize with a low concentration of potassium dichromate standard solution (4.5.2). After heating under reflux, use a low concentration of ammonium ferrous sulfate standard solution (4.6.4). drop.
7.2 The maximum limit for the determination of undiluted water samples is 700 mg/L. When this limit is exceeded, it must be determined after dilution.
7.3 For heavily polluted water samples, select 1/10 of the required volume of sample and 1/10 of the reagent into a 10*150 mm hard glass tube. Shake well and heat with alcohol lamp for several minutes. Observe whether the solution turns blue-green. If it is blue-green, take a small amount of sample and repeat the above test until the solution is not blue-green. In order to determine the appropriate dilution of water samples to be tested.
7.4 Take the sample (6.2) in a conical flask, or take an appropriate amount of water to 20.0Ml.
7.5 blank test: According to the same procedure, replace the sample with 20.0mL blank test, the rest of the reagents and sample determination (7.8) the same, record blank titration when the consumption of milliliters of ammonium ferrous sulfate standard solution ml V1.
7.6 Verification test: The COD value of the 20.0m L potassium hydrogen phthalate standard solution (4.7) was analyzed according to the method provided in the measurement sample (7.8) to check the operation technique and reagent purity.
The theoretical COD value of this solution is 500mg/L. If the result of the calibration test is greater than 96% of the value, the experimental procedure is considered to be basically appropriate. Otherwise, the cause of the failure must be found, and the experiment must be repeated to meet the requirements. .
7.7 De-interference tests: Inorganic reducing substances such as nitrites, sulphides and divalent iron salts will increase the results and it is acceptable to use their oxygen content as part of the COD value of the water sample.
The main interferent in this experiment is chloride, which can be partially removed by the addition of mercury sulfate (4.2). After refluxing, the chloride ion can be combined with mercury sulfate to form a soluble chloromercury complex.
When the chloride ion content exceeds 1000 mg/L, the minimum allowable value of COD is 250 mg/L, and accuracy below this value is not reliable.
7.8 Determination of water sample: In the sample (7.4), add 10.0mL of potassium dichromate standard solution (4.5.1) and several explosion-proof glass beads (4.9), and shake.
Connect the Erlenmeyer flask to the lower end of the condenser (5.1) condenser and switch on the condensate. Slowly add 30mL of silver sulfate-sulfuric acid reagent 4.4) from the upper end of the condenser to prevent the escape of low-boiling organics, and continuously swirl the conical flask to make it evenly mixed. Boil from the start of the solution and reflux for two hours.
After cooling, flush the condenser tube with 20-30 mL of water from the upper end of the condenser tube, remove the Erlenmeyer flask, and dilute it with water to about 140 mL.
After the solution was cooled to room temperature, 3 drops of 1,10-phenanthroline indicator solution (4.8) was added and titrated with ammonium ferrous sulfate standard titration solution (4.6). The color of the solution was changed from yellow to blue-green to red-brown. end. Record the milliliters of consumption V2 of the ammonium ferrous sulfate standard titration solution.
7.9 In special circumstances, the sample to be measured shall be between 10.0mL and 50.0mL, and the volume or weight of the reagent shall be adjusted according to Table 1.
Table 1 The amount of reagent used for different sampling amounts
Sample volume
mL
0.250NK2Cr2O7
mL
Ag2SO4-H2SO4
mL
HgSO4
g
(NH4)2Fe(SO4)2
·6H2O
Mol/L
Pre-titration volume
mL
10.0
20.0
30.0
40.0
50.0
5.0
10.0
15.0
20.0
25.0
15
30
45
60
75
0.2
0.4
0.6
0.8
1.0
0.05
0.10
0.15
0.20
0.25
70
140
210
200
350
8 Representation of results
8.1 Calculation Methods
The chemical oxygen demand of water samples in mg/L is calculated as follows:
C(V1-V2)*8000
COD(mg/L)=
V0
Where: C - concentration of ammonium ferrous sulfate standard titration solution (4.6), mol / L;
V1 - volume of spent ferrous ammonium sulfate standard titration solution consumed in blank test (7.4), mL;
V2 - The volume of consumed ferrous ammonium sulfate standard titration solution consumed in the sample test (7.8), mL;
V0 - the volume of the sample, mL;
The molar mass of 8000-1/4 O2 is converted in mg/L.
The measurement results generally retain three significant figures. For water samples with a small COD value (7.1), when the calculated COD value is less than 10 mg/L, it should be expressed as “COD<10 mg/Lâ€.
The molar mass of 8000-1/4 O2 is converted in mg/L.
The measurement results generally retain three significant figures. For water samples with a small COD value (7.1), when the calculated COD value is less than 10 mg/L, it should be expressed as “COD<10 mg/Lâ€.
8.2 Precision
8.2.1 Precision of standard solution determination
40 different laboratories determined the potassium hydrogen phthalate (4.7) standard solution with a COD value of 500 mg/L, with a standard deviation of 20 mg/L and a relative standard deviation of 4.0%.
8.2.2 Determination of Industrial Wastewater (see Table 2)
Table 2 Precision of COD Determination in Industrial Wastewater
Industrial wastewater type
Number of labs participating in verification
COD mean, mg/L
Relative standard deviation in laboratory, %
Relative standard deviations between laboratories, %
Total relative standard deviations between laboratories, %
Organic waste water
Petrochemical wastewater
Dye Wastewater
Dyeing Wastewater
Pharmaceutical Wastewater
Leather waste water
5
8
6
8
6
9
70.1
398
603
284
517
691
3.0
1.8
0.7
1.3
0.9
1.5
8.0
3.8
2.3
1.8
3.2
3.0
8.5
4.2
2.4
2.3
3.3
3.4