Experiments for the determination of chemical oxygen demand in water samples

Summary

Chemical oxygen demand (Chemical oxygen demand, referred to as COD), refers to the amount of oxidant consumed when treating water samples with potassium dichromate as an oxidizing agent under the condition of strong acid and heating, expressed in mg/L of oxygen. COD reflects the extent to which a body of water is polluted by reducing substances. Reducing substances in water include organic matter, nitrite, ferrous salts, sulfides, etc. Water contaminated by organic matter is common, so the chemical oxygen demand is also used as one of the indicators of the relative content of organic matter, but can only reflect the organic pollution that can be oxidized, can not reflect the pollution status of polycyclic aromatic hydrocarbons, PCBs, dioxin class, and so on.

Operation method

Potassium dichromate reflux method

Principle

In the strong acidic solution accurately add excess potassium dichromate standard solution, heating reflux, the water samples of reducing substances (mainly organic) oxidation, excess potassium dichromate to test ferrous spirit as an indicator, with ferrous ammonium sulphate standard solution back to the drop, according to the amount of consumed ferrous ammonium sulphate standard solution to calculate the amount of oxygen consumed by the reducing substances of the water samples.

Materials and Instruments

Water Sample
Potassium dichromate o-phenanthroline Ferrous sulfate Ammonium ferrous sulfate Silver sulfate Mercury sulfate
All-glass reflux device Electric furnace Burette Pipette Conical flasks

Move

I. Instrumentation
1. 250 mL all-glass reflux device. For water samples of 30 mL or more, use a 500 mL all-glass reflux unit.
2. Electric stoves.
3. 25 mL acid burette.
4. Pipettes.
5. 250 mL conical flask.
II. Pharmaceuticals
Potassium dichromate, o-phenanthroline ( _C12H8N2-H2O ), ferrous sulfate ( _FeSO4-7H2O ), ammonium ferrous sulfate, sulfuric acid, silver sulfate, mercuric sulfate (crystalline or powder).
III. Reagents
1. Potassium dichromate standard solution ( _{C1/6K2Cr2O7=0}.2500 mol/L): weigh 12.258 g of benchmark or high-quality pure potassium dichromate dried at 120°C for 2h, dissolve in water, transfer to 1,000 mL volumetric flask, condense and shake well.
2. Test ferrous spirit indicator solution: weigh 1.485 g of o-phenanthroline, 0.695 g of ferrous sulfate ( _FeSO4-7H2O ) dissolved in water, diluted to 100 mL, stored in a brown bottle.
3. Ammonium ferrous sulphate standard solution (C≈0.1 mol/L): Weigh 39.5 g of ammonium ferrous sulphate ［ ₍ NH4)2Fe ₍ SO4 ₎ 2-6H2O］and dissolve it in water, add 20 mL of concentrated sulphuric acid slowly with stirring, then transfer it to 1 000 mL volumetric flask after cooling, and then condense it with water. Before use, calibrate with potassium dichromate standard solution.
Determination of ammonium ferrous sulfate standard solution: accurately absorb 10.00 mL of potassium dichromate standard solution (reagent 1) in a 250 mL conical flask, dilute with water to 110 mL or so, slowly add 30 mL of concentrated sulfuric acid, mix well. After cooling, add 3 drops of test ferrous spirit indicator solution (reagent 2), titrate with ammonium ferrous sulfate standard solution, the color of the solution from yellow by blue-greento reddish brown is the end point. Calculate the concentration of ammonium ferrous sulfate solution according to the following formula:
C = 0.250 x 10.00/VFormula:C - Concentration of ammonium ferrous sulfate standard solution, mol/L;V - dosage of ammonium ferrous sulfate standard solution, mL.
4. Sulfuric acid-silver sulfate solution: Add 5g of silver sulfate to 500 mL of concentrated sulfuric acid. Leave it for 1~2 days and shake it from time to time to dissolve it.
IV. Experimental steps
1. Take 20.00 mL of well-mixed water sample in a 250 mL reflux conical flask, accurately add 10.00 mL of potassium dichromate standard solution (reagent 1) and several small glass beads or zeolites, connect the reflux condenser tube with a mill mouth, and from the condenser tube, slowly add 30 mL of sulfuric acid-silver sulfate solution (reagent 4), and gently shake the conical flask to make the solution Mix well and heat to reflux for 2 hours (timed from the start of boiling).
2. After cooling, rinse the walls of the condenser tube with 90 mL of water and remove the conical flask. The total volume of solution should not be less than 140 mL, otherwise the titration endpoint will not be obvious due to too much acidity.
3. After the solution is cooled to room temperature again, add 3 drops of test ferrous spirit indicator solution, titrate with ammonium ferrous sulfate standard solution, the color of the solution from yellow to reddish-brown through the blue-green is the end point, record the amount of ammonium ferrous sulfate standard solution.
4. Blank experiment. Measurement of water samples at the same time, take 20 mL of redistilled water, according to the same procedure for the blank experiment. Record the amount of ammonium ferrous sulfate standard solution used in the titration.
(vii) Calculation of resultsCODcr ( _O2, mg/L) = (V0-V1 ) × C × 8 × 1 000/VEquation:C - Concentration of ammonium ferrous sulfate standard solution, mol/L;V0 - the amount of ammonium ferrous sulfate standard solution when titrating the blank solution, mL;V1 - the amount of ammonium ferrous sulfate standard solution when titrating water samples, mL;8 - molar mass of oxygen (1/2 _O2 ), g/mol.

Caveat

1. For high chemical oxygen demand of wastewater samples can first take the above operation required volume of 1/10 of the wastewater samples and reagents, in a hard glass test tube, shaking, heating and observation of whether to turn green. If the solution is green, and then reduce the amount of wastewater samples, until the solution does not change the green, so as to determine the volume of wastewater samples should be taken when analyzing. Dilution, the amount of wastewater samples taken shall not be less than 5 mL, if the chemical oxygen demand is very high, the wastewater samples should be diluted several times step by step.

2. for chemical oxygen demand less than 50 mg / L of water samples, should be changed to 0.025 mol / L potassium dichromate standard solution. Back drop with 0.01 mol / L ammonium ferrous sulfate standard solution.

3. After the water sample is heated and refluxed, the remaining amount of potassium dichromate in the solution should be 1/5～4/5 of the added amount.

4. When checking the quality of reagents and operating techniques with the standard solution of potassium hydrogen o-potassium dichromate, because the theoretical CODcr value of each 1.0 g of potassium hydrogen o-potassium dichromate is 1.176 g, so dissolve 0.4251 g of potassium hydrogen o-potassium dichromate (HOOCC).6 HCOOKHCOOK) in redistilled water, transferred to a 1,000 mL volumetric flask, and then calibrated to make a 500 mg/L CODcr standard solution. Freshly prepared when used.

5. each experiment, should be ammonium ferrous sulfate standard solution for calibration, room temperature is higher, especially should pay attention to its concentration changes.

6. acidic potassium dichromate oxidation is very strong, can oxidize most of the organic matter, add silver sulfate as catalyst, straight chain aliphatic compounds can be completely oxidized, while aromatic organic matter is not oxidized, pyridine is not oxidized, volatile straight chain aliphatic compounds, benzene and other organic matter exists in the vapour phase, can not be in contact with oxidizing liquid, the oxidation is not obvious. Chloride ions can be oxidized by potassium dichromate, and can produce precipitation with silver sulfate, affecting the results of the determination, so before refluxing to the water samples to add mercury sulfate, so that it becomes a complex to eliminate interference. Chloride ion content higher than 1 000 mg / L of the sample should be quantitatively diluted, and then measured.

Common Problems

I. Introduction

Chemical oxygen demand (Chemical oxygen demand, referred to as COD), refers to the strong acid and heating conditions, with potassium dichromate as an oxidizing agent when treating water samples consume the amount of oxidant, expressed in mg / L of oxygen. COD reflects the extent to which a body of water is polluted by reducing substances. Reducing substances in water include organic matter, nitrite, ferrous salts, sulfides, etc. Water contaminated by organic matter is common, so the chemical oxygen demand is also used as one of the indicators of the relative content of organic matter, but can only reflect the organic pollution that can be oxidized, can not reflect the pollution status of polycyclic aromatic hydrocarbons, PCBs, dioxin class, and so on.

Chemical oxygen demand of water samples, can be added due to the type and concentration of oxidizing agent, the acidity of the reaction solution, the reaction temperature and time, as well as the presence or absence of catalysts to obtain different results. Therefore, COD is also a conditional indicator and must be carried out in strict accordance with the operating procedures.

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