Chemical Oxygen Demand

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Principle

Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate [math]\displaystyle{ K_2Cr_2O_7 }[/math]. After digestion, the remaining unreduced [math]\displaystyle{ K_2Cr_2O_7 }[/math] is titrated with ferrous ammonium sulfate to determine the amount of [math]\displaystyle{ K_2Cr_2O_7 }[/math] consumed and the oxidizable organic matter is calculated in terms of oxygen equivalent.

Safety Precautions

Make sure you are wearing a lab coat, latex gloves, and eye protection since you are working with very hazardous chemicals.

Apparatus

1. Digestion vessels: Preferably use borosilicate culture tubes, 20×150mm, with TFE-lined screw caps.
2. Oven : Use an oven for culture-tube digestion only when it has been determined that 2 hours of exposure at 150°C will not damage the caps. Whenever you digest sample with potassium dichromate [math]\displaystyle{ (K_2Cr_2O_7) }[/math], make sure you turn on the hood to prevent hazardous chemicals from leaking.

Reagents

1. Standard potassium dichromate digestion solution, 0.01667 M: Add to about 500 mL distilled water 4.903 g [math]\displaystyle{ K_2Cr_2O_7 }[/math] primary standard grade, previously dried at 150°C for 2 hours, 167 mL concentrated [math]\displaystyle{ H_2SO_4 }[/math], and 33.3 g [math]\displaystyle{ HgSO_4 }[/math]. Dissolve, cool to room temperature, and dilute to 1000 mL.
2. Sulfuric acid solution: Add [math]\displaystyle{ Ag_2SO_4 }[/math], reagent or technical grade, crystals or powder, to concentrated [math]\displaystyle{ H_2SO_4 }[/math], at the rate of 5.5 g [math]\displaystyle{ Ag_2SO_4 }[/math]/kg [math]\displaystyle{ H_2SO_4 }[/math] or 9.7152 g [math]\displaystyle{ Ag_2SO_4 }[/math]/1L [math]\displaystyle{ H_2SO_4 }[/math]. Let stand 1 to 2 days to dissolve [math]\displaystyle{ Ag_2SO_4 }[/math].
3. Ferroin indicator solution: Dissolve 1.485 g 1,10-phenan-throline monohydrate and 695 mg [math]\displaystyle{ FeSO_4 }[/math]•7[math]\displaystyle{ H_2O }[/math] in distilled water and dilute to 100 mL. This indicator solution may be purchased already prepared.
4. Standard ferrous ammonium sulfate titrant (FAS), approximately 0.10M: Dissolve 39.2 g [math]\displaystyle{ Fe(NH_4)_2(SO_4)_2•6H_2O }[/math] in distilled water. Add 20 mL concentrated [math]\displaystyle{ H_2SO_4 }[/math], cool and dilute to 1000 mL. Standardize solution daily against standard [math]\displaystyle{ K_2Cr_2O_7 }[/math] digestion solution as follows:

• Pipet 3 or 6 mL digestion solution into a small beaker. Add 5 or 10 mL distilled water to substitute for sample. Cool to room temperature. Add 1 to 2 drops ferroin indicator and titrate with FAS titrant.
• Molarity of FAS solution=(Volume 0.0167M [math]\displaystyle{ K_2Cr_2O_7 }[/math] solution titrated, mL)/(Volume FAS used in titration, mL) × 0.1

Sample Handling

The samples should have been ground in the grinder and it is recommended that they be ground twice.

Procedure

1. Weigh out samples of between 1.2-1.7 mg using a very precise balance found in room 311 of the National Soil Tilth Laboratory. The steps to operate balance are at the end of this manual.
2. After the tubes and their contents have been labeled/marked, recorded and brought back to the Waste lab you can add the reagents. Using the Pipetman add 5 mL of distilled water to the tubes; try to rinse the interior wall of the tube with the distilled water. Gently shake the tube.
3. Change the tip of the Pipetman and add 3 mL of [math]\displaystyle{ K_2Cr_2O_7 }[/math] solution to the tube. (Always place Pipetman so that the tip is below the top, turn the knob and watch the dial to change the amount of liquid delivered. The red number on the dial indicates mL)
4. Add 7 mL of sulfuric acid reagent to the tube with a regular pipet.
5. Close caps and shake the tube. Don’t shake too vigorously or at yourself.
6. Repeat the above procedure with the 2-3 blank tubes; only they don’t contain any ground material.
7. Place the tubes in oven at 150°C for 2 hours.Caution: These sealed tubes may be under pressure from gases generated during digestion. Wear face and hand protection when handling. If sulfuric acid is omitted or reduced in concentration, very high and dangerous pressures will be generated at 150°C.
8. After 2 hours take the tubes out of the oven and allow them to cool to room temperature. If there are time constraints you can let the tubes sit overnight in their rack.
9. Add a magnetic stirrer to the tube and place it in a holder above the Fisher Flexa-mixer. The Flexa-mixer should also be in a holder so that the mixer and tube will reach a height that the tube will be just below the nozzle of the digital buret. The digit buret is placed on a container of the ferrous ammonium sulfate titrant (FAS).
10. Add 2 drops of Ferroin indicator to the tube. Turn on the stirrer and get a strong swirl speed.
11. First titrate a blank tube. As you add the FAS the color of the liquid in the tube will change from green to blue to orange/red, when the liquid makes the sharp color change to orange/red stop adding FAS and record. Try to titrate all samples until they reach the same color as the first blank did. Caution: The titrated liquid is still very dangerous and can cause damage if the liquid splashes on you. Dispose of the titrated liquid in a special container
12. Catch the stirrer with the long magnet. Use distilled water to rinse off the stirrer while you are holding it in the special container with the long magnet.

Clean up

1. Place a beaker below the nozzle of the digital buret.
2. Wash and scrub the 25mL Pyrex tubes with tap water and rinse with distilled water. Place the rinsed tubes on a rack and place them into an oven.

Calculations

COD (mg/mg) = {8*(a-b)*M/c}

a: mL [math]\displaystyle{ Fe(NH_4)_2(SO_4)_2 }[/math] used for blank

b: mL [math]\displaystyle{ Fe(NH_4)_2(SO_4)_2 }[/math] used for sample

M: molarity of [math]\displaystyle{ Fe(NH_4)_2(SO_4)_2 }[/math]=(Volume 0.01667M [math]\displaystyle{ K_2Cr_2O_7 }[/math] solution titrated, mL)/(Volume FAS used in titration, mL) * 0.1

c: mg of dry sample

Operating Procedure of the Balance

Apparatus

1. Small brush and a tin foil vessel

Operating procedure

1. Calibrate the balance if it has been 2 weeks since the last calibration. The calibration weight is in the top drawer, to the right of the balance, be careful handling the calibration weight so you don’t dent it or touch it with skin. Always have the BREAK ON when you are changing objects being weighed. The balance is very sensitive and the interior wires can break very easily.
2. After calibration, put BREAK ON and place tin foil vessel on the left pan with tweezers.
3. Put BREAK OFF and tare.
4. Put BREAK ON and open the window, carefully take the vessel out with tweezers. Place the vessel on the small iron plate.
5. Put sample in the tin foil vessel. Use the small spatula to transport a small sample into the tin foil vessel; avoid placing the larger particles of the sample onto the vessel.
6. Put BREAK ON and open the window, carefully place sample and vessel unto the center of the left pan with a tweezers DO NOT HIT THE WIRES. Slide window down and take the BREAK OFF. Record the weight (1.2-1.7 mg) after the readout has stabilized.
7. Put BREAK ON, open the window and carefully take the sample and vessel out. Brush the vessel’s contents into a 25 mL Pyrex tube; be careful to get the entire sample into the tube.
8. Place the vessel back into the balance to be reweighed and close the window; take the BREAK OFF. Subtract the vessel weight from the total weight in order to get the actual sample weight delivered.
9. Tare the balance and repeat the steps as many times as necessary.

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