K0016K: Determination of Biological Oxygen Demand

Determination of

Biological Oxygen Demand (BOD)

 Laboration

K0016K Chemical principles

2015-12-10 

Summary

This laboration was performed to measure the contamination of a water sample picked from the canal of lulsund. By using the oxygen’s ability to oxidize manganese(II)ions, a BOD (Biological Oxygen Demand) could be established for the sample. The measured BOD was subsequently compared with a table that indicated the approval stages for water quality which clarified the sample either polluted or clear. From several laborational stages, the result appeared as a failure due to the extra adding of oxygen before the actual test begun. Apparently the BOD showed to be a negative value, close to zero due to the extra oxygen in the sample.

The BOD value measured for the samples is calculated to: -0, 54 mg/dm3, respectively -0, 15 for the two samples. Compared to the table of approval values for water contamination this appears to be unrealistic.

Table of Contents

1 Background

2 Introduction & theory

3 Materials & method

   3.1 Sample titration

   3.2 Iodometric titraion

4 Calculations

5 Results and discussion

References

1 Background

BOD or Biochemical Oxygen Demand is a widely used term of measure the organic quality of water. Although the term “BOD” is not a precise quantitative term, the measurement makes a simple way to determine the organically quality for a water sample. BOD indicates how much oxygen is needed for the microbiological decomposition of the organic material in water. The value of BOD can show how contaminated a water sample is. This contamination could be faecal matter or dissolved organic carbon derived from non-human and animal sources. A high BOD result can lead to complications such as poor eco-system health or human ill health (Penn et al. 2003).

2 Introduction & theory

The aim of this laboration is to determine the concentration of oxygen O2 in a water sample. Table 1 shows (BOD) was determined by setting up several chemical reactions described below to easily decide the amount of oxygen in the sample.

Table 1: Guiding lines for quality of water.

BOD
0 - 1 mg/dm3	Uncontaminated fresh water.
1 – 3 mg/dm3	Relatively uncontaminated water.
3 – 5 mg/dm3	Doubtful quality.
> 5 mg/dm3	Clearly contaminated.

When air is bubbled through water, oxygen will dissolve to a saturation level. The equilibrium will be attained.

To determine the content of we utilize oxygen’s ability to oxidize manganese(II)ions. A with precipitation is formed in the absence of dissolved oxygen by mix the manganese(II) ions with an alkaline iodide solution such as hydroxide or Iodideand hence a white precipitation is formed.

A corresponding fraction of the Mn(II)ions will be oxidized at the addition ofand instead a brownish precipitation is formed out of the white manganese(IV)oxyhydroxide.

The precipitates are dissolved by adding phosphoric acid (H3PO4). The iodide ions are then oxidized simultaneously with the dissolution of theprecipitate, and a dark yellow solution is formed.

The quantitative determination is performed by iodometric titration with a thiosulfate solution, S2O3 2-, using starch solution as a reaction indicator.

3 Materials & method

The materials used in the laboration:

• Two Winkler bottles
• Two 250 ml Erlenmeyer bottles
• One optional pipette
• Two transfer pipettes minimum 1 ml
• One minimum 5 ml pipette
• One Peleus ball
• Two 5 ml volumetric pipettes
• One 50 ml burette
• One funnel
• Two stirring magnets
• A magnetic stirrer
• A scale

Chemicals and solutions used:

• Alkaline potassium iodide solution
• Sodium thiosulfate solution: 0,0100 M
• Manganese(II)chloride solution:
• Phosphoric acid:
• Starch solution: 0.2%
• Sampled water made by laboration assistant

Protective gear:

• Lab coat
• Safety glasses
• Latex gloves

When performing a laboration it is important to prepare all the materials beforehand to ensure that the working process will be as smooth as possible. Considering that different chemicals and solutions are being used in this laboration, both latex gloves and safety glasses are needed in addition to the lab coat. The sampled water had been diluted 10 times by the lab assistant which had then been standing at room temperature for 4 days.

The execution of the laboration can be broken in to two main parts: sample preparation and iodometric titration. The safety glasses and gloves should be used in the former part.

3.1 Sample titration

Start by marking the two Winkler bottles with a whiteboard marker to help keep them apart. Proceed by weighing the bottles together with their plugs. Fill the bottles up carefully with the sampled water with a pipette of your choice. In this step it is important to minimize any additional oxygen that could be added to the water. Put the plugs back on so that there won’t be any air bubbles left in the bottles.

Remove the plugs once again and add 1 ml of the iodide solution to the bottom of each bottle with a transfer pipette. Take the other transfer pipette and add 1 ml manganese(II)chloride solution to the upper part of each bottle. Wipe the bottle dry if some of the content leaks out when the plugs are put back on. To mix the chemicals, hold the plugs down and turn the bottles upside down at least 20 times. This is to assure that the content will get thoroughly mixed. Let it settle for 5 to 10 minutes. During this waiting time, start preparing the titration by filling the burette up with the sodium thiosulfate solution using the funnel up to the 0.00 ml mark.

When the precipitate has settled, remove 5 ml of the clear solution from each bottle with a pipette as shown in image 1. Add 5 ml phosphoric acid to each bottle and mix them until it becomes a homogeneous mixture.

Figure 1: Removing 5 ml of the clear solution with a pipette.

3.2 Iodometric titration

If the previous steps were followed, the burette should be filled with the sodium thiosulfate solution. Pour over the solutions from the Winkler bottles to the Erlenmayer bottles and place a magnet in each. Set the magnetic stirrer under the burette and place one of the E-bottles on top of it.

Turn on the stirrer and add thiosulfate solution until the sample get a light yellow color as seen in image 3. When the sample has taken its light yellowish color, start adding a dozen of drips of starch reaction solution to make the color violet as seen in image 4. This is just too easily decide how much more of the remaining thiosulfate solution should be added to the sample. When the sample has fulfilled its required violet color, start adding thiosulfate solution again until the solution gets completely colorless as seen in picture 5. Now all the remaining Iodide ions has react with the corresponding thiosulfate ions.  Repeat the titration with the remaining E-bottle.

Figure 2: After adding and mixing in the phosphoric acid.

Figure 3: Comparison of the titrated and untitrated solutions.

Figure 4: The color of the solution after adding drips of starch.

Figure 5: Colorless solution after continuing the titration.

4 Calculations

(1) How many moles of sodium thiosulfate was used in each sample?

The number of moles that were used during the titration can be calculated using the formula  , where C is the concentration and V the volume of sodium thiosulfate.

(2) How much water was used before starting the titration?

To calculate the weight of water used in the experiment, we took and calculated the mass difference before and after filling the wrinkle bottles. For this we needed to weigh the bottles while empty and then again after we’d fill them with the water.

Now that the mass of the water is known we can use it together with the density acquired from a table sheet to put up a formula calculating the volume;

(3) How many moles oxygen were present during the titration?

From the equation    we calculated how many moles of oxygen where in the samples. This was possible because the number of moles sodium thiosulfate was known.

(4) What was the concentration of the oxygen before and after the experiment?

For the initial concentration of oxygen we used the formula given by our experiment supervisor,. Where,is the solubility of oxygen at the air pressureinand S is the solubility of oxygen at.

The final concentration of oxygen could be calculated troughwhere the mass,of oxygen was calculated using the mole acquired earlier multiplied with the molar mass of oxygenacquired from the chemical data sheet.

5 Results and discussion

Number of moles sodium thiosulfate used in each titration.

 

Mass, volume and moles of the water used.

   

The table values used when acquiring the volumes whereas follow;which is the density of pure water at the pressure of 1 atm (atmosphere).

The amount of oxygenatoms in the separated samples were;

For (4),  Where the solubility (S) of the oxygen wasat a temperature of 19°C, and the air pressure was given from our supervisor at a value of 102, 4 kPa.

The molar mass for one oxygen atom is approximatelyso for the oxygen in its molecular form we doubled this value according to the formula .

And got that the molar mass for oxygen is

Here on we calculate the mass of the total number of oxygen in each sample.

For the final concentration of oxygen in each sample we reached the values;

Comparing the theoretical value for concentration  with our experimented value C:

The result shows that the test is a failure, the reason of the negative value is that oxygen was added during the resumption of the sample in the Peleus ball. the following actions resulted in a wrongly collected chemical data, and the final calculation stage resulted in a higher concentration of oxygen before the test than after even though the Manganese(II)chloride and thiosulfate ions has reacted with the oxygen and hence decrees the concentration of oxygen.

References

Penn, M., Pauer, J., Mihelcic, J. (2003). Biochemical oxygen demand. Encyclopedia of Life Support Systems. Available from: http://www.eolss.net/sample-chapters/c06/e6-13-04-03.pdf (13 October 2015)