Dissolved Oxygen in water

International Baccalaureate Chemistry

Wasim Khouri Chemistry SL

Chemistry Coursework

By Wasim Khouri y12 Sl Chemistry

Contents

Design

Data Collection and Processing

Conclusion and Evaluation

Design

Research Question: How does the dissolved oxygen content vary at different stages of the sewage treatment plant in Istra?

Hypothesis: Dissolved oxygen in water is very important for many plants and animals so that they can undergo aerobic respiration. At a pressure of one atm and a temperature of 20 °C the maximum solubility in water is only about (9 mg l-1) and therefore, I believe that the dissolved oxygen in the water will not exceed 9 mg l-1 at all stages of the water treatment.

Before I predict what the results will be like, it is vital that you know from what stages of the sewage treatment did I take water samples. The samples are taken from four sections that will be named raw sewage-this is where the water is still very dirty and hasn’t yet been processed at all, air and bacteria tank- here bacteria and air (mainly oxygen) are added to the water, chlorination tank and oxygenation tank.

Oxygen gets into water by diffusion from the surrounding air, by aeration (rapid movement), and as a waste product of photosynthesis. As a result, I believe that raw sewage will have very low dissolved oxygen content due to the fact that it may have a lot of bacteria that will undergo aerobic respiration and that will use oxygen when they decompose. The water taken from the air and bacteria tank is likely to have much higher dissolved oxygen content because air (oxygen) is pumped into the water in order to increase its dissolved oxygen content. However, the bacteria that are added at the same time may be using some of that oxygen and thus will decrease the dissolved oxygen a little. In the chlorination tank, I believe that the dissolved oxygen content will be a little more than the air and bacteria tank because the chlorine will kill any microorganisms that are using the oxygen and hence the dissolved oxygen content will increase as it diffuses from the surrounding air. Finally, I believe that the oxygenation tank will have the highest dissolved oxygen content due to the sheer fact that oxygen is added to the water.

To carry out this experiment we will be using a modified Winkler method. Manganous ions react with oxygen in the presence of potassium hydroxide to form a manganese oxide precipitate (Step 1). An azide is present to prevent any nitrate ions from interfering with the test. On addition of acid, manganese oxide hydroxide oxidizes the iodide to iodine (Step 2). Since the amount of iodine generate is equivalent to the oxygen in the sample the concentration of iodine is calculated by titration of thiosulfate ions that reduce the iodine back to iodide ions.

Step 1: 2Mn2+ + O2 + 4OH- →2MnO(OH)2

Step 2: MnO(OH)2 + 2I- + 4H- → Mn2+ + I2 + 3H2O

Step 3: I2 + 2S2O32- → 2I- + S4O62-

Variables:

Independent variable: The independent variable in the experiment is the different water samples that we are taking i.e. the raw sewage, water from air and bacteria tank, water from the chlorination tank and water from the oxygenation tank.

Dependant variable: The dependant variable is the dissolved oxygen content because it will vary depending on the place from where we take the water sample.

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Dependant variable: The dependant variable is the dissolved oxygen content because it will vary depending on the place from where we take the water sample.

Controlled Variables: The controlled variables in the experiment would be the dissolved oxygen kit which means that we will be using the same glass bottle, same chemicals and the same syringe in order to keep it a fair test. We will control this variable simply by making sure that we use the same dissolved oxygen test kit. This also means that we will use the same volume of the sample each time because the glass bottle fits only a certain volume and this will be easily controlled due to the fact that the glass bottle must be totally filled.

Apparatus needed:

A bucket in which you can put your water samples. This may be an essential piece of equipment due to the fact that you will find it very hard to find the dissolved oxygen content near the sewage plant – there is no space.
Gloves- there are essential for your safety because you will have to put your hand in the dirt water due to the fact that just throwing a bottle in order to get a sample is useless as it will float.
Strings- you may want to tie a bottle to a long stick in order to obtain the samples: this is helpful when you can’t reach with your hand.
Bottles: you can use any really as long as they are very clean and dry but if you have specific water sampling bottles -that is your best option.
A dissolved oxygen test kit.
Clipboard, paper and pen to write down your results.

Method of experiment:

Go to the sewage plant, and from each tank take a water sample.
To take the water samples, place your hand in the water and wait until the bottle fills up. However, with the raw sewage you may encounter problems because it is a deep hole and therefore you will have to throw a bucket, collect water and then fill it into your sampling bottle.
ATTENTION. While collecting your water samples be very careful because it is very easy to trip and fall over, therefore if you have shoelaces or anything that may trip you over make sure that it is out of your way.
When you have collected the samples move away from the sewage plant and now look find the dissolved oxygen content of each sample using the Winkler method.
Step 1: Rinse the glass bottle 3 times with water sample and fill to overflow. Insert stopper and ensure that a small part of the sample spills over.
Step 2: Remove the stopper and add 5 drops each of Manganous Sulphate Solution and Alkali-Azide reagent. (Be very careful with the glass bottle as it is very weak and can shatter easily.)
Step 3: Add some more sample to fill the bottle completely. Carefully stopper the bottle again and ensure that a part of the sample spills over. This is to make sure that no air bubbles have been trapped inside, which would corrupt the reading.
Step 4: Invert several times the bottle. The sample becomes orange-yellow and a flocculent precipitate will form if oxygen is present.
Step 5: Let the sample stand and the flocculent precipitate will start to settle.
Step 6: After approximately 2 minutes, when the upper half of the bottle becomes limpid, add 10 drops of Sulphuric acid solution.
Step 7: Again stopper the bottle and invert it until all particulate material is dissolved. The sample is ready for measurement when it is yellow and completely limpid.
Step 8: Remove the cap from the plastic vessel. Rinse the plastic vessel with the solution in the bottle, fill it to the 5 mL mark and replace the cap. (Make absolutely sure that you are looking at it straight and that the bottom of the meniscus is on the 5 mL mark,)
Step 9: Add 1 drop of Starch Indicator through the cap port and mix by carefully swirling the vessel in tight circles. The solution will turn a violet to blue color.
Step 10: Push and twist pipette tip onto tapered end of syringe ensuring an air tight-fit. Take the titration syringe and push the plunger completely into the syringe. Insert tip into HI 3810-0 Titrant Solution and pull the plunger out until the lower edge of the plunger seal is on the 0 mL mark of the syringe.
Step 11: Place the syringe tip into the cap port of the plastic vessel and slowly add the titration solution dropwise, swirling to mix after each drop. Continue adding titration solution until the solution in the plastic vessel changes from blue to colorless.
Step 12: Read off the milliliters of titration solution from the syringe scale and multiply by 10 to obtain mg/L oxygen.
Take the other samples and repeat Step 1 to Step 12.
When you finish make sure that you dispose of the water into somewhere safe i.e. the toilet. Don’t spill it on the ground or grass or back into the sewage as this is regarded as non-scientific action.
Finally, and most importantly when you packed away all your equipment wash your hands very carefully with soap.

Method of Collecting Data:

We talked about how to carry out the experiment but we didn’t really say how to collect the data.

Take one sample from the raw sewage tank because it is hard to get more due to the fact that the area is very small.
Take three samples from each of the other tanks making sure that each sample comes from a different area of the tank.
When doing the titrations, carry out three titrations for each sample. This means that by the end of the day you will have made 30 titrations because you will have 10 samples.
Record all your data onto a table and find averages for the titrations first and then find averages for the whole tank. This is vital in order to get more accurate results, more relevant data which will then help you make a valid conclusion.

Data Collection and Processing

Data collection:

Table showing the dissolved oxygen content of different samples taken at different stages o f the sewage plant at Istra. (All readings to 2 decimal place)

Data processing:

We have obtained a good set of results but now it is important to process them. To make these results more meaningful we will obtain an average for the dissolved oxygen content in each tank. To find the average, let’s say in tank 2 we add up all the dissolved oxygen readings and divide them by 9.

E.g. Air + bacteria tank → (5.80+5.10+5.50+5.00+5.10+5.50+5.80+5.00+5.10)/9 = 5.32 mg l-1 (±0.15mg l-1)

Table showing the average dissolved oxygen in each tank. (All readings to 2 decimal place)

Data presentation:

Due to the fact that there is no direct relationship between the dissolved oxygen in the water and the tank it is taken from plotting a graph with a line of best fit is useless because it will not tell us anything really. However, what we could do is draw a bar chart in order to understand what’s happening more clearly.

This graph simply shows us that as the water passes through more treatment tanks i.e. it becomes cleaner and more drinkable, the dissolved oxygen in it increases.

Conclusion and Evaluation

Conclusion: In essence it is possible to say that my results partially support my prediction. It appears that indeed the raw sewage had by far the least dissolved oxygen in it and this is explained by the fact that the raw sewage can contain all sorts of bacteria that consume the oxygen. Furthermore, the results show that the chlorination tank has more dissolved oxygen than the air and bacteria tank which is what I predicted and again this can be explained by the fact that the chlorine kills any microorganisms that can use the oxygen. However, what did surprise me was the result of the oxygenation tank because it didn’t quite fit in with my prediction- yes it has more dissolved oxygen than the chlorination tank but only by 0.06 mg l-1 ±0.15 mg l-1. This is a very small difference and I personally imagined it to be much greater due to the fact that oxygen is literally added to the water.

Despite that, there were systematic and random errors that could have distorted our readings. In terms of systematic errors, we can say that the concentration of the chemicals that we used in the Winkler method may have been wrong but it isn’t so much of an error because we used the same kit all the way through. As a result, it may give different readings to a dissolved oxygen probe. However, we certainly encountered many random errors. First of all, the random error could have been when pouring our solution into the plastic vessel- there may have been inaccuracies due to the fact that the meniscus wasn’t always on the 5 mg/L mark. Furthermore, inaccuracies with readings could have occurred when performing a titration on the solution. All of the systematic and random errors could have distorted our readings.

Evaluation: Just like in any experiment, there were certainly limitations and weakness that didn’t allow us to get 100% valid data. First of all, we had a few limitations. Due to the lack of apparatus and time we couldn’t get water samples from the depth of each tank which would then allow us to calculate the average dissolved oxygen throughout the whole cross-section of the water. Furthermore, there wasn’t enough time to do more titrations in order to get more accurate results. Besides, from the first tank where we got raw sewage, we only got one sample. This reason being is that it is very hard to get a water sample because the area is very small, therefore we assume that in this small area the average dissolved oxygen will be enclosed.

In addition, there were weaknesses in the experiment. First of all, we assumed all the way through that the temperature of the water is the same all the time. This is obviously not true because the water was heated while we were trying to find the dissolved oxygen and while it was waiting to be processed. This is a real weakness because the temperature of the water affects the dissolved oxygen a lot: as the temperature increases the solubility of the water decrease. Furthermore, our second biggest problem was that we couldn’t control time. Ideally, the experiment would be done by taking a sample, immediately finding its dissolved oxygen content and then taking another sample and doing the same. However, because of lack of apparatus and more importantly time, I had to take half the samples, leave the plant find the dissolved oxygen content of those and then get the other half and do the same thing. Consequently, the dissolved oxygen content might have changed while the samples were just waiting to be processed.

Improvements: In order to realistically improve this experiment we would have to use a dissolved oxygen probe that can obtain the dissolved oxygen content immediately. That way we can get a sample and immediately find its dissolved oxygen content without exposing it to air and hence not allowing more oxygen to dissolve. Similarly, we can take readings from different sections of each tank including depth samples in order to get more accurate and viable results.

Another interesting aspect for further investigation would be the Biological Oxygen Demand (BOD) which is also a measure of the dissolved oxygen required to decompose the organic matter in water biologically. This would have to be measured over a period of 5 days and a fixed temperature of 20°C.