ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 1 Chemistry IA –Risma Remsudeen Research Question: How does an increase in temperature (from 30°C to 70°C at 10°C intervals) affect the concentration of dissolved oxygen (in ppm) in 250.0 cm3 tap water heated in a water bath for 300 seconds as measured by the number of moles of sodium thiosulfate reacted in an iodine-­‐sodium thiosulfate titration of the Winkler’s method. Introduction: I have always been learning how global warming is taking place and how it causes a rise in air and water temperature that leads to a high mortality rate of organisms. Due to my passion for Ecology, I often read blogs and articles related to this topic. A recent blog I read highlighted the issue of the high mortality rate due to a rise in temperature with an example of salmon, my favourite fish. It stated that one reason for endangered species of salmon especially in dams are due to its increase in temperature (Ives, 2017). According to the blog, more than 250,000 sockeye salmon died as a result of increase in temperature during last summer (Ives, 2017). Although I was aware that temperature would affect the metabolic reactions in the living body, I only recently realised that there were other factors why temperature would kill an aquatic species. This included the depletion of dissolved oxygen levels in water with high temperatures (Climatehotmap.org, 2011). This made me curious to find the relationship of temperature and dissolved oxygen levels. Background Information Oxygen in Water Free oxygen molecules present between water molecules in water are referred to as dissolved oxygen (Environmental Measurement Systems, 2016). Oxygen molecule has a symmetrical structure as shown in figure 1(a) and as a result, it is non-­‐polar. Yet, a London dispersion force and an induced dipole force is formed between the free Oxygen molecule and the water molecule. This is because water molecule is polar. Due to the high electronegativity of the Oxygen atom in the water molecule compared to the hydrogen atoms, the shared electrons are more closer to the nucleus of the Oxygen atom in Water than the hydrogen nuclei. Thus, the water molecule has a negative dipole near the Oxygen atom and a positive dipole near the Hydrogen atom. This polarity of water induces dipoles in the free Oxygen molecules due to random movement of electrons in the molecule (Congress, 2016) as shown in Figure 1 (b). Figure 1: Molecular Structure of Oxygen and Water molecules and the formation of LDF (Congress, 2016) ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 2 Winkler’s Method: (FISH 503 Advanced Limnology, 2017) The Winkler’s method uses redox titration of Iodine and sodium thiosulfate in order to measure dissolved oxygen levels in a solution (typically water). This method was used due to the changes in colour that could help to easily identify the reaction of the colourless oxygen (Web.colby.edu, n.d.). It indirectly measures the concentration of dissolved oxygen through a series of reduction and oxidation reactions by using a fixed ratio of the number of moles of the reagents in a stoichiometric calculation. The three stages to the analysis are: 1. Manganese ion in a manganese salt (eg: manganese sulfate) is oxidized with the hydroxide ion in an alkali (eg: alkaline potassium iodide) and the dissolved free oxygen in the water to form manganese (II) oxide that appears as a black precipitate. (Highlighted in yellow in the equation below) 2Mn2+(aq) + 4OH-­‐(aq) + O2 (g) à 2MnO2 (s) + 2H2O (l) 2. Sulfuric acid is then added to ‘fix’ the number of moles of oxygen as it dissolves the manganese (II) oxide which then immediately is reduced by iodide ions from the alkaline potassium iodide and the H+ ions in the sulfuric acid forming Iodine, changing the colour of the solution to yellow-­‐brown. (Highlighted in blue in the equation). 2MnO2 (s) + 4I-­‐(aq) + 8H+(aq) à 2Mn2+(aq) + 2I2 (s) + 4H2O (l) The number of moles of Oxygen is now fixed. 3. The iodine formed in the reaction is then titrated with Sodium thiosulfate with the thiosulfate ion which oxidizes iodine. As starch is an indicator of Iodine as it turns blue-­‐ black in the presence of Iodine, starch is used in order to identify the volume of sodium thiosulfate required to oxidize all the iodine molecules present in the solution. 4S2O32-­‐ + 2I2 à 2S4O62-­‐ + 4I-­‐ By carrying out stoichiometric calculations using the volume of Sodium thiosulfate used to calculate the volume of iodine liberated in step 2 and the manganese oxide produced in step 1, the number of moles of Oxygen dissolved in water can be calculated leading to the concentration of dissolved oxygen in water. By combining the three equations given above, the stoichiometric ratio of the thiosulfate ion and Oxygen is found to be 4:1 and this is therefore used to find the number of moles of Oxygen that is dissolved in water and reacts with Manganese sulfate. Variables: Independent Variable: Temperature of 250.0cm3 tap water -­‐ The temperature is set by an electrical water bath at 30°C, 40°C, 50°C, 60°C and 70°C and was then checked using a thermometer just before reactants (Manganese (II) Sulfate and Alkaline Potassium Iodide) were added. -­‐ A control apparatus was also set at room temperature of 25°C. The room temperature was ensured to stay the same by setting a fixed temperature using the air conditioner and by measuring the temperature of the solution using a thermometer in between the experiments. Dependent Variable: The amount of Oxygen dissolved in the tap water. -­‐ This is measured using the iodine-­‐sodium thiosulfate titration in Winkler’s method. -­‐ The amount of oxygen dissolved in the tap water can be calculated using the volume of sodium thiosulfate that was reacted in the titration. (See ‘Calculations’) ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen Control Variables: Control How is it controlled? Variables Time the water The samples were kept in samples were a water bath for 300 heated for seconds at the desired temperature, measured using a stopwatch. Volume of 2 cm3 of 0.5 moldm-­‐3 MnSO4 and MnSO4 and 2 cm3 solution Alkaline KI of 0.5 moldm-­‐3 alkaline KI solutions added was added to each sample to the sample using a graduated volumetric pipette Concentration of MnSO4 and Alkaline KI added to the samples Volume of concentrated sulfuric acid added to the samples 0.5 moldm-­‐3 MnSO4, 0.5 moldm-­‐3 KI and 0.5 moldm-­‐3 KOH were used throughout the experiment. 2cm3 sulfuric acid was added to the samples using a 2 cm3 Pasteur glass pipette. Why is it controlled? Different amounts of heat (due to the heating time) provided to the sample may affect the dissolved oxygen in the samples A difference in the volume of MnSO4 and alkaline KI would result in an unknown number of moles of iodine that reacts in the titration. This would make it impossible to find the moles of dissolved oxygen in water using stoichiometric coefficients in reactions. A difference in the concentration of MnSO4 and alkaline KI would result in the same effect as that of a difference in volume of the solutions. Although sulfuric acid can be added in excess, it is important to keep the volume similar across all the samples to avoid difference in the total volume of the solution which can affect the concentration of Oxygen. Mass of Starch 1g starch was added to the To keep the intensity of the colour of added to the yellow-­‐brown solution the solution same in order to avoid a samples during the titration difference in colour subjectivity. 3 Volume of Water 250.0 cm water was With different volumes of water, used as samples measured using a there would be a difference in the measuring cylinder number of moles of Oxygen dissolved in the water which could impact the concentration but there could also be a difference in the amount of heat absorbed with different amounts of water. Type of water Tap water was used as Different types of water: whether used water samples. distilled or tap, have different concentrations of Oxygen. Tap water is known to have more oxygen dissolved than distilled water. Environmental The experiment was With an increase in pressure, Pressure conducted in a controlled solubility of Oxygen increases. lab with the pressure held Therefore, pressure had to be kept constant at 100kPa with a constant in order to avoid factors set room temperature other than temperature affecting the fixed by an air conditioner solubility of Oxygen in water. and by closing the windows in the lab. 3 ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen Method: 4 This method has been adapted from the method used by the University of Idaho (FISH 503 Advanced Limnology, 2017) and the Montana State University (Environmental Sampling, 2017). However, few changes were made due to certain problems that had to be overcome: -­‐ 250cm3 Erlenmeyer flasks were used instead of BOD bottles as the BOD bottles were not available (It was known that Erlenmeyer flasks cannot replace BOD bottles although they were the best alternative that could have been used). When doing this, I had to make sure that the flasks were kept closed most of the time. -­‐ Concentrations of the reagents were all changed from the method by University of Idaho due to a preliminary experiment that was conducted where 0.5 moldm-­‐3 manganese sulfate, 0.5 moldm-­‐3 alkali potassium iodide and 0.05moldm-­‐3 sodium thiosulfate worked the best giving desirable results for the experiment. (There were problems in finding the most suitable concentrations during the preliminary experiment as many concentrations did not give enough changes in colour and the final value after titration that was big enough to be graphed.) -­‐ Alkali potassium iodide (with potassium hydroxide) was used instead of alkali iodide-­‐azide as it served the purpose of the original reagent. Preparation of the Standard Solutions and setting up of the titration apparatus: 1. On a top-­‐pan balance, place a weighing boat and press ‘tare’. Measure 21.13g manganese sulfate by adding it onto the weighing boat using a spatula. 2. In a 250cm3 volumetric flask, add the measured manganese sulphate and rinse the weighing boat and the spatula with distilled water and pour it into the flask. Add distilled water until the solution level reaches more than half of the curved bottom. Make sure the level does not reach the neck of the flask. 3. Invert the flask several times until all the manganese sulphate powder has dissolved. Add distilled water until the meniscus reaches the reference line on the flask. 0.5moldm-­‐3 manganese sulphate standard solution is prepared. 4. Repeat steps 1 to 3 with 20.75g Potassium iodide and 7.01g potassium hydroxide added to another volumetric flask. 5. Repeat steps 1 to 3 for 3.1g Sodium thiosulfate to form 0.05moldm-­‐3 sodium thiosulfate solution. 6. For the titration apparatus, clamp a 50.00cm3 burette onto a clamp stand. 7. Rinse the burette with 30.00 cm3 sodium thiosulfate solution that was prepared. Fill it with the 0.05moldm-­‐3 sodium thiosulfate solution. Heating the water samples: 8. Collect 250cm3 tap water using a 500cm3 measuring cylinder and pour it into an Erlenmeyer’s flask. Ensure that the lid is closed as soon as possible. 9. Fill half of an electrical water bath with tap water and set it to 30°C (Safety: Do not touch the inside of the water bath with hands directly). Wait until the water bath heats up to the temperature set and check the temperature using a thermometer. 10. Place the water sample in the flask in the water bath and set a timer to 300 seconds. Figure 2 : Water sample 11. Remove the water sample after 300 seconds. collected in a 250cm3 (Safety: Use a heat resistant rubber glove to take Erlenmeyer's flask the flasks out) ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 5 Reacting and fixing the dissolved Oxygen: 12. Using a graduated pipette, carefully transfer 2cm3 manganese sulfate to the water sample. Ensure that the tip of the pipette is slightly below the surface level of the water sample so that air is not introduced when transferring. 13. Repeat step 12 with 2 cm3alkali potassium iodide solution. 14. Close the flask with the lid and shake the mixture until brown precipitates are visible. 15. Using a Pasteur Glass Pipette of 2 cm3, add 2 cm3 of concentrated sulfuric acid. Ensure that the pipette tip is just above the surface of the sample to avoid introduction of Oxygen. Close the flask and invert it many times until the precipitates dissolve as shown in figure 3. Figure 3 : Sample after fixing Titration: the dissolved oxygen shown by 16. Place the solution in the Erlenmeyer’s flask under the dissolving of precipitate the burette and note down the initial reading on the burette. Now add the sodium thiosulfate solution drop by drop until the brown colour in the solution becomes yellow. 17. Add 1g of Starch weighed using the top pan balance (as described in step 1). 18. Swirl the flask until the solution turns to a blue-­‐black colour as shown in figure 4. 19. Continue adding the sodium thiosulfate solution drop by drop until the blue-­‐ black colour disappears as shown in Figure 5 20. Note the final reading in the burette. 21. Repeat steps 8 to 20 with 40°C, 50°C, 60°C and 70°C and also repeat each temperature 2 more times to get 3 results for each temperature. Figure 5 : Sample when 1g starch was added Figure 4 : End-­‐point colour o f the titration Safety Issues: 1. According to Cleapps (Student Safety Sheet, 2016), Conc. Sulfuric Acid is corrosive and can cause burns such that gloves must be worn when using the sulfuric acid. In case of spill on the skin, quickly use a dry cloth to wipe the acid and then wash with plenty of water. (Student Safety Sheet, 2016), 2. When heating the flasks in the water bath, take care as it may be hot. Do not touch the water in the water bath nor on the sides of the water bath with bare hands. Use heat resistant rubber gloves to take the flasks out of the water bath. ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 6 Ethical Issue A lot of water has been used in this experiment such that water that may have been particularly in need by people in dry parts of the world is wasted. In order to handle this issue, use a less volume of water for the sample to be tested. Environmental Issue A water bath was used which consumed a lot of energy. In order to deal with environmental issues, turn off the water bath when it is not needed. Raw Data Table 1: Table showing all the measurements of the control variables Concentration of the standard solution of MnSO4 /moldm-­‐3 (±0) 0.50 Concentration of the standard solution of alkaline KI /moldm-­‐3 (±0) 0.50 -­‐3 Concentration of the standard solution of sodium thiosulfate /moldm (±0) 0.05 3 Volume of tap water used as samples /cm (±2.5) 250.0 Volume of Sulfuric Acid /cm3 (±0)** 2 Mass of Starch in each sample/ g (±0.01) 1.00 Time for heated the samples / s (±0.1) 600.0 Note: The concentrations of the standard solutions made (Manganese sulfate, Alkaline potassium iodide and sodium thiosulfate) are given in table 1 instead of mass of the chemicals used and the volume of water used to dilute it as the standard solutions are pre-­‐prepared solutions. Thus, it is also assumed that the uncertainties of the concentrations of all standard solutions are ±0 moldm-­‐3 ** The uncertainty of the Pasteur glass pipette used for sulfuric acid is unknown and is assumed to be ±0. Sulfuric acid is added in excess and uncertainty is not necessary. Table 2: Raw data table for the titration in the Winkler's method Temperature Volume of sodium thiosulfate that reacted in the titration/ cm3 (±0.05) of the Trial 1 Trial 2 Trial 3 sample /°C Initial Final Initial Final Initial Final (±0.1) Reading Reading Reading Reading Reading Reading 25.0* 15.20 18.90 32.60 36.70 17.40 21.30 30.0 3.10 5.90 13.70 17.30 22.40 26.20 40.0 7.30 11.00 17.40 21.30 29.70 33.20 50.0 26.20 29.70 25.70 29.30 36.50 40.00 60.0 29.40 32.60 7.60 11.10 19.40 22.60 70.0 41.00 43.70 43.70 46.70 0.50 3.30 *This was a control test at Room Temperature Note: The temperature of the sample reflects the raw data containing the lowest number of significant figures. Therefore, all the values calculated are rounded to 3 significant figures. Qualitative Observations: -­‐ When the standard solutions were made, the volumetric flask felt hot, suggesting that exothermic reactions were taking place. This may also suggest that heat was lost during the experiment such that the temperature was not kept constant. -­‐ When manganese sulfate and alkaline potassium iodide were added to the water, it turned brown and when kept shaking, it formed precipitate. -­‐ The precipitate dissolved when conc. Sulfuric acid was added as the solution turned to a clear brown colour. ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen -­‐ -­‐ -­‐ 7 When titrated with sodium thiosulfate, the solution turned yellow-­‐brown and adding starch changed the colour to blue-­‐black indicating that the solution contained Iodine. However, it was hard to identify the end point When starch was added, it was insoluble in the solution such that it made it difficult to judge whether the solution had changed its colour during titration. At times, when sodium thiosulfate was added in titration, the reaction was slow that made it difficult to identify the end-­‐point. Processed Data Table 3: Processed data table to calculate the moles of sodium thiosulfate that reacted with the iodine in the sample. Temperature Volume of 0.05moldm-­‐3 Na2S2O3 Average vol. of 0.05 Moles of Na2S2O3 of the sample used as the titre/ cm3 (±0.10) moldm-­‐3 Na2S2O3 used in the samples as titre/cm3 (±0.10) (×10!! )/mol Trial 1 Trial 2 Trial 3 /°C (±0.1) 25.0 3.70 4.10 3.90 3.90 1.95 30.0 2.80 3.60 3.80 3.70* 1.85 40.0 3.70 3.90 3.50 3.70 1.85 50.0 3.50 3.60 3.50 3.53 1.77 60.0 3.20 3.50 3.20 3.20* 1.60 70.0 2.70 3.00 2.80 2.83 1.42 *Note: The non-­‐concordant values are disregarded when averaging. Concordant results are the values within 0.20cm3 difference from each other. Calculation of the absolute uncertainty in the header row: The uncertainty in the header rows is calculated by adding the uncertainties of the raw data involved in calculating the value. For instance, for the volume of the titre used at each trial, the uncertainty is calculated by adding the uncertainties of the ‘initial reading’ and ‘final reading’ in the raw data table. Therefore, the uncertainty of the volume of 0.05moldm-­‐3 sodium thiosulfate used= 0.05+0.05=0.10. (For the uncertainties of the average volume of sodium thiosulfate and moles of thiosulfate, see ‘Uncertainty in Raw Data’) Examples of Calculations: Calculations for preparing the Standard Solutions: The mass required to make the standard solution of Manganese sulfate, alkali potassium iodide and sodium thiosulfate was calculated using their Mr. For example, to prepare 0.5moldm-­‐3 manganese sulfate which when hydrated has the Mr. of 168.90 in a 250cm3 volumetric flask, the Mr. was divided by 4 (as 250cm3 = 1 dm3 divided by 4) and then by 2 (as we need 0.5moldm-­‐3 ). This gives the required mass as 21.13g. Note: All examples given are for the trial done at the room temperature, 25°C 1. Volume of 0.05moldm-­‐3 Na2S2O3 used as the tire: (Trial 1 at 25°C) Volume of titre = final reading – initial reading = 18.90 -­‐ 15.20 = 3.70 dm3 2. Average Volume of 0.05moldm-­‐3 Na2S2O3 used as titre: Average volume = Sum of the concordant volumes in all the trials Number of trials with concordant results !.!"!!.!"!!.!" = ! = 3.90 dm3 ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 8 3. Number of moles of Na2S2O3 in the samples No. of moles = concentration × volume Concentration of Na2S2O3 = 0.05 moldm-­‐3 Moles = 0.05moldm-­‐3 × 0.00390 dm3 = 0.000195mols = 1.95×10!! mols 4. Number of moles of oxygen in the sample As mentioned in the background information, 4 moles of S2O32-­‐ is used when manganese sulfate reacts with 1 mole of O2. Therefore, when 0.000195 moles of Na2S2O3 is used in the titration, !.!!!"#$ ! = 4.88×10!! moles of O2 are present in the samples. Table 4: Processed data table to calculate the concentration of dissolved Oxygen in water at different temperatures. Temperature of the sample /°C (±0.1) 25.0 30.0 40.0 50.0 60.0 70.0 Moles of Na2S2O3 in the samples (×10!! )/mol 1.95 1.85 1.85 1.77 1.60 1.42 Moles of Oxygen in the samples (×10!! )/ mol 4.88 4.63 4.63 4.43 4.00 3.55 Mass of Oxygen in the water / g 0.00156 0.00148 0.00148 0.00142 0.00128 0.00114 Concentration of dissolved Oxygen in water /ppm 6.24 5.92 5.92 5.67 5.12 4.54 5 Mass of Oxygen in 250cm3 water sample: Mass = Moles × Mr. Mr. of O2 = 32.00 gmol-­‐1 (taken from the IB data booklet) Thus, mass of O2 = 4.88 ×10-­‐5 × 32.00 = 1.56 ×10-­‐3 g 6 Concentration of dissolved O2 in ppm: Mass of O2 in mg = 1.56 mg parts per million means mass of a solute in mg per 1 dm3 solution Therefore, concentration = !.!"!" !.!"!" ! = 6.24 ppm Table 5: Uncertainty table to calculate the uncertainties in the raw data due to precision of apparatus Temp eratur e /°C (±0.1) % uncer tainty /% Vol. of % Water unce sample rtai s/ cm3 nty/ (±2.5) % 25.0 0.37 250.0 1.0 30.0 0.33 250.0 1.0 40.0 0.25 250.0 1.0 50.0 0.20 250.0 1.0 60.0 0.17 250.0 1.0 70.0 0.14 250.0 1.0 Average Percentage Uncertainty Vol. of MnSO4 / cm3 (±0.05) 2.00 2.00 2.00 2.00 2.00 2.00 % unce rtai nty/ % 3 3 3 3 3 3 Vol. of Alkaline KI /cm3 (±0.05) 2.00 2.00 2.00 2.00 2.00 2.00 % unce rtai nty/ % 3 3 3 3 3 3 Ave. Vol. of Na2S2O 3 3 /cm (±0.10) 3.90 3.70 3.70 3.53 3.20 2.83 % unce rtai nty/ % 3 3 3 3 3 4 Total % uncert ainty/ % 10 10 10 10 10 10 10 ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 9 The percentage uncertainties have been rounded to 1 s.f if uncertainty > 2 and to 2s.f if uncertainty < 2. !"#$%&'( !"#$%&'("&) The percentage uncertainty is calculated by !"#$%&"'"() !"#$% ×100 For example, for 25°C, !.! Percentage uncertainty of the temperature = !".! ×100 = 0.37 The total percentage uncertainty is then calculated by adding all the separate percentage uncertainties. For 25°C, total % uncertainty = 0.37+1+3+3+3 = 10.37 which is then rounded to 10 (1s.f) as it is greater than 2. For the total absolute uncertainty shown in table 6, the total percentage uncertainty for each temperature is multiplied with the concentration of Oxygen in the samples and then divided by 100 !" For example, for 25°C, the absolute uncertainty = !"" ×6.24 = 0.624 The absolute uncertainty with the concentration of dissolved oxygen in the 250.0cm3 water sample at each temperature is shown in table 6. Table 6: Table showing the absolute uncertainties for each of the concentrations of dissolved oxygen in the sample at different temperatures. Temperature of the Concentration of dissolved oxygen in water /ppm sample /°C (±0.1) 25.0 6.24±0.624 30.0 5.92±0.590 40.0 5.92±0.590 50.0 5.67±0.565 60.0 5.12±0.510 70.0 4.54±0.455 Figure 6: The relationship between temperature and the concentration of dissolved Oxygen in water Concentration of dissolved Oxygen/ppm 6.5 6 y = -­‐0.0344x + 7.1448 R² = 0.91932 5.5 5 4.5 4 25 30 35 40 45 50 55 60 65 70 Temperature/°C ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 10 Analysis of the Graph: The graph shows that as temperature increases, the concentration of dissolved oxygen decreases. There is a strong, negative correlation between the variables as shown by the R2 value of 0.91932 which is very close to 1. There are no clear outliers in the graph and hence, my results seem accurate. Conclusion The results of this experiment supports my hypothesis and explanation given in the introduction and background information. There is a strong negative correlation between temperature and the concentration of dissolved oxygen in water. The concentration at room temperature is 6.24 ppm which when heated to a temperature of 60°C becomes 5.12 ppm. This supports the explanation that as temperature increases, the free dissolved oxygen molecules gain more kinetic energy such that they move faster. This would lead to a lesser possibility for the induction of dipoles by the polar water molecules such that the free oxygen molecules are liberated. Furthermore, this data seems to be reliable as the graph has an R2 value of 0.91932, a value very close to 1, suggesting the strong relationship between my variables. This makes me very confident with the result. In addition, the percentage uncertainty of the results is 10% which is not a very large uncertainty. This further makes me confident with the result. However, the uncertainties that are present, may be caused due to the limited amount of repeats and methodological errors in my experiment (See Limitations and Improvements). The theoretical value found in the website of the Environmental Protection Agency (Archive.epa.gov, 2012) is 8.24 ppm at 25°C (the temperature of the control trials) whereas in my experiment, I got 6.24 ppm at 25°C. This may suggest that my experiment result is slightly different to the theoretical value such that it may indicate unreliability of my results to some extent. To calculate the percentage error in this result: !!!"#!$%&'( !"#$%!!"#!$%&!'()* !"#$% % error = !!!"#!$%&'( !"#$% x 100 = 8.24−6.24 ×100 8.24 = 24.3% Since my random error is 10% and my percentage error for the experiment is 22%, it may mean that my systematic error for this experiment would be 24.3% -­‐10% = 14.3%. This % error seems reasonable considering all the limitations of the experiment (see ‘limitations’). For instance, although I calculated the concentration of dissolved oxygen in each of the samples at different temperatures, the results would have been influenced by the uncertainties due to the precision of apparatus that has low uncertainty. When possible, apparatus with high precision such as a burette and a 10cm3 pipette was used in order to reduce the effect of uncertainties on the results. Thus, I am fairly confident with my results. To conclude, the results show that as temperature increases, oxygen concentration in water decreases which could be a reason for the extinction of aquatic species such as salmon with an increase in temperature as a result of global warming. Thus, this investigation shows that it is important to limit the increase in atmospheric temperature so that aquatic species get access to dissolved oxygen. ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen 11 Strengths The strengths of this experiment lies on controlling many of the control variables as mentioned earlier and by using many precise apparatus such as burette and pipette to eliminate the possibility of high uncertainties and random errors due to the precision of the apparatus. Another strength would be the careful selection of the range with respect to the context as mentioned in the introduction so that the research question is answered within the context. Limitations and Improvements Limitations Number of Repeats for each temperature The use of Erlenmeyer’s flask instead of BOD bottle Oxygen exposure was not controlled perfectly Heat was lost in the experiment The oxygen was not fixed right when the flasks were taken out of the water bath. The water bath’s temperature was inconsistent The end point of the titration was subjective Significance High significance: Only 3 repeats were done with some of the temperatures having only 2 concordant results. The random error could have been minimized with a greater number of repeats. High Significance: Use of the Erlenmeyer’s flask may have increased the possibility of oxygen entering the flask and the sample. BOD bottles would have prevented exposure of oxygen to the sample. High Significance: Oxygen may have dissolved in water during the experiment such as when titrating and transferring of liquid Improvements Do more repeats for each temperature with more number of concordant results. The temperature of the waterbath fluctuated when it was opened to take flasks out. This would mean that the samples were not heated at a constant temperature for 300 seconds The end-­‐point of the titration was prone to bias as sometimes, the colour seemed to be light blue-­‐ black/ purple which also looked white. Use a more precise apparatus such as a hot plate which can heat the samples at a constant temperature Use a BOD for titration using Winkler’s method Use a BOD bottle for titration and also ensure that the flask is kept closed most of the time so that the exposure is lowered. High Significance: Temperature Insulate the bottles and may have fluctuated that when the keep the lid closed at all result was taken, the temperature times. may not have been the one when it was checked using a thermometer. High Significance: Temperature Do the experiment may have changed and oxygen quickly solubility would have changed before fixing the oxygen. Use a colour chart and mark the colour so that the decided end-­‐colour can be compared with the other trials. ALL TEXTS BELONG TO OWNERS TAKEN FROM https://internalassessments.wordpress.com/ Risma Remsudeen Limitations Reaction occurred slowly that there was a change in colour sometime after the addition of sodium thiosulfate during the titration. Significance Sometimes, the colour of the sample after adding sodium thiosulfate changed with time suggesting that a slow reaction may be taking place. Tap water may contain other ions. The ions may also affect the reactions in the Winkler’s method such that the results obtained may not be exactly for the dissolved oxygen in the water. 12 Improvements Add the titre slowly and swirl after adding each drop. Allow time for the solution to change colour and ensure that the colour would not change before adding more titre or recording the results. Use distilled water but pump Oxygen to the maximum so that it would contain oxygen and would avoid any ions that can affect the results. Further Experiments Further experiment that could be done could be to find the effect of other factors such as salinity and pressure on the oxygen level in water as the world pollution affects the salinity of water. Bibliography Archive.epa.gov. (2012). 5.2 Dissolved Oxygen and Biochemical Oxygen Demand | Monitoring & Assessment | US EPA. [online] Available at: https://archive.epa.gov/water/archive/web/html/vms52.html [Accessed 28 May 2017]. Climatehotmap.org. (2011). Global Warming Effects on Lakes and Rivers. [online] Available at: http://www.climatehotmap.org/global-­‐warming-­‐effects/lakes-­‐and-­‐ rivers.html [Accessed 28 May 2017]. Congress, J. (2016). Health and Medicine | BCA Chemistry | Page 6. [online] Bcachemistry.wordpress.com. Available at: https://bcachemistry.wordpress.com/category/health-­‐and-­‐medicine/page/6/ [Accessed 28 May 2017]. Environmental Measurement Systems. (2016). Dissolved Oxygen -­‐ Environmental Measurement Systems. [online] Available at: http://www.fondriest.com/environmental-­‐measurements/parameters/water-­‐ quality/dissolved-­‐oxygen/ [Accessed 28 May 2017]. Environmental Sampling. (2017). Dissolved Oxygen by the Winkler Method. [online] Available at: http://serc.carleton.edu/microbelife/research_methods/environ_sampling/oxygen. html [Accessed 28 May 2017]. FISH 503 Advanced Limnology. (2017). 1st ed. [ebook] Idaho, Moscow: University of Idaho, pp.2-­‐4, 8-­‐9. Available at: http://www.webpages.uidaho.edu/fish503al/002%20Oxygen/FISH%20503%20Wi nkler%20titration%20lab%20day%20I.pdf [Accessed 28 May 2017]. Ives, S. (2017). Protect Wild Salmon From Rising Water Temperatures. [online] ForceChange. Available at: https://forcechange.com/185103/protect-­‐wild-­‐salmon-­‐ from-­‐rising-­‐water-­‐temperatures/ [Accessed 28 May 2017]. Web.colby.edu. (n.d.). Winkler Titrations – Measuring Dissolved Oxygen | Colby at Sea. [online] Available at: http://web.colby.edu/colbyatsea/2011/02/11/winkler-­‐ titrations-­‐measuring-­‐dissolved-­‐oxygen/ [Accessed 20 Jul. 2017].
