Chemistry • Year 12 • Module 8 • Lesson 7
Monitoring Dissolved Oxygen & BOD
Apply Winkler stoichiometry, interpret real-data graphs and analyse a Murray-Darling case study to connect chemistry to ecological risk.
1. Winkler titration — multi-step calculation
Show every step. Include units throughout. 9 marks
Background. A 200.0 mL water sample is collected from a site downstream of a small sewage treatment plant near Sydney. After the Winkler reagents are added and the acid–iodide step is performed, the liberated iodine requires 12.40 mL of 0.0200 mol L−1 sodium thiosulfate to reach the colourless endpoint.
1.1 Calculate the moles of sodium thiosulfate used. 1 mark
1.2 Using the Winkler stoichiometric ratio (1 mol O2 : 4 mol Na2S2O3), calculate the moles of dissolved oxygen in the sample. 1 mark
1.3 Convert moles of O2 to mass (in mg) and calculate the dissolved oxygen concentration in mg L−1. (M(O2) = 32.00 g mol−1) 3 marks
1.4 The Australian Water Quality Guidelines suggest that a healthy freshwater river should maintain DO above 6 mg L−1. Assess whether this site meets that guideline and identify one reason this value might be characteristic of a site receiving treated sewage discharge. 2 marks
1.5 Explain why the Winkler titration must be performed on a freshly collected, sealed sample with no air bubbles. 2 marks
2. Interpret the dissolved oxygen vs temperature graph
Study the figure below, then answer the sub-questions. 8 marks
2.1 Describe the overall trend shown in the graph. 2 marks
2.2 Estimate the dissolved oxygen saturation at 25°C. 1 mark
2.3 Using the graph, identify above what temperature the dissolved oxygen saturation drops below the healthy river threshold of 6 mg L−1. Explain the chemical reason why this occurs. 3 marks
2.4 In January 2019, river water temperatures in parts of the Darling River (NSW) reached 28–32°C. Use the graph to predict the likely dissolved oxygen range and explain why this, combined with a high BOD, made conditions lethal for native fish. 2 marks
3. Case study — Murray–Darling fish kill, 2018–2019
7 marks
In late 2018 and early 2019, hundreds of thousands of Murray cod, golden perch and other native fish died in the lower Darling River near Menindee, NSW. The Australian Institute of Marine Science (AIMS) and independent hydrologists attributed the kills to a combination of factors: an extreme drought had reduced river flow and concentrated nutrients from agricultural and pastoral run-off; warm summer temperatures elevated in some reaches to 28–32°C; cyanobacterial (blue-green algae) blooms developed and subsequently died; and DO levels at affected sites were measured at 0.5–2 mg L−1 during kill events.
3.1 Use the data above to calculate a possible BOD5 range for a Darling River sample if the initial DO was 7.8 mg L−1 and the final DO ranged from 0.5 to 2.0 mg L−1. Classify both values. 2 marks
3.2 Explain, using the concepts of dissolved oxygen, BOD and the eutrophication sequence, how the combination of warm temperatures, nutrient loading and algal decomposition led to the fish kills. 5 marks
4. Predict and justify
4 marks
Q4. A river monitoring officer takes a DO reading on Monday and records 6.2 mg L−1 — just above the healthy threshold. A BOD5 test on the same sample shows a value of 7.8 mg L−1. The officer notes that a large cyanobacterial bloom is visible upstream and the weather forecast is for five days of temperatures above 30°C.
Predict what will happen to dissolved oxygen levels at this site by Friday and justify your prediction using the lesson framework. In your answer, refer specifically to the implications of the BOD5 value and the temperature forecast.
Q1 — Winkler calculation
1.1 n(Na2S2O3) = cV = 0.0200 × 0.01240 = 2.48 × 10−4 mol.
1.2 n(O2) = 2.48 × 10−4 ÷ 4 = 6.20 × 10−5 mol.
1.3 m(O2) = nM = 6.20 × 10−5 × 32.00 = 1.984 × 10−3 g = 1.984 mg in 200.0 mL. Scale: DO = 1.984 × (1000/200) = 9.92 mg L−1.
1.4 9.92 mg L−1 is above 6 mg L−1; the site meets the guideline at this sampling point. However, downstream of treated sewage discharge the BOD may still be elevated; DO may fall if decomposition of residual organic matter accelerates.
1.5 Any dissolved oxygen that escapes as gas or enters from the atmosphere changes the sample's DO before analysis. A sealed, air-bubble-free sample preserves the original oxygen concentration.
Q2 — Graph interpretation
2.1 As water temperature increases from 0°C to 35°C, dissolved oxygen saturation decreases continuously (inverse relationship). The curve is steep between 0 and 15°C and flattens at higher temperatures.
2.2 Approximately 8.2 mg L−1 at 25°C (accept 8.0–8.4).
2.3 DO saturation falls below 6 mg L−1 above approximately 33–35°C on the graph. Chemically, higher temperatures give water molecules more kinetic energy, reducing the strength of the intermolecular interactions that hold O2 in solution (Henry’s Law — solubility of a gas decreases with increasing temperature).
2.4 At 28–32°C, graph shows DO saturation of approximately 7.5–7.9 mg L−1. However, with high BOD (microbial decomposition consuming additional oxygen), actual DO can fall well below saturation, potentially to hypoxic levels below 4 mg L−1, causing fish stress and death.
Q3 — Murray–Darling case study
3.1 BOD range: 7.8 − 2.0 = 5.8 mg L−1 (moderate–heavy pollution) to 7.8 − 0.5 = 7.3 mg L−1 (heavy pollution, approaching critical). Both values exceed the moderate-pollution threshold (>2) and the upper end indicates conditions near heavily polluted (>8).
3.2 Award up to 5 marks. Key points: (1) drought concentrated nutrients (nitrates/phosphates), promoting cyanobacterial bloom; (2) warm temperatures (28–32°C) reduced DO solubility per graph; (3) algal bloom blocked light, killed submerged plants; (4) dead algae and plants decomposed by aerobic bacteria, raising BOD and consuming remaining dissolved oxygen; (5) DO fell to 0.5–2 mg L−1 — hypoxic, below survival threshold for Murray cod and golden perch, causing mass fish kill.
Q4 — Predict and justify
Dissolved oxygen will very likely fall below the 6 mg L−1 threshold by Friday, and possibly to hypoxic levels. Justification: (1) BOD5 = 7.8 mg L−1 indicates high microbial decomposition activity that will continue to consume oxygen over the next five days; (2) temperatures above 30°C will reduce DO solubility (graph shows saturation drops to ~7.5 mg L−1 at 30°C); (3) the upstream cyanobacterial bloom will begin dying and decomposing, further elevating BOD; the combined effect is that dissolved oxygen demand will exceed replenishment, likely causing hypoxia. Award 4 marks for correctly referencing BOD value (demand), temperature effect on solubility, and the eutrophication/decomposition driver with a clear prediction.