Chemistry • Year 12 • Module 8 • Lesson 7
Monitoring Dissolved Oxygen & BOD
Build the core vocabulary, the Winkler titration sequence and the BOD5 formula — recall and structural understanding before problem-solving.
1. Term–definition match
Match each term in the right-hand column by writing the correct letter (A–J) next to each definition. Each term is used once. 10 marks (1 each)
| # | Definition | Term (write letter) |
|---|---|---|
| 1.1 | The concentration of oxygen gas dissolved in water, measured in mg L−1 and essential for aquatic life. | |
| 1.2 | The amount of O2 consumed by microorganisms decomposing organic matter in water over 5 days at 20°C. | |
| 1.3 | A classical wet-chemistry method that converts dissolved oxygen into a measurable iodine titre via a chain of redox reactions. | |
| 1.4 | An electronic field instrument that measures dissolved oxygen electrochemically and requires calibration. | |
| 1.5 | The reagent in the Winkler method that is titrated against the iodine released during the redox sequence. | |
| 1.6 | The state of water described as having very low dissolved oxygen; the condition responsible for mass aquatic deaths. | |
| 1.7 | BOD5 classification for water with a value below 2 mg L−1. | |
| 1.8 | BOD5 classification for water with a value above 8 mg L−1. | |
| 1.9 | The stoichiometric ratio linking dissolved oxygen to sodium thiosulfate in the Winkler method. | |
| 1.10 | The process by which excess nutrient input causes algal growth, followed by decomposition, falling dissolved oxygen and possible fish kill. |
2. Fill-in-the-blank — Winkler titration sequence
Complete the paragraph by writing one word or phrase in each blank. 8 marks (1 each)
In the Winkler titration, the first step is to add a (a) _____________ reagent to a fixed volume of water sample in a sealed bottle. The dissolved oxygen in the sample oxidises Mn2+ to form (b) _____________, chemically trapping the oxygen signal. In the second step, acid and (c) _____________ ions are added. The oxidised manganese species then oxidises the iodide to produce (d) _____________, releasing an amount that is proportional to the original dissolved oxygen. In the third step, the released iodine is titrated with (e) _____________, which is the standard reductant used at this stage. The colour change at the endpoint is from brown to (f) _____________ when a few drops of starch indicator are used. The critical stoichiometric shortcut for HSC calculations is that 1 mol O2 corresponds to (g) _____________ mol of sodium thiosulfate. Unlike the DO meter, which gives a (h) _____________ field reading, the Winkler method is based on quantitative redox chemistry.
3. True or false — with correction
Circle T or F. If false, write the corrected version on the line. 8 marks (1 T/F, 1 correction where needed)
3.1 Dissolved oxygen (DO) measures how much oxygen bacteria will consume over five days. T / F
3.2 In the Winkler method, sodium thiosulfate reacts directly with dissolved oxygen. T / F
3.3 A BOD5 of 1.5 mg L−1 indicates clean water. T / F
3.4 Dissolved oxygen decreases as water temperature increases. T / F
4. Function recall
Answer each question in 1–2 sentences using precise lesson terms. 10 marks (2 each)
4.1 Why is dissolved oxygen the most urgent water-quality parameter when a fish kill is suspected?
4.2 What is the function of the manganese (Mn2+) reagent in the Winkler titration?
4.3 What does a high BOD5 value reveal about the biological activity in a water sample?
4.4 Why should BOD5 incubation be conducted in the dark?
4.5 Name two environmental factors (other than biological demand) that can cause dissolved oxygen to fall in a natural waterway.
5. Sequence the eutrophication chain
The events below are shuffled. Write the correct order (1–6) in the “Order” column to trace eutrophication from nutrient input to fish kill. 6 marks (1 each)
| Order | Event |
|---|---|
| Dissolved oxygen falls to hypoxic levels, causing aquatic organisms to die. | |
| Excess nitrate and phosphate enter the river from agricultural run-off. | |
| Bacterial decomposition of the dead biomass intensifies, raising oxygen demand. | |
| Algal bloom develops rapidly, reducing light penetration to the river bed. | |
| Submerged plants and algae die due to light deprivation and resource competition. | |
| BOD5 of the water rises significantly above 8 mg L−1. |
Q1 — Term–definition match
1.1 A · 1.2 B · 1.3 C · 1.4 D · 1.5 E · 1.6 F · 1.7 G · 1.8 H · 1.9 I · 1.10 J
Q2 — Cloze paragraph
(a) manganese (Mn2+) • (b) MnO2 • (c) iodide • (d) iodine (I2) • (e) sodium thiosulfate • (f) colourless • (g) 4 • (h) rapid/immediate
Q3 — True / false
3.1 False. Correction: DO measures the oxygen currently dissolved in water. BOD5 measures the oxygen consumed by microorganisms over five days.
3.2 False. Correction: Sodium thiosulfate titrates iodine, not dissolved oxygen directly. Oxygen is first chemically converted into iodine via the Winkler redox chain.
3.3 True.
3.4 True.
Q4 — Function recall
4.1 Dissolved oxygen is the most urgent parameter because aquatic organisms require it for cellular respiration; when DO drops below critical thresholds (typically <4 mg L−1 for most fish), organisms experience stress or die rapidly.
4.2 The Mn2+ reagent traps dissolved oxygen by being oxidised to MnO2, fixing the oxygen signal in the sample chemically before any further steps.
4.3 A high BOD5 indicates that a large amount of biodegradable organic matter is present, supporting intense microbial decomposition that rapidly consumes dissolved oxygen.
4.4 Incubation in the dark prevents photosynthesis by algae, which would produce oxygen and artificially lower the measured BOD5 value.
4.5 Accept any two: increased water temperature (reduces solubility of O2); increased salinity; reduced water mixing/turbulence; physical shading reducing photosynthetic O2 production.
Q5 — Eutrophication sequence
Correct order: 1 — Excess nitrate and phosphate enter the river from agricultural run-off. • 2 — Algal bloom develops rapidly, reducing light penetration to the river bed. • 3 — Submerged plants and algae die due to light deprivation and resource competition. • 4 — Bacterial decomposition of the dead biomass intensifies, raising oxygen demand. • 5 — BOD5 of the water rises significantly above 8 mg L−1. • 6 — Dissolved oxygen falls to hypoxic levels, causing aquatic organisms to die.