📚 IB Chemistry: Determination of Biochemical Oxygen Demand (BOD) in Water by the Winkler Method | IB化学：温克勒法测定水中生化需氧量(BOD)
The Winkler method is a classical iodometric titration used to measure dissolved oxygen (DO) in water. In environmental chemistry it provides the basis for determining biochemical oxygen demand (BOD), a key indicator of organic pollution. This article explains the full procedure and underlying redox chemistry, with attention to stoichiometry and IB-style calculations.
温克勒法是一种经典的碘量滴定法，用于测量水中的溶解氧（DO）。在环境化学中，它是测定生化需氧量（BOD）的基础，而BOD是衡量有机污染的关键指标。本文将完整介绍操作流程和背后的氧化还原化学，并重点讨论化学计量关系和IB风格的运算。
1. What Is BOD and Why Does It Matter? | 什么是生化需氧量及其重要性
Biochemical oxygen demand (BOD) measures the amount of oxygen consumed by microorganisms when they decompose organic matter in water. It is a standard pollution index used to assess the quality of rivers, lakes, wastewater and industrial effluents.
生化需氧量（BOD）衡量水中的微生物在分解有机物时消耗的氧气量。它是评估河流、湖泊、废水和工业排放水质的标准污染指标。
In the standard five-day test, a sample is incubated in the dark at 20 °C for five days. The dissolved oxygen concentration is measured before and after incubation; the difference is the BOD₅ value.
在标准的五日试验中，样品在20 °C的黑暗条件下培养五天。培养前后分别测定溶解氧浓度；两者之差即为BOD₅值。
High BOD values indicate high levels of organic pollution, which can deplete oxygen in natural waters and harm aquatic life. That is why accurate DO measurements are essential for environmental monitoring.
高BOD值表示有机污染严重，可能消耗天然水体中的氧气并危害水生生物。因此，准确测量DO对环境监测至关重要。
2. Principle of the Winkler Method | 温克勒法的定量原理
The Winkler method converts dissolved oxygen into manganese dioxide through a series of redox reactions. The manganese dioxide then liberates iodine from iodide under acidic conditions, and the iodine is titrated with sodium thiosulfate.
温克勒法通过一系列氧化还原反应，先把溶解氧转化为二氧化锰，然后在酸性条件下使二氧化锰从碘化钾中释放出碘，再用硫代硫酸钠滴定碘。
The amount of iodine produced is chemically equivalent to the original dissolved oxygen. Therefore, by measuring the volume of thiosulfate required, the DO concentration can be calculated accurately.
释放出的碘量在化学计量上等同于原来的溶解氧。因此，通过测量消耗的硫代硫酸钠体积，就可以精确计算溶解氧浓度。
This is a classic iodometric titration and is still widely used in environmental chemistry laboratories.
这是一种经典的碘量滴定法，至今仍广泛应用于环境化学实验室。
3. Reagents and Equipment | 试剂与仪器
The table below summarises the main reagents and their roles in the Winkler method.
下表总结了温克勒法中的主要试剂及其作用。
| Reagent | 试剂 | Role in the method | 在该方法中的作用 |
| Manganese(II) sulfate, MnSO₄ | 硫酸锰 MnSO₄ | Provides Mn²⁺ ions to form Mn(OH)₂ precipitate | 提供Mn²⁺离子生成Mn(OH)₂沉淀 |
| Alkaline iodide solution: NaOH/KI | 碱性碘化钾溶液：NaOH/KI | Provides OH⁻ to precipitate Mn(OH)₂ and I⁻ for later iodine release | 提供OH⁻沉淀Mn(OH)₂，并供给后续释放碘所需的I⁻ |
| Concentrated sulfuric acid, H₂SO₄ | 浓硫酸 H₂SO₄ | Acidifies the sample to convert MnO₂ back to soluble Mn²⁺ and release I₂ | 酸化样品，使MnO₂重新溶解为Mn²⁺并释放I₂ |
| Standard sodium thiosulfate, Na₂S₂O₃ | 标准硫代硫酸钠 Na₂S₂O₃ | Reduces I₂ to I⁻ during titration | 滴定过程中将I₂还原为I⁻ |
| Starch indicator | 淀粉指示剂 | Forms a blue complex with iodine to show the endpoint | 与碘形成蓝色配合物指示终点 |
Important equipment includes BOD bottles, burettes, pipettes, volumetric flasks and a dark incubator maintained at 20 °C.
重要仪器包括BOD瓶、滴定管、移液管、容量瓶和保持在20 °C的黑暗培养箱。
4. Sample Collection and Pre-treatment | 样品采集与预处理
Water samples must be collected without trapping air bubbles. The bottle is filled completely and capped underwater to prevent atmospheric oxygen from dissolving into the sample.
采集水样时不能混入气泡。瓶子要完全装满并在水下加盖，防止空气中的氧溶解进入样品。
If the sample contains residual chlorine, it must be removed before the test because chlorine can oxidize iodide and give a false positive result. Sodium thiosulfate is often added to eliminate chlorine.
如果样品含有余氯，必须在实验前除去，因为氯会氧化碘离子而产生假阳性结果。通常可加入硫代硫酸钠以去除氯。
Samples with very high organic content require dilution with aerated, oxygen-saturated water so that enough dissolved oxygen remains after five days of incubation. The dilution factor must be recorded carefully.
有机含量非常高的样品需要用充氧至饱和的稀释水进行稀释，以确保培养五天后仍有足够的溶解氧。稀释倍数必须仔细记录。
5. Fixation of Dissolved Oxygen | 溶解氧的固定
Immediately after sampling, solutions of manganese(II) sulfate and alkaline iodide are added to the BOD bottle. Under alkaline conditions, white manganese(II) hydroxide precipitate forms:
取样后立即向BOD瓶中加入硫酸锰溶液和碱性碘化钾溶液。在碱性条件下，生成白色的氢氧化锰沉淀：
Mn²⁺(aq) + 2OH⁻(aq) → Mn(OH)₂(s)
The dissolved oxygen in the sample then oxidizes the Mn(OH)₂ precipitate to brown manganese(IV) oxide or manganese oxyhydroxide:
样品中的溶解氧随即把Mn(OH)₂沉淀氧化为棕色的锰(IV)氧化物或锰的羟基氧化物：
2Mn(OH)₂(s) + O₂(aq) → 2MnO(OH)₂(s)
This step “fixes” the oxygen, meaning that oxygen is now trapped as a solid material that can be handled and stored without changing its amount. Fixation must occur at the sampling site.
这一步将氧气“固定”，即氧气以固体物质形式被捕获，可以安全处理和存放而不会损失。固定操作必须在采样现场完成。
6. Acidification and Iodine Release | 酸化与碘释放
After fixation, concentrated sulfuric acid is added to acidify the mixture. The brown precipitate dissolves, and the Mn(IV) species acts as an oxidizing agent to oxidize iodide ions to iodine:
固定完成后，加入浓硫酸使混合物酸化。棕色沉淀溶解，其中Mn(IV)作为氧化剂将碘离子氧化为碘单质：
MnO(OH)₂(s) + 2I⁻(aq) + 4H⁺(aq) → Mn²⁺(aq) + I₂(aq) + 3H₂O(l)
The amount of iodine released is stoichiometrically equivalent to the dissolved oxygen originally present. The solution becomes yellow-brown due to iodine.
释放出的碘量在化学计量上与原溶解氧相等。溶液因含碘而呈黄褐色。
Acidification must be complete before titration, and the flask should be stoppered and mixed well to ensure all the oxidised manganese has reacted with iodide.
滴定前必须酸化完全，并塞紧瓶塞充分摇匀，确保所有氧化态锰都与碘离子反应完毕。
7. Titration with Sodium Thiosulfate | 硫代硫酸钠滴定
The liberated iodine is titrated with a standard solution of sodium thiosulfate. The reaction is:
释放的碘用标准硫代硫酸钠溶液滴定，反应为：
I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq)
Iodine is reduced to iodide while thiosulfate is oxidized to tetrathionate. This is a clean redox titration with no proton transfer involved.
碘被还原为碘离子，硫代硫酸根被氧化为连四硫酸根。这是一个不涉及质子转移的纯净氧化还原滴定过程。
Starch solution is added only when the solution has turned pale yellow, near the endpoint. Starch forms a deep blue complex with iodine; at the endpoint the blue colour disappears permanently and the solution becomes colourless.
淀粉溶液只能在溶液变为淡黄色、接近终点时加入。淀粉会与碘形成深蓝色配合物；达到终点时蓝色永久消失，溶液变为无色。
At least three consistent titrations should be performed, and the average volume used in calculations.
至少应进行三次平行滴定，取平均值用于计算。
8. Stoichiometry of the Winkler Method | 温克勒法的化学计量关系
From the reactions shown above, the overall stoichiometric relationship can be deduced as follows.
根据上述反应，可以推导出总体的化学计量关系。
One mole of oxygen produces two moles of MnO(OH)₂, and each mole of MnO(OH)₂ releases one mole of iodine. Therefore:
1 mol氧气生成2 mol MnO(OH)₂，而每1 mol MnO(OH)₂释放1 mol碘。因此：
1 mol O₂ ≡ 2 mol I₂
Each mole of iodine reacts with two moles of thiosulfate in titration:
滴定中每1 mol碘与2 mol硫代硫酸根反应：
1 mol I₂ ≡ 2 mol S₂O₃²⁻
Combining the relationships gives the key conversion factor:
合并上述关系得到关键换算因数：
1 mol O₂ ≡ 4 mol S₂O₃²⁻
Thus the amount of dissolved oxygen is one quarter of the amount of thiosulfate consumed in the titration.
因此，溶解氧的物质的量是滴定消耗的硫代硫酸根物质的量的四分之一。
9. Calculating Dissolved Oxygen and BOD | 溶解氧与BOD的计算
Suppose V mL of sodium thiosulfate solution of molar concentration M mol L⁻¹ is used to titrate a sample of Vs mL. The number of moles of thiosulfate consumed is:
假设滴定Vs mL水样消耗了浓度为M mol L⁻¹的硫代硫酸钠溶液V mL，则消耗的硫代硫酸根物质的量为：
n(S₂O₃²⁻) = M × V / 1000
Since one mole of O₂ corresponds to four moles of thiosulfate, the moles of dissolved oxygen are:
由于1 mol O₂对应4 mol硫代硫酸根，溶解氧的物质的量为：
n(O₂) = M × V / 4000
Multiplying by the molar mass of oxygen gas, 32000 mg mol⁻¹, gives the mass of oxygen in mg. The concentration in mg L⁻¹ is:
乘以氧气的摩尔质量32000 mg mol⁻¹，得到氧的质量（单位mg）。浓度以mg L⁻¹表示为：
DO(mg L⁻¹) = (8 × M × V × 1000) / Vs
For an undiluted BOD test, the BOD₅ is simply the difference between initial DO and final DO:
对于未稀释的BOD试验，BOD₅就是初始DO与最终DO之差：
BOD₅ = DO₀ − DO₅
When the sample has been diluted, the dilution factor must be included:
若样品经过稀释，则必须乘上稀释因子：
BOD₅ = (DO₀ − DO₅) × (V_total / V_sample)
Example: 2.0 mL of wastewater is diluted to 300 mL in a BOD bottle. The initial DO is 6.2 mg L⁻¹ and the DO after five days is 3.4 mg L⁻¹. The dilution factor is 300 / 2 = 150, so BOD₅ = (6.2 − 3.4) × 150 = 420 mg L⁻¹.
例：将2.0 mL废水稀释至300 mL于BOD瓶中。初始DO为6.2 mg L⁻¹，五天后DO为3.4 mg L⁻¹。稀释因子为300 / 2 = 150，因此BOD₅ = (6.2 − 3.4) × 150 = 420 mg L⁻¹。
10. Interferences and Limitations | 干扰与局限性
Several interfering substances can affect the accuracy of the Winkler method.
多种干扰物质会影响温克勒法的准确性。
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Nitrite ions, NO₂⁻, oxidize iodide to iodine and produce falsely high DO results. Sodium azide is added to remove nitrite interference.
亚硝酸根离子NO₂⁻会把碘离子氧化成碘，导致DO结果偏高。可加入叠氮化钠消除亚硝酸盐干扰。
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Ferric ions, Fe³⁺, also oxidize iodide. Fluoride or phosphoric acid can be used to complex iron and prevent this interference.
三价铁离子Fe³⁺也会氧化碘离子。可以用氟离子或磷酸配合铁离子以消除干扰。
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Sulfide ions consume iodine and lead to low results. They must be removed or precipitated before analysis.
硫离子会消耗碘导致结果偏低，分析前必须去除或沉淀。
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Residual chlorine, strong oxidising or reducing agents in industrial effluents can also interfere with the redox reactions.
工业废水中的余氯、强氧化剂或还原剂也会干扰氧化还原反应。
Another important limitation is that the Winkler method only works well when the DO concentration is at least about 1 mg L⁻¹. Samples with very high BOD require dilution to prevent complete oxygen depletion during incubation.
另一个重要局限是温克勒法只有在溶解氧浓度不低于约1 mg L⁻¹时才能准确工作。BOD极高的样品需要稀释，避免培养过程中氧气完全耗尽。
11. IB Examination Tips | IB考试要点
In IB Chemistry, questions about the Winkler method often focus on stoichiometry, redox equations and titration calculations.
在IB化学中，关于温克勒法的题目通常聚焦于化学计量关系、氧化还原方程式和滴定计算。
Memorise the key mole ratio: 1 mol O₂ ≡ 4 mol S₂O₃²⁻. This is the most commonly tested calculation.
务必记住关键摩尔比：1 mol O₂ ≡ 4 mol S₂O₃²⁻。这是最常见的计算考点。
Be careful with units. The volume of thiosulfate is often given in mL, while concentration is in mol L⁻¹. Convert mL to L consistently before calculating moles.
注意单位换算。硫代硫酸钠体积常以mL为单位，而浓度以mol L⁻¹为单位。计算物质的量时要把mL统一换算为L。
When writing redox equations, identify the oxidation and reduction half-reactions. In the iodine-thiosulfate titration, iodine is reduced and thiosulfate is oxidised.
书写氧化还原方程式时，先确定氧化半反应和还原半反应。在碘-硫代硫酸钠滴定中，碘被还原，硫代硫酸根被氧化。
For BOD questions, always state clearly whether the sample was diluted and remember to apply the dilution factor. Also note that the BOD test uses dark incubation at 20 °C for five days to prevent photosynthesis and ensure standard conditions.
解答BOD问题时，必须明确说明样品是否稀释并记得乘以稀释因子。同时要注意BOD试验在20 °C下黑暗培养五天，以防光合作用并保证标准条件。
12. Summary | 总结
The Winkler method is a reliable iodometric titration for measuring dissolved oxygen and is central to the determination of BOD in water analysis. The oxygen is first fixed as manganese hydroxide, then converted to iodine, and finally titrated with sodium thiosulfate.
温克勒法是一种可靠的碘量滴定法，用于测定溶解氧，是水质分析中测定BOD的核心方法。氧气先被固定为氢氧化锰，再转化为碘，最后用硫代硫酸钠滴定。
The essential stoichiometric relationship is 1 mol O₂ : 4 mol S₂O₃²⁻. Using this, students can calculate DO and BOD with confidence if they carefully handle units and dilution factors.
关键在于化学计量关系1 mol O₂ : 4 mol S₂O₃²⁻。只要认真处理单位和稀释因子，学生就可以自信地计算DO和BOD。
Understanding the redox chemistry, interferences and practical steps of the Winkler method equips IB students to answer both theoretical and experimental questions accurately.
理解温克勒法的氧化还原化学、干扰因素和操作步骤，能帮助IB学生准确回答理论性和实验性题目。
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