一、先分清三个概念
- 游离性余氯：以次氯酸（HOCl）与次氯酸根（OCl⁻）形式存在，消毒能力强，是通常关注的消毒指标。
- 化合性余氯：氯与氨氮反应生成氯胺，消毒能力弱，是产生氯味刺激的主要原因。
- 总氯：游离氯与化合氯之和。
选型前必须先明确要测的是游离氯还是总氯，两者的试剂或电极配置不同。
二、DPD 比色法
水样中的余氯与 DPD 试剂反应生成红色化合物，颜色深浅与余氯浓度成正比，通过光度计或目视比色盘读取。这是实验室与现场便携检测的主流方法。
优点是准确度高、成本低、可作为在线仪表的比对基准；缺点是需要消耗试剂、人工操作、无法实现连续在线测量，且显色有时间窗口需按时读数。
三、电极法（安培法）
常见有覆膜电流法与恒电压法。覆膜电流法中，余氯透过选择性膜在工作电极上被还原，产生与浓度成正比的电流；恒电压法则通过控制电极电位实现测量。
优点是连续在线、响应快、无需试剂；缺点是受流速、温度、pH 影响明显，需要定期更换膜片与电解液，且电极法测量结果通常需要定期用 DPD 法比对校正。
四、影响因素
- pH：pH 决定 HOCl 与 OCl⁻ 的比例，既影响消毒效果也影响电极响应；多数电极法要求在一定 pH 范围内使用或配套 pH 补偿。
- 温度：影响电极响应与扩散速率，需保证温度补偿。
- 流速：覆膜电极需要稳定流速，流速过低读数偏低；需配流通池并控制流量。
- 氧化剂干扰：二氧化氯、臭氧等会对电极产生交叉响应，需核对电极的选择性。
五、怎么选
- 自来水厂、管网、二次供水在线监测：以电极法在线监测为主，定期用 DPD 法做比对验证。
- 实验室、抽检、比对：用台式或便携式 DPD 光度计。
- 泳池水：可用电极法在线监测，但需关注温度与消毒剂类型。
- 污水消毒、中水回用：介质复杂，需核对电极抗污染能力与清洗方式。
告诉我们要测游离氯还是总氯、介质类型与浓度范围，我们可以帮您匹配电极或试剂配置与配套流通池。
1. Three concepts first
- Free residual chlorine: present as hypochlorous acid (HOCl) and hypochlorite (OCl-); strong disinfection power and normally the parameter of interest.
- Combined residual chlorine: chloramines formed with ammonia; weak disinfection power and the main cause of chlorinous odour and irritation.
- Total chlorine: the sum of free and combined chlorine.
Before selecting, decide whether you need free or total chlorine - the reagents or electrode configuration differ.
2. DPD colorimetry
Residual chlorine reacts with DPD reagent to form a red compound whose intensity is proportional to concentration; it is read with a photometer or a visual comparator disc. This is the mainstream laboratory and field method.
Advantages: high accuracy, low cost, usable as the reference for checking online instruments. Disadvantages: reagents are consumed, it is a manual operation, continuous online measurement is not possible, and the colour must be read within a defined time window.
3. Electrode (amperometric) method
The common variants are the membrane-covered amperometric and potentiostatic methods. In the first, chlorine diffuses through a selective membrane and is reduced at the working electrode, giving a current proportional to concentration; the second controls the electrode potential to achieve measurement.
Advantages: continuous online, fast response, no reagents. Disadvantages: markedly affected by flow, temperature and pH; membrane and electrolyte need periodic replacement; results normally need periodic cross-checking against the DPD method.
4. Influencing factors
- pH: determines the HOCl / OCl- ratio, affecting both disinfection efficiency and electrode response; most electrode methods require a pH range or pH compensation.
- Temperature: affects electrode response and diffusion, so temperature compensation is needed.
- Flow: membrane-covered electrodes need stable flow; too little flow reads low. Use a flow cell and control the flow rate.
- Oxidant interference: chlorine dioxide and ozone can cross-respond; check electrode selectivity.
5. How to choose
- Waterworks, distribution networks, secondary supply: electrode-based online monitoring with periodic DPD cross-checks.
- Laboratory, spot checks, comparison: benchtop or portable DPD photometer.
- Swimming pools: electrode-based online monitoring works, but watch temperature and disinfectant type.
- Sewage disinfection, water reuse: complex media - check the electrode fouling resistance and cleaning method.
Tell us whether you need free or total chlorine, the medium type and the concentration range, and we will match the electrode or reagent configuration and the flow cell.