一、COD 在线分析仪的两条技术路线
重铬酸钾消解比色法：水样与重铬酸钾、硫酸银（催化剂）、硫酸汞（掩蔽氯离子）及浓硫酸混合，在高温下消解（常见约 165℃ 至 175℃，具体按仪器说明书），有机物被氧化，通过测定 Cr(VI) 的减少量或 Cr(III) 的增加量换算 COD。
UV 法（紫外吸收法）：利用有机物在 254 nm 处的紫外吸收换算 COD 或换算成等价指标，无需化学试剂，响应快，但必须用现场水样与标准方法做相关性标定，且浊度会造成干扰，需要浊度补偿或前端过滤。
二、氨氮在线分析仪的常见方法
- 纳氏试剂分光光度法：经典方法，灵敏度高，但试剂含汞，废液需专门收集处置。
- 水杨酸分光光度法：不含汞，废液处置压力小，适合对环保要求较高的项目。
- 氨气敏电极法：无需显色试剂，维护相对简单，但需定期补充电解液与更换膜，且受干扰离子影响。
三、试剂与废液：选型时最容易被忽略的成本
湿化学法仪器的长期成本主要来自三块：试剂消耗、泵管等易损件更换、废液处置。含铬、含汞废液属于危险废物，必须按当地环保要求分类收集并交由有资质单位处置，不能直排。
选型时建议明确：单次测量的试剂消耗量、测量频次、试剂保质期、每月试剂成本，以及废液的收集方式与处置渠道。
四、维护周期（常见经验范围）
- 试剂更换：视试剂稳定性与测量频次，通常数周至数月。
- 泵管更换：蠕动泵管是易损件，通常约 1 至 3 个月更换一次，具体取决于材质与运行频次。
- 标定：通常用标准溶液做零点与量程标定，频次多为每 1 至 4 周一次，验收或精度要求高的场合会更频繁。
- 管路与消解池清洗：出现结晶、堵塞或基线漂移时及时清洗。
以上均为常见经验范围，实际周期请以仪器说明书与实际工况为准。
五、选型核对清单
- 量程与检出限是否满足排放限值要求；
- 单次测量周期（消解时间）是否满足控制需求；
- 氯离子浓度是否需要掩蔽，掩蔽能力上限；
- 试剂配方是否含汞、含铬，废液处置是否可落实；
- 是否具备自动清洗、自动标定、断样报警功能；
- 采样预处理（过滤、均质）是否配套。
把水样特点（氯离子、悬浮物、浓度范围）、排放限值要求与测量频次告诉我们，我们可以帮您对比不同方法的适配性与长期成本。
1. Two routes for online COD
Potassium dichromate digestion colorimetry: the sample is mixed with potassium dichromate, silver sulphate (catalyst), mercury sulphate (chloride masking) and concentrated sulphuric acid and digested at high temperature (commonly about 165 to 175 C - follow the instrument manual). Organic matter is oxidised and COD is derived from the decrease of Cr(VI) or the increase of Cr(III).
UV absorption method: organic matter absorbs UV at 254 nm and COD (or an equivalent index) is derived from that absorption. No chemical reagents are needed and response is fast, but a correlation calibration against the reference method using local samples is mandatory, and turbidity interferes, so turbidity compensation or upstream filtration is required.
2. Common methods for online ammonia
- Nessler reagent spectrophotometry: the classic method, highly sensitive, but the reagent contains mercury and the waste must be collected and disposed of separately.
- Salicylic acid spectrophotometry: mercury-free, less waste-disposal burden, suitable for projects with stricter environmental requirements.
- Ammonia gas-sensing electrode: no colour reagent and relatively simple maintenance, but electrolyte must be topped up and the membrane replaced periodically, and interfering ions affect the result.
3. Reagents and waste: the cost most often overlooked
The long-term cost of wet-chemical analysers comes from three areas: reagent consumption, wearing parts such as pump tubing, and waste disposal. Chromium- and mercury-containing waste is hazardous and must be collected separately and disposed of by a licensed contractor; it must never go to drain.
When selecting, pin down reagent consumption per measurement, measurement frequency, reagent shelf life, monthly reagent cost, and how the waste will be collected and disposed of.
4. Maintenance intervals (typical rules of thumb)
- Reagent replacement: depending on stability and measurement frequency, typically several weeks to several months.
- Pump tubing: peristaltic tubing is a wearing part, typically replaced every one to three months depending on material and duty cycle.
- Calibration: normally zero and span with standard solutions, often every one to four weeks, more often during commissioning or where accuracy demands it.
- Cleaning of tubing and digestion cell: clean promptly when crystallisation, blockage or baseline drift appears.
These are typical rules of thumb; always follow the instrument manual and your actual duty.
5. Selection checklist
- Do range and detection limit meet the discharge limit?
- Is the measurement cycle (digestion time) short enough for control purposes?
- Is chloride masking needed, and what is the masking capacity?
- Do reagents contain mercury or chromium, and can waste disposal actually be arranged?
- Are auto-cleaning, auto-calibration and sample-loss alarms available?
- Is sampling pre-treatment (filtration, homogenisation) included?
Tell us the sample characteristics (chloride, suspended solids, concentration range), the discharge limit and the measurement frequency and we will compare the suitability and long-term cost of each method.