一、测量原理
pH 电极由指示电极（玻璃电极）与参比电极组成，两者之间的电位差遵循能斯特方程。在 25℃ 时，每变化 1 个 pH 单位，电位差变化约 59.16 mV，这一数值称为斜率。
参比电极提供一个稳定的参考电位，通过液接界与被测介质形成回路。因此 pH 测量的稳定性，取决于玻璃球泡的状态、参比电位是否稳定、以及液接界是否通畅。
二、结构选型要点
- 玻璃球泡：低离子强度的纯水测量要选低阻玻璃或流动参比结构；高温工况需选高温玻璃。
- 液接界：陶瓷芯适合一般水质；PTFE 或开放式液接抗污染能力强，适合污水、含油、含悬浮物的介质。
- 参比系统：常规为 Ag/Acl 与 KCl 电解液；含硫化物、蛋白质、强氧化剂的工况要选抗污染参比或双液接结构。
- 电缆与接头：常见 VP、S8、BNC 等接口，长距离传输优先选带前置放大器或数字型的电极。
- 材质：接液部分按介质选玻璃、PPS、PVDF、不锈钢或钛。
三、两点标定怎么做
标准做法是两点标定：先用 pH 6.86 或 7.00 的标准缓冲液定零点，再用 pH 4.01（偏酸工况）或 pH 9.18 / 10.00（偏碱工况）定斜率。
标定结果的判断参考（常见经验范围，具体以仪表说明书为准）：
- 斜率：正常约 95% 至 105%；低于 90% 说明响应明显变差，低于 85% 通常建议更换。
- 零点（不对称电位）：一般在 ±30 mV 以内；超过 ±50 mV 说明参比或球泡已明显老化。
- 稳定时间：新鲜电极在缓冲液中数十秒内应稳定；若几分钟仍缓慢漂移，多为液接堵塞或球泡污染。
标定要注意缓冲液的温度。多数仪表带自动温度补偿，但仍建议使用与工况温度接近的缓冲液，并定期更换缓冲液（开瓶后易受污染）。
四、清洗方法（按污染类型选）
- 一般污染：中性洗涤剂温水浸泡后冲洗。
- 无机结垢：稀盐酸短时间浸泡后充分冲洗。
- 油脂与有机物：用酒精或专用清洗液短时间浸泡。
- 蛋白质污染：用含蛋白酶的清洗液浸泡。
- 液接堵塞：按说明书用温水加压冲洗或更换液接部件。
清洗后必须重新标定。禁止用硬物刮擦玻璃球泡，也禁止用强碱长时间浸泡。
五、保存与寿命
短期与长期保存都应浸泡在 3 mol/L KCl 溶液（或电极配套的保存液）中，绝对不能用纯水或蒸馏水长时间浸泡，否则参比电解液会被稀释流失。
电极寿命受工况影响很大：清洁常温水可长达一到两年，高温、强酸强碱、含油含悬浮物的工况可能只有几个月。
六、故障速查
- 读数缓慢漂移：参比污染、液接堵塞、缓冲液失效。
- 响应变慢：球泡老化或表面污染，先清洗再标定，无改善则更换。
- 读数跳变：电缆接头受潮、屏蔽层接地不良、附近有强干扰源。
- 标定斜率一直偏低：电极接近寿命终点，或缓冲液配制/保存不当。
- 读数恒定不变：电极电缆断路、接线脱落或变送器故障。
如果您手上有电极型号与工况描述，我们可以帮您核对适配的电极规格、保存液与标定液配置。
1. Measuring principle
A pH electrode combines a glass indicating electrode and a reference electrode. The potential difference between them follows the Nernst equation: at 25 C, one pH unit corresponds to about 59.16 mV, a value known as the slope.
The reference electrode supplies a stable potential and completes the circuit through the liquid junction. Stability therefore depends on the condition of the glass bulb, the stability of the reference potential and whether the junction is clear.
2. Structural selection
- Glass bulb: for low ionic strength pure water choose low-resistance glass or a flowing reference; high-temperature duties need high-temperature glass.
- Liquid junction: ceramic suits general water; PTFE or open junctions resist fouling and suit sewage, oily or solids-laden media.
- Reference system: normally Ag/AgCl with KCl electrolyte; sulphide, protein or strong oxidising media need an anti-fouling or double-junction reference.
- Cable and connector: VP, S8, BNC and others are common; for long runs prefer electrodes with a preamplifier or digital output.
- Materials: choose glass, PPS, PVDF, stainless steel or titanium for wetted parts according to the medium.
3. Two-point calibration
The standard method is two-point calibration: set the zero point with pH 6.86 or 7.00 buffer, then set the slope with pH 4.01 (acidic duties) or pH 9.18 / 10.00 (alkaline duties).
Typical acceptance figures (common rule of thumb - always follow the instrument manual):
- Slope: normally about 95% to 105%; below 90% the response has visibly degraded and below 85% replacement is usually advised.
- Zero point (asymmetry potential): normally within plus or minus 30 mV; beyond plus or minus 50 mV the reference or bulb is clearly aged.
- Stabilisation time: a healthy electrode settles within tens of seconds in buffer; drift lasting several minutes usually means a blocked junction or a fouled bulb.
Watch buffer temperature. Most transmitters compensate automatically, but it is still worth using buffers close to the process temperature and replacing them regularly, since opened bottles contaminate easily.
4. Cleaning by fouling type
- General fouling: soak in warm water with neutral detergent, then rinse.
- Inorganic scale: brief soak in dilute hydrochloric acid, then rinse thoroughly.
- Grease and organics: brief soak in alcohol or a dedicated cleaning solution.
- Protein fouling: soak in a cleaning solution containing protease.
- Blocked junction: flush with warm water under pressure as the manual describes, or replace the junction part.
Always recalibrate after cleaning. Never scrape the glass bulb and never soak it in strong alkali for long periods.
5. Storage and service life
Both short and long-term storage should be in 3 mol/L KCl solution (or the storage solution supplied with the electrode). Never store in pure or distilled water, which dilutes and strips the reference electrolyte.
Life depends heavily on the duty: clean ambient-temperature water can give one to two years, while high temperature, strong acid or alkali, oily or solids-laden duties may give only a few months.
6. Troubleshooting
- Slow drift: contaminated reference, blocked junction, exhausted buffer.
- Sluggish response: aged or fouled bulb - clean, recalibrate, replace if no improvement.
- Jumping readings: damp connector, poor shield earthing, nearby interference source.
- Persistently low slope: electrode near end of life, or buffers prepared or stored incorrectly.
- Frozen reading: broken electrode cable, loose terminal or transmitter fault.
Send us your electrode model and duty description and we will help confirm the matching electrode specification, storage solution and buffer set.