pH Meter - 12 Practical Steps to Troubleshoot Calibration Problems

pH is a very important parameter that needs to be measured in almost every water quality application. It plays a role in taste (acid = fresh, neutral = mild, alkaline = inedible) and in the preservation of food. In environmental sampling and monitoring, high or low pH can indicate contamination. In wastewater treatment, pH is adjusted as part of discharge permits and many treatment processes are dependent on pH. In biotechnology, the pH needs to be closely monitored during the production of immunoassay solutions. These are just a few of the many applications where pH measurement is a valuable tool.

You need good pH data. Who hasn't You should get good data, but to get good pH data you need to calibrate. You need to calibrate well and calibrate often.

In order to determine the slope and zero point of the electrode, an electrode calibration is required. Since both of these can change over time, frequent calibration is required.

Calibration frequency depends on the application, some applications require daily calibration while others may only require weekly or monthly calibration. More frequent calibration is recommended when measuring in heavily polluted, low ionic, strongly acidic and high temperature solutions.

Ok, so we know we need to calibrate, but for some reason your pH probe won't calibrate.

If you are having problems with pH calibration, there are a few things you should check.

1. Check the life of the probe

Probes for pH meters typically last 12-18 months. This is true whether the probe is used or not. The lot number will determine the life of your pH probe. Lot numbers are two numbers followed by a letter. The numbers indicate the year of manufacture and the letters indicate the month, ie - A = January, B = February, C = March etc. Note that the letter "I" is not used, which means H=August, J=September, etc.

Lot number example: 12A *

*probe produced in January 2012

2. Perform routine maintenance

Keeping your pH probe clean can also help eliminate pH calibration problems. pH electrodes typically require weekly or monthly cleaning. Always check the meter and electrode manuals for calibration and routine maintenance information.

Before cleaning the pH electrode, care should be taken that distilled or deionized (DI) water should not be used for electrode storage or soaking. This may cause permanent damage to the electrodes. However, it is recommended to rinse the electrodes with deionized water when cleaning and/or transferring to other solutions. Also, preferably, the pH electrode is cleaned chemically rather than mechanically. 

If the reference junction on the probe is dirty, the probe may become unresponsive.

Field electrodes:

Periodically soak the electrode in a 1:1 bleach solution for 15-30 minutes to reduce the chance of reference junction clogging.

If you have hard deposits (e.g. barnacles) accumulating on your probes, you can clean them by soaking the pH probe in vinegar or 1M (molar) HCL (hydrochloric acid) for about 3 minutes. A vinegar soak may require a longer soak to remove hard deposits.

Laboratory electrodes:

Place the electrode in 0.1M HCl or 0.1M NaOH (sodium hydroxide) for 5 minutes. If the buildup is not removed, the solution should be carefully heated to about 50°C (122°F) before increasing the acid or base concentration.

3. Check the probe for physical damage

If the probe is damaged, glass bulb cracked, glass cracked, etc., the probe will need to be replaced.

The term "glass electrode" does not denote the material used to construct the electrode body, since the electrodes can have plastic or glass electrode bodies. Instead, "glass electrode" is used to describe the membrane material (i.e. glass membrane).

Plastic electrodes are stronger and less prone to breakage than glass electrodes, which typically have a wider operating temperature range. Glass electrodes are also usually refillable.

Regardless, replacing the pH sensor is a good option if there is obvious visual physical damage.

4. Verify that the pH probe has never dried out

Always store the pH probe in a humid environment or immersed in buffer 4. If you notice that your pH probe is drying out, you may need to replace it. Storing pH values ​​in pH 4 buffer helps prevent growth or buildup in the storage environment, which could cover the sensor and affect your readings.

All pH electrodes can suffer permanent damage if they are not stored properly. Recommendations for storage of pH electrodes vary depending on the length of time the electrode is stored and the type of electrode. Regardless of the length of storage or the type of electrode, pH electrodes must not be allowed to dry out. Likewise, pH electrodes should never be stored in deionized water. Use a pH 4 buffer as it also helps eliminate electrolyte leaching.

What to do if the pH probe dries out? If this is the case, and you have allowed your pH electrode to dry out, immersing the electrode in a suitable solution may successfully rehydrate it. YSI field electrodes should be placed in pH 4 buffer and laboratory electrodes should be placed in 3 M KCl (Ag+ free) solution. Both laboratory and field electrodes should be soaked for at least 24 hours before calibration. If the electrode performs poorly after rehydration and subsequent cleaning procedures are unsuccessful, the electrode may need to be replaced.

5. Check the temperature probe used by the instrument

Check the temperature specification of the probe. If the temperature probe is out of specification, the pH will not perform accurately because the electrode slope depends on the temperature of the solution. The accuracy of the pH data depends on the accuracy of the temperature data. Thermistors cannot be calibrated, so if your temperature sensor is no longer good, the electrodes will need to be replaced.

6. Always use fresh, unused, unexpired pH buffers for calibration

You never want to reuse buffers for calibration. Once buffers have been used for calibration, they are assumed to be contaminated and should not be used again. Reusing buffers can cause pH probes to degrade or not calibrate at all. This reuse can also make it difficult to determine whether the probe or buffer caused the pH calibration failure.

A good way to use a reuse buffer is to do a probe rinse only.

Expired buffer solutions should not be used (you see a theme here), and buffer bottles should not be opened. Carbon dioxide in the air changes the pH of the alkaline buffer solution, so the alkaline buffer bottle should only be opened briefly. Use opened buffer containers as soon as possible.

7. Perform at least two-point calibration

Better to perform at least a 2 point calibration, and the pH 7 buffer needs to be one of those points. The pH buffers used should differ by at least two pH units and should encompass the expected in situ pH conditions. Alkaline buffers should not be used unless the expected pH of the sample is higher than 7, as their pH changes rapidly through CO2 absorption.

A 3-point calibration is usually done if the pH conditions of the sample are not well known. Determine asymmetry and slope for 2-point and 3-point calibrations.

A 1-point calibration will only determine the zero point, not the electrode slope. 1-point calibration is of limited use and can only be done with pH 7 buffer. The pH values ​​obtained can be used for comparison with previous results, but are not absolute values.

Electrodes and calibration containers should be rinsed with subsequent solutions between calibration points, as any displacement buffer carried over may cause measurement errors. For example, if you have finished calibrating to pH 7 and are about to calibrate to pH 10, rinse with pH 10 buffer. Alternatively, the electrode can be rinsed with deionized water and carefully wiped dry. Calibration containers should always be kept clean. The size of the calibration vessel is usually not critical as long as the electrode junction is submerged in water.

8. Buffer 7 should be the first

Although not always necessary (and some instruments have automatic buffer recognition), it is better to start calibration with a pH 7 buffer and can easily be part of a standard operating procedure (SOP).

pH calibration does two things –

Establish a new electrode slope according to the Nernst equation to set the zero point.

由于这两者都可能随时间变化，因此需要频繁进行校准。

零点，也称为不对称电位/点，通常是将电极置于pH 7缓冲液中时的mV值。毫不奇怪，理论上的零点为0 mV。这是正确的，因为参比电极通常处于pH值为7的电解质溶液中。

如果参比电极和感应电极都处于具有相同pH的溶液中，则理论上  它们的电势应该  没有差异，导致pH计显示0 mV。一个  新的  电极将有一个不对称电位通常只有几毫伏如果已经精心准备。

9.如果可能，将校准重置为出厂默认设置

并非所有仪器都具备此功能，但是恢复为默认校准可以帮助消除校准错误警告。更好查阅用户手册。用户手册还将提供执行此任务的正确过程，因为该过程可能会因仪器而异。（用户手册可在我们的资源库中找到）

10.确认每个缓冲区中的探测响应时间

响应时间应不超过60秒，并且良好的电极应在该时间内保持稳定。该响应时间可能取决于探头的寿命和清洁度，而响应速度慢则表明电极已旧或变脏。

校准期间的问题可能是电极老化的结果。即使不使用电极，通常也可以使用12-18个月。如果在极端操作条件（强酸，高温）下使用，电极的老化速度会更快，从而导致电极响应变慢，斜率变化和不对称变化（即零点）。 

11.检查每个缓冲器中的毫伏

在检查传感器性能或校准时，更好在仪器上显示mV值，而不是显示pH值。并非所有仪器都具有此功能，但是某些仪器具有。

每个缓冲液中观察到的mV值应如下：

缓冲器7应为0至+/- 50 mV。

缓冲区4的正方向应与缓冲区7 mV值相差165至180 mV。

缓冲器10在负方向上应与缓冲器7 mV值相差165至180 mV。

12.绝不接受超出范围的校准

如果您接受超出范围的校准，则您的探头将无法正确校准。如果接受超出范围的校准，很可能不会收集任何可用的pH数据。

13.检查电解液液位

对于可填充电极，请确保电解质的填充液位至少比测量溶液液位高2 cm。如果电解液被污染，请更换电解液。

14.打开笔芯滑块/止动器

For YSI laboratory electrodes with a fillable reference, the first step in calibrating and/or taking measurements is to open the fill opening. Depending on the model, the fill port can be a slide (pictured left) or a plug (pictured right). The filler port needs to be kept open during calibration and measurement.

15. The junction temperature sensor should be immersed in

The reference junction needs to be fully immersed in the solution. The temperature sensor also needs to be in solution in order to accurately compensate the pH for temperature.

16. Check the selected buffer set

The pH of buffer solutions depends on temperature (as mentioned earlier), and the response varies by manufacturer. Likewise, the pH of the buffers within the buffer set can vary between sets. Modern pH meters automatically adjust for the respective temperature profile once the buffer settings used are properly set.