Q. If I order a pH meter, what accessories do I need to use with it?

A. You need a pH electrode and at least two pH buffers, one at pH7 and the other at either pH4 or pH10.

Q. How often do I need to calibrate my pH meter?

A. Before each use or set of uses.

Q. Can I use any pH electrode with any pH meter? 

A. Generally, Yes. The output of a pH electrode is a DC mV level which is (theoretically) the same for all pH electrodes. The impedance of pH electrodes is pretty high (50Mohm or higher) and some pH electrodes using special glass can be as high as 1000Mohm. Some of the cheaper pH meters do not have a high enough input impedance to cope with the special electrodes. Check the meter specifications and look for an input impedance of at least 10Gohm (10 000Mohm) if you want to use high impedance electrodes. 

Q. How long do pH electrodes last? 

A. How long is a piece of string? The lifetime of a pH electrode depends on many factors including storage, cleaning and the solutions it is used in.
For normal laboratory work I would expect a life of around a year if the electrode is kept clean and stored with a wet bulb in a cool environment.
A similar lifetime could be expected for an industrial electrode in a non aggressive environment. In very aggresive applications an electrode life measured in days is possible.
If your electrodes are not lasting more than three months then you probably have not got the correct type for your application. Some electrodes have been known to last several years.

Q. Is there any significant difference between a glass body electrode and a plastic or epoxy body electrode?

A. Basically no. They both use a glass membrane and ceramic junction to perform the measurement and either should give similar accuracy. Plastic electrodes normally work up to temperatures of around 80°C whereas glass can get to 100°C or higher. The choice is usually one of cost and/or ruggedness for the type of use. An epoxy body laboratory style electrode would probably be the better choice for field use because it is harder to break. Glass is often preferred in the laboratory because it is easier to clean. Some chemicals can soften or melt epoxy plastics which can limit their use. Epoxy plastic electrodes are usually the least expensive choice and are ideal for household type measurements like fish tanks, ponds & swimming pools.

Q. How should I store my electrode?

A. The best solution for electrode storage is 4M KCl. pH 4 buffer, pH 7 buffer, or tap water are also acceptable. Never store your electrode in distilled water.

Q. When do I need automatic temperature compensation? 

A. A pH electrode gives a temperature dependant mV output which is positive below 7 pH and negative above. For pH values close to 7 the output is close to zero so that temperature effects are minimal. The further away from 7 you go, the worse things become. If the solution temperature is stable then a manual adjustment is all that is necessary but if the solution temperature changes substantially, and the pH is very low or very high, then temperature compensation is a must.
Different instrument manufacturers use different temperature sensors for automatic temperature compensation so be careful to specify the correct one for your meter.
This chart gives the approximate readings one would expect on a pH meter calibrated at 25°C without temperature compensation.

Q. Should I use an electrode with an integral temperature sensor or a separate electrode and sensor? 

A. Either option should give you equally accurate results. The considerations are cost and convenience. 
When buffering your electrode to your meter both pH electrode and temperature sensor should be at the same temperature for best accuracy. The integral design is obviously more convenient when regular buffering is required. 
Temperature sensors do not have a limited operating life like a pH electrode so that only the pH electrode needs replacing from time to time. A pH electrode with integral temperature sensor normally costs at least as much as the two combined which makes it a more expensive choice.

Q. What is a double junction electrode and when should I use one? 

A. Double junction electrodes have an extra chamber of electrolyte with a second junction between the reference electrode and the solution under test. This electrolyte is chosen to minimise transfer of ions from the solution under test which may react with the reference junction which is usually Ag/AgCl. If the solution under test contains anything which reacts with silver then a double junction is essential. Problem chemicals include halides, cyanides, proteins, tris buffers and heavy metals. 
The only drawback to double junction electrodes is that they tend to respond slowly and have a wider hysterisis than single junctions, otherwise all electrodes would probably be made as double junction. 
A new technology double junction with the accuracy and response of single junction electrodes is now available from Phoenix Electrode called the Red Rod electrode.

Q. Can I measure the pH of a gas?

A. The only way to measure the pH of a gas is to dissolve it into a distilled water and measure the mixture. Technically, the pH of the distilled water/gas mixture will be that of the gas.

Q. How far can by pH electrode be from my meter? What if it is too far?

A. 200 feet (61 metres) is normally regarded as the maximum distance an electrode can be from a pH meter, but this can vary depending on the amount of electrical noise in the vicinity. Try to keep the electrode cable away from power cables.
If the distance is greater, you will need a pre-amplifier or a 4~20mA transmitter. A pre-amplifier is normally battery powered and mounts right next to the electrode or can even be mounted in the cap. The output is the same mV signal as the electrode but at a lower impedance which increases noise immunity. The output from a pre-amplifier can be fed into the normal meter input socket.
A 4~20mA transmitter will allow you to use your electrode at much geater distances - even kilometers but requires an instrument with a 4~20mA input socket.

Q. How do I extend the ATC wires on my industrial pH electrodes?

A. Most ATC devices are resistive (such as thermistors) and use ordinary copper wire, so extension is easy. The accuracy of low resistance devices like Pt100s (in 2 wire configuration) can be compromised if too much extra resistance is added. Some devices may require a special extension cable so check with your supplier.

Q. Why will my pH system no longer autocalibrate?

A. When the pH system will not autocalibrate, the meter, pH electrode and pH buffers should be checked systematically.
If your meter has a millivolt mode, measure the electrode millivolts in pH buffers. In order for ORION meters to recognize pH buffers for autocalibration, millivolt (mV) limits have been placed on each buffer (other manufacturers use similar values):

• The electrode millivolts in a pH 7 buffer should be 0mV ± 30 mV.
• The electrode millivolts in a pH 4 buffer should be 140 to 210 mV.
• The electrode millivolts in a pH 10 buffer should be -140 to -210 mV.
• If the millivolt values are outside of the above ranges, clean the pH electrode. If cleaning does not return the mV to an acceptable range, either manually calibrate or replace the electrode.
  Note: As long as the pH electrode has a slope between 92% and 102%, the electrode should be working properly.
  The pH buffers should be replaced if the measured millivolts are outside of the acceptable ranges. Contaminated buffers may slightly contribute to shifted millivolt values.

Q. How do I take soil measurements?

A. Use a rugged bulb electrode like the 5773521 or a pHree-flow electrode for easy cleaning. Prepare the sample by combining a 5g soil sample with 5g distilled water, mixing thoroughly, and allowing the mixture to settle for 10 minutes. Carefully insert probe so bulb is in the soil part and the junction is in the supernatant. Allow reading to stabilize.

Q. When should I use a half cell?

A. When you are measuring a pressurized flow in a stream or pipe. The reference half cell would be mounted upstream, the measuring electrode would be mounted down stream.

Q. How does a high concentration of surfactants affect pH measurements and what can be done about it?

A. I would expect that any pH electrode will in time become coated by the surfactants in the sample causing slow response, drifting, and lowered slope. If you wish to remove the coating, try some reasonably hot tap water and gently rub the coating off the pH bulb and reference junction. If this works, buy two electrodes and put one in service while the other one is being cleaned. Basically, you will need a pH electrode with a low resistance pH bulb, and the reference portion of the electrode should have a double junction design with an outer chamber that is refillable, e.g. pHoenix Cat. No. 5732703. The low resistance pH glass helps to improve the response time of pH glasses which slow down in surfactant-concentrated samples. Considering the filling solutions available, the best one would be Lithium Chloride in Ethylene Glycol.  But I prefer that you take a 90 ml aliquot of the regular 4 M KCl fill solution and dilute it to volume with the sample mixture in a l00 ml volumetric flask.  Use this solution to fill the reference chamber of the electrode. This solution will help reduce junction potentials which form between an aqueous reference fill solution and surfactant-concentrated samples. There is also the problem of calibrating the electrode with aqueous buffers like pH 7, 4, & 10, since you are not calibrating the electrode with surfactant-concentrated buffers.  Please be aware that you may have as much as a 0.5 pH unit error just from calibration.