pH Measurement FAQs

FAQs about pH measurement

1. When ordering a pH meter, what accessories do I need to start measuring? You need a pH electrode and at least two pH buffers, one at pH 7 and the other at either pH 4 or 10. Doing a 3-point calibration offers the best accuracy, otherwise for a 2-point calibration the 2nd point selected should be the closest to the sample value.

2. Two different ORP electrodes measuring the same solution are giving two different readings; is one of them faulty? No. Because ORP is a relative measurement, it is almost impossible to compare two ORP electrodes directly. ORP electrodes come equipped with bands made up of platinum or gold, for example. Each band type will give you a different reading in the same solution. Even if the electrodes are of the same band type, the leak rate through the reference junction will affect your readings.

Instead, simply measure two solutions and note the difference between the two electrodes. Once again, the difference between two solutions should be compared. You should be looking for a change of state, rather than an absolute measurement. You can check your electrode using pH buffer and quinhydrone, or one of the available prepared ORP calibration standards.

3. Can I measure the pH of a gas? The only way to measure the pH of a gas is to dissolve it in distilled water and measure the mixture. Technically, the pH of the distilled water/gas mixture will be that of the gas.

4. How should I store my electrode? The best solution for short-term electrode storage is a prepared storage solution. Alternatives in descending order are: pH 4 buffer, pH 7 buffer, or tap water. Never store your electrode in distilled/deionized water as it will leach out the electrolytes.

Long-term storage is best done by storing the electrode dry and rehydrating it prior to use by soaking in a pH 4 buffer for a minimum of a half an hour. If it is a refillable electrode, empty the fill solution, then refill and hydrate in pH 4 buffer for a minimum of half an hour.

5. What is the difference between a combination electrode and a sensing electrode with a reference half-cell? A combination electrode works in virtually all pH applications. It combines the reference and sensing elements into a single electrode. The separate sensing electrode with reference half-cell combination is rarely used as it is bulky and not convenient.

6. How does one take soil pH measurements? Use our soil electrodes (EW-05992-62 or EW-05992-60). Prepare the sample by combining a 5-g soil sample with 5 g of 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. It is important to follow the same procedure each time in order to produce consistent results.

7. When is a double-junction electrode better than a single-junction electrode? In dirty samples containing particulates, proteins, sulfides, and Tris buffers, a double-junction electrode is protected from having the second junction becoming clogged. Single-junction electrodes should be considered primarily for use with clean water samples.

8. Is automatic temperature compensation (ATC) really necessary? Temperature influences pH. Using ATC is always recommended and makes the reading more accurate as the meter compensates for errors associated with the electrode, but it does not correct the pH value to any specific temperature. All samples respond to temperature differently, so it’s impossible to program the meter to adjust for pH changes with temperature for all samples. A pH buffer can be standardized to a particular temperature because it has been tested and this test determined how it will change with temperature. The table of temperature vs pH on the buffer bottle will show this.

Temperature effects are nonlinear and should be considered whenever high accuracy is required or when there are wide variances from the standard of the calibration buffers (typically 25ºC).

9. How do I connect my electrode with BNC connector to wire terminals? Although some controller manuals may suggest cutting the end off, this is not recommended. The best way is to use an adapter to go from the BNC to wire ends.

10. How can you unclog a pH electrode? To unclog a clogged junction, soak the electrode in a pH 4 buffer solution or KCl solution heated to 60 to 80°C for approximately 10 minutes. Allow it to cool in unheated solution, and then rinse with distilled water. If these steps do not restore the performance, replace the electrode.

11. How far can my pH electrode be from my meter? 50 feet (15 meters) is the maximum distance an electrode can be from a pH meter. If the distance is greater, you will need a transmitter. Use either a 4 to 20 mA transmitter (make sure your pH meter accepts 4 to 20 mA signal) or purchase an industrial electrode and a preamplified cable, which will allow you to use your electrode up to 500 feet (152 meters) from your meter provided you are not in a noisy environment.

12. How do I extend the ATC wire on my industrial pH electrodes? You can extend the ATC wire with a shielded copper wire or a special extension cable.

13. What is dead band? The term “dead band” refers to an area of a signal range (or band) where no action occurs. In pH control systems, the purpose is to prevent frequent turning on and off of pumps or valves which tends to shorten the life of the equipment. Dead band is different from hysteresis. With hysteresis there is no dead zone, the output is always in one direction or another.

14. If measuring the entire range of pH, what slope should be used? It is recommended to use the upper end or a pH 10 buffer. Be aware that some meters average the total slope, whereas others use the slope per segment.

15. Should the hole in a refillable electrode be left open or closed? Use the easy-to-remember phrase "Open it when you use it, close it when you don't." For a refillable electrode to work properly, the filling solution must weep out of the junction. If the fill hole is closed, it will slow the amount of filling solution that is able to leave the junction.

16. What is the best pH electrode for viscous fluids? Generally, a flushable junction is the best option. The liquid-to-liquid junction ensures stable and accurate readings in viscous and dirty samples.

17. When do I need to use a calomel electrode? It is never necessary. Calomel electrodes were once widely used, however since they contain trace amounts of mercury, a substance which has become increasingly banned, it is being phased out. A double-junction or a Thermo Scientific™ ROSS™ electrode will fulfill the same application requirements without the hazards of mercury.

18. What would I use to measure hydrofluoric acid (HF)? Since hydrofluoric acid etches glass, it can be difficult to measure. A toroidal conductivity probe can be used with a conductivity meter. The readings can be used to correlate the percentage of HF in a solution. If only an actual pH reading will suffice, there are special probes such as a nonglass design and a flat surface probe with a special glass formulation that may be used. Contact Cole-Parmer for a recommendation for which will be best suited for your particular application. Standard pH probes should not be used even for short periods as the adverse effect on the glass is immediate.

19. How often should I calibrate my pH meter? Calibrate your meter prior to each use or set of uses at least once per four hours of use. Also, changing electrodes requires recalibration. Calibration corrects for inconsistencies inherent in all electrodes, particularly as they age or become contaminated.

20. How can you restore a dry pH electrode? Restore a dry electrode by soaking it in pH 4 buffer or tap water for a minimum of half an hour. If it is a refillable electrode, rinse out the refill chamber with distilled water and refill with the proper solution, then soak it in pH 4 buffer or tap water for a minimum of half an hour.