pH measures the acidity/alkalinity of a solution on a scale of 1 to 14 pH, with 1 being the most acidic and 14 being the most alkaline or basic. pH is a common measure of water quality used in environmental testing, food & beverages, cosmetics, pharmaceuticals and other fields.
Three basic styles of pH meters are available for varying applications:
pH pens are easy to carry, simple to operate, inexpensive, and also provide digital resolution and accuracy not available with test strips. For most applications, a pH pen will fulfill the need for precise, reliable pH measurements. pH pens are available with higher or lower resolutions and accuracies, and are priced accordingly. All are waterproof and float if accidentally dropped into water.
When seeking additional functions, such as datalogging and computer output, a portable pH meter’s larger case, keypad and display enable more complex menus, functions and outputs. Despite these sophisticated features, portable pH meters are still compact enough to be carried in the field. They accept a wide variety of replaceable electrodes, including probes reading mV and ORP (oxygen reduction potential).
Benchtop pH meters are preferable for long-term lab tests. They offer larger display than portable meters, an adjustable electrode holder arm, and can be permanently connected to AC power and a computer.
All pH probes drift and should therefore be calibrated prior to any series of readings. Fortunately, calibration is a simple process that uses standard pH buffer solutions. The user simply brackets the point he expects to measure by calibrating to buffer solutions above and below that point. For example, if expecting a reading of about pH 6, one would first calibrate the meter by placing the probe in a buffer of pH 7, and then pH 4. Many of our meters offer automatic buffer recognition, which recognizes and calibrates to the buffer solution with the push of a button.
ATC (Automatic Temperature Compensation)
pH is directly related to temperature, e.g. the pH reading of the same solution will vary (slightly) according to its temperature. This can be corrected manually or automatically. Manual correction requires the user to measure the temperature of the solution with a thermometer and enter the results into the pH meter. The meter then corrects the pH reading accordingly. Many pH meters and pH pens, however, feature automatic temperature compensation. The ATC pH probe contains its own temperature sensor and the meter automatically corrects for temperature without any input from the user.
Q: Does my pH electrode have to be wet?
A: All electrodes have a bulb which must be kept hydrated and a reference junction which must be kept wet to prevent excess leakage of the internal electrolyte solution from the reference junction.
Q: What should I store my pH electrode in?
A: Ideally, storage solution since it has the same chemical make-up of what is in the electrode itself, but if that is not available use buffer 4 or 7 solution. NEVER STORE IN PURE (DISTILLED) WATER!!! In the case of electrodes that are continuously immersed, storage is not an issue since they are constantly wet.
Q: What if I see white crystals on my pH electrode?
A: It is simply electrolyte (salt) and by soaking the electrode for a couple of hours the electrode will be fine unless it has been dry for long periods of time.
Q: How should I maintain my pH electrodes?
A: Periodically clean your electrode with mild acid or professionally formulated cleaning solutions and, of course always properly store your electrodes.
Q: How often should I calibrate my pH meter?
A: It can vary. However, by performing periodic checks with fresh calibration solution you can determine when and how often calibration is necessary.
Q: Which buffer solutions should I use to calibrate my pH meter with?
A: 99% of the time buffer 7 and buffer 4 solution are the two you need to perform a calibration. If you are measuring mostly in the alkaline scale, then you might use buffer 7 and buffer 10, although buffer 10 is not as stable a solution as buffer 4, and therefore has a much shorter life once exposed to air.
Q: Is it always necessary to perform a 2-point calibration or is a 1-point calibration sufficient?
A: If the last calibration was performed on the same day, a control calibration with one buffer is sufficient. Then only the zero potential is adjusted, the old slope will remain as it was. Otherwise, a 2-point calibration is recommended, because only then an actual probe status can be determined and taken into account for the measurements to come.
Q: Do dirty and wet cables have an influence on the pH reading?
A: Due to the extremely small currents which pass through the pH electrode, the cable, plug and connector must be kept clean and dry if reliable measurements are to be obtained.
Q: What is the lifetime of a pH probe?
A: The lifetime of a pH electrode depends on several factors including storage conditions, correct maintenance and the type of sample measured. Under normal laboratory conditions, for aqueous samples, the average lifetime is between 12 and 18 months, supposing of course that the electrode is kept clean and kept hydrated during storage. If the probe is used with dirty samples (e.g. stirred solutions with particles), is subjected to mechanical abrasion or used at high temperature or high pressure, the lifetime may be only a few weeks. In hot alkaline solutions, pH probes can be damaged after only a few hours. Regular maintenance helps pH probes keep working efficiently.
Q: How do I test my pH probe to see if it needs replacing..?
A: A simple test can be performed using Windex glass cleaner with ammonia, and any type of soft drink. The procedure is as follows; turn your meter or tester on and place the probe in a soft drink, which is acidic, and the reading will be from 2.7 to 3.9. Then go to the Windex with ammonia which is alkaline. Your display should move very quickly up the scale to a point above 10.0. If the probe slowly moves up the scale, then it is time to consider replacing it.
Q: Is it possible to use a pH probe in alcoholic solutions?
A: Yes, but short-term only. Longer exposure to high percentage alcoholic solutions leads to dehydration of the glass membrane which then has to be conditioned again.