pH Measurement

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 pH 7 and the other at either pH 4 or 10.

Q. My co worker is using an ORP electrode to measure the same solutions as I, but our readings are not even close. Could there be something wrong with my electrode?

A. 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.

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 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. What is the difference between a combination electrode and a sensing electrode with a reference cell?

A. A combination electrode is more convenient and requires a smaller container and volume. The sensing electrode with reference cell combination allows you to select the reference cell most compatible with your solution. You can select the double junction, calomel, or ------ half cell. You will probably get better life from this combination, and can replace each cell individually.

Q. How do I take soil measurements?

A. Use our soil electrodes ----- or -----. 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 the stabilize.

Q. Why is a double junction electrode better than a single junction electrode?

A. A double junction electrode is less likely to become clogged because the second junction is located higher up in the probe out of contact with the sample.

Q. Is automatic temperature compensation (ATC) really necessary?

A. The necessity of ATC depends on the required accuracy of a pH reading. pH readings vary with temperature. For example, a sample with a pH of 7 at 25 deg C, may have a pH of 7.08 at 5 deg C and a pH of 6.98 at 60 deg C.

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

A. Before each use or set of uses.

Q. How can you unclog a pH electrode? How can you restore a dry pH electrode?

A. First check the interior wire. If corrosion is evident, replace the electrode. If not, then soak the electrode in pH 4 buffer solution at 50 degrees C for 2 - 4 hours. Restore a dry electrode by soaking it in tap water after rinsing out the refill chamber with distilled water and refilling with the proper solution.

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

A. 200 feet (61 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 with a built-in-transmitter. A transmitter will allow you to use your electrode up to 1000 feet (305 meters) from your meter provided you are not in a noisy environment.

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

A. You can extend the ATC wire with copper wire or a special extension cable.

Q. What is dead band?

A. When the input is varied to a devise until a detectable output charge is observed then the input is varied in the opposite direction until a detectable output charge is seen, the increment through which the input signal is varied is the deadband.

Q. If measuring the entire range of pH what slope should be used?

A. The upper end or pH 10 buffer.

Q. What is the difference between blue glass and amber glass, and what does that have to do with pH measurement?

A. The valance of Na+ is much larger than H+. Amber glass has a smaller pore size thus possibly discriminating between H+ and Na+ allowing only the smaller H+ to enter the ----- greatly eliminating Na+ interference.

Q. What pH electrode do I use for a specific application?

A. Follow the general rules below for selecting the right pH electrode:

Glass bodied pH electrodes may be used in most sample types.
Epoxy bodied pH electrodes are designed for rugged environments, multiple-user situations, and field or plant applications. Epoxy bodied pH electrodes should not be used in organic solvents.
For situations containing proteins, sulfide, and TRIS, use ----- electrodes.
For viscous or dirty samples, use pHoenix Free-FLow pH electrodes for best results and easy cleaning.
Refer to the ----- for a complete overview of pHoenix's pH electrode lines.

Q. How do I store my pH electrode?

A. Proper electrode storage maximizes electrode performance and extends electrode life. There are two ways to store a pH electrode:

Use pHoenix Electrode Company's DEDICATED PH STORAGE SOLUTION CAT NO. R001000
As a temporary substitute use 200 mL of pH 7 buffer to which 1 gram of potassium chloride (KCl) has been added.
For long-term storage information refer to the appropriate pHoenix pH electrode instruction manual.

Q. What filling solution do I use?

A. The recommended filling solution depends on the type of electrode. Some pHoenix electrodes have sealed references and do not require filling solution. For refillable pH electrodes, use the guide below to choose the correct filling solution:

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Q. How do I clean my pH electrode?

A. The solution used to clean a pH electrode depends on the possible contaminants. Use the guide below to choose the appropriate solution:

For general cleaning soak the electrode in 0.1 M HCl or 0.1 M HNO₃ for 30 minutes.
For removing stubborn deposits and bacterial growth, soak the electrode in a 1:10 dilution of household laundry bleach for 15 minutes.
For removal of protein deposits soak the electrode in 1% pepsin in 0.1 M HCl for 15 minutes.
For removal of inorganic deposits soak the electrode in 0.1 M tetra sodium EDTA for 15 minutes.
For removal of oil and grease rinse the electrode with mild detergent or methanol.
After any of the cleaning procedures, thoroughly rinse the electrode with deionized water, drain and refill the reference chamber, and soak the electrode in storage solution for at least 1 hour.

Q. Do pH buffers and filling solutions have a shelf-life?

A. The typical shelf-life for pH buffers and filling solutions is 2 years unopened and 6 months open. For best results, the pH buffer bottles should be sealed promptly to avoid carbon dioxide absorption.

Q. What is a good pH electrode slope range?

A. The acceptable slope range is 92% to 102%. Slopes below 92% indicate that the electrode may require cleaning or if cleaning does not help, the electrode should be replaced. Slopes above 102% indicate that the pH buffers are contaminated.

Q. Do I need an Automatic Temperature Compensation (ATC) probe?

A. The most common cause of error in pH measurements is temperature. The slope of a pH electrode is highly dependent of temperature and pH buffer values and sample values change with temperature. For the most accurate results an ATC probe is always recommended.
There are three advantages for using an ATC probe. The meter recognizes a particular pH buffer and autocalibrates with the correct pH value at the current temperature. The meter calculates and stores the correct slope value. The meter automatically adjusts the stored slope in memory to display the temperature adjusted pH value of the sample.

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:

The electrode millivolts in a pH 7 buffer should be 0+/-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. My pH electrode is drifting. What should I do?

A. There are three possible causes for electrode drift:

If the pH electrode is "out-of-box" and drifting, the electrode may not be properly conditioned. Refer to the appropriate electrode instruction manual for details.
If the pH electrode is stable in buffers but not in the sample, the electrode may be incompatible with the sample or application. For application assistance contact pHoenix Electrode.
If the pH electrode is drifting in buffers and samples, the electrode may require cleaning.