Does Your pH Reading Measure Up?
Simply stated, pH is the measurement of the relative acidity or alkalinity of a sample based upon the level of hydrogen ion activity in a
water solution. The pH scale ranges from 0 to 14, with a measurement below 7.0 pH being an acid, at 7.0 pH a neutral, and above 7.0
pH an alkaline. So wine with a pH of 3.2 is an acid, milk with a pH of 6.6 is almost neutral slightly acidic, and seawater with a pH of 8.3
is alkaline.
Pure water is defined as having a 7.0 pH at 25 deg C, while surface water ranges from about 6.5 to 8.5 pH and ground water ranges
from 6.0 to 8.5. When we get outside these ranges, water can get very nasty. Water with a pH less than 6.5 is acidic, soft and
corrosive. It can leach ions such as iron, copper, lead, zinc, and manganese, resulting in water containing elevated levels of toxic
metals. It can hurt pipes, cause corrosion and have a sour taste. Water with a pH greater than 8.5 is considered hard. Hard water
does not pose a health risk, but it causes scale and deposits, adds an alkali taste to the water and makes for bitter coffee.
In 1945 Dr. Beckman developed the first pH meter and started a revolution in analytical testing. The measurement of pH is one of the
easiest and most frequently run tests in analytical chemistry. If done correctly, this simple test can provide important information on
practically every step of water treatment: neutralizations, water softening, precipitation, coagulation, disinfection, corrosion control, etc.
Unfortunately, too many people try to cut corners and skip steps, thereby jeopardizing the accuracy of their reading.
The pH Measurement System
A good pH measurement system consists of four key elements: a pH meter, a pH combination electrode, a temperature probe (ATC),
and NIST traceable pH buffers. All four elements must be present to ensure accurate readings.
1.
pH Meter: This consists of a potentiometer with a user interface that converts a mV signal into a pH reading and calculates
and confirms an acceptable electrode slope. Either a benchtop or a portable unit can be used to meet your requirements.
2.
pH Electrode: The electrode sends a mV signal to the meter based upon the difference in hydrogen ion concentration
between your sample and the reference electrode. (Today most pH electrodes are combination electrodes—they include an
indicating and reference electrode combined into a single compact body.)
Dirty and dehydrated electrodes account for 90+% of pH problems: drift, sluggish response, and unstable values. Depending
upon the type of electrode and samples, electrodes have a life of about 6 to 24 months.
Electrodes over 2 years old do not measure up! Get a replacement.
3.
Temperature Probe (ATC): pH readings vary with temperature (for example, a sample with a pH of 7.0 at 25 deg C will
have a pH of 7.08 at 5 deg C and a pH of 6.98 at 60 deg C), so temperature compensation is required to standardize pH
values. A “triode” is a pH electrode that includes a temperature sensor—so you do not need a separate ATC probe.
Otherwise, a temperature probe is required with your pH electrode to have an EPA compliant pH measurement.
4.
NIST Traceable Buffers: Your pH buffers are the only part of the measurement system that are certified and traceable. It is
important to perform a two- or a three-point calibration with fresh pH buffer at least once every eight hour shift to ensure the
accuracy of your pH readings.
Beakers filled with buffer covered with watch glasses and parafilm do not measure up! Cross-contamination is the
least of your worries. As a buffer solution is exposed to the air, carbon dioxide is absorbed by the solution, slowly changing
the pH. We once monitored the change of our pH 10 buffer during an afternoon—and the pH 10 buffer changed more than a
half pH unit. No getting around it—if you calibrate your meter with a reused buffer that has been sitting out in the lab, you
lose accuracy.
With these elements in place, you are off to a good start to ensure your pH readings will measure up!
Selecting a Probe to suit your Application
Yes, your pH electrode is the number one source of trouble. To get the most from your pH electrode, make sure you select an
electrode to meet the requirements of your application.
•
Glass verses Epoxy- Epoxy body electrodes are generally used in the field. They are a good choice where rough handling
and breakage are a problem. Do not use for samples with a high concentration of organic solvents. Not suggested for
samples with temperatures over 60 deg C.
Glass is ideal for laboratory usage. Although very fragile, they can withstand higher temperatures and organic solvents.
Generally glass has better accuracy and a longer life—as long as Bubba does not break it.
•
Sealed verses Refillable- Sealed electrodes are low cost and low maintenance—but also have a shorter life (usually less
than a year).
Refillable electrodes give a quicker response and will last up to 24 months with proper care.
•
Single Junction verses Double Junction- Single junction electrodes are suited for clean, potable water—nothing else.
Double junction electrodes are less likely to get clogged and can handle influent, well water, and process streams. The
electrode has an “internal buffer zone” that protects against heavy metals and other contaminations.
We recommend the following electrodes for the suggested applications:
Clean Water Lab- glass, refillable, single-junction electrode.
Dirty Water Lab- glass, refillable, double-junction electrode
Clean Water Field- epoxy, sealed, single junction electrode.
Process and Influent-epoxy, refillable, double-junction electrode
Wastewater- epoxy, refillable or flushable, double junction electrode
Sludge-epoxy, flushable, double junction electrode
pH Maintenance
You have the components for a great pH measuring system, and you have selected an electrode to meet your application. You
are on your way—but do not forget that a little maintenance can go a long way to ensure the life and accuracy of your system.
•
Condition a new probe - Fill refillable electrode with fill solution (not required for sealed electrodes). Soak in pH 4 or pH 7
buffer for 1 to 2 hours to hydrate the glass bulb.
•
Electrode Storage - When you are done with your measurements, rinse and soak the electrode in electrode storage
solution. pH electrodes must always be stored wet. (For long term storage, add storage solution in boot or electrode storage
bottle before sealing and packing away.)
NEVER store your electrode in DI water, solvents, acids or pH 10 buffer! This will kill your electrode.
•
Fill solution - Replace fill solution in refillable electrodes every 2 to 3 weeks. Keep the fill hole open during measurement
and storage (the only exception is for long term storage).
•
Electrode Cleaning
For oil or grease, soak in a warm alconox solution; then rise and soak in storage solution.
For bacterial growth, soak in a 1:10 mixture of bleach and water for 15 minutes; rinse and soak in storage solution.
For inorganic deposits, soak in 0.1M EDTA for 15 minutes; rinse and soak in storage solution.
For a clogged reference junction and dry gel electrodes, heat a .1M KCl solution to about 50deg C, and let slowly
return to room temperature with electrode in solution; again, rinse and soak in storage solution.
If you do not have storage solution, 4M KCl solution, pH 4 or 7 buffer work great to store electrodes. In a worst-case
situation, an electrode can be stored in tap water for a short period of time.
•
Buffers, Storage and Fill Solutions-- The manufacturer of your pH meter should not dictate your brand of pH solutions and
buffers. Shop and save some money; any NIST traceable pH buffer or pH solution will provide accurate results.
In summary, good maintenance, selecting the proper electrode for the application, and having the components of a complete pH
measuring system should get you on the road to good pH measurements. There are many quirky samples, like ultra-pure water,
but we do not have time to cover all the potential problems. You can always call Orion’s Technical Service to get additional
support for your application questions.
For customer service, call 1300-735-295
Email: [email protected]
Visit us online: www.thermofisher.com.au
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