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Use, Care and Calibration of pH Testing Tools
Document Number: 149
Introduction
Maintaining and controlling proper pH levels is essential to many agricultural,
industrial and environmental processes. In agriculture, proper pH of the
soil ensures the best growing crop. For drinking water, processing plants
have to control pH in order to produce safe drinking water. Environmentally,
pH monitoring and control are critical to prevent damage or deterioration
of the quality of all plant, animal and human life.
pH is a numeric scale used to express a solution's acidity or alkalinity.
Acidity or alkalinity are terms that refer to a solution's concentration
of hydrogen ions. The greater the concentration of hydrogen ions, the
more acidic the solution; fewer hydrogen ions and a solution is considered
alkaline. The pH scale ranges from 0 to 14. Seven is the mid-point at
which a solution (such as distilled water) is neither acid nor alkaline.
A pH lower than seven indicates more acidic solutions like vinegar (3.0
pH), sulfuric acid (1.2 pH) or orange juice (3.7 pH). pH values higher
than seven represent alkaline or "basic" solutions such as sodium hydroxide
(pH 13), ammonia base household cleaners (pH 12), and potassium hydroxide
(pH 14).
Testing
Tools
The first step in controlling pH is measuring it. Today pH can be measured
by various methods including pH paper, digital readout pocket testers,
portable and bench top meters.
pH
paper is an inexpensive and quick way to check pH. Strips of paper
are impregnated with a chemical indicator. This indicator, when immersed
in a sample liquid, turns the paper test strip a specific color. The pH
of the sample liquid is determined by visually comparing the color of
the test strip to a standard chart that is provided. These indicators
are available for every pH range but have limitations to their accuracy.
They can be particularly difficult to interpret when dealing with colored
or turbid samples.
An alternative to pH paper is the pocket
tester. Convenient and easy to carry, these testers are ideal for
filed use, pH tests can be made quickly without having to take a sample
back to the laboratory. Pocket testers provide enough accuracy for general
applications and are relatively inexpensive.
When more accuracy is required, portable
meters are a good choice over the pocket tester. Most portable meters
are larger in size than pocket pH testers. They are durable and well suited
for accurate pH readings in the field. They use a wide variety of replaceable
pH electrodes for individual applications and conditions.
Bench
top meters are found mainly in testing or research laboratories. They
are generally more expensive but offer increased accuracy and more testing
options. Bench top meters also use a wide variety of replaceable pH electrodes.
Types of Electrodes
pH
electrodes consist of a general purpose glass electrode (measuring
electrode) and a reference electrode. They are built into one electrode
and referred to as a combination pH electrode. Combination pH electrodes
are filled with a salt solution: a mixture made up of Potassium Chloride
(KCl) saturated with a silver/silver chloride (Ag/AgCl) reference solution.
The combination electrode is ideal for general purpose applications.
Because standard electrodes have silver in the reference solution inside
of the electrode, there are a number of applications where this type of
electrode cannot be used. The following are solutions which cannot be
measured with general purpose electrodes.
- Heavy metals such as silver, iron and lead.
- Proteins.
- Organics such as acetone.
- Low ion solutions such as distilled water.
- High sodium concentrations such as solutions containing large amounts
of salts.
- Sulfides.
If a sample contains any of these contaminants, the pH electrode may
work for only a short period of item before it fails.
To check the pH of the above mentioned solutions, the following electrodes
can be used:
Colomel Reference electrodes are designed to work in
solutions containing proteins, organics, low ion activity and heavy metals.
They are filled with Mercury/Mercury Chloride (Hg/Hg2Cl2) reference solution.
Double-Junction Reference electrodes are designed to
work in the same applications as the colomel electrode as well as work
in highly concentrated solutions. This is due to the presence of two reference
junctions to filter out any potential contamination of the reference electrode.
Teflon Junction Reference electrodes are designed for applications where
the solutions to be measured can clog the reference of a standard electrode.
Teflon Junction Reference electrodes are recommended
when dealing with solutions like paints, gels and pastes.
Electrode Care-Preparing the pH Electrode to
Measure Solutions
The pH electrodes are shipped moist with storage solution. On occasion,
after an electrode has been sitting in storage for a period of time, white
crystals may appear near the bulb of the electrode or around the electrode
cap. These crystals result from the potassium chloride solution (KCl)
that has evaporated and crystallized. Prior to using the electrode for
the first time, rinse the electrode with some distilled water, and condition
it in a buffer solution for about 10-20 minutes.
Electrode Conditioning
- Remove the protective cap from the bottom of the sensor and rinse
the electrode with distilled or deionized water.
- Place the electrode in a beaker of one of the following liquids for
one hour to rehydrate the electrode.
- Potassium Chloride (KCl), 3.8 or 4.0 molar concentration.
- 4.01 Buffer Solution.
- 7.00 Buffer Solution.
- Tap water.
- After one hour, rinse the electrode with distilled water. You are
now ready to calibrate the meter and take measurements.
Calibration
How do you calibrate? To properly calibrate a pH meter, you must choose
a minimum of two buffers for a two point calibration (except pH meters
that have only a 1-point calibration feature), one always being pH 7,
and a second point which covers the expected pH of your sample. For example,
if you are measuring an acidic solution (pH below 7), you would choose
the pH 4.01 and 7.00 buffers to calibrate the meter. Likewise, if you
were measuring mostly basic samples (pH above 7), you would choose pH
7.00 and pH 10.00 buffers for calibration.
Some meters refer to calibration as standardized and slope. To standardize
a pH meter means to calibrate at a pH of 7.00. After doing this, you slope
the meter to either the pH 4.01 or 10.00 buffers.
Electrode
Storage
After measurements have been made, the electrode should be stored in
a manner which will keep the bulb of the electrode moist or hydrated.
All electrodes are shipped with a protective cap, also called a "rubber
boot". This cap can be used to hold a small amount of liquid. Add a few
drops of potassium chloride (KCl) or 4.01 buffer solution to the cap.
Then place the cap back on the electrode. This method works for long or
short term storage.
The electrode can be placed in a beaker or an electrode saver bottle
filled with KCl solution. Buffers with a pH of 4.01 or 7.00 are also good
for storage. For short term storage, regular tap water can be used. Do
not use distilled or deionized water.
Commonly Asked Questions
| Q. |
What is ATC? |
| A. |
ATC (Automatic Temperature Compensation) is an option found on pocket
testers and handheld and benchtop meters. Click here to read
more on temperature compensation for pH instruments. |
| Q. |
Why is it used? |
| A. |
ATC allows the meter to compensate for temperature variation that
may affect the accuracy of pH measurements. |
| Q. |
How is this done? |
| A. |
Meters with ATC accept one of several types of temperature probes
or have a temperature sensor built into the electrode. These probes
or electrodes measure the temperature of the sample and determine
the pH value at that temperature. |
| Q. |
Do pH meters need calibration? |
| A. |
Yes. Before taking the pH measurement of your solution, you must
calibrate the pH meter. Particularly if it has been stored or used
to test a pH range that vastly differs from the one you currently
need to test. When you calibrate a meter, you are simply checking
and fixing the pH meter and probe ensuring that it reads pH correctly. |
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Please Note: The information contained in this
publication is intended for general information purposes only. This publication
is not a substitute for review of the applicable government regulations
and standards, and should not be construed as legal advice or opinion.
Readers with specific questions should refer to the cited regulation or
consult with an attorney.
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