All about pH
The following, as written, applies to the
Ultrameter™, TechPro™ and pDS meter instruments. However, the
basic operation and maintenance are the same for all pH or ORP sensors.
What is pH?
The negative logarithm of
hydrogen ions in a solution. Ratio of Hydrogen ions (H+) and Hydroxyl
ions (OH-) determine the pH value of a solution. Any hydrogen
activity will produce a 59.16 mV/pH unit across the glass membrane.
The measurement is expressed on a scale of 0.0 to 14.0. Water with a pH of
7 is considered neutral (H+ ions = 10-7 and OH- ions =10-7). A
solution is considered Acidic when the hydrogen ions (H+) exceed the
Hydroxyl ions (OH-) and a solution is considered an Alkaline (base) when
the Hydroxyl ions (OH-) exceed Hydrogen ions (H+).
How is pH measured?
A pH instrument consists of
three main components, refer to fig 1.
Teflon ® Dupont Company
A properly hydrated glass sensor will
produce a "Gel Layer" on the inside and outside of the glass
membrane. The "Gel Layer" enables hydrogen ions to
develop an electrical potential across the pH glass sensor and the
millivolt signal varies with hydrogen ion activity on the glass membrane
while submerged in the solution being tested.
The pH measuring cell:
Hydrogen sensitive glass is blown onto the end of an inert glass
stem. A silver wire treated with silver chloride (Ag/AgCl) is
sealed inside the glass (cell) with a solution of potassium chloride
saturated with silver chloride. The measuring solution has a
neutral pH level of 7 or 0 mV.
The Reference cell:
A silver wire treated with silver chloride (Ag/AgCl) is sealed inside
an inert glass housing (cell) with a solution of potassium chloride
saturated with silver chloride. The inert glass prevents
hydrogen ion activity from test solutions to influence the reference
cells constant millivolt signal. The combination of the
reference electrode silver-silver chloride wire and the saturated
potassium chloride solution develops a constant 199-millivolt
reference signal. The millivolt signal produced inside the
reference electrode does not vary as long as the chloride
concentration remains constant. The reference voltage is used as
a baseline to compare variations or changes in solution being
tested. The reference cell is in contact with the test solution
through a reference junction that is commonly made of porous Teflon ®
, ceramic or a wick type material called a pelon strip. This
junction completes the measuring circuit of the pH sensor.
Display meter: When the pH
sensor is placed in a solution, the pH-measuring cell develops a
millivolt signal that reflects the hydrogen ion activity of the test
solution. A high impedance meter accurately measures the small
millivolt changes and displays the results in pH units on either an
analog meter or digital display.
The pH glass membrane is sensitive to the
temperatures of solutions being tested. Prolonged use and/or
exposure to temperatures (above 35° C) will accelerate the aging and
increase the chemical attack to the glass membrane which will shorten the
overall service life of the sensor. ELEVATED
TEMPERATURES WILL SHORTEN THE SERVICE LIFE OF A pH SENSOR.
Increased temperatures will also decrease
the impedance of the glass membrane. The decrease of the impedance
effects the millivolt output of the glass membrane. Temperature
changes close to neutral (pH 7) usually do not effect pH levels, however,
when levels are < pH 3 and > pH 11 a dramatic error
may occur. This problem is resolved using a built in ATC (Automatic
Temperature Compensation) which uses a mathematical formula
(Nernst equation) to correct pH errors due to temperature factors.
Other factors, that effect
the life of a sensor:
Because standard glass electrodes are
manufactured using a silver/silver chloride electrode inserted into a
potassium chloride/silver chloride solution, the following list of
solutions cause the reference solution to precipitate. If the
following solutions are tested, it is recommended that the pH sensor well
be thoroughly rinsed. The testing of these
solutions will severely reduce the service life of the pH sensor:
Heavy metals - silver, iron, and lead
Low ion solutions - distilled water
High sodium concentrates
Fluorides (in high concentrations or
Note: This is not a complete list of
solutions that can cause the reference solution to precipitate.
Sodium ion error:
As solutions approach and exceed the
pH level of 12.0, the high concentration of sodium ions interfere with the
standard glass membrane and cause pH levels to be displayed lower than
actual pH levels. If solutions being tested are normally high
alkaline, (>12 pH) a probe manufactured with special glass may be
required. The special glass may be used throughout the pH range of 0
to 14, but due to the natural high resistance of the glass, it will
significantly increase the overall time needed to analyze a sample.
Constant use in solutions with pH levels higher than 12 will
reduce the life of the probe.
The break down of the pH sensor electrodes
and the depletion and/or saturation of the reference solution require your
pH instrument to be re-calibrated. This should normally be performed
twice a month, but depending on the actual use of the instrument it may be
necessary to increase the intervals between calibrations.
Refer to your
Myron L® Company instruction manual for detail instructions on your specific
instrument calibration procedures. (Get one here -
The calibration should be performed using
at least two pH buffer standards. The initial calibration should use
Myron L® pH buffer solution 7, this will check and allow the instrument to
be adjusted so its output reflects 0 millivolts, neutral, or pH 7. A
second calibration using a standard solution that reflects the normal
range of solutions being analyzed is needed. If acidic solutions are
normally tested, a Myron L® pH buffer solution 4 should be used. If
solutions to be tested are normally alkaline a Myron L® pH buffer solution
10 should be used. It is not necessary to calibrate your instrument
over three standards (4, 7, and 10) unless during normal daily use of the
instrument the solutions being tested varies from low to high pH
ranges. In this case, an increase of calibration intervals is also
How to maximize the life of
your Myron L® pH or pH/ORP sensor:
The Myron L® Company uses a general
purpose glass pH sensor. This glass sensor may be used in most
applications. To ensure maximum life of your Myron L® pH test
instruments, the following information should be considered whether you
are a distributor or an end user.
It is the experience of our repair
technicians that 90% of all premature pH sensor failures can be prevented
with the incorporation of a few maintenance procedures. The
following should be performed after using the Myron L® test instrument or
if you plan to store your test instrument for an extended period of time.
The pH sensor well (fig. 2) must
be filled with Myron L® storage solution (preferred), Myron L® pH buffer 4
or tap water with table salt added and its protective cap (with foam
insert) firmly installed.
Failure to do so will:
Allow the glass membrane to dry out.
A de-hydrated glass membrane will not produce the necessary "Gel
layer" on the sensor surface, which is essential to allow the
exchange of hydrogen ions (measure pH).
Allow airborne contaminants to settle on
the glass membrane surface. Once contaminants dry onto the
surface of the glass membrane it will inhibit the transfer of hydrogen
ions. (See Factory Approved Cleaning Process
Allow the reference junction to dry
out. The reference junction material is usually a wick or fiber
type material that completes the electrical circuit between the
reference electrode cell and the solution being tested.
Dehydration causes the reference solution to leach out of the
electrode cavity and form crystals in the junction. This is
normally referred to as the "Bridging effect".
Repeated dehydration of the pH or pH/ORP
sensor will cause the instrument to have a slower response time and be
more difficult to calibrate. Dehydration will significantly reduce
the normal service life of the sensor.
Store spare pH or pH/ORP sensors in a
refrigerator. "DO NOT FREEZE"
Take proper precautions not to allow the temperature to fall below
freezing, this will cause the solution to expand and may damage the
electrodes inside the sensor. Storage in a refrigerated environment
will slow the evaporation of the storage solution, but not prevent
evaporation. Always inspect and replace the storage solution in your
spare sensors on a regular basis.
using the Myron L® storage solution, it is common for white crystal
formations to form around the seal of the pH sensor well and protective
cap. This is a normal occurrence as the solution evaporates. Never
store the sensor in high purity water (distilled or de-ionized).
Cleaning Process for the pH sensor:
During the normal use of your Myron L® hand
held pH or pH/ORP instruments, the cleaning of your pH sensor bulb will
have to be performed. The cleaning is usually due to the deposits of
organic and inorganic contaminates left on the sensor from the solutions
being tested. If you suspect your instrument is inaccurate, the
display value drifts or the response is slow and sluggish perform the
Rinse the sensor well (three times) and
fill with Myron L® pH buffer 4 solution. If the pH continues
to drift below the pH 4 level (i.e. 3, 2, or 1) repeat the test using
Myron L® buffer 10. If the pH level drifts beyond the pH level of 10
(i.e. 11, 12 etc.) the cleaning procedure outlined below may be performed
to increase the performance and accuracy of your test instrument.
While performing the above tests, if the pH
levels of the buffer solutions 4 and 10 actually drift toward pH 7 this is
an indication that the pH sensor is damaged and needs to be replaced.
Wear proper eye protection and
gloves during the following cleaning procedures.
The Myron L® Company recommends the
following procedure to clean and recover your pH or pH /ORP sensor.
NOTE: Not all pH or pH/ORP
sensors can be recovered.
Fill the pH/ORP sensor well with 100%
alcohol isopropyl alcohol, if not available use rubbing alcohol
this will remove any oils.
Allow the sensor to soak for 10 minutes.
Rinse with RO or DI water.
Rinse the sensor well (three times) and
fill with Myron L® storage or Myron L® pH buffer 4. Replace the
protective cap and allow the sensor to recover over night.
Re-calibrate the instrument according to the Myron L® instruction manual that was
provided with your instrument. (Get it here -
Downloads) If the instrument fails to calibrate properly,
to the next step.
If the above procedure did not recover the
pH sensor function, perform the following:
Fill the pH or pH/ORP sensor well with a
hot salt solution 60° C (140° F), potassium chloride (KCI preferred)
or hot tap water with table salt (NaCl). Allow the solution to
Re-calibrate the instrument according to
the Myron L® instruction manual that was provided with your
instrument. If the instrument fails to calibrate properly, the
pH or pH/ORP sensor must be replaced.
The Myron L®
Company warrants the Ultrameter™, TechPro™ and pDS meter pH and pH/ORP sensor assemblies against manufacturer
defects for 6 months from the date the instrument or the
replacement sensor assembly was purchased and put into service by the end
user. Failure to maintain proper hydration of the glass pH sensors or the
use of the instrument in any manner not described in the operation manual
supplied with the instrument will void the factory warranty.
you do not use your Myron L® instrument on a regular basis or if you
are a stocking distributor, the storage solution in the pH or pH/ORP
sensor well will evaporate over time and must be replenished. To prevent
premature pH glass sensor failure, The Myron L® Company suggests the
implementation of a preventative maintenance program. Failure to do so
could void the factory warranty. The measuring of concentrated chlorinated
solvents such as Acetone, Xyline, and other similar harsh chemicals in
your Myron L® instrument is not recommended. The use of chlorinated
solvents may void the manufacturer warranty.