PH 183 -- pH Electrode
BIOPAC
WWW.biopac.com
Systems, Inc.
Application Note PH 183
SS15L -- pH
Electrode
pH Electrode with 3 Different Buffer Solutions
SS15L Specifications
BIOPAC Software
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BSL PRO v.3.6.6
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BIOPAC Hardware
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pH Electrode SS15L
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Other Hardware
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Double junction
Refillable electrode
Glass body
Measures pH within the range of 0-14
3.5 ounces
3.25m in length
1.2cm in diameter
IMPORTANT: This electrode is not temperature compensated. Your
sample will have different pH values depending on the temperature of
the sample. See Temperature Compensation for details.
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PH 183 -- pH Electrode
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Rubber Gloves
Squirt bottle of deionized
or distilled H2O
2 medium test tubes or
small beakers
1 large beaker ½ filled
with water (to be used as
rinse beaker)
2 standard buffer
solutions; one preferably
neutral (pH 7)
Objective:
This Application note will give an overview of pH theory, Calibration procedures, tips for taking good
measurements, cleaning and storage recommendations/requirements, and how to replace the reference fluid
in the pH Electrode.
Abstract:
Basics about pH, Acids, Bases and Buffers:
pH
pH is an abbreviation symbol used to indicate the concentration of hydrogen ions
in a solution. By definition, pH stands for the negative logarithm (base 10) of the
hydrogen ion concentration:
pH = - log10 [H+]
The brackets denote hydrogen ion concentration in grams/liter.
Examples:
If
= 10-8 gm/liter, pH = 8
If [H+] = 10-7 gm/liter, pH = 7
pH = 7 is an easier way to say [H+] = .0000001 gm/liter.
pH = 8 is an easier way to say [H+] = .00000001 gm/liter.
[H+]
Notice that a change of one whole pH number (e.g., 7 to 8) represents a tenfold change in H+
concentration.
The pH scale extends from zero to fourteen (0-14). pH values less than 7.00 are indicative of
an acid solution. pH values greater than 7.00 are indicative of an alkaline (basic) solution. The
pH value of 7.00 is considered neutral (neither acidic nor basic).
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PH 183 -- pH Electrode
Reaction:
(more H+)
pH : 0 1 2 3 4 5 6
Acidic
Neutral
(less H+)
7
8 9 10 11 12 13 14
Basic
Acid
An acid is a substance that can donate (give up) free hydrogen ions to a solution. When placed into a
solution, an acid increases the concentration of H+ in the solution, thereby decreasing the pH of the
solution.
Base
A base is a substance that can accept (take up) free hydrogen ions from a solution. When placed into a
solution, a base decreases the concentration of H+ in the solution, thereby increasing the pH of the
solution.
Chemical Buffer
Chemical buffers are substances that prevent marked changes in the pH of a solution when acids or
bases are added to it. A buffer system is composed of a weak acid and its conjugate base, together
called an acid-base buffer pair.
Note about the electrode values: The electrode provides approximately a single digit pH value change for
every 5mV change in the electrode reading, either positive or negative depending on whether the pH is
above 7 or below it. A neutral buffer solution of pH 7 will have a reading of about 0mV. A solution with a pH of
10 will read about –15mV, and a solution with a pH of 3 will read about 20mV.
A brief formula to find the reading in mV from pH is as follows:
Subtract the pH from 7 and multiply by 5.
5(7-pH) = mV reading
If the pH is 10:
7-10 = -3 -3 x 5= -15 mV
If the pH is 3:
7-3 = 4 4 x 5 = 20 mV
Notes For Taking Good Measurements:
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Each pH electrode will give a slightly different reading, and it is only by calibrating the pH electrode
with buffer solutions of known pH that the pH electrode reading will be most accurate. If comparing the
pH of solutions, it is suggested that you use the same pH electrode for taking the measurements in all
solutions.
Solutions which are to be used as a reference (such as your pH neutral buffer solution and your high
or low pH buffer solution that will be used to calibrate the pH electrode) need to be fresh, and as pure
as possible. They should be poured from the bottle and used within a few minutes. If left sitting out,
over time they may change pH slightly by reacting with the open air. This also means you should keep
the lid tightly closed on the bottle of buffer solution, and make sure that the buffer is still good.
Solutions typically last about 2 years unopened, and 6 months opened.
Hold the pH electrode as vertical as possible. Use a narrower test tube so you can lean the electrode
against the side and not change the angle too much. The angle may have a slight affect on the
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PH 183 -- pH Electrode
reading, so moving it more than 15º from straight vertical is not advised.
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Give the solution a slight stir when inserting the pH electrode in the measured solution, this will give
you a more accurate reading when the recording smoothes out in about a minute. Once you have
given the solution a stir, let the electrode sit completely motionless and resting either on a stand or
leaning as vertically as possible in the test tube or beaker.
Rinse the electrode tip with dH2O and gently dry it before taking measurements to keep all solutions
undiluted and to give you a more accurate reading.
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When calibrating, set the reference buffer solutions to 25ºC.
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Replace the reference fluid in the electrode every 4-5 days of use.
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Calibrate before every use.
· Temperature Compensation
The error of the pH electrode's readings is small, but can be noticeable. There isn't much one
can do that is practical for everyday use to counteract this error -- error-preventive procedures
are complicated and vary from electrode to electrode. Setting solutions to 25°C will help give
more accurate pH reading, and understanding the factors responsible will aid in acquiring more
useful data.
There are two different factors that may change the pH electrode's reading in a solution with a
different temperature. The first, and most dramatic, is the result of the solution actually
changing pH with different temperatures. The second is due to the pH electrode's ability to read
the pH of the solution and is a result of the pH's response and sensitivity to the solution at
different temperatures. The further the solution is from pH 7 the more the temperature will
affect the pH electrode's ability to read the pH accurately.
Setup:
· Equipment:
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Rubber Gloves
Squirt bottle of dH2O
2 medium test tubes or small beakers
1 large beaker ½ filled with water (to be used as rinse beaker)
2 standard buffer solutions; one preferably neutral (pH 7)
· Software:
Launch BSL PRO v. 3.6.6 (or better).
Procedure:
· Calibration:
Preferred method:
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PH 183 -- pH Electrode
Calibration of the pH electrode with a neutral buffer solution and with a standard buffer
solution of known pH (can be either base or acid.)
1. Remove the electrode from the protective covering and rinse down with dH2O to remove any dried
material and leftover residue. Clean if necessary (see cleaning instructions below.)
2. Place the electrode in a buffer solution of pH 4 or pH 7. Let it soak for a few minutes.
3. Set up the large beaker of water, and the two smaller beakers of known pH solutions (it is suggested
you don’t pour the buffer solutions until right before use to keep pH as accurate as possible).
4. Remove the electrode from the solution, rinse with dH2O in the beaker filled with water and dry off
gently with a tissue to remove all excess liquid.
5. Plug the electrode into channel 1 on the MP30 unit.
6. Immerse around 2 inches into the neutral pH, giving it a brief stir and letting the electrode sit
motionless in the solution for a minute.
7. Select MP30>Set up channels>View/change parameter>Scaling to generate the Change Scaling
Parameters dialog.
8. Set the scale value for Cal 1 at 7.
9. Click Cal 1, which will should change the Cal1 value to slightly above or below 0.
10. Remove the electrode from the neutral solution and rinse with dH2O into water beaker. Lightly pat dry
with a tissue.
11. In the second test tube or beaker, pour your acidic or basic buffer solution. Immerse the electrode in
the solution, give it a light stir, and then let sit motionless in the solution for a few minutes.
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PH 183 -- pH Electrode
12. After a few minutes have passed click Cal 2.
13. Set the scale value of Cal 2 to the pH value of the buffer solution you are using.
14. Change the units label to pH.
15. Remove from the solution, rinse with dH2O and pat dry.
16. Store the electrode in a neutral pH solution until ready for next use.
Alternate method:
Calibration with one solution of known pH:
If using a pH 7 neutral buffer:
1. Repeat as above up to step 7. Make sure that the scale value for Cal 1 is set at 7.
2. For Cal 2 use the value you got for Cal 1 plus 10.
If Cal 1 = .6375, then you will enter 10.6375.
3. In the Cal 2 scale value, enter 5.
4. Change the units label to pH.
If using a solution with a pH other than 7.
1.
2.
3.
4.
Repeat above procedure up to step 6.
Under MP30>Set up channels>View/change parameter>Scaling click Cal 1.
Enter the pH value of the buffer solution in Cal 1 Scale Value box.
Determine the Cal 2 values based on the pH of your buffer.
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If the pH is between 2 and 14
A. For Cal 2 value: add 10 units to the Cal 1 value and enter in the Cal 2 box.
Example:
If Cal 1 input = –5.3492; Cal 2 input = 4.6508.
If Cal 1 input = 9.4924; Cal 2 input = 19.4924.
B. For the scale value: take the scale value of Cal 1 (the pH of your solution) and
subtract 2 units. Enter this number in your Cal 2 scale value box.
Example:
If pH = 2; Scale value of Cal 2 = 0.
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If the pH is between 2 and 0 (pH electrode cannot measure below 0)
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PH 183 -- pH Electrode
A. For the Cal 2 value: subtract 10 units from the Cal 1 value and enter in the Cal 2 box.
Example:
If Cal 1 input= 30.5028, Cal 2 = 20.5028.
B. For the scale value: take the scale value of Cal 1 (the pH of your solution) and add 2
units. Enter this number in your scale value Cal 2 box.
Example:
If pH = 1, scale value of Cal 2 = 3.
Analysis:
Typical recording of a Neutral buffer solution, a Basic solution and an Acidic
solution. Note the rough recording in between solutions that is a result of
cleaning.
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PH 183 -- pH Electrode
Break down of typical cleaning of electrode and recording a sample of tap water.
Care of the pH Electrode:
Cleaning the pH Electrode:
Different cleaning solutions are used depending on what you have been using the pH electrode with and
what has contaminated it. The table below offers some brief guidelines:
Type of Contaminant
Cleansing Solution
General
0.1 M HCl or 0.1 M HNO3 Soak 30 minutes
Bacteria Deposits
1:10 dilution of house
hold laundry bleach
Soak 15 minutes
Protein Deposits
1% pepsin in 0.1 M HCl
Soak 15 minutes
Inorganic Deposits
0.1 M tetrasodium EDTA Soak 15 minutes
Salt Deposits
KCl 0.1 M
Then NaOH 0.1M
Cleaning Time
Soak 5 minutes
each
Oil and Grease
Mild detergent or
methanol
Clogged electrode
pH 4 buffer solution at 50 Soak 2-4 hours
degrees C.
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Rinse Thoroughly
PH 183 -- pH Electrode
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Post cleansing, rinse completely with dH2O and replace reference fluid inside the electrode.
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Soak the pH electrode in storage solution for no less than an hour prior to next use.
Do NOT use acetone or alcohol for cleaning as they dehydrate the pH glass and reference junction. If either
is used, the electrode needs to be rehydrated in pH 4 buffer preferably with some (~1g/200 mL) potassium
chloride added.
Long Term Care and Storage:
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This type of electrode is refillable, and will most likely need to be replaced every 4-5
days with fresh reference fluid.
The electrode should be stored in a vertical position.
Do not store the electrode in distilled or deionized water for long periods of time, as the
water will eventually damage the glass. A commercially prepared solution of 4 M KCl
with pH 4 or pH 7 is recommended for storage. Tap water is an acceptable substitute.
As a temporary substitute, use 200 mL of pH 7 buffer to which 1 gram of potassium
chloride (KCl) has been added.
Replacing the Reference Fluid:
Basic Replacement
Reference fluid chamber parts
1. Remove the plug on the top of the reference chamber.
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PH 183 -- pH Electrode
2. Drain out reference chamber fluid and dispose.
3. Fill the electrode chamber with solution to just below the filling hole.
4. Replace rubber plug. Tip: try twisting the plug and changing angles to get the plug back into the
hole.
If the Electrode has Gone Dry
1. Rinse out inside chamber with dH2O.
2. Make sure all water is removed from inside chamber. Shake dry, but be careful of using a tissue to
dry as it may leave fiber residue.
3. Replace appropriate fluid as guided above.
4. Soak in tap water for an hour.
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