PH 183 -- pH Electrode
BIOPAC WWW.biopac.com Systems, Inc. Application Note PH 183
SS15L -- pH Electrode
pH Electrode with 3 Different Buffer Solutions
BIOPAC Software SS15L Specifications
● BSL PRO v.3.6.6
● Double junction BIOPAC Hardware ● Refillable electrode ● pH Electrode SS15L ● Glass body ● Measures pH within the range of 0-14 Other Hardware ● 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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● 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 [H+] = 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.
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 : 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Reaction: Acidic Neutral Basic (more H+) (less H+) 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: If the pH is 3: 7-10 = -3 -3 x 5= -15 mV 7-3 = 4 4 x 5 = 20 mV
Notes For Taking Good Measurements:
■ 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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reading, so moving it more than 15º from straight vertical is not advised.
■ 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.
■ When calibrating, set the reference buffer solutions to 25ºC.
■ Replace the reference fluid in the electrode every 4-5 days of use.
■ 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: ■ 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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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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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. Repeat above procedure up to step 6. 2. Under MP30>Set up channels>View/change parameter>Scaling click Cal 1. 3. Enter the pH value of the buffer solution in Cal 1 Scale Value box. 4. Determine the Cal 2 values based on the pH of your buffer.
● 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.
● If the pH is between 2 and 0 (pH electrode cannot measure below 0)
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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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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 Cleaning Time
General 0.1 M HCl or 0.1 M HNO3 Soak 30 minutes
Bacteria Deposits 1:10 dilution of house Soak 15 minutes hold laundry bleach
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 Soak 5 minutes Then NaOH 0.1M each
Oil and Grease Mild detergent or Rinse Thoroughly methanol
Clogged electrode pH 4 buffer solution at 50 Soak 2-4 hours degrees C.
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● Post cleansing, rinse completely with dH2O and replace reference fluid inside the electrode.
● 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:
● 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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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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