A calibration equation is stored on each pH Sensor before it is shipped. For the most accurate measurements with this sensor, we recommend you perform your own 2-point calibration with buffer solutions.
As the pH Sensor ages, the performance of the electrode will change and drift from the saved calibration. Good maintenance and recalibration of the pH Sensor will ensure the readings are accurate.
Preparing the pH Sensor for Calibration
First, remove the storage bottle and rinse the tip with DI water. Never wipe the sensing tip. Instead of wiping the sensing glass, you may blot the tip with a lint-free paper towel to remove excess moisture, but be extra careful not to rub the surface of the glass.
Preparing the Calibration Solutions
To do a two-point calibration for a pH sensor you will need two different pH buffer solutions. Your calibration is only as good as your knowledge of the reference values. For best results, the two calibration points should be widely separated and bracketing the range you anticipate in your experiment. For most chemistry experiments, we recommend buffer solutions of pH 4, 7, and 10. There are multiple ways to obtain these solutions:
Vernier sells a pH buffer kit as pH Buffer Capsule Kit. The kit contains 10 tablets each of buffer pH 4, 7, and 10, and a small bottle of buffer preservative. Each tablet is added to 100 mL of distilled water to prepare the respective pH buffer solutions.
Buffer capsules and prepared buffer solutions are also commonly available through a variety of chemical suppliers.
You can prepare your own buffer solutions using the following recipes:
pH 4.00: Add 2.0 mL of 0.1 M HCl to 1000 mL of 0.1 M potassium hydrogen phthalate.
pH 7.00: Add 582 mL of 0.1 M NaOH to 1000 mL of 0.1 M potassium dihydrogen phosphate.
pH 10.00: Add 214 mL of 0.1 M NaOH to 1000 mL of 0.05 M sodium bicarbonate.
Performing pH Sensor calibration
For the step-by-step instructions to calibrate the pH Sensor, see the list below.
Due to potential safety concerns, Vernier is conducting a voluntary recall of the Vernier Circuit Board 2 (order code VCB2). The issue is that it is possible to place the switch between two positions. When this happens, the power can short circuit and overheat the switch to dangerous temperatures. While no injuries have been reported, this affects Vernier Circuit Board 2 units that were purchased between August 2015 and April 2018.
Those who own one or more Vernier Circuit Board 2 units need to:
Stop using the Vernier Circuit Board 2 immediately.
Remove all batteries from the battery holders immediately.
Arrange for a retrofit of the circuit board to eliminate this problem by providing your information in the Vernier recall form.
Once the recall form is filled out, Vernier will send the customer a prepaid UPS label to return the unit(s) for a free upgrade.
This recall was conducted, voluntarily by Vernier, under the Consumer Protection Safety Commission’s Fast Track Recall process. Fast Track recalls are initiated by firms who commit to work with CPSC to quickly announce the recall and remedy to protect consumers.
pH is a quantitative unit of measure that describes the degree of acidity or alkalinity of a substance. It is measured on a scale of 0 to 14. The formal definition of pH is the negative logarithm of the hydrogen ion concentration (i.e., pH = –log10[H+]). In practice, it is the hydrogen ion activity that is measured, rather than its concentration. The activity is a measure of the “effective concentration”.
How are pH values measured? pH is a potentiometric measurement where an electrical signal is converted to a pH reading. The signal produced and measured is a potential difference between the sensing and reference electrodes. The theoretical potential at pH 7 is 0 mV and the slope of the line is ~59 mV. This means that, in theory, the pH sensor will change its output by 59 mV for every change in a pH unit. The relationship between the potential and hydrogen ion activity in the sample is described by the Nernst equation.
E = Eo – 2.3 (RT/nF) log aH+
where: E = total potential (in mV) developed between the sensing and reference electrodes
Eo = standard potential of the electrode at aH+ = 1 mol/L
R = gas constant
T = temperature in K
n = number of electrons
F = Faraday constant
aH+ = activity of the hydrogen ion in solution
The term 2.3RT/nF is referred to as the Nernst slope. For an ideal electrode the slope at 25°C is 59.16 mV per decade change in hydrogen ion activity. In reality, the behavior is slightly different than in theory. Calibrating the sensor compensates for this by determining the actual slope and offset using buffers and updating the data-collection software accordingly.
Vernier pH Sensors are combination electrodes. This means the sensor contains both the reference and measuring electrode in one body. When the sensor is placed in the solution, the glass bulb senses the hydrogen ions and the internal electrolyte solution picks up the signal from the glass bulb. The silver/silver chloride/ (Ag/AgCl) reference electrode containing electrolyte generates a constant potential. The difference between the reference and measuring electrodes is a function of the pH value of a solution.