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Chemistry

Why You Want an ORP Sensor

Graph of redox titration with Go Direct ORP Sensor

An Oxidation Reduction Potential (ORP) Sensor measures the activity of oxidizers and reducers in an aqueous solution. It is a potentiometric measurement from a two-electrode system similar to a pH sensor. Sometimes it is also referred to as a redox measurement. Unlike a pH sensor, an ORP sensor measures the ratio of oxidized to reduced forms of all chemical species in solution.

The ORP sensor is made up of two electrochemical half cells where the reference electrode is generally Ag/AgCl and the measurement electrode is commonly Pt. The potential difference between the two electrodes represents the redox potential of the solution being measured and can be described by the Nernst equation.

E = Eo – 2.3 (RT/nF) x (log [Ox] / [Red])

Where:

  • E = total potential developed between the measurement and reference electrodes
  • Eo = a voltage specific to the system
  • R = gas constant
  • T = temperature in K
  • n = the number of electrons involved in the equilibrium between the oxidized and reduced species
  • F = Faraday constant
  • [Ox] = concentration of the oxidized species
  • [Red] = concentration of the reduced form of that species

The output of the ORP sensor is relative to the reference electrode. For example, a reading of +100 mV indicates the potential is 100 mV higher than the potential of the reference half cell and suggests an oxidizing environment. Likewise, a –100 mV reading indicates a potential 100 mV lower than the reference half cell and is a reducing environment. In some applications, redox potential may be reported as Eh which is the voltage reading with respect to the Standard Hydrogen Electrode (SHE). By taking into account the offset of the reference electrode used in the ORP sensor, the potential can be converted into Eh readings. Vernier ORP sensors use a Ag/AgCl saturated KCl reference electrode.

In education, a common application for an ORP sensor is a potentiometric titration. Similar to an acid-base titration, a titrant is added to a sample incrementally until all the sample has reacted and the end-point is reached. One example where students can apply their understanding of redox is by using a Vernier Go Direct ORP Sensor to determine the concentration of H2O2 by titrating the solution with KMnO4. To correctly calculate the concentration, students must understand the balanced redox reaction between KMnO4 and H2O2.

Vernier Tip: Check out two additional experiments from Vernier using an ORP Sensor.

Three Tips For Keeping Your pH Sensor Healthy

Tip #1: Keep Your pH Sensor Hydrated

Storing your pH Sensor in storage solution is important for preventing the reference electrolyte from leaching out, keeping the junction clear, and keeping the glass tip hydrated. If you’re out of pH storage solution, Vernier sells premade pH storage solution. As an alternative, you can prepare your own storage solution using a pH 4 buffer. It is recommended that you replace the pH Sensor storage solution annually.

If your pH Sensor was stored dry, immerse the tip in the pH storage solution for a minimum of 8 hours prior to use and then check the response in known buffer solutions. If the reading is close to the known pH of the buffer solution, recalibrating the sensor is recommended. If the readings are off by several pH values, the pH readings do not change when moved from one buffer solution to another different buffer, or the sensor response seems extremely slow, the problem may be more serious. Sometimes a method called “shocking” is used to revive pH electrodes.

Tip #2: Clean Your pH Sensor

Generally, rinsing the tip with DI water should suffice. If the glass tip looks dirty, you can rinse the tip with warm water and a mild household dishwashing detergent (not Alconox detergent). If the tip and bottle appear to have mold growing in or on them, you can clean it with a dilute bleach solution to remove the mold.

  1. Fill the storage bottle with a mixture of 1 part chlorine bleach to 3 or 4 parts water and soak the electrode for 8 minutes.
  2. Thoroughly rinse both in cold or lukewarm water.
  3. Refill the bottle with pH Storage Solution and return the sensor to the bottle.

In biological labs where proteins are used, the glass tip can get fairly dirty. In this case, do the following:

  1. Soak the tip for 10–15 minutes in an acidic pepsin mixture. Approximately 5% pepsin in a 0.001 M HCl solution should work.
  2. Thoroughly rinse the tip in warm tap water.
  3. Refill the storage bottle with pH Storage Solution and return the sensor to the bottle.

Tip #3: Maintain Your pH Calibration Solutions

A new pH Sensor is shipped with a default calibration, but as the sensor ages, it may need to be recalibrated. It is important to use good buffer solutions for calibrating.

To prevent contamination of your buffer solutions, never submerge your sensor right into the bottle. Pour out just what is needed into a container that has been rinsed with DI water and use that for your calibration. Never pour used buffer back into the bottle.

Vernier Tip: Instead of purchasing premade buffer solutions, consider buying buffer capsules. The buffer capsules have a longer shelf life than premade solutions.

Do I Need to Calibrate My pH Sensor?

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:

  1. 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.
  2. Buffer capsules and prepared buffer solutions are also commonly available through a variety of chemical suppliers.
  3. You can prepare your own buffer solutions using the following recipes:

    1. pH 4.00: Add 2.0 mL of 0.1 M HCl to 1000 mL of 0.1 M potassium hydrogen phthalate.
    2. pH 7.00: Add 582 mL of 0.1 M NaOH to 1000 mL of 0.1 M potassium dihydrogen phosphate.
    3. 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.

The Theory Behind pH Measurements

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.

Graph showing Nernst equation relationship
Theoretical Nernst behavior for pH

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.

Price Reduction on Gas Chromatograph

Simultaneous graphs of the normal force and friction force plus a graph of the calculated coefficient of kinetic friction

Introducing your students to gas chromatography has just become more affordable. We have lowered the price of the Vernier Mini GC Plus from $2289 to $1989, giving you a $300 per unit cost savings. And remember, it uses room air as the carrier gas, so no gas tanks are required!

For more information on this unique instrument, See Vernier Mini GC Plus

Organic Chemistry Products and Free Experiments

Download 16 free experiments for use with Vernier’s new organic chemistry data-collection technology.

See products and experiments for organic chemistry »

Use the SpectroVis Plus to Explore Fluorescence Spectroscopy

Fluorescent molecules are compounds that absorb light of one wavelength, then re-emit light at a longer wavelength. This emitted light can be quantified using fluorescence spectroscopy. Molecular and cellular biologists use fluorescent compounds to label proteins, gels, and even cellular organelles. In many ways, fluorescent compounds have revolutionized research in the life sciences.

Continue reading Use the SpectroVis Plus to Explore Fluorescence Spectroscopy

Using the Wide-Range Temperature Probe with MEL-TEMP®

If your school owns MEL-TEMP units, this is for you. Our Wide-Range Temperature Probe fits perfectly into the thermometer slot of a MEL-TEMP unit. This is a great option, especially if your school is no longer using mercury thermometers. The Wide-Range Temperature Probe can be used safely to 330°C, and its RTD (resistance temperature detection) technology ensures accuracy to ±0.1°C.

Continue reading Using the Wide-Range Temperature Probe with MEL-TEMP®

Periodic Table Graphing

When we originally created the popular Periodic Table application in LabQuest, we realized that there was a gold mine of information in the database that could eventually be used to create periodic table plots, such as atomic radius vs. atomic number, first ionization energy vs. atomic number, etc. Most introductory chemistry courses introduce periodic trends by having students create these kinds of plots and look for recurring trends in many chemical and physical properties. You can do this in LabQuest App.

Continue reading Periodic Table Graphing

SpectroVis Plus

Vernier has updated its popular spectrometer with improved features:

  • Improved range: 380-950 nm (VIS-NIR)
  • 1 nm between reported values
  • Improved optical resolution (~2.5 nm)
  • New support for fluorescence

Available March 2010

Learn more about the SpectroVis Plus »

SpectroVis Plus
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