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Chemistry

Pivot Interactives Adds Chemistry

Have you ever done an experiment that you wish you could repeat with different chemicals or concentrations but lacked the time and materials? This is where Pivot Interactives new activities for chemistry can become a valuable teaching tool.

Pivot Interactives is a browser-based collection of videos and analysis tools that enable students to control real results—not simulations. The videos come with appropriate tools for measuring volume, mass, temperature, time, and even color intensity. There are additional tools to carefully control the progression of the video and experiment. Online tables and graphs are used for students to graph relationships between the variables being studied. Calculated columns can be built and graphed.

Students use Beer’s Law to measure the concentration of solutions.
Students use Beer’s law to measure the concentration of solutions.

Topics include Beer-Lambert law, acid-base titration, kinetics, rate laws and activation energy, gas laws, density, specific heat, and more.

Wait until you see how the “black box” around a colorimeter or spectrometer is stripped away to show the essence of how the device measures transmittance and absorbance. The students use their own eyes as the detector with a clever combination of tools and filters to determine the appropriate wavelength to use for the experiment.

See how Pivot Interactives can enhance your instruction and bring to life many concepts in your chemistry curriculum by signing up for a free 30-day trial.

Flash Photolysis 101

Instrumentation is used in the undergraduate chemistry curriculum to help demonstrate the fundamental aspects of chemical reactions and demonstrate how it can be used to determine certain properties of a chemical system. For example, absorbance spectroscopy teaches students about transmission and absorption of radiation by a compound and how these measurements can be used to determine concentration or chemical reaction order. Chromatography illustrates to students how the structure of compounds can help isolate them from others. When certain techniques are coupled together, the concepts are layered and even more can be learned about the system being studied.

Flash photolysis spectroscopy is a type of time-resolved absorbance spectroscopy that helps students investigate chemical reaction order as well as the basics of photochemistry. Flash photolysis is often referred to as a “pump-probe technique” because it involves an excitation source or a “pump” and a detection source or a “probe”. This technique was so groundbreaking that the 1969 Nobel Prize in Chemistry was awarded to the scientists who developed it.

The diagram below shows a typical flash photolysis setup. In this system, white light from an LED light source probes any spectral changes made in the system by the excitation light pulse. A xenon flash lamp provides the photo-excitation pulse. The white light from the LED source is focused on the sample. From the sample this beam goes through a wavelength filter as it is focused on a photodiode, which detects this light’s intensity. When the xenon lamp is flashed, an intense near-UV, white light pulse enters the sample. If it causes changes in the absorption of the sample at the filter’s wavelength, the detector measures these changes. The voltage from the detector is collected, digitized, and stored as a function of time.

A figure showing a typical flash photolysis setup

With recent advances in photochemistry in a number of disciplines, understanding photo-induced chemical kinetics is quickly becoming an essential part of the undergraduate chemistry curriculum. Due to limitations in affordable instrumentation, photochemical kinetics is often left to the textbook alone. The Vernier Flash Photolysis Spectrometer is an affordable option available to instructors to help students get hands-on experience with this important technique. We provide a number of free experiments to get you started, including one that involves exploration of a simple light-induced, cis-trans isomerization of Congo Red. Congo Red is a diazo dye that is a derivative of azobenzene. When light excites the ground state trans- form at its visible broadband absorbance, some ground state molecules are converted to a higher energy cis- form instantaneously (on this time scale, at least). The cis- state is metastable with respect to the trans- ground state resulting in slow conversion back to this trans- ground state, as shown in the state diagram below. The loss of the absorbance at 600 nm observed by the Vernier Flash Photolysis Spectrometer gives students the opportunity to follow the progress of a thermal cis-trans isomerization and measure its rate on timescales that cannot be achieved by traditional mixing methods.

A figure showing The cis- state is metastable with respect to the trans- ground state resulting in slow conversion back to this trans- ground state

The data and analysis provides an opportunity for discussion with students about various topics, including perturbation kinetics, photochemistry, fast kinetics, and bimolecular rate constants. This experiment, and others like it, allow for easy incorporation of time-resolved spectroscopy into the undergraduate physical chemistry, biochemistry, organic chemistry, and inorganic chemistry curriculums.

The Vernier Advantage for College Chemistry

Vernier Go Direct sensors for college chemistry

Whether you are teaching general or upper-level college chemistry courses, our affordable sensors and instrumentation make it possible for every student to participate in hands-on learning. Our combination of sensors, software, college-level experiments, and instructional resources engage students and instructors in scientific discovery. We have assembled a collection of products and experiments for commonly taught college chemistry courses.

  • General Chemistry: Complete an acid-base titration with our pH probes that have 0.1 pH unit accuracy and a drop counter that accurately converts drops to volume.
  • Organic Chemistry: Measure and analyze the GC retention times of a Fischer esterification reaction mixture using the Mini GC Plus Gas Chromatograph with room air as the carrier gas.
  • Biochemistry: The Vernier UV-VIS Spectrophotometer can be used to measure the 260/280 nm ratio when purifying proteins and DNA. Its range, 220 nm to 850 nm and 3 nm optical resolution, makes it ideal for biological applications.
  • Analytical Chemistry: Investigate redox reactions with a potentiometric titration using an ORP (oxidation-reduction potential) sensor.
  • Physical Chemistry: Explore excited-state dynamics with one of our free experiments that walks students through the heavy-atom quenching of quinine fluorescence using the Vernier Fluorescence/UV-VIS Spectrophotometer.

At Vernier, we believe that the use of our sensors and lab equipment should serve to enhance your teaching, not get in your way. Take a look at Periodic Elements—our college chemistry blog where our chemists share their knowledge and ideas. Sign up for Periodic Elements and find a full list of recommendations for college chemistry.

5 Popular Tools for Teaching Environmental Chemistry

Do you teach environmental chemistry? Are you looking for lab experiment ideas and equipment?

Students taking environmental chemistry will learn basic techniques for chemical analysis of environmental samples, including air, water and soil. Many of these experiments may take place in the field requiring rugged and portable equipment. These Go Direct® Sensors connect directly to your mobile device, Chromebook, or computer using our free Graphical Analysis 4 app or Spectral Analysis 4 app. The sensors can be used wired via USB or wirelessly via Bluetooth® wireless technology, allowing you to choose the best solution.

Here are five products from Vernier, selected by our experts, for environmental chemistry.

  1. Go Direct Tris-Compatible Flat pH Sensor

    Go Direct® Tris-Compatible Flat pH

    The flat glass shape of the Go Direct Tris-Compatible Flat pH Sensor is useful for measuring the pH of semisolids such as soil slurries. It features a sealed, gel-filled, double-junction electrode, making it compatible with Tris buffers and solutions containing proteins or sulfides.

    Example experiments:

    • Measure soil pH
    • Investigate the effect of acid rain on soil
    • Understand the role of buffers
  2. Go Direct Dissolved Oxygen Probe

    Go Direct® Optical Dissolved Oxygen Probe

    The Go Direct Optical Dissolved Oxygen Probe combines the power of multiple sensors to measure dissolved oxygen, water temperature, and atmospheric pressure. The Go Direct Optical Dissolved Oxygen Probe uses luminescent technology to provide fast, easy, and accurate results. This waterproof probe is perfect for the field or for the laboratory.

    Example experiments:

    • Investigate the relationship between temperature and dissolved oxygen in water
    • Measure primary productivity or biological/biochemical oxygen demand
    • Monitor watersheds over time
  3. Go Direct Ion-Selective Electrodes (ISEs)

    Go Direct® Nitrate Ion-Selective Electrode

    Our Go Direct family of Ion-Selective Electrodes are great for monitoring five environmentally-important ions: calcium (Ca2+), chloride (Cl), ammonium (NH4+), nitrate (NO3), and potassium (K+). These are combination-style, non-refillable, gel-filled electrodes with the option to report measurements in mV or mg/L.

    Example experiments:

    • Measure changes in nitrate concentration due to acidic rainfall or fertilizer runoff from fields
    • Study changes in levels of ammonium ions introduced from fertilizers
    • Quantify chloride in ocean water
    • Use the calcium concentration to evaluate water hardness
    • Investigate potassium levels in NPK fertilizers
  4. Go Direct Conductivity

    Go Direct® Conductivity Probe

    Go Direct Conductivity Probe determines the ionic content of an aqueous solution by measuring its electrical conductivity. It features a built-in temperature sensor to simultaneously read conductivity and temperature. Automatic temperature compensation allows students to calibrate the probe in the lab and then make measurements outdoors without temperature changes affecting data. The Go Direct Conductivity Probe has a range of 0 to 20,000 μS/cm (0 to 10,000 mg/L TDS) to provide optimal precision in any given range.

    Example experiments:

    • Use conductivity to study soil salinity
    • Measure total dissolved solids (TDS)
    • Investigate the difference between ionic and molecular compounds, strong and weak acids, or ionic compounds that yield different ratios of ions.
  5. Go Direct SpectroVis® Plus Spectrophotometer

    Go Direct® SpectroVis® Plus Spectrophotometer

    The Go Direct SpectroVis Plus is a spectrophotometer with a range of 380–950 nm. This device can quickly collect a full spectrum (absorbance, percent transmission, or fluorescence).

    Example experiments:

    • Select a single wavelength for colorimetric assays (e.g., total phosphates, lead, iron)
    • Use the Spectrophotometer Optical Fiber to measure emissions from flame tests or other light sources

Check out Environmental Science and Water Quality for additional options. Everything we offer includes our unparalleled customer service, technical support, and resources, so you are always supported when integrating our technology.

Decoding Your Absorbance Readings

In an effort to help you and your students better understand spectrometer absorbance readings, we’ve collected a few commonly asked questions:

Why don’t absorbance readings have units?

Absorbance readings are unitless because they are calculated from a ratio of the intensity of light transmitted through the sample (I) to the intensity of light transmitted through a blank (Io). This ratio results in a unitless value.
Absorbance = log (Io/I)

Why are absorbance readings most accurate between 0.1 and 1?

Remember that absorbance is the logarithm of the transmission of light through a sample. Transmission (T) is the ratio of the intensity of light transmitted through the sample (I) to the intensity of light transmitted through a blank (Io). Therefore, absorbance = log (Io/I).

At an absorbance of 2 you are at 1%T, which means that 99% of available light is being blocked (absorbed) by the sample. At an ABS of 3 you are at 0.1% T, which means that 99.9% of the available light is being blocked (absorbed) by the sample. Such small amounts of light are very difficult to detect and are outside the meaningful range of most spectrometers.

Vernier array spectrometers and colorimeters have a useful absorbance range between 0.1 and 1.0. Any absorbance reading above 1 can be inaccurate. There are spectrometers that will report meaningful values at absorbance ranges above 1.0, but these are research instruments that are also quite expensive. In most classroom settings, the best option is to simply dilute your samples to ensure they are in this range.

How important is it to use a quartz cuvette for absorbance readings in the UV?

It depends on how accurate you want your absorbance readings to be. UV plastic cuvettes are less expensive and have practical applications when working with students, but they lose transparency quickly in the UV. Most are only rated to 280 nm. If you want the most accurate data possible below 280 nm, a quartz cuvette is the best option. Another unfortunate side effect of using UV-plastic cuvettes is that students commonly confuse them with visible-only plastic cuvettes. This cuts out all UV light, so data will be very poor. If you are going to use UV-plastic cuvettes, make sure you are using them for the proper applications.

Why You Should Attend Your Local ACS Meetings

Are you a member of the American Chemical Society (ACS)? I’ve been a member for 10 years and currently serve as the Secretary for the local Portland Section. ACS is a scientific society of chemistry professionals that includes students, educators, and industrial chemists.

Each year ACS hosts two national meetings; one in the spring and one in the fall. The national meetings offer the opportunity to discover and share knowledge through posters, presentations, and training workshops. The expo features hundreds of exhibitors showcasing new technological developments. The ACS Career Fair at the national meetings offer access to ACS career consultants and a career fair for job seekers and employers. I found my current career here at Vernier Software & Technology through the ACS Career Fair.

There are also several regional meetings that are organized by ACS Local Sections. These meetings also feature technical programs on a variety of topics, poster sessions, expositions, and social events. The smaller size of an ACS regional meeting allows for a greater opportunity for interactions and costs less to attend than a national meeting. Take advantage of the opportunity to attend a regional meeting.

I attended the Northwest (NORM) Regional Meeting from June 24–27 on the campus of the Pacific Northwest National Lab in Richland, WA. The theme was Powering the Future: Energy, Environment, Education. I co-presented a talk with Prof. Karen Goodwin from Centralia College about Data Acquisition in the Chemistry Lab. It highlighted the benefits of using data logging for several common general chemistry experiments such as gas laws, acid-base titrations, and electrochemistry. Our goal was to show that using data-acquisition tools results in fewer possibilities of transcription errors and combines the power of graphical visualization and mathematical data analysis.

Boyle's Law

My next talk will be at the Fall National Meeting that takes place in Boston, Aug 19–23, 2018. I will be presenting a talk on using kitchen chemistry and technology to engage K–12 and college students. I hope to meet you there. If you are interested in attending, here are the details.

  • PAPER TITLE: Using kitchen chemistry and technology to engage K–12 and college students (CHED 137)
  • DAY & TIME OF PRESENTATION: Monday, August, 20, 2018 from 3:45 PM–4:05 PM
  • ROOM & LOCATION: Cambridge 1/2 – Seaport World Trade Center

Calibrating a Drop Counter

Calibrating a Drop Counter

Did you know that you really can’t calibrate a Vernier Drop Counter? Instead, you are actually calibrating the tip of the titrant reservoir.

The Vernier Drop Counter is a modified photogate designed, through the software, to monitor drops of liquid as they pass through the slot. A beam of infrared light passes through the slot continuously. If anything blocks that beam, the software interprets that as a drop. The result is either a drop, or no drop.

More accurately, when you calibrate a Vernier Drop Counter, you are determining the drop size for titrants as they are delivered from the reservoir tip. The drop size will vary depending on the composition of the titrant, the intermolecular forces between the molecules of the titrant and the plastic in the tip, the height of liquid in the reservoir, the rate at which the drops are allowed to flow out of the tip, and even the temperature. The tip is made of formed plastic. The drop size will also vary with different tips.

For the absolute best results, different titrants should be calibrated individually. For aqueous solutions with concentrations of 1 M or less, a reasonable calibration of the Vernier Drop Counter can done with distilled water at room temperature. While the software is running, drops are allowed to pass through the slot in the Drop Counter, collected in a graduated cylinder, and counted by the software. A drip rate of around 1 drop every 2 seconds is recommended to allow the drops to reach their maximum size. (This also allows the pH, conductivity sensor, or oxidation/reduction potential sensor time to react between drops.) It is recommended that the reagent reservoir be filled to 60 mL and a total volume of just under 10 mL be collected during the calibration. This ensures that the pressure on the drops at the tip is reasonably constant. The larger the volume of titrant used and number of drops, the more accurate the value for the drop size.

Once a prescribed volume of water is collected, the valve is closed, the volume is entered into the software, and the value of drops/mL is calculated automatically. This value can be written on the reservoir system (reagent reservoir, valves, and tip) for future use. All Vernier software allows the user to enter the known value without having to completely carry out the calibration again.

Vernier Tip: Check out our Thingiverse webpage for a free, 3D-printable design for a widget that centers the reagent reservoir system above a Drop Counter. It comes in handy for aligning the tip to ensure the drops are detected by the Drop Counter.

3D-printed burette-centering widget

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.

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