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Chemistry

Demystifying Ion-Selective Electrodes

Vernier Go Direct ISEs

Traditionally visual techniques are used to measure the concentration of ions in solution. Concentration is determined by comparing colors of solutions with charts and tables. Vernier ion-selective electrodes (ISEs), offer a much easier and more reliable method to measure ammonium, calcium, chloride, nitrate, and potassium ions in solution. By adhering to a few best practices, students can consistently get good data with our ion-selective electrodes.

Common customer questions about the use of ion-selective electrodes.

1. My ion-selective electrode is not reading correctly or will not calibrate.

The most common reason for this is the age of the module in the electrode. All of our ion-selective electrodes have replaceable modules, with the exception the chloride electrode.

Go Direct Nitrate ISE module

To replace the module, carefully, unscrew the end of the electrode and extract the module from the body. The replaceable module will have a date stamped on the side. The modules are warranted for 1 year past the date of purchase. Under typical classroom use, you should expect to replace the module after a year. For this reason, we recommend purchasing modules as close to the time you will use them as possible.

The chloride specific electrode uses a solid state membrane that does not need to be replaced with time. However, the response of this electrode may slow with use. Cut a 1 in2 piece of the polishing strip that came with the electrode, Thoroughly wet the dull side of the polishing strip and electrode with distilled water. Gently polish the end of the electrode to remove accumulated material that is impeding the performance. Rinse the electrode with distilled water and calibrate it.

Replacement modules can be found on our Replacements and Accessories web page.

2. My ion-selective electrode loses its calibration right after I calibrate it.

Other than age, the most common reason for an ISE to lose its calibration right away is that the calibration data was not saved to the memory of the sensor.

LabQuest ISEs

When collecting data with LabQuest ISEs (those with a white, plastic BTA connector), you can store the calibration to the sensor itself. After the calibration procedure, look for the Storage tab in the software. Click or tap the Storage tab, and select the option to “Save the Calibration to the Sensor” or “Set Sensor Calibration.” This will ensure that, after the sensor is disconnected, the most recent calibration will load automatically when the sensor is used again—even if connected to a different LabQuest or computer. For more information about calibrating and storing calibrations with various sensors and interfaces, visit How do I calibrate my sensor? Note: When collecting data with LabQuest ISEs and Graphical Analysis 4, the calibration cannot presently be stored; calibrate your ISEs each time you use them.

Go Direct ISEs

When using Go Direct ISEs and Graphical Analysis 4 app, the calibration information is automatically stored in the the memory of the sensor; there is no need to do additional steps to store the calibration.

3. My ISE is reading off from the calibration standards, even right after I calibrate it.

The response time of ISEs is much slower than most of our other sensors. This means that both the calibration and data collection must be done slowly and consistently:

  1. Make sure to soak your ISE in the high standard solution for at least 30 minutes before calibrating.
  2. When performing the calibration wait at least 90 seconds to 2 minutes in each standard solution before keeping the calibration point.
  3. When using the sensor to read the concentration of an ion in solution, make sure to wait the same amount of time you did when you calibrated the probe.

Vernier Ion-Selective Electrodes can offer another way to enhance your chemistry and water quality studies. With a little foreknowledge you will be able to do some interesting experiments with these sensors.

For more tips on using Vernier ion-selective electrodes, see General tips for using Ion Selective Electrodes (ISE).

Tips for Better Fluorescence Data

Elaine using the Fluorescence spectrometer

Fluorescence spectroscopy is a very sensitive and delicate technique. It often requires a few attempts before getting great data. In our chemistry department, we have come across a few common problems and would like to share some solutions that fix or avoid them.

While these helpful tips are designed for improving your Vernier Fluorescence/UV-VIS Spectrophotometer data, the majority of them can be used with any combination fluorescence/absorbance spectrometer.

  1. Collect an absorbance spectrum of your sample first. This will help you decide which excitation wavelength will work the best for your application. It will also help you narrow down the concentration of your sample for fluorescence measurements.
  2. If the absorbance reading where you plan to excite it is greater than 0.1 absorbance units, dilute your sample until it is around 0.1. Fluorescence is a very sensitive technique and requires samples to be more dilute than absorbance measurements. If samples are too concentrated, you may see low fluorescence emission peaks and/or distorted band shapes. This is due to the inner filter effect which is the reabsorption of emitted radiation.
  3. Try different sample time/integration times. The sample time is the time that the individual diodes (pixels) in the array are allowed to respond to light before they are “read out” and reset to zero. They respond linearly to light until they approach saturation. When students increase and decrease this value, the signal will get larger and smaller proportionally. In Logger Pro, you can adjust this value during live data collection to see the result.
  4. Change the LED Intensity. This is similar to adjusting the slit width in a typical fluorescence spectrometer. Increasing the intensity should increase the signal.
  5. Change the Samples to Average. This command sets the number of discrete spectral acquisitions that are accumulated before a spectrum is displayed. The higher the value, the better the signal to noise ratio. The drawback here is that the more samples your students are averaging, the longer they are going to have to wait for a stable spectrum.
  6. Make sure you are using Logger Pro 3.15 or newer, or LabQuest 2.4.2 or newer. In older versions, you are forced to calibrate in fluorescence mode. There is a bug during fluorescence calibration that does not honor the sample time you typed in before the calibration. This will cause spectra to look much smaller than you may anticipate. If you chose to calibrate even in later versions, you will see this bug; the short-term fix is to change the sample time after calibration.

To get acquainted with the Vernier Fluorescence/UV-VIS Spectrophotometer software and instrument, we suggest following the free lab instructions for “A Guided Inquiry Approach to Understanding Fluorescence Spectroscopy“.

The experiment walks through a lot of common sticking points and really helps students understand what they are looking at when taking fluorescence data.

Want to see more? I will be showing off the Vernier Fluorescence/UV-VIS Spectrophotometer at the Fall Regional ACS shows (MWRM, SERMACS, and SWRM) as well as at WCCTA. See a full list of the college chemistry conferences we attend »


Upcoming Conference Schedule for Vernier

Vernier booth

By participating in many regional and national trade shows each year, Vernier Software and Technology provides current and future customers with many opportunities to personally experience our products and services. It’s a great way to review the features of the latest Vernier products, or to bring any technical matters or applications questions to our attention. See our upcoming chemistry conference schedule below.

We also offer free Professional Development Workshops that include hands-on Vernier equipment training to help you integrate data-collection technology into your science curriculum.

Get Your 260/280 Ratio with Logger Pro

Molecular biologists and biochemists are always talking about their sample’s 260/280 ratio. It is so commonly used that entire instruments are dedicated to displaying this value when analyzing a sample. The A260/A280 ratio is a procedure that tells scientists the extinct of contamination of their nucleic acid solution by proteins, carbohydrates, and other organic molecules.

The basis of this test relies on the Beer-Lambert law: A = εbc; where A is absorbance, e is the molar extinction coefficient, b is the cell path length, and c is the sample concentration. The commonly accepted average extinction coefficients for a 1 mg/mL nucleic acid solution at 260 nm and 280 nm are 20 and 10, respectively. In proteins, the extinction coefficient values at 260 nm and 280 nm at a concentration of 1 mg/mL are 0.57 and 1.00, respectively. Therefore, nucleic acid samples would be expected to have a higher absorbance at 260 nm than at 280 nm; in a protein sample, the opposite is true. Using these extinction coefficients, pure nucleic acid samples would have an A260/A280 ratio of 2.0, while protein would be 0.57.

In order to make the determination of a 260/280 ratio easier with a Vernier UV-VIS Spectrophotometer or a Vernier Fluorescence/UV-VIS Spectrophotometer, we have developed a Logger Pro 3 template that you can use to easily see the ratio without having to go through the entire process every time. A screenshot of the Logger Pro template is shown below.

A screenshot showing Logger Pro's 260/280 ratio template

To use this template, first download the template file. Connect the spectrometer to your computer and open the file in Logger Pro. After you have properly calibrated your spectrometer and prepped your sample, insert your sample into the spectrometer. Press the Collect button and the A260/A280 ratio value will be displayed live in the meter. The table is also displayed in the Logger Pro template. If you double click on the 260/280 header in the table, you can see the calculations used to generate the value and make modifications, if desired.

Vernier has great options for biochemistry, be sure to visit our biochemistry solution page for more free resources and great tips.

Download our Logger Pro template for completing a 260/280 ratio »

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
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