Application of Ion-Selective Electrodes in the Classroom: Water Quality and Generation

Application of Ion-Selective Electrodes in the Classroom: Water Quality and Generation of an Action Potential

Erik Helleson

Cheney High School

Cheney, WA

Carisa Fore

Lewis & Clark State College

Orofino, ID

Washington State University Mentors

Professor Bernard Van Wie

Chemical Engineering

Sarah Haarsma

Graduate Research Assistant

July, 2009

The project herein was supported by the National Science Foundation Grant No. EEC-

0338868: Dr. Richard I. Zollars, Principal Investigator and Dr. Donald C. Orlich, co-PI.

The module was developed by the authors and does not necessarily represent an official

endorsement by the National Science Foundation.

Table of Contents

Page #

Summary 3

Introduction 3

Rationale 5

Science 5

Engineering 5

Goals 5

Activities 6

·  Calibration curves of potassium and sodium

·  Differences in permeability between potassium and sodium

·  Water Quality Analysis

Appendices 15

·  Sample Calibration Graphs

·  Permeability graphs

·  Student Data Tables

References 22

Project Summary

Overview of Project

The purpose of this module is to increase awareness to middle and high school students of the variety of engineering applications ion-selective electrodes (ISE’s) possess. The activities in this module will allow students to analyze a variety of ions (e.g. potassium, sodium, calcium, and chloride) and their important role in water quality and the generation of action potentials. The students will gather data to create a calibration curve that they can extrapolate to determine the unknown concentrations of a variety of substances. In addition students will have an opportunity to determine how selective the potassium and sodium membranes are to portray the importance different permeabilities play to generate an electric potential in our cells.

Intended Audience

The water quality activities are appropriate for both middle and high school classrooms. The activities can be utilized with a minimal introduction to the chemistry and electronics involved. The activities relating to action potentials should be reserved for upper-level biology courses. General biology rarely goes in-depth into the electric potential of cells; therefore the activities will have little relevance.

Estimated Duration

The activities range from only a few days (2-4) to a full two weeks, depending on the depth and amount of activities you choose. If you have the students construct the ISE’s and/or circuit boards an additional week will be necessary due to the drying, soaking and testing requirements.

Introduction

With the exponential increase in lab technology available to the K-12 sector, students have surely been exposed to a variety of sensors by the time they reach the high school level. Students have probably used sensors that calculate pH levels, temperature, oxygen and carbon dioxide levels, and possibly even levels of common ions such as sodium or potassium. However, there is often a disconnect between the technology and learning because the students plug in the sensor into a computer and the results are graphed for them. Although the results are incredibly convenient for educators with the too-much material and not enough time dilemma, but do the students really understand how the sensors work and what their results mean?

This module will allow students to help make the sensors, (ISE’s) learn about how they work, record and graph their own data, and finally interpret the data to come to conclusions. The goal is that the results will be more meaningful because they have had a role in the whole process.

ISE’s are used in research and the classroom because they are inexpensive sensors that are selective for only one particular ion. The electrodes have a membrane with a particular compound called an ionophore that is selective for a particular ion. For example, the ionophore for the potassium electrodes is called dibenzo-18-crown-6, as show in Diagram 1. The crown part of the molecule attracts potassium electrodes, as illustrated in Diagram 2. This space-filled model shows the potassium (purple), being surrounded by the crown, composed of the six oxygen molecules (red). When the electrodes are immersed in a solution containing the ion they are selective for, the ions will pass through the membrane via the ionophore until a charge builds up where it resists further flow. This charge can be measured as a change in voltage. However, the voltage can only be measured in a system that includes a reference electrode that allows all ions to pass through its membrane.

Because we are dealing with dilute concentrations the change in voltage will be small, measured in millivolts (mV) with a sensitive multimeter.

The experiments in this module begin with constructing a calibration curve for each electrode, graphing the Log of known concentrations against the voltage reading. The calibration curves can then be used to determine the concentrations of unknown solutions, such as testing the concentration of calcium in milk. The relationship between concentration and the electrode potential is illustrated in the nernst equation Ex = RT/zF ln [X]1/[X]2 where [3]:

Ex = Equilibrium potential for any ion

R = gas constant

T= absolute temperature (Kelvin)

z= electrical charge of the ion

[X] the concentrations of the ion on each side of the membrane

This equation can be simplified to Ex = 58/z log [X]2/[X]1

The nernst equation is very useful in systems where only one ion exists, however in both biological systems and bodies of water, multiple ions exist creating a more complicated system because of interference between ions. The nernst equation can be expanded to include multiple ions, in the Goldman equation:

V= 58 log Pa[A]2 + Pb[B]2 + Pc[C]2 …

Pa[A]1 + Pb [B]1 + Pc[C]1 …

where, V is the voltage across the membrane and

P is the permeability of the membrane to each ion. [3]

One important application of the Goldman equation is the difference in permeability between sodium and potassium that allows the generation of the action potential. One goal of this module is to calculate the difference in permeability of the membranes to mimic the conditions of the neuron.

Rationale For Module

The Summer at WSU Engineering Experiences for Teachers (SWEET) program was developed to give science and mathematics teachers the opportunity to design and implement engineering curriculum into their classrooms. The main purpose of this module is to introduce engineering to junior high and high school students in order to develop interest as well as open the door for a future career in the engineering field. This module was constructed to enhance teaching of content standards in various forms of science using engineering for overall integrated learning. This project is funded by the National Science Foundation and takes place at Washington State University.

Science

Various forms of science were used in construction of this module; chemistry, biology and environmental science. Chemistry was used to show students how ions (their size and charge) interact with other ions to create an electrical charge. Biology was introduced by showing how membranes can display selective permeability and how this leads to action potentials within the body. Environmental science was displayed by testing water quality.

Engineering

For a holistic learning experience students need to be able to obtain new knowledge and be able to apply it. This is where the engineering comes in. In this module we used ion selective electrodes (ISE) to demonstrate how to take scientific knowledge, apply it and then be able to analyze the results. This module used potassium and sodium ISEs to demonstrate the effects of membrane permeability and their relation to action potentials. For the water quality testing we used the previous two along with calcium and chloride. Ion selective electrodes have shown to be an effective and inexpensive way to display these effects as well as introduce engineering skills.

Goals

·  Students will be able to describe how ISE’s work

·  Students will be able to construct a calibration curve for each of the ions.

·  Students will be able to extrapolate data from unknown sources and determine the concentration of the ion in the substance.

·  Students will be able to determine the differences in permeability between sodium and potassium

Activities

Experiment 1 Creating Calibration Curves for Sodium and Potassium ISE’s

Introduction: The generation of an action potential is a very important, yet abstract concept introduced in upper-level high school and introductory college courses. The most important concept that leads to the generation of the action potential is the differences in permeability between sodium and potassium. This is a very abstract concept because common sense would ask how that is possible when sodium and potassium have like charges and are of similar size. This laboratory experience will help visualize that membranes can be selective for particular ions, thus leading to the generation of the action potential in our neurons.

Safety

1.  Students are required to wear goggles and aprons at all times.

2.  Although none of the solutions used are toxic, they need to be handled carefully.

3.  NO food or drink allowed in the laboratory classroom.

Pre-Requisite Skills/Knowledge

·  Familiarity with ISE’s and how they work

·  Use of a multimeter

·  Data collection skills

·  Familiarity with Microsoft Excel and the Chart Wizard

Instructional Strategies

Students will work in groups of three, due to the limitation of supplies. One student will be responsible for taking care of the ISE;s and reference electrodes, putting them on the electric board and rinsing them between tests. Another student will set up the board and make sure all electrical connections are in place. The final student is in charge of the different solutions and recording the voltages.

You can either have half the class do potassium and the other half sodium or have each group make the calibration curves for both.

It is pertinent that the students take great care with their results because the data will be used for further experiments.

Materials/Equipment

1.  Ion selective electrodes, (Na+ and K+, one per group).

2.  Reference electrodes (one per group).

3.  Electrical digital multimeter able to read milli-volts (one per group).

4.  Power source that will provide 3 volts (2 AA barriers, 2 AAA batteries, or electronic power source).

5.  Battery holders (one per group).

6.  Circuit board specific to ion selective electrode testing (one per group).

7.  KCl and NaCl solutions (10-5, 10-4, 10-3, 10-2, 10-1 M)

8.  Small Petri dishes for holding solutions buffers (5/10 per group).

9.  Squeeze bottle filled with de-ionized (DI) water for rinsing electrodes between tests (one per group).

10.  250 ml beaker to catch water when rinsing electrodes.

Procedure

1.  Connect the digital multimeter to the circuit board as per Diagram #1. (An additional technique to ensure proper installation of wiring would be to color code the red connections with red marker and the black connections with black marker. Warning: connecting wires to the wrong sides (i.e.; reversing polarity) will damage the circuits.)

2.  Connect the power supply to the circuit board as per Diagram #1.

3.  Connect the pH electrode and the reference electrode as per Diagram #1. (Make sure you wash both electrodes with DI before connecting).

4.  Place wired circuit board into clamp/ring stand assembly.

5.  Turn the digital multimeter to mV (reading either 2000 or 200)

6.  Set up ring-stand apparatus to hold the circuit board connections with room underneath to change out the solutions (Hint: We had the best success with clamping one of the multimeter wires)

7.  Measure out at least 30 mL of each solution into a petri dish and label

8.  Place Petri dish of 10-5 solution of either KCl (if using K+ ISE) or NaCl (if using Na+ ISE) underneath circuit board and lower the electrodes down until the membranes are in the solution. Make sure the electrodes are not touching the bottom of the Petri dish.

9.  Allow the multimeter to stabilize for approximately 30 seconds and then record the voltage in data table provided (See Appendix 4) If the voltage readings continue to fluctuate choose the median voltage.

10.  Repeat the procedure steps 8-9 with the other concentrations. As long as you are increasing in concentration you do not need to rinse the electrodes with DI.

11.  Allow for at least three tests starting with lowest concentration and moving in order to get the most accurate readings. Be sure to rinse the electrodes before starting each new test.

12.  After you have completed the tests for one electrode you may proceed to the next, following the same steps unless the teacher has divided the class into two groups.

13.  Clean up per instructions of the teacher.

Part II Construction of calibration curves

14.  Open up Microsoft Excel

15.  Input the data from Appendix 3 . Be sure to use the log of the concentration (i.e. Log 10-7 = -7)

16.  Use Average function to calculate the averages of three trials

17.  Using chart wizard create a line graph with Log [Concentration] on the x-axis and Voltage (mV)

18.  Have students extrapolate graphs to concentration of 0.

Analysis:

See sample graphs in Appendix 1

Conclusions

1.  Find the slope of a trend line from concentration of -2 to 0.

2.  Compare experimental values to known 58mV change/tenfold concentration difference.

Discussion Questions

1.  What does the slope of the trendline represent?

2.  What are possible errors to account for the discrepancy between experiemental and known values of mV change/tenfold concentration?

3.  Of what use is the calibration curve data by itself?

Lab #2 Comparing Permeability of Na+ and K+

Introduction: This lab is a continuation of the student’s previous work as they now compare how permeable the membranes truly are. The students will be placing the K+ electrodes in NaCl solutions and Na+ electrodes in KCl.

Procedure:

1.  Make sure the students are using the same electrode, reference electrode and circuit board as the earlier experiments to eliminate variables.

2.  Set up the circuit board as previously shown.

3.  Place a Na+ electrode in 10-5 KCl solution, let multimeter stabilize and record voltage in the data table (See Appendix 5)