USING ION-SELECTIVE ELECTRODES TO STUDY CONCENTRATIONS OF ELECTROLYTES IN COMMON BEVERAGES Kelly Bell, MS Darington High School Darington, WA & Alec Olschner, MiT West Valley City School Spokane Valley, WA & Jim Stewart, MEd Garfield-Palouse High School Palouse, WA Washington State University Mentors Professor Bernard Van Wie, Ph.D. Chemical Engineering & Sarah Haarsma, MS Graduate Research Assistant July, 2010 The project herein was supported by the National Science Foundation Grant Award No. EEC-0808716: Dr. Richard L. Zollars, Principle Investigator. This module was developed by the authors and does not necessarily represent an official endorsement by the National Science Foundation. 1 Project Summary O ver view of Pr oj ect This module uses ion-selective electr odes (ISEs) to stimulate student interest in engineering and its applicability to everyday life. Students, both in middle school and in high school, will test the concentration of electr olytes, (biologically significant ions) in common beverages. Students will learn how to create a calibration curve using data collected from solutions of known concentration and then interpolate points on the curve that match data gathered about the specific drinks being tested. Students will also test the selectivity of the ISEs to see how well one ion is detected in a solution with multiple ions of the same charge. These activities can be used as a supplement to a variety of content topics or can stand alone as a complete unit, providing an overview or review of many inter-related chemistry topics. I ntended A udience This module is intended for students studying chemistry and can be geared towards students at any level, from an introductory middle school physical science class to an advanced high school chemistry class. The activities will be presented for a high school-level chemistry course and adaptations for students with less background knowledge or readiness will be noted. The concept for the module is such that it could lend itself to a collaboration with the mathematics (algebra or pre-calculus depending on level) and/or health teacher. E stimated Duration This module is to take place over at least five 50-minute long class periods or three 90- minute long block periods. The timeline can be expanded based on the depth of material to be taught and the availability of more class time. The approximately 250 minutes of total class time is a minimum to be able to complete all three learning activities. A jig-saw activity will occur during the first class period in which students will collectively read and discuss an article explaining the basic ideas surrounding ions, electrolytes, the role of electrolytes in the body, and what happens to these ions/electrolytes during exercise. The second and third periods will be used to introduce ISEs and complete the first hands-on activity. The fourth and fifth days will be spent learning about the mathematics involved with the ISEs and students will take part in another hands-on activity to determine the accuracy and selectivity of the ISEs. (Duration for middle school students could be shorter, the math involved with the second hands- on activity is above the level that most middle school students will have encountered. This activity could be done as a qualitative rather than quantitative lab.) Note: The circuit boards and ISEs needed for the hands-on activities should already be assembled. 2 I ntr oduction Most students will have heard of sodium and potassium, they are necessary nutrients for the human body and are more or less common in the food we eat. Table salt is a common ionic compound comprised of sodium (Na+) and chlorine (Cl-), NaCl. Students will also most likely be familiar with sports drinks because of advertising and their prevalence in our culture as refreshments. In fact, many sports drink advertisements are now highlighting the amount of electrolytes contained in each drink, which are vital nutrients for athletes as they exercise, train, and compete. The activities in this module are based around learning more about electrolytes and how the concentrations of these electrolytes in sports drinks might be measured. Electrolytes are salts that conduct electricity and are found in the body fluid, tissue, and blood. Examples of electrolytes include sodium, potassium, magnesium, and calcium. Sodium (Na+) is concentrated outside of the cell wall in the extracellular fluid. Potassium (K+) is concentrated in the intracellular fluid, inside the cells. Proper balance of these two electrolytes is essential for muscle coordination, heart function, fluid absorption and excretion, nerve function, and concentration. The kidneys regulate fluid absorption and excretion and maintain a narrow range of electrolyte levels. Normally, sodium and potassium are filtered from the blood and excreted in the urine and feces depending on the bodies needs. During exercise electrolytes are lost through sweat as well, increasing the rate that electrolytes are lost from the body. Too much sodium is called hypernatremia; too little is called hyponatremia. Too much potassium is called hyperkalemia; too little is called hypokalemia. Each of these conditions has a set of symptoms, all of which hinder the body’s optimized performance. Recovering lost electrolytes after exercise is vital to the health, performance, and recovery of an athlete. Students will use Ion-Selective Electrodes (ISEs) to measure concentrations of electrolytes in popular sports drinks as well as investigate the nature of the electrodes, how they work and why they are used in many real-life applications. The investigation of electrolytes and ISEs will illustrate characteristics of ions, electrochemistry, and the connection of science to everyday life. The activities in this module will also help students develop skills such as reading scientific literature, calculating solution concentration, graphing data, and using mathematic equations to model relationships between compounds and solutions. Ion-Selective Electrodes work because of a membrane that coats the end of the electrode. This membrane has a specific compound called an ionophore that targets a specific ion and moves that ion from one side of the membrane to the other. The current through the system from the power source moves the ions across the membrane because of the charge associated with the ion. The concentration of that ion is determined by the electric potential (voltage) measured across that membrane. The mathematical relationship between concentration of the ion and the voltage difference across the membrane is logarithmic, the voltage doubles for every 10-times increase in concentration. pH is a common scale that has this relationship and in fact pH simply means the potential of H+ ions in a solution. Acidity is based on the concentration of H+ ions. pH meters are one type of ISE, they selectively detect H+ ions and report the concentration based on the electric potential created by moving the ions across a membrane. 3 R ationale for M odule It is a goal of all teachers to offer a program that provides excitement and relevance to their students. In an effort to help teachers realize this goal, Washington State University and National Science Foundation Institute for Science and Mathematics Education through Engineering Experiences allows teachers the opportunity to work with professors in some aspect of the professor’s research. Teachers in the program use what they have learned to create modules that can be shared with students in the middle and high school classroom. This module is the culmination of our part in that process. The module is based on work being done by Dr. Bernard Van Wie and Sarah Haarsma. The basis for this research is the design and utilization of inexpensive bio-sensing electrodes (ISEs). The cost of the electrodes is minimal (although the time necessary to build a set can make the endeavor daunting initially). Using the same technology scientists have been using in the field, our students will analyze the claims of sport drink manufacturers in regards to electrolytes in their drinks. The interest of many students in sports and sports nutrition will be the “hook” to get them excited about science that may be relevant to their lives. An example of ISEs in the “real world” would be the US Naval Research Laboratory use of the ISEs to monitor for copper (II), which leaches from the paint of ship hulls. The paint is used to reduce the growth of barnacles reducing drag thus increasing fuel efficiency. The copper (II) is however toxic to crustaceans. [1] Science The foundational scientific idea behind this module is that ionic compounds dissociate in water and each ion carries a charge that allows it to be drawn across a membrane and produce a current. It is the electric potential of this current that is measured and, through data collection and analysis, will indicate the concentration of certain ions in a solution. This module also explores the biological necessity of certain ions as nutrients, or electrolytes, that aid in primary bodily functions. Students will learn about these ions and the role of these ions in the body. E ngineering This module is designed to give students a motivational and hands-on science activity. It could be argued that the students will actually be performing an engineering activity, in the sense that they will be using an established scientific method to find a solution to a problem that is unique (at least to them). In either case students will be exposed to the work