1 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.
Transcript
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USI NG I ON-SE L E C T I V E E L E C T R ODE S T O ST UDY C ONC E NT R A T I ONS OF
E L E C T R OL Y T E S I N C OM M ON B E V E R A G E S
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.
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Pr oject Summar y
Over view of Pr oject 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 calibr ation cur ve 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 Dur ation 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.
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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.
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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 ngineer ing
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 being done presently by chemical engineers and can lead to many extensions, discussion or research opportunities.
G oals
*Introduce students to ionization and electro-chemistry *Motivate students using a realistic and hands on lab. *Student can determine validity of sports drink bottle claims *Establish a link between health and science/engineering. *Students understand safe and efficient use of lab (pay attention to detail)
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*Link mathematics to science (graphing, logarithms) *Students use of lab apparatus (volt meters, circuit boards) to collect data *For “advanced” students – use of trend line; logarithms to analyze data *For “advanced” students – use of lab apparatus to concoct concentration mixtures - math needed to concoct concentration mixtures & chemistry involved in ionization *Use of technology (EXCEL) to extrapolate data: enter data; graph; trend line *Use Nernst equation to calculate the concentration coefficient.
E quipment
I on-Selective E lectr odes This project relies on the functionality of ion selective electrodes to determine the concentration of specific ions present in a solution. The electrode consists of a copper wire surrounded by a PVC sheath with one end connecting to a circuit board and the other end covered by a membrane (Figure 1). This membrane contains an ionophore that favors specific ions and will carry the selected ion across the membrane. This movement of charged ions creates a current through the copper wire, through the circuit board and back down through the reference electrode (also submerged in the solution) completing the circuit. Different ionophores are used for different ions, meaning that ISEs can be made for a variety of ions. The activities will use two different types of ISEs, one type that uses valinomycin as an ionophore which is highly selective for K+ and another type that uses NaX, which is selective for Na+. R efer ence E lectr odes Figure 1: ISE Reference electrodes are made of silver wire coated in AgCl and soaking in 3M NaCl. The wire is housed in 1/16” Tygon(c) tubing with one end sealed and the other end filled with a non-selective membrane (Figure 2). These electrodes are named as such because they provide a reference resistance that helps determine the voltage across our testing electrode. Figure 2: Reference Electrodes
PVC
Cu wire
solution membrane
Ag wire 3M NaCl filled
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C ir cuit B oar ds The circuit boards can be constructed by hand using a soldering iron and PC board purchased from an electronics store. The circuit board had pin receptacles for a power source, the two electrodes, and two (2) voltmeter probes (Figures 3, 4). Large circles represent voltmeter pin receptacles; small circles are either power source or electrode receptacles. Each receptacle is labeled either “Red/Black” for power source and volt meter or “Ref./ISE” for electrode receptacles. “R” labels a resistor. The black box is an operational amplifier (Op. Amp).
Figure 3: Circuit Board (Top side) Figure 4: Circuit Board (Bottom side) V oltmeter s The voltmeters required are any basic voltmeter that can read at the 2000mV level. If higher precision is desired, and voltage readings are low enough, the 200mV level can be used.
Pr er equisite Student Skills/K nowledge
Science Ionization (Ion) Dimensional analysis
M ath
pH Graphing (scale, plot, labeling) Mole vs. Molarity Linear equations (trend line) Prefixes (milli – micro) Logarithm (log scale) Electrode Exponential graph ISE Ratios Membrane Ionophore UNITS Electrical current Chemical families Electrolyte Circuit board Multi-meter DI water EXCEL (entering data; graphing; trend line)
Red
Red
Black
Black ISE R
Ref. R
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Activity One/ Introduction
A ctivities
In class jigsaw activity: Why Drinking Too Much Water is Dangerous & The Idea That Launched an Industry
Activity Two: 1st hands-on activity
“Can I believe the bottle?”
Activity Three: 2nd hands-on activity
“How selective IS an ion-selective electrode?”
Jigsaw Activity: Introduction to Ions and Electrolytes
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Directions: Use the articles Why Drinking Too Much Water Is Dangerous By Dr. Ben Kim [2] and Gatorade The Idea that Launched an Industry by Joe Kays & Arline Phillips–Han to do a jigsaw activity [3].
1. Divide the class into teams of four to six students, depending on the reading
level.
2. Assign each student one section of the articles to read.
3. Give students time to read over their section and become familiar with it.
4. Form temporary "expert groups" by having one student from each jigsaw group join other students assigned to the same section. Give students in these expert groups time to discuss the main points of their segment and to rehearse the presentations they will make to their jigsaw group.
5. Bring the students back into their jigsaw groups.
6. Ask each student to present her or his section to the group. Encourage others
On January 12, 2007, a 28-year old Californian wife and mother of three children died from drinking too much water. Her body was found in her home shortly after she took part in a water-drinking contest that was sponsored by a local radio show. Entitled "Hold Your Wee For A Wii," the contest promoters promised a free Wii video game machine to the contestant who drank the most water without urinating.
It is estimated that the woman who died drank approximately 2 gallons of water during the contest. When she and other contestants complained of discomfort and showed visible signs of distress, they were laughed at by the promoters and even heckled.
This tragic news story highlights the importance of understanding why drinking too much water can be dangerous to your health.
Whenever you disregard your sense of thirst and strive to ingest several glasses of water a day just because you have been told that doing so is good for your health, you actually put unnecessary strain on your body in two major ways:
1. Ingesting more water than you need can increase your total blood volume. And since your blood volume exists within a closed system - your blood circulatory system - needlessly increasing your blood volume on a regular basis puts unnecessary burden on your heart and blood vessels.
2. Your kidneys must work overtime to filter excess water out of your blood circulatory system. Your kidneys are not the equivalent of a pair of plumbing pipes whereby the more water you flush through your kidneys, the cleaner they become; rather, the filtration system that exists in your kidneys is composed in part by a series of specialized capillary beds called glomeruli. Your glomeruli can get damaged by unnecessary wear and tear over time, and drowning your system with large amounts of water is one of many potential causes of said damage.
Putting unnecessary burden on your cardiovascular system and your kidneys by ingesting unnecessary water is a subtle process. For the average person, it is virtually impossible to know that this burden exists, as there are usually no obvious symptoms on a moment-to-moment basis. But make no mistake about it: this burden is real and can hurt your health over the long term.
Forcing your body to accept a large amount of water within a short period of time - say, an hour or two - as several contestants did during the "Hold Your Wee for a Wii" contest can be fatally dangerous to your health. Here's why:
If you force large amounts of water into your system over a short period of time, your kidneys will struggle to eliminate enough water from your system to keep the overall amount at a safe level.
As your blood circulatory system becomes diluted with excess water, the concentration of electrolytes in your blood will drop relative to the concentration of electrolytes in
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your cells. In an effort to maintain an equal balance of electrolytes between your blood and your cells, water will seep into your cells from your blood, causing your cells to swell.
If this swelling occurs in your brain, the bones that make up your skull hardly budge. The result is an increase in intracranial pressure i.e. your brain gets squeezed. Depending on how much water your drink in a short period of time, you could experience a wide variety of symptoms, ranging from a mild headache to impaired breathing. And as occurred recently in the tragic water-drinking contest, it is quite possible to die if you drink enough water in a short enough period of time.
This information is particularly important for parents to pass on to their children. Foolish water-drinking contests are not uncommon among high school and university students, especially while playing cards.
So how much water should you drink to best support your health?
The answer to this question depends on your unique circumstances, including your diet, exercise habits, and environment.
If you eat plenty of foods that are naturally rich in water, such as vegetables, fruits, and cooked legumes and whole grains, you may not need to drink very much water at all. If you do not use much or any salt and other seasonings, your need for drinking water goes down even further.
Conversely, if you do not eat a lot of plant foods and/or you add substantial salt and spices to your meals, you may need to drink several glasses of water every day.
Regardless of what your diet looks like, if you sweat on a regular basis because of exercise or a warm climate, you will need to supply your body with more water (through food and/or liquids) than someone who does not sweat regularly.
Ultimately, the best guidance I can provide on this issue is to follow your sense of thirst. Some people believe that thirst is not a reliable indicator of how much water you need, since many people suffer with symptoms related to dehydration and don't seem to feel a need to drink water on a regular basis. My experience has been that most people who are chronically dehydrated have learned to ignore a parched mouth. If you ask such people if they are thirsty and would like a piece of fruit or a glass of water, they will almost always realize that they are indeed thirsty.
Some people suggest observing the color of your urine as a way of looking out for dehydration. The idea is that clear urine indicates that you are well hydrated, while yellow urine indicates that you need more water in your system. While this advice is somewhat useful, it is important to remember that some chemicals (like synthetic vitamins) and heavily pigmented foods (like red beets) can add substantial color to your urine. Thumbs down for synthetic vitamins, and thumbs up for red beets and other richly colored vegetables and fruits.
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The main idea that I wish to share through this article is to beware of mindlessly drinking several glasses of water per day without considering your diet, exercise habits, climate, and sense of thirst. And when you do find yourself in need of water, remember that you can get it from liquids and/or whole foods.
The Sports Drink that Evolved into an Industry Joe Kays & Arline Phillips–Han
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After playing a game of basketball in the hot sun, or mowing your yard in the
sizzling heat, you might grab a cold bottle of Gatorade™ to quench your thirst. By drinking this non-carbonated drink, you're not only relieving your thirst, but you're also replacing lost hydration from sweating. Gatorade also supplies your body with carbohydrates and electrolytes. You may already have known that from reading the bottle. But, did you know, that, health benefits aside, there is an interesting history of Gatorade? It all started in the fall of 1965. The University of Florida's football team, the "Gators", were having a hard time practicing and playing in the intense southern heat. The players sweated so much in the heat and humidity that they easily became rundown. The "sports drinks" of today weren't invented yet, and plain water didn't work to revive the exhausted football players. The Gators' coaches were perplexed. Not knowing what else to do, they asked the Science Department of the Florida university if they could come up with an antidote. That September, Dr. Robert Cade, Dr. Dana Shires, Dr. Alex DeQuesada, and Dr. Jim Free began to work after hours to come up with a remedy for their football team's frequent dehydration problem. The four put their minds together and quickly invented a drinkable liquid they thought would work. Their brew had no color to it, but it had a bad taste. So, the scientists added lemon juice to the liquid in an effort to hide its putrid taste.
On October first, Ray Graves, who was head coach of the Florida Gators, allowed the new sports drink to be tested on his freshman players. The results showed promise. So, the very next day, the entire varsity team drank the colorless liquid during their game against the Louisiana State University Tigers. The temperature was a searing one hundred degrees. But the Gators chugged Gatorade and fought their way to victory, beating the Tigers 14 to 7.
By November of the same year, the scientist's invention, now named "Gatorade" after their football team, was an important part of the Gator's practice sessions and games. The team furiously guzzled it to avoid dehydration.
In 1966, the Florida Gators's football record was a victorious 9-2. The team succeeded in winning a chance to play in the prestigious Orange Bowl. Not only did the team play in the Orange Bowl on January 1, 1967, for the first time, but they went on to beat Georgia Tech. That same year, University of Florida quarterback Steve Spurrier took home the coveted Heisman Trophy.
It wasn't surprising that Gatorade became the "Official Sports Drink of the NFL" in 1967.
The Kansas City Chiefs won the Super Bowl title in 1969. That team also credited Gatorade with giving them the winning edge.
Originally, the winning formula of Gatorade contained water, sugar, glucose-fructose syrups, citric acid, salt, sodium citrate, monopotassium phosphate, flavoring, and coloring ingredients.
The group of University of Florida scientists who invented the sports drink sold their part to Stokely-VanCamp. Stokely-VanCamp began to distribute Gatorade to the
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public in the summer of 1968. Tennis star Arthur Ashe is said to have started glugging the sports drink at this time. Ashe won the U.S. Open in 1968. He also helped the United States Davis Cup team win that year.
Stokely-VanCamp was bought by The Quaker Oats Company in 1983. The new owners of Gatorade opened up the "Gatorade Sports Science Institute (GSSI) in Illinois in 1988. The purpose of this laboratory was to support the claim that Gatorade is the "best thirst-quenching product in the world." The Institute studies the "Science of Sweat."
Today, check the label of a Gatorade Thirst Quencher sports drink that's Strawberry Lemonade flavored, and it reads, "Water, sucrose syrup, glucose-fructose syrup, citric acid, natural and artificial flavors, salt, sodium citrate, monopotassium phosphate, ester gum, red 40."
A 20-ounce bottle supplies the drinker with 270 milligrams of sodium, 75 milligrams of potassium, 35 grams of carbs, and 35 grams of sugar. Gatorade contains no fats and no protein.
Originally, Gatorade was only available in Lemon-Lime. But it now comes in a wide variety of flavors such as Cool Blue, Berry Citrus, Passion Fruit, Fierce Melon, Mandarine, Gatorade Rain, Orange, and Fruit Punch, just to name a few of their thirty flavors. Athletes and common people alike drink this popular sports drink in order to rehydrate their bodies after during sports or other activities that make them sweat. First Hands-on Activity:Purpose
“Can I Believe the Bottle?”
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Students will learn about Ion-Selective Electrodes (ISEs) by using them to determine the concentration of Sodium and Potassium in sports drinks. Safety Precautions 1. Safety goggles should be worn at all times while any lab materials are in use. 2. No food or beverages (including the sports drinks to be tested) will be present or
consumed during the lab. 3. Students will follow the instructions given by this lab handout and the lab instructor. 4. Lab protocol, etiquette, and courtesy will be observed whenever in lab. Equipment & Materials • Ion-selective electrode specific to either sodium ions (Na+) or potassium ions (K+), one
per group. • Reference electrode • Circuit board • 3V power source (2 AA batteries, 2 AAA batteries, or electronic power source) • Digital multimeter able to read values in millivolts (mV) • Stock solutions of sodium and potassium: 1.0x 10-1M; 1.0x 10-2M; 1.0x 10-3M; 1.0x 10-4M; 1.0x 10-5M. Each stock solution should be placed in a small petri dish. • Sample of “Sports Drink A” with a small petri dish • Sample of “Sports Drink B” with a small petri dish • Squeeze bottle filled with de-ionized water for rinsing both electrodes between tests • 250 mL beaker to catch rinse water from electrodes • Ring stand and three-fingered clamp to hold circuit board with electrodes. • OPTIONAL: beaker upside down to act as a stand to set solutions on. Instructional Strategies Students will work in groups of two or three. One student is responsible for changing solutions and ensuring all electrical contacts are maintained, another for rising the electrodes between solutions, and the third for recording data. If only two students are present in a group the student who rinses the electrodes should also record the data. Pre-Lab:
(For classes consisting of students with less math and chemistry background it would be appropriate for the pre-lab to be completed as a whole class, with the teacher walking the students through each step)
• Using the information on the bottles, calculate the molarity of sodium and potassium for Sports Drink A and Sports Drink B.
Calculated Concentration: Calculated Concentration: of Sodium in Sports Drink A: _____M of Sodium in Sports Drink B: _____M of Potassium in Sports Drink A:_____M of Potassium in Sports Drink B:_____M
1. Connect the digital voltmeter and power supply to the top of the circuit board. Your instructor will demonstrate the proper places to plug in each wire.
Data Collection:
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2. Use the three-fingered clamp and ring stand to hold the circuit board about two inches above the top of the upside-down beaker. This beaker will act as a stand for you to set your stock solutions and samples on.
3. Once this has been properly set up and checked by the instructor, you will receive a reference electrode and an ion-selective electrode. Your group’s recorder should write down the code of your reference electrode, ISE, and circuit board on your data sheet. Also, be sure to record the type of ion (either Sodium or Potassium) that your ISE is supposed to measure.
4. The group member responsible for ensuring the electrical connections should insert the wires of the electrodes in the appropriate receptacles and double check that all other connections are still touching.
IMPORTANT: Reference electrodes should only be touched by the silver wire when pushing them into the circuit board. If the plastic tubing is held when pushing it in the silver wire will slide down further into the tube and could ruin the electrode.
5. Fill five petri dishes a little over half full with one of each of the stock solutions. Make sure that these solutions contain the ion that matches your ISE. The 1.0x 10-5M solution should be placed in the petri dish labelled “1.0x 10-5M”, and so forth with each of the rest of the solutions.
6. Once the electrodes are in place and the stock solutions have been gathered, the group member responsible for rinsing the electrodes should spray water over each electrode, catching the rise water with the 250mL beaker. This insures that contaminants from the air or other contaminants that may be present on the electrode will not affect the data collected.
7. Lift the 1.0x 10-5M stock solution up so that both the ISE and reference electrode are submerged in the solution then place your spare beaker upside-down underneath the petri dish to hold it in place.
8. Wait for about 30 seconds or until the measurement on the voltmeter stabilizes. The recorder should then write down the number in the data table for the appropriate stock solution for Trial 1.
9. Remove the beaker and solution and rinse both electrodes, collecting the rinse water. 10. Again, place the electrodes in the 1.0x 10-5M stock solution and take another
voltage reading once the reading has stabilized. This is the reading for Trial 2. 11. Repeat the test one more time with the 1.0x 10-5M solution and record the
voltage reading. This is Trial 3 for the 1.0x 10-5M stock solution. 12. Average the voltage readings values of Trials 1-3. Record this in the Average
Voltage 13. Repeat steps 7-12 for each of the remaining four stock solutions in order:
1.0x 10-4M, 1.0x 10-3M, 1.0x 10-2M, 1.0x 10-1M. Be sure to rinse the electrodes after EACH trial. Be careful not to get any water into your stock solutions.
14. Pour samples of “Sports Drink A” and “Sports Drink B” into two new petri dishes. 15. Test each sports drink sample, using steps 7-12 as a reminder. At this point the
data table should be complete with all trials recorded and all averages taken. Data Analysis:
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(If your classroom has access to computers with EXCEL, students may complete the graphing portion of the data analysis on the computer) 1. Using the graphing paper provided, graph the average readings for each stock
solution. 2. A single straight line should be drawn through the data points as closely as possible.
This line is your calibration curve. ***Graphing Help:
The x-axis of the graph is the log(x) of the concentration of the ion in the solution tested. The axis will already be labeled with the concentrations of the stock solutions: 1.0x 10-5M; 1.0x 10-4M; 1.0x 10-3M; 1.0x 10-2M; 1.0x 10-1M. The y-axis of the graph is the voltage reading (in millivolts). The numbers on this axis will depend on the data gathered. The bottom of the graph should be labeled with a number just below your lowest average data, the top of the graph should be labeled with a number just above the highest average data.
3. Place the average voltage data for the samples of “Sports Drink A” and “Sports Drink B” on the calibration curve drawn on the graph.
4. The x-value of these two points are the concentration of the ion in the two samples. Sports Drink A Sports Drink B Concentration:__________________ Concentration: __________________ 5. Compare the concentration measured to the concentration of the ions that were
calculated in the pre-lab. Are these two numbers similar?_________________ 6. Calculate the % error of your measurement. Remember: (calculated concentration) x100 = % Error
% Error = ___________________% 8. How accurate were your measurements? Describe any possible sources of error that
may have affected your value. Do you think the data on the bottle is correct? Why? ______________________________________________________________________ ______________________________________________________________________ Data Sheet: Reference Electrode Code #:_______ Ion-Selective Electrode Code #:_____ Circuit Board Code #:_____________
“How Selective Are ISEs?” Purpose Students will deepen knowledge of Ion-Selective Electrodes (ISEs) and characteristics of ions by determining the selectivity if each of the two types of ion-selective electrodes.
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Safety Precautions 1. Safety goggles should be worn at all times while any lab materials are in use. 2. No food or beverages will be present or consumed during the lab. NO GUM! 3. Students will follow the instructions given by this lab handout and the lab instructor. 4. Lab protocol, etiquette, and courtesy will be observed whenever in lab. Equipment & Materials • Two (2) ion-selective electrodes, one specific to sodium ions (Na+) and the other
specific to potassium ions (K+), one pair per group. • Reference electrode • Circuit board • 3V power source (2 AA batteries, 2 AAA batteries, or electronic power source) • Digital multimeter able to read values in milli-volts (mV) • Stock solutions of sodium and potassium: 1.0x 10-1M; 1.0x 10-2M; 1.0x 10-3M; 1.0x 10-4M; 1.0x 10-5M. Each stock solution should be placed in a small petri dish. • Squeeze bottle filled with de-ionized water for rinsing both electrodes between tests • 250 mL beaker to catch rinse water from electrodes • Ring stand and three-fingered clamp to hold circuit board with electrodes. • OPTIONAL: beaker upside down to act as a stand to set solutions on. Instructional Strategies Students will work in groups of two or three. One student is responsible for changing solutions and ensuring all electrical contacts are maintained, another for rising the electrodes between solutions, and the third for recording data. If only two students are present in a group the student who rinses the electrodes should also record the data. The two part data collection can be done in either order, some students may wish to start testing the sodium selective electrode first and the potassium-selective electrode second. This flexibility reduces the need of a large class set of electrodes, once groups have finished with one electrode they can trade with another group that has the opposite type.
1. We will be measuring the concentration of sodium and potassium ions. What are the charges associated with these two ions?
Initial Set-Up 1. Connect the digital voltmeter and power supply to the top of the circuit board. Your
instructor will demonstrate the proper places to plug in each wire. 2. Use the three-fingered clamp and ring stand to hold the circuit board about two
inches above the top of the upside-down beaker. This beaker will act as a stand for you to set your stock solutions and samples on.
3. Once this has been properly set up and checked by the instructor, you will receive a reference electrode and two (2) ion-selective electrodes; one for sodium and one for potassium. Your group’s recorder should write down the code of your reference electrode, ISEs, and circuit board on your data sheet.
4. The group member responsible for ensuring the electrical connections should insert the wire of the reference electrode into the appropriate receptacle and double check that all other connections are still touching.
IMPORTANT: Reference electrodes should only be touched by the silver wire when pushing them into the circuit board. If the plastic tubing is held when pushing it in the silver wire will slide down further into the tube and could ruin the electrode.
Data Collection Part 1: Testing the Potassium-Selective Electrode 1. Plug the potassium-selective electrode into your circuit board. 2. Fill five Petri dishes a little over half full with one of each of the sodium stock
solutions. The 1.0x 10-5M solution should be placed in the Petri dish labeled “1.0x 10-5M”, and so forth with each of the rest of the solutions.
3. Once the electrodes are in place and the stock solutions have been gathered, the group member responsible for rinsing the electrodes should spray water over each attached electrode, catching the rise water with the 250mL beaker.
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4. Lift the 1.0x 10-5M sodium stock solution up so that both the ISE and reference electrode are submerged in the solution then place your spare beaker upside-down underneath the Petri dish to hold it in place.
5. Wait for about 30 seconds or until the measurement on the voltmeter stabilizes. The recorder should then write down the number in the data table for the appropriate stock solution for Trial 1.
6. Remove the beaker and solution and rinse both electrodes, collecting the rinse water.
7. Again, place the electrodes in the 1.0x 10-5M sodium stock solution and take another voltage reading once the reading has stabilized. This is the reading for Trial 2.
8. Repeat the test one more time with the 1.0x 10-5M sodium stock solution and record the voltage reading. This is Trial 3 for the 1.0x 10-5M stock solution.
9. Average the voltage readings values of Trials 1-3. Record this in the Average Voltage.
10. Repeat steps 4-9 for each of the remaining four sodium stock solutions in order: 1.0x 10-4M, 1.0x 10-3M, 1.0x 10-2M, 1.0x 10-1M. Be sure to rinse the electrodes after
EACH trial. Be careful not to get any water into your stock solutions. 11. Pour out each stock solution and wash the Petri dishes thoroughly. Dry with a paper
towel or Kimwipe. 12. Fill your Petri dishes with one each of the potassium stock solution. The 1.0x 10-5M
potassium solution should be placed in the Petri dish labeled “1.0x 10-5M”, and so forth with each of the rest of the solutions.
13. Once the electrodes are in place and the potassium stock solutions have been gathered, the group member responsible for rinsing the electrodes should spray water over each attached electrode, catching the rise water with the 250mL beaker.
14. Lift the 1.0x 10-5M potassium stock solution up so that both the ISE and reference electrode are submerged in the solution then place your spare beaker upside-down underneath the petri dish to hold it in place.
15. Wait for about 30 seconds or until the measurement on the voltmeter stabilizes. The recorder should then write down the number in the data table for the appropriate stock solution for Trial 1.
16. Remove the beaker and solution and rinse both electrodes, collecting the rinse water.
17. Place the electrodes in the 1.0x 10-5M potassium stock solution and take another voltage reading once the reading has stabilized. This is the reading for Trial 2.
18. Repeat the test one more time with the 1.0x 10-5M potassium stock solution and record the voltage reading. This is Trial 3 for the 1.0x 10-5M stock solution.
19. Average the voltage readings values of Trials 1-3. Record this in the Average Voltage.
20. Repeat steps 7-12 for each of the remaining four potassium stock solutions in order: 1.0x 10-4M, 1.0x 10-3M, 1.0x 10-2M, 1.0x 10-1M. Be sure to rinse the electrodes after
EACH trial. Be careful not to get any water into your stock solutions. Data Collection Part 2: Testing the Sodium-Selective Electrode 1. Remove the potassium-selective electrode from the circuit board and insert the
sodium-selective electrode.
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2. Testing the sodium-selective electrode will happen in the exact same way that the potassium-selective electrode was tested. Take three trial reading for each stock solution for both types of ions. Follow the steps from Part 1, but instead the sodium selective-electrode is in place.
Data Analysis 1. Graph the average voltages for each type of stock solution on the graph paper
provided. Use two different colors, one for sodium and one for potassium. Each of the two graphs should have two set of data. Now, similar to the previous lab, draw a best-fit line for each set of data (2 per graph).
2. The Log(concentration) values of the two lines on each graph must be compared to find how selective each electrode is for each ion.
For example: If the Potassium-Selective Electrode reads 1.0x 10-4M for sodium at a certain voltage and 1.0x 10-2M for potassium we can find that: 1.0x 10-2M = 1.0x 10-4M 1
100
This means that the PSE is 100x more selective for potassium than for sodium. 3. For the two electrodes find the ratio of selectivity between the ion that should be
selected and the ion that could also (mistakenly) be selected.
4. Look at the ratios of selectivity for each electrode. Do they have the same level of
selectivity? How can you tell which is more selective for its desired ion? Explain. __________________________________________________________________________________________________________________________________________________________________________________________________________________ 5. During the previous lab we used data from our ISEs to find concentrations of sodium and potassium in sports drinks. Sports drinks contain both sodium and potassium. Now knowing that other ions can be selected, and thus inflate our data, what can we say about the accuracy of our data from the previous lab? Which data (for sodium or for potassium) were more skewed by the presence of other ions in the sports drinks? Why? ____________________________________________________________________________________________________________________________________________ ______________________________________________________________________
Data Sheet: Reference Electrode Code #:________ Circuit Board Code #:__________
Potassium-Selective Electrode Code #:_____ Part 1: Testing the Potassium-Selective Electrode
Sodium Stock Solution Data:
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Na+ 1.0x10-5 1.0x10-4 1.0x10-3 1.0x10-2 1.0x10-1
Trial 1
Trial 2
Trial 3
Average Potassium Stock Solution Data:
K+ 1.0x10-5 1.0x10-4 1.0x10-3 1.0x10-2 1.0x10-1
Trial 1
Trial 2
Trial 3
Average
Sodium-Selective Electrode Code #:_____ Part 2: Testing the Sodium-Selective Electrode
Sodium Stock Solution Data:
Na+ 1.0x10-5 1.0x10-4 1.0x10-3 1.0x10-2 1.0x10-1
Trial 1
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Na+ 1.0x10-5 1.0x10-4 1.0x10-3 1.0x10-2 1.0x10-1
Trial 2
Trial 3
Average Potassium Stock Solution Data:
K+ 1.0x10-5 1.0x10-4 1.0x10-3 1.0x10-2 1.0x10-1
Trial 1
Trial 2
Trial 3
Average R efer ences [1] Kirking, S & Nordquist E (2005) Introduction to Ion-Selective Electrodes in the Classroom
and Their Application to the Real World. 2-5.
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[2] Kays, J & Phillips–Han, A. (2003) Gatorade: The Idea That Launched an Industry. 8(1). Retrieved July 21, 2010, from: http://www.research.ufl.edu/publications/explore/v08n1/gatorade.html
[3] Kim, B. (2009). Why Drinking Too Much Water Can Be Dangerous. Retrieved July 21,
