OBSERVATION of Ph SHIFTS USING the PROCESS of IONIC EXCHANGE

OBSERVATION OF pH SHIFTS USING THE PROCESS OF IONIC EXCHANGE

Teri Alvarez-Ziegler

PRIDE High School

Sunnyside, WA

Sandra Poisson

Lewis-Clark State College

Lewiston, ID

Washington State University Mentors

Dr. Neil Ivory

Chemical Engineering

Ann Marie Hardin

PhD Candidate

Washington State University

Pullman, WA

July, 2006

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.

SUMMARY

Overview of Project:

This module is an active learning module, which can be adapted to a teacher’s time schedule and laboratory equipment. The overall focus is to gain an understanding of the effect of ion concentration effect on a changed surface by observing pH shifts that occur during ionic exchange chromatography. The students will also gain a better understanding of such scientific concepts as: Molarity, pH scale, pH indicators, buffers, diffusion, and charged surfaces. Engineering concepts, such as chromatography, are discussed and performed within this lab. These are demonstrative of, and applicable to, the concept and performance of a home water softener system.

Intended Audience:

This teaching module is intended for high school classes, grades 9-12. This module could be used in Biology, Chemistry, or Physical Science classes.

Estimated Duration:

This module is estimated to last a week, based on five 50-minute class periods. Day 1-consists of a pre-test, review of the concepts, worksheets, and discussion. Day 2-the students are divided up into groups of two, as pre-determined by the teacher. The students will prepare the resin needed for the lab. Day 3-the students will make the color standards and stock buffer. Day 4- the students will adjust their buffer to the required NaCl concentrations and perform the resin pH experiment. Day 5-the students compare data/findings. They will also check if all NaCl concentration buffer samples remain at 6.0 pH. They will do their lab write-up and the student evaluation form. Note: The teacher could adjust this module to fit into three block class periods.

INTRODUCTION

Rationale for Module:

The purpose of this module is to expose students to engineering activities. The main focus of this module is to show how the salt concentration affects the surface charge of the resin, and its consequent effect on the pH shift. Students will be able to have hands-on experiences, and develop their laboratory skills. This module will solidify basic scientific concepts in the students as they work on this more complex activity. The students will gain familiarity with proper laboratory procedures, and handling of chemicals and equipment. This module meets various learning standards, see Learning Objectives.

SCIENCE

The following are concepts related to this module:

Molarity (M):

A measurement of concentration. M= moles of solute/liters of solution

pH Scale:

pH= -log[H+], a way to measure the concentration of Hydrogen, and relative acidity or alkalinity. The lower the pH, the higher the Hydrogen concentration is. The pH scale is logarithmic, which means that moving 1 unit either way on the pH scale results in a 10 fold increase in hydrogen ion concentration either direction. The pH scale range is 0 to 14, with 7.0 being neutral. Numbers below 7.0 on the pH scale are acidic. Numbers above 7.0 on the pH scale are basic.

pH Indicators:

These are colored substances that can be either in an acidic or basic form. The acidic and basic forms have different colors. The ranges of different indicators are known and can be found in various literature sources (Brown, LeMay, Bursten, 1997, pp. 579-580). For Bromothymol blue (the indicator used in this module) the active pH range is 6.0 (yellow color) to 7.6 (blue color) (Bishop, 1972, pp. 109).

Buffers:

These are solutions, usually composed of a weak acid or weak base with salts of that acid or base, that resist changes in pH with the addition of an acid or base. The acid form neutralizes the OH- and the base form neutralizes the H+. The buffer effectively stabilizes when the pH of the solution is within ±1.0 of the buffer’s pKa.

If OH- is added to the buffer solution the following reaction takes place:

OH-(aq) + HA (aq) à H2O(l) + A-(aq)

If H+ is added to the buffer solution the following reaction takes place:

H+(aq) + A-(aq) à HA(aq)

Buffer capacity: the quantity of acid or base the buffer can neutralize before the pH begins to change drastically.

To calculate the pH of a buffer, use Henderson-Hasselbach equation:

pH= pKa + log ([base]/[acid])

(Brown, LeMay, Bursten, 1997, pp. 624 - 626).

Bis-Tris properties: Useful pH range is 5.8-7.2 and the pKa of the buffer is 6.5 (www.sigmaaldrich.com, 2006).

Diffusion:

This is a process where one substance spreads throughout a space or another substance to reach equilibrium. Diffusion is driven by a difference in concentration across space. Over time, the substance that is not evenly distributed throughout a space will diffuse until an equilibrium condition is reached.

Charged Surfaces:

Positive charges on the surface of materials will attract negatively charged molecules and vise versa (like charges repel each other). The negative molecules will bind to the surface of the positively charged material while positively charged molecules will be repelled from the surface. This is demonstrated by the positively charged resin used in this module. The pH shift is an indication of the exclusion of hydrogen ions from the internal surfaces of the resin.

ENGINEERING

Chromatography:

This is the process of separating substance components (different dyes and/or proteins) based on their differing abilities to adhere to the surface of another substance (Brown, LeMay, Bursten, 1997, p. 9).

Ion Exchange Chromatography:

This process relies on charge to charge interactions between the proteins in a sample and the charges on the resin’s surface (Ion, 2006). The resin used in ion exchange is a matrix composed of small beads, which are made from organic polymers (Ion Exchange resin, 2006). This process can be subdivided into cation exchange, in which positively charges ions bind to a negatively charged resin; and anion exchange, in which the binding ions are negative and the immobilized functional group is positive (Ion, 2006).

The process used in this module is the anion exchange system. The resin used in this module is Q-Sepharose Fast-Flow.

Real Life Application - Home Water Softener Systems:

What is Hard Water?

Hard water is water containing a great deal of the minerals Ca and Mg (both carry a positive charge) (Water Softening FAQ’s, 2006).

Why are people concerned with Hard Water?

Hard water creates soap curd and scale. Soap curds dull the colors of fabrics and shorten the life of the fabric. To get the clothes completely clean, more detergent is needed, which can rack up money (Water Softening FAQ’s, 2006).

Scale is formed when the hard water is heated. This can build up in the pipes forming “rocks.” This causes the water heater to work harder since it has to heat up the rock as well as the water (Water Softening FAQ’s, 2006).

The following are additional problems resulting from hard water: clogged water pipes; water heater inefficiency and scaling; decreased water flow or pressure; frozen valves and faucets; scale spots on glassware and/or silverware; crust rings in toilet tanks and rings in toilet bowl; white spotting on bath tubs, tile, and chrome; white deposits on shower heads; hard to remove film on shower tiles and doors; and poor sudsing of soap and shampoo.

How does the Water Softener Work?

Most water softeners work by using the cation exchange system. This system uses resin beads to remove the Ca and Mg ions from the hard water by the following process. The resin beads attract the positively charged Na or K ions from the saline solution surrounding the beads. The Na or K ions are released when the beads come in contact with the Ca and/or Mg ions in the hard water. The Ca and/or Mg ions will adhere to the surface of the resin beads (Water Softening FAQ’s, 2006).

There reaches a point in the system where all the Mg or Ca have taken up the positions held by the Na or K ions. At this point, recharging the system is required. This is done by adding more salt to the solution which causes the release of the Mg or Ca ions. These ions are released as waste, while the Na or K ions take back their original place, allowing the system to start again (Water Softening FAQ’s, 2006).

What is Soft Water?

Soft water is water with only the Na or K cations remaining. It is the end product of the water softening process. Soft water is a step closer to pure water.

GOALS

Students will use prior knowledge learned from science classes to complete the lab. This teaching module will expand their knowledge and the scope of what their perception of engineering is. Students will learn that engineering involves all aspects of science, and their effects on the world around us.

LEARNING OBJECTIVES

Students will be able to define the following terms (Cognitive; Knowledge):

·  Molarity

·  diffusion

·  pH

·  ionic exchange

·  ionic exchange chromatography

·  chromatography

·  pH scale

·  pH indicators

·  buffers

·  charged surfaces

Students will be able to:

1.  understand that there are different areas of study in engineering.

( cognitive; comprehension )

2.  understand that salt concentration affects the surface charge of the resin.

( cognitive; comprehension )

3.  understand what causes the pH shift.

( cognitive; comprehension )

4.  have hands-on laboratory experience.

( cognitive; application )

5.  develop their laboratory skills.

( cognitive; application )

6.  solidify basic scientific concepts.

( cognitive; comprehension )

7.  gain familiarity with proper laboratory procedures.

( cognitive; application )

8.  gain familiarity with handling of laboratory equipment and chemicals.

( cognitive; application )

Science GLEs:

1.1.1  Understand the atomic nature of matter, how it relates to physical and chemical properties, and serves as the basis for the structure and use of the periodic table.

1.2.1  Analyze how systems function, including the inputs, outputs, transfers,

transformations, and feedback of a system and its subsystems.

2.1.3  Synthesize a revised scientific explanation using evidence, data, and

inferential logic.

2.1.4  Analyze how physical, conceptual, and mathematical models represent

and are used to investigate objects, events, systems, and processes.

Reading GLEs:

1.3.2  Understand and apply content/academic vocabulary critical to the meaning

of the text, including vocabularies relevant to different contexts, cultures, and communities.

3.2.2  Apply understanding of complex information, including functional

documents, to perform a task.

Mathematic GLEs:

1.2.3  Understand how to convert units of measure within systems.

1.4.3  Apply appropriate methods and technology to collect data or evaluate

methods used by others for a given research question.

1.5.6  Apply procedures to solve equations and systems of equations.

2.2.2  Apply mathematical tools to solve the problem.

EQUIPMENT

0.1 M NaOH / Parafilm
1 M HCl / pH meter (or pH paper)
100 mL beakers / Pipettes or Droppers
15 mL Falcon centrifuge tubes w/lids / Q-Sepharose fast-flow, 300 mL or 100 mL
50 mL Falcon centrifuge tubes w/lids / Refrigerator unit
600 mL beakers / Ruler
Bis-Tris / Scale
Bromothymol Blue / Small covered containers
Calculator / Stir plate w/ magnetic stirrers (or glass stir rods)
Distilled (DI) water / Stock bottles
Graduated cylinders, 100 mL / Test tube racks
Graduated cylinders, 500 mL / Test tubes
Graduated eppendorf microcentrifuge tubes / Volumetric Flask, 100 mL
Graph paper / Vortex mixer
Labels/Tape / Waste beakers
NaCl stock / White surface for color comparison (paper)

TEACHER PREP

Estimated time:

The estimated time for teacher prep is approximately 1 ½ to 2 hours. Of course, this depends on the availability of the materials and equipment needed. This should be enough time to prepare enough material for 5 groups.

Background:

Q Sepharose FF is a positively charged anion exchange resin commonly used in protein purification processes. The positive charge excludes hydrogen and other positively charged ions from the resin phase. The extent of this exclusion is determined by the density of the active surface charges. The resin will be contacted with a 20 mM Bis- Tris buffer at pH 6.0 containing different NaCl concentrations. The chlorine ions in the solution interact with the positively charged resin surface and effectively “turn off” some of the charge sites. This moderates the extent to which hydrogen ions are excluded from the resin phase, and consequently, the magnitude of the pH shift.

Procedures:

Start resin tubes: Total time: 10 minutes for each group.

1. Obtain the stock resin. DO NOT MIX STOCK BOTTLE!
2. Gently stir stock resin while pipetting out 8 mL of settled resin per group, place in 15mL Falcon tube. The goal here is to introduce enough storage alcohol into a region of settled resin to be able pull it into a pipette, not to completely mix the resin with the storage solution.
3. Let the resin settle and adjust to get 4 mL settled resin per tube as follows:
4. If the settled resin volume is too high, add storage solution from the stock bottle to the tube until the liquid volume is equal to the settled resin volume. Invert the tube repeatedly to completely mix the resin and alcohol. Remove resin mixture from the falcon tube until the total volume in the tube is 8 mL. If you have used a clean falcon tube and pipette, the excess can be returned to the stock bottle.
5. If the settled resin volume is too low, repeat step 2 above.
6. Cap tubes and store at room temperature.
7. This should end up equaling 4 mL of settled resin per each group.

Make Bromothymol Blue solution (0.04%): Total time: 20 minutes for whole class.

1. Mix 0.64 mL of 1.0 M NaOH and 0.04 g of Bromothymol blue, in an eppendorf tube.
2. Mix until dissolved, either using a vortex mixer or by inverting the tube.
3. Pour the mixture in a volumetric flask (100 mL).
4. Rinse the eppendorf tube with DI water, until all of the blue solution is removed, placing each rinse in the flask.
5. Fill the volumetric flask to 100 mL with DI water.
6. Mix well.
7. Divide into stock bottles, one for each group.
8. This will be enough for the whole class.

Measure out NaCl: Total time: 25 minutes for each group.