Introduction: (Initial Observation)
Most water contains some dissolved minerals. If water has a high concentration of calcium and magnesium it is called hard water. Hard water is not a health risk, but a nuisance because of mineral buildup on fixtures and poor soap and/or detergent performance. Hard water interferes with almost every cleaning task, from laundering and dishwashing to bathing and personal grooming.
Clothes laundered in hard water may look dingy and feel harsh and scratchy. Dishes and glasses may be spotted when dry. Hard water may cause a film on glass shower doors, shower walls, bathtubs, sinks, faucets, etc. Hair washed in hard water may feel sticky and look dull. Water flow may be reduced by hard water deposits in pipes.
A known process of transforming hard water to soft water is ion exchange. During this process water goes through a filter containing ion exchangers that may trap or substitute different ions.
Although the description seams simple, there are many questions that can be studied. For example I want to know:
- What are ion exchanger material?
- How many different ion exchangers exist?
- What type of ions do they trap? Anions or cations?
- After how much filtering do ion exchangers become saturated and stop working?
- Can we recover ion exchanger material and reuse them?
- Ion exchangers substitute one type of ions with another type of ions? What is the benefit of substituting ions?
- How can we determine or test the hardness of water before and after going through an ion exchanger.
In this project I will experiment ion exchange so that I will better understand and demonstrates how the chemical process of ion exchange can remove undesired ions from water.
Find out about hard water and soft water. Search the Internet for “effects of hard water” to see how excessive amount of minerals in water can be annoying and inconvenient. Read books about water treatment and ask professionals who might know in order to learn about the chemistry of ions and the process of ion exchange in water treatment. Keep track of where you got your information from.
The following are samples of information that you may find.
Hard Water and our lives
Dealing with hard water problems in the home can be a nuisance. The amount of hardness minerals in water affects the amount of soap and detergent necessary for cleaning. Soap used in hard water combines with the minerals to form a sticky soap curd. Some synthetic detergents are less effective in hard water because the active ingredient is partially inactivated by hardness, even though it stays dissolved.
Bathing with soap in hard water leaves a film of sticky soap curd on the skin. The film may prevent removal of soil and bacteria. Soap curd interferes with the return of skin to its normal, slightly acid condition, and may lead to irritation. Soap curd on hair may make it dull, lifeless and difficult to manage.
When doing laundry in hard water, soap curds lodge in fabric during washing to make fabric stiff and rough. Incomplete soil removal from laundry causes graying of white fabric and the loss of brightness in colors. A sour odor can develop in clothes. Continuous laundering in hard water can shorten the life of clothes.
In addition, soap curds can deposit on dishes, bathtubs and showers, and all water fixtures.
Hard water also contributes to inefficient and costly operation of water-using appliances. Heated hard water forms a scale of calcium and magnesium minerals that can contribute to the inefficient operation or failure of water-using appliances. Pipes can become clogged with scale that reduces water flow and ultimately requires pipe replacement.
Potential Health Effects
Hard water is not a health hazard. In fact, the National Research Council (National Academy of Sciences) states that hard drinking water generally contributes a small amount toward total calcium and magnesium human dietary needs. The further state that in some instances, where dissolved calcium and magnesium are very high, water could be a major contributor of calcium and magnesium to the diet.
Researchers have studied water hardness and cardiovascular disease mortality. Such studies have been “epidemiological studies,” which are statistical relationship studies.
While some studies suggest a correlation between hard water and lower cardiovascular disease mortality, other studies do not suggest a correlation. The National Research Council states that results at this time are inconclusive and recommends that further studies should be conducted.
If you are on a municipal water system, the water supplier can tell you the hardness level of the water they deliver. If you have a private water supply, you can have the water tested for hardness. Most water testing laboratories offer hardness tests for a fee.
Many companies that sell water treatment equipment offer hardness tests. When using these water tests, be certain you understand the nature of the test, the water condition being measured, and the significance of the test results.
An approximate estimate of water hardness can be obtained without the aid of outside testing facilities. Water hardness testing kits are available for purchase through water testing supply companies. If more accurate measurements are needed, obtain a laboratory test.
Interpreting Test Results
The hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.
The Environmental Protection Agency (EPA) establishes standards for drinking water which fall into two categories — Primary Standards and Secondary Standards.
Primary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosiveness, foaming, and staining properties of water. There is no Primary or Secondary standard for water hardness.
Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:
|Classification||mg/l or ppm||grains/gal|
|Soft||0 – 17.1||0 – 1|
|Slightly hard||17.1 – 60||1 – 3.5|
|Moderately hard||60 – 120||3.5 – 7.0|
|Hard||120 – 180||7.0 – 10.5|
|Very Hard||180 & over||10.5 & over|
Other organizations may use slightly different classifications.
Water softening methods
There are two ways to help control water hardness: use a packaged water softener, or use a mechanical water softening unit.
Packaged water softeners are chemicals that help control water hardness. They fall into two categories: precipitating and non-precipitating.
Precipitating water softeners include washing soda and borax. These products form an insoluble precipitate with calcium and magnesium ions. The mineral ions then cannot interfere with cleaning efficiency, but the precipitate makes water cloudy and can build up on surfaces.
Precipitating water softeners increase alkalinity of the cleaning solution and this may damage skin and other materials being cleaned.
Non-precipitating water softeners use complex phosphates to sequester calcium and magnesium ions. There is no precipitate to form deposits and alkalinity is not increased.
If used in enough quantity, non-precipitating water softeners will help dissolve soap curd for a period of time.
Water softening units can be permanently installed into the plumbing system to continuously remove calcium and magnesium.
Water softeners operate on the ion exchange process. In this process, water passes through a media bed, usually sulfonated polystyrene beads. The beads are supersaturated with sodium. The ion exchange process takes place as hard water passes through the softening material. The hardness ions (calcium and magnesium) attach themselves to the resin beads while sodium on the resin beads is released simultaneously into the water.
A basic water softener unit uses a cationic resin to extract hard cations (such as calcium and magnesium) and substitute them with soft cations (such as hydrogen, sodium or potassium). In other words if there are some calcium sulfate in water, it will be changed to sodium sulfate. This is good because sodium sulfate does not form scale in boiler and hot water pipes.
Resins can be recharged
When the resin becomes saturated with calcium and magnesium, it must be recharged. The recharging is done by passing a salt (brine) solution through the resin. The sodium replaces the calcium and magnesium which are discharged in the waste water.
Hard water treated with an ion exchange water softener has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water.
For example, if the water has a hardness of 10 grains per gallon, it will contain about 80 mg/liter of sodium after being softened in an ion exchange water softener if all hardness minerals are removed.
Because of the sodium content of softened water, some individuals may be advised by their physician, not to install water softeners, to soften only hot water or to bypass the water softener with a cold water line to provide unsoftened water for drinking and cooking; usually to a separate faucet at the kitchen sink.
Softened water is not recommended for watering plants, lawns, and gardens due to its sodium content.
Although not commonly used, potassium chloride can also be used to create the salt brine. In that case potassium rather than sodium is exchanged with calcium and magnesium.
If the ion exchanger contains both anionic and cationic resins, it will extract both anions and cations. That is why in many applications, deionized water can be used like distilled water.
Principle of Operation
Ion exchange is a reversible process in which a dissolved ionic species is taken up by a solid in a stoichiometric manner. Every ion removed from the solution is replaced by an equivalent amount of charge in the form of another ionic species. Sites on the exchange material have a fixed charge, either positive or negative.
The most common type of ion exchange material is ion exchange resin, which consist of an irregular, macromolecular, three-dimensional network of hydrocarbon chains. This matrix carries ionic functional groups such as
-SO3– -COO– -PO32- -AsO32-
in cation exchangers, and
-NH3+ =NH2+ =N+= =S+–
in anion exchangers.
The equilibrium relations governing the activity for each kind of exchange resin are as follows:
Cation exchange: R–H+ + X+ « R–X+ + H+
Anion exchange: R+OH– + Y– « R+Y– + OH–
Neutralization: H+ + OH– « H2O
· R– and R+represent the cation and anion exchange resins respectively.
· X+ and Y–represent cationic and anionic impurities respectively
Usually ion exchange units consist of a column packed with the ion exchange resin (Figure below). Mixed beds, which are a mix of anion and cation exchanger resins, are commonly used. The solution to be treated passes through the column where the undesired ion is exchanged for a resin ion, which has lower affinity for the resin than the undesired ion. The released ion (usually H+ and OH- that react to form H2O) becomes part of the solution. The treated effluent is free of undesired ions.
Applications and Limitations
The most common application of ion exchange resin is in mixed bed packed reactors. In this case, cation and anion exchange resins are mixed together in a packed bed to simultaneously remove both unwanted cations and anions from solution. Since both cation exchange and anion exchange occur simultaneously in this configuration, any released H+ or OH- ions will be immediately neutralized to form water and maintain a neutral pH.
Ion exchange is a unit operation mainly used in water treatment processes as softening, dealkalizing, demineralizing, organic compound removal, nitrate removal, and degasification. These treatments are used in municipal supplied water conditioning, boiler feedwater conditioning, ultrapure water generation for electronics manufacture, pharmaceutical-grade water, and other applications. Ion exchange is also used in separation processes (i.e. purification of juices and syrups, production of dextrose and fructose, metal recovery), reaction engineering (i.e. catalysis), etc.
ION EXCHANGE AND ZEOLITES
The name zeolite comes from the two Greek words “zein” and “lithos” which mean “boiling stone”. It was first applied by Granstedt, a Swedish geologist, in 1756 to describe a certain class of natural minerals which when heated, swelled and gave off their water of hydration. These zeolites are hydrated double silicates consisting of an alkali or alkali earth oxide, alumina, silica and water. more...
Some natural zeolites are being sold as cat litter freshener, carpet deodorizer and pet odor eliminator.
FUNDAMENTALS OF DEIONIZATION
BY ION EXCHANGE
All naturally-occurring water contains dissolved mineral salts. In solution, salts separate into positively-charged cations and negatively-charged anions. Deionization can reduce the amounts of these ions to very low levels through the process of ion exchange.
Cations are removed by cation exchange resin. It replaces sodium, calcium, magnesium and other cations with hydrogen ions (H). This exchange produces acids which must be removed or neutralized by anion exchange resin.
Cation: An ion or group of ions having a positive charge and characteristically moving toward the negative electrode in electrolysis.
Two types of anion resin are used for deionization: weak base resin and strong base resin. Weak base resin absorbs strong acids, while strong base resin exchanges chloride, sulfate, and alkaline anions for hydroxide ions (OH). The hydrogen ions from the cation exchange process combine with the hydroxide ions from the anion exchange process to form water (HOH or HO).
Because the deionization process is so effective, the water quality is usually measured by the water’s resistance to electric current (in Ohm-cm). Deionized water quality depends on a variety of factors, including raw water composition, ion exchange resin types and quantities and the number of resin tanks in the system.
Two-bed deionizers use separate tanks, one containing cation resin, the other containing anion resin. A two-bed weak base deionizer typically produces water with electrical resistance of about 50,000 Ohm-cm. A two-bed strong base deionizer typically produces water with electrical resistance of about 200,000 Ohm-cm.
In a mixed-bed deionizer, cation and anion resins are thoroughly mixed in a single tank. The mixed resins act like a series of alternating cation and anion exchange tanks to produce very high quality water. A mixed-bed deionizer typically produces water with greater than 10,000,000 Ohm-cm resistance, which is equivalent to less than 0.05 mg/L of sodium chloride.
The resins need regeneration when they no longer produce the desired quality water. In the case of a two-bed deionizer, the cation tank is backwashed for 5 to 10 minutes, then washed with a 6 percent solution of hydrochloric acid. Then the anion tank is backwashed and washed with a 5 percent solution of sodium hydroxide. After rinsing residual chemicals from each tank, water flows through both tanks to drain until the water reaches the desired quality.
In a mixed-bed deionizer, the resins have to be separated before regeneration. After regeneration and rinsing they have to be remixed, using air, before returning to service.
Although the process is fairly simple in concept, its application is complicated by variables in raw water composition, treated water quality needs, resin selection, chemical dosages and control system requirements.
How effective or efficient are different ion exchangers?
The purpose of this project is to separate aqueous cations from an aqueous by use of some synthetic resin or natural zeolite in order to determine their efficiency.
Your question or purpose can be different, however you can still use the same basic information and the same experiments that are proposed here.
For example you may want to find out what material are ion exchangers? So you may test different synthetic and natural material to see if they can exchange ions.
You may want to compare different ion exchangers to see how fast they can exchange ions. Or you may compare ion exchanger resins with different beads size to determine what size of beads are most efficient.
Research can be done on methods of identifying the mineral content of water.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
The independent variable is the type of ion exchanger that we choose to test. Possible values are natural zeolites (powder and granules, cat litter freshener), synthetic cathionic resins (different brands or beads size).
Dependent variable is the efficiency, effectiveness or performance of the ion exchanger.
Controlled variables are water type, temperature and experiment procedures including the amount of ion exchanger and it’s exposure time to certain amount of water.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following is a sample hypothesis.
Among one type of zeolite and two different types of cationic resins that I am testing, the synthetic cationic resin with smaller beads will be more efficient.
My hypothesis is based on my gathered information and the fact that smaller beads have a larger surface area per unit weight.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: Using cationic ion exchangers
In this experiment you will use one type of cationic ion exchanger to soften some hard water. The effectiveness of the process can be determined by the solubility of soap in water and the amount of lather. Before beginning the experiment, it may be helpful to review the definition of ions as particles with an electrical charge and the concept that particles with opposite electrical charges are attracted to one another.
Most municipal water systems in the United States have hard water. However, if your municipal water (tap water) is naturally soft, you will need to make your own hard water for this activity. You can make a representative of hard water by dissolving 3.3 grams (0.1 oz.) of Epsom salts (MgSO 4 . 7H 2 0) in one liter (approximately 1 quart) of distilled water or your softened tap water.
- Gather 100 grams of each ion exchanger that you want to test.
- Before use, the cation exchange resin should be soaked in tap water for at least 24 hours.
- Prepare your ion exchanger tank by connecting a PVC valve to a 1 foot long PVC pipe. Insert a foam or cotton in the pipe to act as a filter, so the ion exchanger material can not pass through the valve.
- Pour the 100 grams ion exchanger that you have prepared in the pipe.
- Make sure the valve is open and slowly pour some soft water into the pipe until the water comes out off the valve. Leave the valve open until this water will fully drain. You do that to make sure that the foam and the entire system are soaked and will not hold any of your hard water.
- Start adding one liter of your hard water to the inlet of the pipe and let it drain from the outlet (valve). Collect this in a separate container. Label the container with the name of the ion exchanger now in the pipe.
- If you have more than one ion exchanger tank, use a different tank for each ion exchanger that you want to test; otherwise, wash this ion exchanger tank and prepare it for other ion exchangers.
- Repeat the ion exchange with each of the ion exchangers that you want to test and collect the results in different labeled containers.
- Cut identical same size soap cubes about the size of a dace (almost 1.5 cm x 1.5 cm)
- Drop one soap cube in each softened water sample. Stir the water and compare the solubility of soap and the amount of lather in water samples softened using different ion exchangers.
This experiment assumes that the zeolite or ion exchange resin that you test is a cation exchanger in the sodium (Na + ) form; i.e., the resin comes saturated with sodium cations. It is possible that the resin supplier will offer you a cation exchanger in the hydrogen (H + ) form. This will make no difference in the result of the experiment. But point out that Ca 2+ and Mg 2+ are replaced in the water by H + .
After you complete the experiment, you should understand that ions of calcium and/or magnesium, which react with the soap molecules to form an insoluble material, have been replaced by ions of sodium, which do not react with the soap molecules, and that the source of the sodium ions is the cation resin. Note that an ion exchange resin is a man-made product that works the same way as naturally occurring zeolites. The visual result of the ion exchange is the forming of soap bubbles.
After use, the resin may be recharged by soaking it in salt water and storing it in a sealed container.
Before using the resin again, rinse it several times with fresh water.
Description: Soap is added to a sample of hard tap water and it does not produce suds when shaken. Another sample of tap water is passed through an ion exchange column. Soap is added to the second sample and it produces lots of suds since the calcium ions have been removed.
Materials and Equipment:
- 100 grams of each cation exchange resin or zeolite
- 3 bottles or flasks that will hold one liter of water
- PVC pipe and valve to act as a tank
- caps or stoppers for the bottles
- soap (not liquid detergent)
- Pieces of foam or cheesecloth
- measuring cup
- hard water (or one liter [2 pints] tap water + 3.3 grams [0.12 ounces] Epsom salts for each test)
- Some dealers who sell ion exchange water softeners may sell or give small quantities of cation exchange resins to schools at no charge.
- The availability of specific resin varies from one geographic area to another.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Record your results in a table like this:
|Suds/ Bubbles||undessolved soap after one hour||Soap curd|
|Cation Exchanger 2|
Soap curd can be filtered, dried and weighted.
Unresolved soap can be dried and weighted.
Suds and bubbles can be measured or estimated by volume.
No calculation is required.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
- We did not recharge the ion exchanger resins and zeolites before use. We could possibly get a different result if we would charge all samples at identical condition.
Sienko, M.J., and Plane, R.A.,Chemistry, Principles and Application, McGraw-Hill, Inc., New York, 1979, p. 469. This work discusses the basics of ion exchange.
Westmeyer, P., Successful Devices in Teaching Chemistry, J. Weston Walch, Portland, ME, p. 213.
Attention Chemists, Schools, & Colleges
ChemicalStore.com offers a large selection of chemicals for research and education at affordable price and convenience of online ordering.
Visit ChemicalStore.com today.