Assignment directions for a project using water hardness

Chem Projects Research Team

CheMystery Labs

380 South Buzz Blvd

Bee Hive, UT 84302

 

Dear Team:

 

The County Council of Governments would like to know how the total water hardness compares between various communities in the county. Some soft water costumers think their systems are being affected by excessive hard water provided by the community.  The County Council of Governments has a budget of $60,000 to answer their question.

 

 Before you begin your work, I will need the following information from you so that I can approve your work.  The planning space for these items is found in your project packet.

 

Ø       a detailed summary of your hypothesis, experimental

      design table, a plan for the procedures along with all

       necessary data tables

Ø       a description of necessary calculations

Ø       an itemized list of equipment with costs and chemical with

      their costs, include labor costs of $80 / hour and cost for

      laboratory space $10,000/day, and a standard disposal

      fee of $2000 / gram of material. 

 

After you complete the analysis, prepare a report for the County Council of Governments.  Remember that this report will be seen by a variety of people, so be certain it projects the image we want to present.  The items that should be included in the report are found in your project packet.

 

Good luck with the project.  Please keep in close contact me as you proceed with the project.  You will need a data verification signature, by having me observe one of the trials in the experiment.

 

Sincerely

 

Your teacher

 

President and CEO

CheMystery Labs Inc.

 

The Total Hardness Of Water In Local Water Supplies

       Background Information

              Hard water contains dissolved salts, especially those of calcium and magnesium.  Hard water often contains magnesium and
              iron ions also.  These ions are responsible for an assortment of problems ranging from aesthetic ones, such as
              bathtub ring and soap scum, to more critical ones like plugged steam lines and damaged water heaters. Most of the
              dissolved calcium ions in the natural waters of rivers, lakes, and streams originate from
              limestone, which is principally calcium carbonate, CaCO₃.  The limestone is dissolved by rain water or by water
              contaminated by mining and industrial wastes containing hydrochloric and sulfuric acids.
 
 

    CaCO₃      +      H⁺    --------->       Ca²⁺     +    HCO₃^-

              The reaction of water with atmospheric carbon dioxide makes rain water naturally slightly acidic.  Unnatural acidic
              rain can result from the reaction of atmosphere water with industrial pollutants like sulfur dioxide and nitrogen
              dioxide.
 
 

H₂O   +   CO_{2 (g)  ---------}>   H₂CO_{3 (aq)}

H₂O     +    SO_{2 (g)  ---------}>   H₂SO_{3 (aq)}

H₂O    +    2NO_{2 (g)}   ------------>    HNO_{2 (aq)}     +     HNO_{3 (aq)}

 

              The location of limestone deposits varies throughout the country, as do the amounts of industrial acids exposed to
              rivers and the acidity of rainfall.  Thus, the amounts of dissolved calcium ion varies, and so does the resultant
              hardness of water in varius locations.

              Calcium ions, while nontoxic, present a number of significant household and industrial problems.  For example,
              calcium ion reacts with the ingredients of many soaps and shampoos to form an insoluble scum that you know as a
              "bath tub ring."  The formation of this precipitate causes the soap to lose its effectiveness.
 
 

Ca²⁺    +   2CH₃(CH₂)₁₅COO^-Na⁺  ---------->   2Na⁺    +    (CH₃(CH₂)₁₅COO^-)₂Ca

                                                    sodium stearate                                                        calcium stearate
                                                    a typical soap                                                           an insoluble scum

              Hard water also accounts for the buildup of "boiler scale" on the insides of teapots, water heaters, and industrial
              boilers.  The scale consists of calcium carbonate that has precipitated from hard water heating:
 
 

Ca(HCO₃)_{2 (aq)}       +      heat    ---------->     CaCO_{3 (s)}    +      CO_{2 (g)}     +   H₂O

 

              The calcium carbonate scale must be removed periodically because it reduces heating efficiency by acting as an
              insulator.  Calcium carbonate also promotes corrosion and block pipes.

               The analysis for water hardness is based on the chemical reaction of Ca²⁺ ( and Mg²⁺) with an ion called
               dihydrogenethylenediaminetetraacetate, C₁₀H₁₂N₂O₈^2-.  Since the name and formula are complicated, it is easier
               to simply called it "EDTA".    

                 The analytical procedure is a titration.  In this titration, a known amount of water is measured out and EDTA

                 solution is added drop-wise until just enough is present to completely react with all of the calcium and

                 magnesium ions in the sample.

                 In the project you will measure the total amount of calcium and magnesium ions in
               various water samples from around your area and compare them to a standard.  You will add a particular kind of
               solution, called a buffer, to each water sample which will maintain the pH at about 10.5.  You will then titrate each
               sample with EDTA until the indicator Ero-T turns from wine red to purple-blue at the end point.

               The indicator is red in the presence of Mg2+.  EDTA reacts with both Ca 2+ and Mg 2+.  The indicator turns blue
               when all of the Mg 2+ and Ca2+ are combined with EDTA.

              Equations:
                                      Mg²⁺   +   2In^-   =  MgIn₂                        EDTA^2-   +   Ca²⁺   ------>  CaEDTA
                                      blue                        red                            EDTA^2-   +   Mg²⁺  ------>  MgEDTA
 
 

              You will report the results as either parts per million (ppm) of calcium carbonate, CaCO₃, even though some of the
              hardness may be due to the presence of magnesium ions, or mg of hardness per liter of sample or grains of
              hardness per gallon of sample.  The higher the number of ppm, mg/L or grains per gallon of CaCO₃, the more
              calcium ions are in the water and the harder the water is.
 

         Experimental Procedures for measuring the
         amount of hardness in water
 

            General Testing for Water Hardness
 

                1.  Add 2.5 mL of water to a 10 mL graduated cylinder

                2.  Add 6 drops of hardness #1  [ a buffer ]

                3.  Add 3 drops of hardness #2

                4.  Add drop by drop hardness #3 until the color
                     changes from red color tints to purple-blue color tints.
                     [ EDTA ].  Start adding EDTA solution slowly, one drop at a time, counting drops and
                    stirrring after each addition.  Continue until the color starts to change from red to
                    purple.  Wait a few moments to see if the color continues to change ( the reaction is
                    slow); then slowly add additional drops until the color becomes pure purple-blue.
                    Keep track of the total number of drops used.  When you reach the end point,
                    enter the number of drops in your data table.
 
 

                Types of tests that must be conducted

                    1.  Test the color change of the indicator.  Use 2.5 mL of distilled water to conduct this test.
                                The color should be a clear purple-blue color.  If not, add i drop of the EDTA solution
                                to produce this color.  This is the color you will be looking for at the end of each titration.
                                It should be pure purple-blue with no trace of red.

                    2.  Three trials of the calcium reference solution.

                 3.  Three trials of each local water sample.
 

                Calculations

                    1.  Calculate the average number of drops of EDTA used for each set of titrations.

                    2.  Calculate the mg of hardness per drop of EDTA used for the reference solution.
 

                   .5 mg hardness in ref. sol'n        _X         2.5 mL of ref. sol'n        ₌         mg of hardness
                         1 mL ref. sol'n                                ave. drops of EDTA                1 drop of EDTA
 

                    3.  Calculate  the mg of hardness for each local water supply sample.

                        drops of EDTA            _X            mg of hardness             ₌            mg of hardness
                          2.5 mL sample                          1 drop of EDTA                           1 mL sample

                    4.    Finally, convert this into milligrams of hardness per liter of water.  This is the normal way of reporting
                           water hardness.
 
 
                            mg of hardness            _X            1000 mL             ₌               mg of hardness
                             1 mL sample                                1 L                                     1 L sample
 
 
 

           Hardness Chart
 

Description	Concentration (mg/L) or (ppm)	Concentration (grains per gallon)**
Very Hard	over 300 mg/L or ppm	over 10.5 gpg
Hard	150 to 300 mg/L or ppm	7.1 to 10.5 gpg
Moderately Hard	50 to 150 mg/L or ppm	3.5 to 7.0 gpg
Soft	0 to 50 mg/L or ppm	0 to 3.4 gpg

                 ** Every drop of hardness #3   =  1 grain of total hardness per gallon of water

 

             Materials Used

             Hardness #1-Buffer
                    2.3 g NH4Cl
                    16.75 mL NH3 (concentrated)
                    250 mL Distilled water

              Hardness #2-Dye
                    .1 g Eriochrome Black T in 200 mL of hot distilled water

              Hardness #3-Titrate Solution
                    .015 M EDTA

               Calcium carbonate reference solution 500 mg / L or .5 mg / mL
 
 


 

Other Ideas for Water Hardness Research Projects
 

 
 All the solutions and apparatus required for the Water Hardness Point studies and analysis will be provided, except that you must provide the water samples from various communities or water sources.  Here are some general suggestions for the types of projects that you might consider.

                 

                  How effective are various water softeners at removing the total water hardness in our community?