How Hard Is Your Water?
Student Laboratory Kit
Materials Included In Kit
Buffer solution, pH 10, 150 mL
Calmagite indicator solution, 0.1%, 50 mL
EDTA, disodium salt, 0.010 M, 250 mL
Pipets, Beral-type, microtip, 15
Pipets, Beral-type, thin-stem, 15
Additional Materials Required
Water, distilled or deionized
Beakers, 50-mL, 4
Graduated cylinder, 10-mL
The pH 10 buffer solution is mildly toxic by ingestion, a severe body tissue irritant and produces harmful vapors; avoid breathing the fumes. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Final products may be disposed of according to Flinn Disposal Method #26b.
Typically, the titration of calcium and magnesium by EDTA is carried out in the presence of Erochrome Black-T, but endpoints tend to not be as sharp. Experience in developing this procedure has shown that calmagite gives sharp, easily identified endpoints. Calmagite solution is blue in basic solution. However, when calmagite forms a complex with calcium or magnesium, the solution is pink in color.
- In a basic solution, the acidic hydrogens, that is the hydrogens attached to oxygen, are pulled away from the EDTA molecule. That leaves each of these four oxygen atoms with three nonbonding pairs electrons and a negative charge. The two nitrogen atoms each have a nonbonding pair of electrons. The EDTA4– (as shown in Figure 1) can bond to calcium or magnesium ions in six places: the four singly bonded oxygen atoms and the two nitrogen atoms, each donating a pair of electrons to the metal ion. The resulting Ca–EDTA and Mg–EDTA complexes both have a minus two charge.
- During the titration the following processes occur:
- Tap water containing Ca2+ and Mg2+ is placed in a beaker. Some pH 10 buffer is then added. The buffer maintains a basic pH so that the singly bonded oxygen atoms in EDTA are not bonded to hydrogen atoms and can bond to the metal ions.
- A small amount of calmagite solution is added. The calmagite immediately bonds with some of the free metal ions forming a red complex.
- EDTA is then added dropwise until all of the metal ions have been pulled away from the calmagite. This is seen when the calmagite indicator solution changes from red (metal ions attached to the calmagite) to blue (no metal ions attached to calmagite). This signals the end of the titration.
- Water with a range of 0 to 60 parts per million is considered soft, 60 to 120 parts per million—medium hard, 120 to 180 parts per million—hard, and 180 parts per million and above is considered very hard.
- Smaller beakers or plastic cups may also be used if you are short on glassware.
- If there is some question on the purity of your distilled water, you may want to run a trial test on your own, since significant amounts of dissolved minerals will affect student results.
- In order to set up an accurate control (beaker 4), it is imperative that students rinse both the graduated cylinder and their beakers with distilled or deionized water.
- Presence of the relatively small amounts of copper, iron, and/or aluminum ions in solution may result in interference problems. These may be masked effectively or removed from solution by the addition of stronger complexing agents. Potassium tartrate will mask aluminum, while the pH 10 buffer will precipitate iron as the hydroxide. Copper requires either fluoride ion or cyanide ion (which are very toxic and not commonly used in high school labs). If you have hard water due to the presence of bicarbonates, no masking is needed, since the solutions do not sit long enough before titration for interference to occur.
- Very soft water (<50 ppm) may require a change in the concentration of EDTA solution. Try a 0.005 M solution of EDTA if this is the case.
- If your kitchen (or other) faucet has a purification filter, try analyzing both the filtered and the unfiltered water. Discuss the results.
- Repeat the experiment as described in the procedure, but use 2 mL of water and 1 mL of buffer, instead of 5 mL and 2 mL, respectively. Compare both the hardness values obtained and the consistency of data and discuss the results.
- Try substituting volumes of 10 mL for the water and 4 mL of buffer in the original procedure. Compare the hardness values obtained and the consistency of data. Discuss the results.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Analyzing and interpreting data
Developing and using models
Planning and carrying out investigations
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
MS-ESS2.C: The Roles of Water in Earth’s Surface Processes
HS-PS1.A: Structure and Properties of Matter
Structure and function
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-ESS2-5. Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
Average value of calcium in Beakers 1, 2 and 3 (in ppm) ___188___
Answers to Questions
- Convert the number of drops of EDTA used in each titration to milliliters of EDTA used.
(Drops of EDTA)/(Drops of water per mL) = mL of EDTA used
- Convert the milliliters of EDTA used in each titration to liters of EDTA used. Record these values in the data table.
(mL of EDTA)/(1000 mL/L) = Liters of EDTA used
- Calculate the number of moles of EDTA used in each titration, using the volumes calculated in the previous question and the molar concentration of the EDTA, 0.010 M. Record these values in the data table.
(Liters of EDTA used) x 0.010 M EDTA = moles of EDTA used
- Given that one mole of EDTA reacts with one mole of calcium, calculate the number of grams of calcium that were present in each of 5.0 mL samples. (Hint: Use the molar mass of calcium.) Record these values in the data table.
(moles of EDTA used) x 40.1 g calcium/mol = grams of calcium per sample
- Taking the density of water to be 1.0 g/mL (or 1000g/L), a calcium ion concentration of 1.0 g/L would correspond to 1000 ppm. Convert the results from Question 4 from grams of calcium per 5.0 mL sample to grams of calcium in 1.00 L. Then multiply this value by 1,000,000 to get parts per million of calcium. Record each individual sample values and the average value of parts per million calcium in the data table.
(grams of calcium per sample)/5.00 mL = grams calcium in 1.00 mL
(grams of calcium in 1.00 mL) x (1 x 106) = parts per million calcium
- Identify two major sources of experimental error in this analysis and suggest ways in which the procedure could be modified to minimize those errors.
The biggest problems center around the impossibility of getting quantitative transfer of liquids from the graduated cylinder into the reaction vessel. A secondary difficulty involves the tendency of Beral pipets to give an overflow of drops. Another is finding a consistent endpoint.
Special thanks to John G. Little, St. Mary’s High School, Stockton, CA, for providing the idea and instructions for this activity.
Dick, J. G. Analytical Chemistry; McGraw-Hill: New York, 1973.
Lindstrom, F., and Diehl, H., Analytical Chemistry 1960, 32, 1123.
Pierce, W. C. Haenisch, E. L., and Sawyer, D. T., Quantitative Analysis; John Wiley and Sons: New York, 1958.
Scwartzenbach, G., and Flaschka, H., (tran. by Irving, H.M.N.H.), Complexometric Titrations; Methuen and Co. Ltd.; London, 1969.
Welcher, F. J., The Analytical Uses of Ethylenediamine Tetraacetic Acid; D.Van Nostrand Co.: Princeton, NJ, 1958.