TAMUCC Department of Chemistry

Determining the Hardness of Water . . . Click Here for print version.

The "Total Hardness is defined as the sum of the calcium and magnesium concentrations, both expressed as calcium carbonate, in milligrams per liter." [Standard Methods for the Analysis of Water and Wastewater, 19th Ed., 1995, p 2-35.] This essentially means that we assume that any Mg⁺² ion present is Ca⁺² ion and that the source of all calcium ion is CaCO₃ which has a formula weight of 100.09 g/mol or mg/mmol. The instructions which follow are similar to the procedure laid out in Standard Methods, but are somewhat simplified. The instructions are based upon Experiments 5.6 and 5.7 of Day and Underwood, Laboratory Manual for Quantitative Analysis, 5th Ed., Prentice-Hall.

Preparation of Sodium-EDTA Solution and Standard Calcium Solution: (Students should expect to find this EDTA solution pre-prepared. The following instructions are given for completeness.) Weigh about 4 g of disodium dihydrogen EDTA dihydrate and about 0.1 g of MgCl₂^. 6H₂O into a clean 400 mL beaker. Dissolve the solid in DI water and transfer the solution to a clean 1 L plastic bottle. Add additional DI water rinsing the beaker until the total volume of solution is about 1 liter. Mix and label. (The standard calcium chloride solution described next should also be pre-prepared.) It is made by weighing accurately about 0.4000 g of primary standard calcium carbonate, CaCO₃. The solid is then transferred to a 500 mL volumetric flask using about 100 mL of DI water. Then 1:1 hydrochloric acid is added to the flask until there are no more bubbles of carbon dioxide and the solution is clear. The reaction is CaCO₃ + 2 HCl --> CaCl₂ + H₂O + CO₂ . Then DI water is added to fill the 500 mL volumetric. (The molarity of the calcium chloride solution should be about 0.008000 M. The precise value will be supplied. )

Standardization of EDTA Solution: Each group of students should perform at least one standardization titration. The results from three or more student groups should be averaged to determine the concentration of the EDTA solution. To standardize the EDTA solution pipet a 50 mL aliquot of the calcium solution into a 250 mL Erlenmeyer flask and add 5 mL of the ammonia-ammonium chloride buffer solution. Also add 5 drops of the Eriochrome Black T indicator. Using a 50 mL buret titrate with the EDTA solution to the endpoint where the color changes from wine-red to pure blue. No tinge of red should remain. It is expected that approximately 40 mL will be required. Both the buffer and the indicator should be pre-prepared from the following directions. For 100 mL of the buffer dissolve 6.75 g of ammonium chloride in 57 mL of concentrated ammonia and dilute to 100 mL. The indicator solution is 0.5 g of Eriochrome Black T in 100 mL of 95% ethanol. Now calculate the molarity of the EDTA solution and its calcium carbonate titer. A sample calculation follows: Assume the molarity of the calcium solution is 0.008052. The 50 mL aliquot contains 50 mL * 0.008052 mmol/mL = 0.4026 mmol. Now assume that 39.65 mL of EDTA solution are required. Since moles of EDTA = moles of Ca, the molarity of the EDTA is 0.4026 mmol / 39.65 mL = 0.01015 mmol / mL = 0.01015 M. The calcium carbonate titer is mass of calcium carbonate that each mL of EDTA solution will titrate. The CaCO₃ titer = the molarity of the EDTA times the molar mass of the CaCO₃. For example, 0.01015 mmol/mL * 100.09 mg/mmol = 1.016 mg/mL

Determination of the Total Hardness of Water: Pipet 100 mL of your fresh water sample into a 250 mL Erlenmeyer flask. Add only 1 mL of the buffer solution and 5 drops of the E. Black T indicator solution. Titrate with the EDTA solution to the same blue endpoint. Your titration should require at least 20 mL and not more than 50 mL (a buretful). If you have to refill the buret, do so in order to estimate the volume required. Adjust the volume of your sample of water so that 20 - 40 mL of EDTA are required. You will need at least two satisfactory titrations to average for your hardness. Assume that your 100 mL sample requires 42.67 mL of EDTA. Here is a sample hardness calculation: 42.67 mL * 1.016 mg/mL = 43.35 mg of CaCO₃ in the 100 mL sample. Since ppm is mg/L, this is one situation where we divide by the volume in liters, 100 mL = 0.100 L and hardness = 43.35 mg / 0.100 L = 433.5 mg/L. Please understand that this sample calculation is an illustration only and is not meant to suggest the results you will obtain.

Further Directions and Explanations: Include in your report a calculation of the pH of the buffer solution described above. See slide 8 of the Power Point lecture. Use 15.4 M for the molarity of the 57 mL of concentrated ammonia. One aspect of this experiment which can be confusing concerns the 0.1 g of MgCl₂^. 6H₂O which is added to the 1 L of EDTA titrant solution. It seems odd that we deliberately add Mg⁺² ions to a solution which will be used to titrate unknown amounts of Ca⁺² and Mg⁺² ions. The magnesium is added to insure that the indicator works in the first standardization titrations where only Ca⁺² ions are being titrated. Once the EDTA titrant solution enters the buffered flask, it contains two forms of EDTA -- free, uncomplexed Y^-4 and a small amount of complexed MgY^-2. The standardization determines the amount of free, uncomplexed Y^-4 only. The free ligand reacts directly with the Ca⁺² ions of the standard calcium solution forming the CaY^-2 complex. The magnesium bound ligand also reacts: MgY^-2 + Ca⁺² --> Mg⁺² + CaY^-2. The magnesium ion which is liberated binds to the indicator giving the wine-red color. Just before the endpoint all of the calcium ions have been complexed. The last amounts of Y^-4 added strip the magnesium ions away from the indicator and all of the Mg⁺² ions return to their original complexed state of MgY^-2. Except to ensure that the color change occurs, the magnesium added has no effect on the standardization or hardness titrations.

		Chemistry


Home Degrees People Faculty Staff Assistants WorkStudies Researchers Events Courses Laboratories Instrumentation NMR Equipment Safety Research Welch SSC Workshop Chemistry Database MSDS Links Chemistry Club ACS South Texas Section Physical & Environmental Sciences TAMUCC	Determining the Hardness of Water . . . Click Here for print version. The "Total Hardness is defined as the sum of the calcium and magnesium concentrations, both expressed as calcium carbonate, in milligrams per liter." [Standard Methods for the Analysis of Water and Wastewater, 19th Ed., 1995, p 2-35.] This essentially means that we assume that any Mg⁺² ion present is Ca⁺² ion and that the source of all calcium ion is CaCO₃ which has a formula weight of 100.09 g/mol or mg/mmol. The instructions which follow are similar to the procedure laid out in Standard Methods, but are somewhat simplified. The instructions are based upon Experiments 5.6 and 5.7 of Day and Underwood, Laboratory Manual for Quantitative Analysis, 5th Ed., Prentice-Hall. Preparation of Sodium-EDTA Solution and Standard Calcium Solution: (Students should expect to find this EDTA solution pre-prepared. The following instructions are given for completeness.) Weigh about 4 g of disodium dihydrogen EDTA dihydrate and about 0.1 g of MgCl₂^. 6H₂O into a clean 400 mL beaker. Dissolve the solid in DI water and transfer the solution to a clean 1 L plastic bottle. Add additional DI water rinsing the beaker until the total volume of solution is about 1 liter. Mix and label. (The standard calcium chloride solution described next should also be pre-prepared.) It is made by weighing accurately about 0.4000 g of primary standard calcium carbonate, CaCO₃. The solid is then transferred to a 500 mL volumetric flask using about 100 mL of DI water. Then 1:1 hydrochloric acid is added to the flask until there are no more bubbles of carbon dioxide and the solution is clear. The reaction is CaCO₃ + 2 HCl --> CaCl₂ + H₂O + CO₂ . Then DI water is added to fill the 500 mL volumetric. (The molarity of the calcium chloride solution should be about 0.008000 M. The precise value will be supplied. ) Standardization of EDTA Solution: Each group of students should perform at least one standardization titration. The results from three or more student groups should be averaged to determine the concentration of the EDTA solution. To standardize the EDTA solution pipet a 50 mL aliquot of the calcium solution into a 250 mL Erlenmeyer flask and add 5 mL of the ammonia-ammonium chloride buffer solution. Also add 5 drops of the Eriochrome Black T indicator. Using a 50 mL buret titrate with the EDTA solution to the endpoint where the color changes from wine-red to pure blue. No tinge of red should remain. It is expected that approximately 40 mL will be required. Both the buffer and the indicator should be pre-prepared from the following directions. For 100 mL of the buffer dissolve 6.75 g of ammonium chloride in 57 mL of concentrated ammonia and dilute to 100 mL. The indicator solution is 0.5 g of Eriochrome Black T in 100 mL of 95% ethanol. Now calculate the molarity of the EDTA solution and its calcium carbonate titer. A sample calculation follows: Assume the molarity of the calcium solution is 0.008052. The 50 mL aliquot contains 50 mL * 0.008052 mmol/mL = 0.4026 mmol. Now assume that 39.65 mL of EDTA solution are required. Since moles of EDTA = moles of Ca, the molarity of the EDTA is 0.4026 mmol / 39.65 mL = 0.01015 mmol / mL = 0.01015 M. The calcium carbonate titer is mass of calcium carbonate that each mL of EDTA solution will titrate. The CaCO₃ titer = the molarity of the EDTA times the molar mass of the CaCO₃. For example, 0.01015 mmol/mL * 100.09 mg/mmol = 1.016 mg/mL Determination of the Total Hardness of Water: Pipet 100 mL of your fresh water sample into a 250 mL Erlenmeyer flask. Add only 1 mL of the buffer solution and 5 drops of the E. Black T indicator solution. Titrate with the EDTA solution to the same blue endpoint. Your titration should require at least 20 mL and not more than 50 mL (a buretful). If you have to refill the buret, do so in order to estimate the volume required. Adjust the volume of your sample of water so that 20 - 40 mL of EDTA are required. You will need at least two satisfactory titrations to average for your hardness. Assume that your 100 mL sample requires 42.67 mL of EDTA. Here is a sample hardness calculation: 42.67 mL * 1.016 mg/mL = 43.35 mg of CaCO₃ in the 100 mL sample. Since ppm is mg/L, this is one situation where we divide by the volume in liters, 100 mL = 0.100 L and hardness = 43.35 mg / 0.100 L = 433.5 mg/L. Please understand that this sample calculation is an illustration only and is not meant to suggest the results you will obtain. Further Directions and Explanations: Include in your report a calculation of the pH of the buffer solution described above. See slide 8 of the Power Point lecture. Use 15.4 M for the molarity of the 57 mL of concentrated ammonia. One aspect of this experiment which can be confusing concerns the 0.1 g of MgCl₂^. 6H₂O which is added to the 1 L of EDTA titrant solution. It seems odd that we deliberately add Mg⁺² ions to a solution which will be used to titrate unknown amounts of Ca⁺² and Mg⁺² ions. The magnesium is added to insure that the indicator works in the first standardization titrations where only Ca⁺² ions are being titrated. Once the EDTA titrant solution enters the buffered flask, it contains two forms of EDTA -- free, uncomplexed Y^-4 and a small amount of complexed MgY^-2. The standardization determines the amount of free, uncomplexed Y^-4 only. The free ligand reacts directly with the Ca⁺² ions of the standard calcium solution forming the CaY^-2 complex. The magnesium bound ligand also reacts: MgY^-2 + Ca⁺² --> Mg⁺² + CaY^-2. The magnesium ion which is liberated binds to the indicator giving the wine-red color. Just before the endpoint all of the calcium ions have been complexed. The last amounts of Y^-4 added strip the magnesium ions away from the indicator and all of the Mg⁺² ions return to their original complexed state of MgY^-2. Except to ensure that the color change occurs, the magnesium added has no effect on the standardization or hardness titrations.