Calcium and Water Hardness

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Ca 2+ 3. Prepare the Calcium Ion-Selective Electrode (ISE) for data collection. a. The ISE should be soaking in the High Standard. Make sure that it is not resting on the bottom of the container, and that the small white reference contacts are immersed. b. Plug the ISE Amplifier into the Port 1 or Channel 1 of the Vernier computer interface. Connect the Calcium ISE to the ISE Amplifier. 4. You are now ready to prepare the computer and Calcium ISE for calibration. First Calibration Point a. Choose Calibrate from the Experiment menu and then click . b. Type Ò1000Ó (the concentration in mg/LÊCa 2+ ) in the edit box. c. When the displayed voltage reading for Input 1 stabilizes, click Keep . Test 13 Second Calibration Point d. Rinse the ISE thoroughly with distilled water and gently blot it dry with a tissue. Be very gentle when blotting the membrane. Important: Failure to carefully rinse and dry the ISE will contaminate the standard. e. Place the tip of the ISE into the Low Standard (10 mg/L Ca 2+ ). Be sure that the ISE is not resting on the bottom of the bottle and that the small white reference contacts are immersed. Make sure no air bubbles are trapped below the ISE. f. After briefly swirling the solution, hold the ISE still and wait approximately 30 seconds for the voltage reading displayed on the computer screen to stabilize. g. Enter Ò10Ó (the concentration in mg/LÊCa 2+ ) in the edit box. h. When the displayed voltage reading for Input 1 stabilizes, click Keep , then click OK . 5. You are now ready to collect calcium concentration data. a. Rinse the ISE with distilled water and gently blot it dry. b. Place the tip of the probe into the stream at Site 1, or into a cup with sample water from the stream. Make sure the ISE is not resting on the bottom and that the small white reference contacts are immersed. Make sure that no air bubbles are trapped below the ISE. c. After briefly swirling the solution, hold the ISE still and wait approximately 30 seconds. d. If the calcium concentration value appears stable, simply record it on the Data & Calculations sheet and proceed to Step 7. 6. If the calcium concentration value displayed in the Meter window is fluctuating, determine the mean (or average) value. To do this: a. Click Collect to begin a 10-second sampling run. Important: Leave the probe tip submerged for the 10 seconds that data is being collected. b. Click on the Statistics button,Ê , to display the statistics box on the graph. c. Record the mean calcium concentration value on the Data & Calculations sheet. 7. Return to Step 5 to obtain a second reading. 8. Convert the calcium concentration (mg/L Ca 2+ ) to units of calcium hardness (mg/L as CaCO 3 ). The calculation takes into account the difference in the molar masses of calcium (40Êg/mol Ca 2+ ) and calcium carbonate (100 g/mol CaCO 3 ). To convert to units of calcium hardness, perform the following calculation: calcium hardness as CaCO 3 = (mg/L Ca 2+ ) 5 (100 g CaCO 3 / 40 g Ca 2+ ) = (mg/L Ca 2+ ) 5 2.5 Record this value on the Data & Calculations sheet (round to the nearest 0.01 mg/L). 13 - 4 Water Quality with Computers
Calcium and Water Hardness DATA & CALCULATIONS Calcium and Calcium Hardness Stream or lake: ___________________________ Site name: ______________________________ Site number: ____________________________ Date: __________________________________ Ca 2+ Time of day: __________________________ Student name: ________________________ Student name: ________________________ Student name: ________________________ A B Reading Calcium (mg/L Ca 2+ ) Calcium hardness (mg/L CaCO 3 ) 1 2 Average Column Procedure: A. Record the calcium concentration (mg/L Ca 2+ ) from the computer. B. Multiply calcium concentration (mg/L Ca 2+ ) by 2.5 to obtain calcium hardness (as CaCO 3 ). Field Observations (e.g., weather, geography, vegetation along stream) _________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Test Completed: ________________ Date: ______ Water Quality with Computers 13 - 5
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Calcium and Water Hardness

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