Total Water Hardness

Transcription

1 Test 14 INTRODUCTION When water passes through or over deposits such as limestone, the levels of Ca 2+, Mg 2+, and HCO Ð 3 ions present in the water can greatly increase and Hard-Water Cations cause the water to be classified as hard water. This term results from the fact that calcium and magnesium ions in Calcium, Ca 2+ water combine with soap molecules, making it ÒhardÓ to get suds. In Test 13, Calcium and Water Hardness, an Magnesium, Mg 2+ Ion-Selective Electrode was used to determine calcium Others: Fe hardness, in mg/l as CaCO 3. In this test, total hardness 3+, Sr 2+, Zn 2+, Mn 2+ will be determined. Total hardness is defined as the sum of calcium and magnesium hardness 1, in mg/l as CaCO 3. In addition to total hardness, the test described here will allow you to determine the concentration of Mg 2+, in mg/l. Total Hardness Soft: 0-30 Moderately soft: Moderately hard: Hard: Very hard: > 180 High levels of hard-water ions such as Ca 2+ and Mg 2+ can cause scaly deposits in plumbing, appliances, and boilers. These two ions also combine chemically with soap molecules, resulting in decreased cleansing action. The American Water Works Association indicates that ideal quality water should not contain more than 80 mg/l of total hardness as CaCO 3. High levels of total hardness are not considered a health concern. On the contrary, calcium is an important component of cell walls of aquatic plants, and of the bones or shells of aquatic organisms. Magnesium is an essential nutrient for plants, and is a component of chlorophyll. Expected Levels Total hardness in freshwater is usually in the range of 15 to 375 mg/l as CaCO 3. Calcium hardness in freshwater is in the range of 10 to 250 mg/l, often double that of magnesium hardness (5 to 125 mg/l). Typical seawater has calcium hardness of 1000 mg/l, magnesium hardness of 5630 mg/l, and total hardness of 6630 mg/l as CaCO 3. Total water hardness, in mg/l as CaCO 3 1 Even though Fe 2+, Fe 3+, Sr 2+, Zn 2+, and Mn 2+ may contribute to water hardness, their levels are typically much less than Ca 2+ and Mg 2+. Their levels are not usually included in total hardness measurements. Water Quality with Computers 14-1

2 TWH Test 14 Table 1: Ca Hardness, Mg Hardness, and Total Hardness in Selected Sites Site (fall season) Ca hardness Mg hardness Total hardness Merrimack River, Lowell, NH Mississippi River, Memphis, TN Rio Grande River, El Paso, TX Ohio River, Grand Chain, OH Willamette River, Portland, OR Missouri River, Garrison Dam, ND Sacramento River, Keswick, CA Hudson River, Poughkeepsie, NY Platte River, Louisville, NE Colorado River, Andrade, CA Summary of Method The sample is first adjusted to a ph of 10 using a buffer solution. The sample is titrated to its equivalence point using a standard solution. 2 draws the calcium and magnesium ions into a complex, so neither one has free ions in solution. The Calmagite indicator initially turns red in the presence of magnesium, then turns blue when enough solution has been added to combine with all calcium and magnesium ions The total hardness of the sample is calculated using the precise volume of solution added when the indicator changes color, as well as the concentration, in mol/l. If calcium concentration was determined in Test 13, magnesium concentration can also be calculated (in mg/l Mg 2+ ) Here is a summary of the measurements and calculations in Test 14. Sample values are included. a. Total hardness as CaCO 3, obtained using the titration = 120 mg/l (sample value) b. Mg hardness as CaCO 3 = total hardness Ð calcium hardness (Test 13) = 120 Ð 70 = 50 mg/l c. Mg 2+ = (50 mg/l as CaCO 3 ) 5 (24 g Mg 2+ / 100 g CaCO 3 ) = 12 mg/l as Mg 2+ 2 is an abbreviation for ethylenediaminetetraacetic acid Water Quality with Computers

3 TOTAL WATER HARDNESS TWH Materials Checklist sampling bottles 100-mL graduated cylinder 250-mL Erlenmeyer flask 25- or 50-mL buret Hardness 1 Buffer Solution two Calmagite 3 indicator powder pillows 0.01 M 4 titrant (TitraVer Standard Solution) water bottle with distilled water Collection and Storage of Samples 1. This test must be conducted in the lab. Collect at least 300 ml of sample water. 2. It is important to obtain the water sample from below the surface of the water and as far away from the shore as is safe. If suitable areas of the stream appear to be unreachable, samplers consisting of a rod and container can be constructed for collection. Refer to page Intro-4 of the Introduction for more details. Testing Procedure 1. Obtain a clean buret and rinse it with a few ml of the M titrant (TitraVer Standard Solution). Fill the buret a little above the 0-mL level with the solution. Drain a small amount of the solution so it fills the buret tip and leaves the solution at the 0- ml mark (or just below it). Record the buret level on the Data & Calculations sheet, to the nearest 0.01 ml. 2. Prepare the water sample for titration. a. Use a graduated cylinder to measure 50 ml of your water sample into a 250-mL Erlenmeyer flask. b. Add 1 ml of Hardness 1 Buffer Solution to the Erlenmeyer flask using the 1-mL calibrated dropper. Gently swirl the contents of the flask to mix. c. Add the contents of one Calmagite 3 powder pillow to the Erlenmeyer flask. Gently swirl the contents of the flask to mix. The solution should now be red in color. 3. Titrate the sample you prepared in Step 2. a. Slowly add 0.01 M titrant to the sample in the Erlenmeyer flaskñstart with 1-mL additions. Swirl the sample after each addition of titrant. b. Near the equivalence point, the red color will start to fade, and become more violet (mixture of red and blue). You should reduce the addition volume to 1 drop at a time. Note: The reaction at the equivalence point is very slow (~1Êsecond), so take your time. c. When the last tinge of red starts to disappear and turn to a pure blue color, you have reached the equivalence point. Record the final buret volume (to the nearest 0.01 ml) on your Data & Calculations sheet. d. If you are unsure whether you have reached the equivalence point, simply record the buret volume, then add another drop of titrant. If the color becomes a truer blue color, use the new buret reading; if not, use the previous buret reading. 3 Calmagite is contained in ManVer 2 Hardness Indicator Powder Pillows. 4 Hach TitraVer Hardness Titrant is labeled as 0.01 M, and also as an equivalent concentration of 0.02 N. Water Quality with Computers 14-3

4 TWH Test Perform a second titration trial. To do this, discard the first titration mixture, as directed by your instructor. If you have enough titrant left for a second titration, simply record the current buret level; if not, refill the buret and record the buret level. Rinse the 250-mL Erlenmeyer flask twice with distilled water, then repeat Steps 2 Ð 3. Calculations 1. To determine the titrant volumes used, subtract the initial volume from the final volume for each trial. Record the titrant volumes for the two trials on the Data & Calculations sheet. Important: If the titrant volumes for the two trials are not in close agreement, you should repeat StepsÊ1 Ð 3 of the Testing Procedure until you have two trials that are close. 2. Calculate the value for total water hardness. a. For each trial, determine total water hardness using the equation total water hardness as CaCO 3 (mg/l) = (titrant volume) b. Record the total water hardness value on the Data & Calculations sheet. Optional Calculations 3. Calculate magnesium hardness and magnesium concentration (mg/l Mg 2+ ). a. On the second table of the Data & Calculations sheet, record the calcium hardness value you obtained in Test 13, and the total water hardness value from the first part of this test. b. Calculate magnesium hardness. Use the equation magnesium hardness = total hardness Ð calcium hardness c. Calculate Mg 2+ concentration, using the formula Mg 2+ = (magnesium hardness as CaCO 3 ) 5 (24 g Mg 2+ / 100 g CaCO 3 ). d. Record the values for magnesium hardness and Mg 2+ concentration on the Data & Calculations sheet Water Quality with Computers

5 DATA & CALCULATIONS Total Water Hardness Stream or lake: Site name: Site number: Date: Time of day: Student name: Student name: Student name: TWH Column A B C D Total water hardness Initial buret reading (ml) Final buret reading (ml) Titrant volume (ml) Total water hardness Example 0.10 ml 9.22 ml 9.12 ml 182 mg/l 1 2 Column Procedure, total water hardness: A. Initial buret reading (ml) B. Final buret reading (ml) C. Titrant volume (ml) = B A D. Total water hardness (mg) = C Average total water hardness Column A B C D Magnesium calculations (optional) Calcium hardness (from Test 13) Total hardness (from 14) Magnesium hardness Magnesium concentration (mg/l as Mg 2+ ) Example 132 mg/l 182 mg/l 50 mg/l 12 mg/l Column Procedure, magnesium concentration: A. Record value from Test 13 B. Record value from Test 14 (above) C. Magnesium hardness = B A D. Magnesium concentration (mg/l as Mg 2+ ) = C Field Observations (e.g., weather, geography, vegetation along stream) Test Completed: Date: Water Quality with Computers 14-5

6 TWH ADDITIONAL INFORMATION Test 14 Tips for Instructors 1. If students pass the equivalence point on the first trial, have them keep the resulting blue solution to use as a color comparison for the next titration. Having a solution with the actual blue color is very helpful in subsequent trials. 2. If you do not have a Vernier Calcium Ion-Selective Electrode, Test 14 can be done without having previously performed Test The following reagents can be ordered directly from Hach Company (5600 Lindbergh Drive, Loveland, CO, Tel: , Hardness 1 Buffer Solution (118 ml, 100 tests) ManVer 2 Hardness Indicator Powder Pillows (100 tests) TitraVer Hardness Titrant (0.010 M, or N, 946 ml) A reagent kit can also be ordered from Hach Company that includes all three reagents: Hardness (Total) Reagent Sets (~100 tests) , ethylenediaminetetraacetic acid or H 4 C 10 H 12 N 2 O 8, is a weak acid that can lose four H + ions on complete neutralization. In this section, we will represent in a simplified form, H 4 X, where X = C 10 H 12 N 2 O 8. Hach Titraver Standard Solution contains the disodium salt of, Na 2 H 2 X, which dissociates according to the equation Na 2 H 2 X(aq) 2 Na + (aq) + H 2 X 2Ð (aq) The titration is carried out near a ph of 10 in an organic amine-amide buffer system (Hardness 1 Buffer Solution). The buffer system keeps the mainly in the HY 3Ð form by the reaction H 2 X 2Ð (aq) + OH Ð (aq) HX 3Ð (aq) + H 2 O(l) where it complexes with Group 2 ions very well, but does not tend to react as readily with other cations such as Fe 3+ that might be present in the water sample. The chelating reactions between this form of and calcium or magnesium ions during the titration are Ca 2+ (aq) + HX 3Ð (aq) CaX 2Ð (aq) + H + (aq) Mg 2+ (aq) + HX 3Ð (aq) MgX 2Ð (aq) + H + (aq) The 1:1 mole ratio between either Mg 2+ or Ca 2+ and HX 3Ð will be represented in calculations below using the following mole ratio 1 mol CaCO 3 / 1 mol 5. In Step 3 in the Calculations section, students are instructed to calculate total water hardness of the sample by multiplying the titrant volume by a conversion factor of This conversion factor is derived from the following equation Vol Total Water Hardness = Vol CaCO3 Vol Total Water Hardness = 50 ml moles moles Total Water Hardness = Vol 20.0 CaCO3 Molarity molar mass CaCO3 1mol CaCO mol 100 g 1000 mg 1mol 1L 1mol 1g 1000 mg 1g 14-6 Water Quality with Computers

7 6. The student procedure is used to determine total water hardness in the range of 0 to 500 mg/l as CaCO 3. If values greater than 500 mg/l are encountered or anticipated, use one of these two options: If students are using a 25-mL buret, and still have a red color after delivering 25 ml of titrant, they can refill their buret and continue the titration. When they have reached the equivalence point, they can simply use the sum of the two titrant volumes used. Another option for handling amounts in excess of 500 mg/l is to measure out a water sample of 25 ml in Step 1 of the procedure (instead of 50 ml), then use a conversion factor of 40 (instead of 20) in Step 2a of the calculations. Add 25 ml of distilled water to the sample to bring the total volume to 50 ml. With a 25-mL buret, this method extends the range to 1000 mg/l. 7. The Hach TitraVer Standard Solution has a concentration of M. Remind students that this allows them 3 significant figures in their calculationsñanswers less than 100 mg/l can be expressed to the nearest 0.1 mg/l (e.g., 84.5 mg/l), and answers greater than 100 mg/l to the nearest 1 mg/l (e.g., 145 mg/l). If more accuracy is required, the TitraVer solution can be standardized using the same titration procedure, except using a mg/l total CaCO 3 standard solution in place of the water sample. This way, the concentration of the standard solution can be determined to 4Êsignificant figures, using the formula molarity, M, of = mol / volume 8. You can prepare 0.01 M titrant using solid, disodium salt, dihydrate, Na 2 C 10 H 14 N 2 O 8 2H 2 O. Add g of the disodium salt to enough distilled water to prepare 1 liter of solution. TWH Water Quality with Computers 14-7