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Hardness

Hardness. WQT 134 Aquatic Chemistry II Standard Methods 20 th ed #2340 Hardness Applied Water and Spent water Manual Chapter 14. Week 5 Objectives. Reading assignment:

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Hardness

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  1. Hardness WQT 134 Aquatic Chemistry II Standard Methods 20th ed #2340 Hardness Applied Water and Spent water Manual Chapter 14

  2. Week 5 Objectives Reading assignment: American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF). 1999.  Standard Methods for the Examination of Water and Wastewater, 20th edition 1. Understand the role and function of hardness in water treatment. 2. Understand how to measure hardness (SM #2340) 3. Relate hardness to total alkalinity

  3. Hardness #2340 Carbonate hardness: Occurs when the hardness numerically is equal to or less than the sum of carbonate and bicarbonate alkalinity Noncarbonate hardness:Occurs when the hardness numerically is more than the sum of carbonate and bicarbonate alkalinity.

  4. Hardness #2340 Temporary or Carbonate Hardness : is hardness that can be removed by boiling or hydrochloric acid. It is caused by a combination of calcium ions and bicarbonate ions in the water. Permanent hardness:is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises.

  5. Hardness #2340 • What is hardness? • This test is designed to measure Hardness in water which is mainly due to the presence of ions of the calcium (Ca 2+), magnesium (Mg 2+), and iron (Fe 2+). • Why do we care? • USGS, 85% of US homes have hard water. • Hard water require considerable amounts of soap to produce a lather • Hard water mineral deposits form an insoluble precipitate such as scale (coats pipes, boilers, fixtures, heaters, etc.) • Soft water =poor coagulation • Secondary MCl for aesthetic purposes

  6. Hardness is defined as the sum of the _____ and ____ ions, although any divalent metal ion can contribute to hardness. • Calcium and Magnesium • Magnesium and Sodium • Calcium and Sulfate • Struvite

  7. Hardness #2340 • How is it done? • Sample needs to be buffered to pH 10 with a salt and an organic amine. An organic dye or indicator (Eriochrome Black T) is added which complexes the calcium and magnesium and turns the water red. • 2. The sample is then titrated with 0.01 M EDTA which complexes all the free calcium and magnesium ions in the sample. At the endpoint the free calcium and magnesium are no longer present and the EDTA will produce a blue color. • 3. The volume of EDTA consumed is used to determine the hardness in mg/L CaCO3 or the total calcium and magnesium combined hardness. You go from a wine red color to a sharp blue color.

  8. Hardness #2340 How is it done? Before w/ indicator After EDTA titration To endpoint

  9. Hardness #2340 • What are disadvantages of the procedure? • Some metal ions and organics interfere by causing fading or indistinct end points • Suspended or colloidal organic matter also may interfere with the end point. • Must be done at room temperature

  10. Hardness #2340 • What are the units and conversions? • hardness in mg/l as CaCO3 Calculations and Formulas? Hardness as CaCO3 mg/L= (ml of EDTA (sample) – ml of EDTA (blank))(0.01 M EDTA)(100 mg=CaCO3 milliMole)(1000 ml/L) ml of sample volume titrated

  11. Hardness #2340 Example Problem? The results for the hardness of a sample are as follows: Volume of sample titrated = 50 ml Volume of EDTA consumed by the sample = 4.4 ml Volume of EDTA consumed by the blank = 0.2 ml Hardness as CaCO3 mg/L= (ml of sample used – ml of blank)(0.01 M EDTA)(100 mg =CaCO3 milliMole)(1000 ml/L) ml of sample volume titrated Total hardness = (4.4 ml – 0.2 ml) (0.010M) (100) (1000) 50 ml Total hardness = 84 mg/L as calcium carbonate

  12. Hard Waters in the USA

  13. Hardness #2340 • What are typical values in nature? • Classification mg/L Soft 0 - 17.10 Slightly hard 17.1 - 60 Moderately hard 60 - 120 Hard 120 – 180 Very Hard 180 & over Drinking water average is about 250 mg/L as calcium carbonate hardness

  14. Hardness #2340 • Tips and Suggestions? • Titrate color blank VERY SLOWLY 1-3 drops!!! • Sample volume (~100 ml??); its trial and error • When making buffer solution do it in the fume hood ammonium hydroxide= ammonia odor • Add EDTA slowly! • Keep track of EDTAadded for the (blank and sample)! • Rely solely on indicators or can use pH probe!

  15. Hardness and Total Alkalinity • Summary? • Accuracy of total hardness as calcium and magnesium from 1 to 800 mg/L CaCO3.

  16. Calmagite turns blue in the absence of these ions. • Calcium and Magnesium • Magnesium and Sodium • Sodium and Calcium • All carbonates

  17. This is the titrant used for the Hardness analysis. • EDTA - A Chelating Agent • 0.03 N Sulfuric acid • 0.125 N Hydrochloric acid • Sodium hydroxide

  18. Alkalinity and hardness are both expressed analytically as this. • Calcium Carbonate - mg/L • Calcium carbonate • Calcium carbonate, mg/L • mg/L as Calcium carbonate • CaCO3

  19. A test on a water supply showed a hardness of 232 mg/L. A certain dosage of polyphosphates will theoretically reduce this hardness by 21 percent. What should the water hardness be after treatment? • 49 mg/L • 211 mg/L • 183 mg/L • 174 mg/L

  20. Alkalinity and hardness are both analyzed by adding a known reagent to the sample. This process results in a ______ change. • Color Change

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