Experiment 14 Determination of Water Hardness

Note All Erlenmeyer and volumetric flasks used in this experiment must be rinsed thoroughly with distilled water prior to use. Ordinary tap water contains hardness minerals that will contaminate. The pH = 10 ammonia buffer required can be prepared by dissolving 35 g of NH4C1 and 285 mL of concentrated ammonium hydroxide in water and diluting to 500 mL. The EBT indicator should be fresh and prepared by dissolving 200 mg in a mixture of 15 mL of triethanolamine and 5 mL of ethyl alcohol. 

Weigh 4.0 g of disodium dihydrogen EDTA dihydrate and 0.10 g of magnesium chloride hexahy-drate into a 1-L glass bottle. Add one pellet of NaOH and fill to approximately 1 L with water. Shake well. These ingredients will require some time to dissolve, so it is recommended that this solution be prepared one lab session ahead of its intended use. 

Dry a quantity (at least 0.5 g) of primary standard CaC03 for 1 h. Your instructor may choose to dispense this to you. While drying, continue with the procedure below. 

Obtain a water sample. It will be contained in a 1-L volumetric flask. Dilute to the mark with distilled water. Shake well. 

Pipet 100.0-mL aliquots of the water sample (an aliquot is a portion of a solution) into each of three clean 500-mL Erlenmeyer flasks. Add 5.0 mL (graduated cylinder) of the pH =10 buffer and three drops of EBT indicator to each flask. 

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