# W ATER H ARDNESS. Definition Concerns Scaling Cleaning Public Health.

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W ATER H ARDNESS

Definition Concerns Scaling Cleaning Public Health

C AUSES Cations causing hardness Anions Ca 2+ HCO 3 - Mg 2+ SO 4 2- Sr 2+ Cl - Fe 2+ NO - Mn 2+ SiO 3 2- Table 19.1. Principle cations causing hardness in water and the major anions associated with them

S OURCES From USGS

W ATER H ARDNESS R ATING mg/L as CaCO 3 Degree of Hardness 0-60Soft water 61-120Moderately hard water 121-180Hard water >180Very hard water From USGS

M ETHODS OF D ETERMINATION 1. Complete Cation Analysis Most accurate Calculation of the hardness caused by each ion is performed by use of the general formula: Hardness (mg/L) as CaCO 3 = M 2+ (mg/L) X 50 EW of M 2+ where M 2+ represents any divalent metallic ion and EW represents equivalent weight

E XAMPLE OF C OMPLETE C ATION A NALYSIS Calculate the hardness of a water sample with the following analysis: Cation Concentration (mg/L) Anion Concentration (mg/L) Na + 20Cl - 40 Ca 2+ 15SO 4 2- 16 Mg 2+ 10NO 3 - 1 Sr 2+ 2 Alkalinity 50

E XAMPLE OF C OMPLETE C ATION A NALYSIS CationEWHardness, mg/L as CaCO 3 Ca 2+ 20.0(15)(50)/(20.0) = 37.5 Mg 2+ 12.2(10)(50)/(12.2) = 41.0 Sr 2+ 43.8(2)(50)/(43.8) = 2.3 Total hardness = 80.8

M ETHODS OF A NALYSIS 2. Titrimetric Method Burette A piece of glassware designed to deliver known amounts of liquid into another container

EDTA Ethylenediaminetetracetic acid Chelating agent Substance whose molecules can form several bonds to a single metal ion Multi-dentate ligands Many clawed; holding onto the metal ion to form a very stable complex 4-6 bonds

EBT Eriochrome Black Indicator

A W ORD ABOUT M OLES A mole used in chemistry is something like the term dozen we use every day. The atomic weight of any element, expressed in grams, contains 6.02 x 10 23 atoms. There are 6.02 x 10 23 atoms of oxygen in 16 g of oxygen There are 6.02 x 10 23 atoms of hydrogen in 1 g of hydrogen A mole simply means that you have 6.02 x 10 23 of whatever youre talking about Avogadros number

M OLARITY Defined as the number of moles of solute divided by the number of liters of solution Molarity (mol/L; M) = moles of solute liters of solution The molar concentration of EDTA in this experiment is 0.005 mol/L.

E XAMPLE OF EDTA C ALCULATION 50.0 mL tap water sample 13.75 mL EDTA used in the titration a) Calculate the moles of EDTA used. b) Calculate the molarity of metal ion present in the tap water sample (Remember: EDTA binds to metal ions on a 1:1 molar ratio). c) Find the ppm calcium ion concentration in the sample based on your results from question b).

E XAMPLE OF EDTA C ALCULATION a) Calculate the moles of EDTA used Molarity of EDTA = 0.005 mol/L Moles of EDTA = (Molarity)(Liters of EDTA used)

E XAMPLE OF EDTA C ALCULATION a) Calculate the molarity of metal ion present (Remember EDTA binds to metal ions on a 1:1 ratio) Molarity of metal ion = moles of EDTA liters of water sample

E XAMPLE OF EDTA C ALCULATION Calculate concentration of CaCO 3 in ppm CaCO 3 (ppm) = (Molarity of metal ion)(FW of Ca 2+ )(1000 mg/g) CaCO 3 (ppm) =

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