WATER HARDNESS

WATER HARDNESS

Definition

Concerns Scaling Cleaning Public Health

CAUSES

Cations causing hardness

Anions

Ca2+ HCO3-

Mg2+ SO42-

Sr2+ Cl-

Fe2+ NO-

Mn2+ SiO32-

Table 19.1. Principle cations causing hardness in water and the major anions

associated with them

SOURCESFrom USGS

WATER HARDNESS RATING

mg/L as CaCO3 Degree of Hardness

0-60 Soft water

61-120 Moderately hard water

121-180 Hard water

>180 Very hard water

From USGS

METHODS OF DETERMINATION

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 CaCO3 = M2+ (mg/L) X 50

EW of M2+

where M2+ represents any divalent metallic ion and EW represents equivalent weight

EXAMPLE OF COMPLETE CATION ANALYSIS

Calculate the hardness of a water sample with the following analysis:

Cation Concentration (mg/L)

Anion Concentration (mg/L)

Na+ 20 Cl- 40

Ca2+ 15 SO42- 16

Mg2+ 10 NO3- 1

Sr2+ 2 Alkalinity 50

EXAMPLE OF COMPLETE CATION ANALYSIS

Cation EW Hardness, mg/L as CaCO3

Ca2+ 20.0 (15)(50)/(20.0) = 37.5

Mg2+ 12.2 (10)(50)/(12.2) = 41.0

Sr2+ 43.8 (2)(50)/(43.8) = 2.3

Total hardness = 80.8

METHODS OF ANALYSIS

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 WORD ABOUT MOLES 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 1023 atoms. There are 6.02 x 1023 atoms of oxygen in 16 g of

oxygen There are 6.02 x 1023 atoms of hydrogen in 1 g

of hydrogen

A mole simply means that you have 6.02 x 1023 of whatever you’re talking about Avogadro’s number

MOLARITY

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.

EXAMPLE OF EDTA CALCULATION

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).

EXAMPLE OF EDTA CALCULATION

a) Calculate the moles of EDTA used

Molarity of EDTA = 0.005 mol/L

Moles of EDTA = (Molarity)(Liters of EDTA used)

EXAMPLE OF EDTA CALCULATION

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

EXAMPLE OF EDTA CALCULATION

Calculate concentration of CaCO3 in ppm

CaCO3 (ppm) =

(Molarity of metal ion)(FW of Ca2+)(1000 mg/g)

CaCO3 (ppm) =