HARDNESS of WATER

An EDTA Titration

INTRODUCTION

Water as the pure substance, H₂O, obviously contains only hydrogen and oxygen. However, because of its polarity and hydrogen bonding, water is an excellent solvent, especially for ionic solids. Since the water we normally use has been sitting in contact with other materials, (the ground, reservoirs, pipes etc.) some of this foreign material has been dissolved. These impurities could be solids, liquids or gasses. To eliminate dissolved solid impurities, water is distilled to give rise to a higher purity water called "distilled water". This eliminates solids and most of the gasses including much of the dissolved oxygen and nitrogen. Water is also purified through a combination of reverse osmosis and chromatography to produce a high purity "deionized water". Deionized water differs from fresh distilled water in that it contains more dissolved oxygen and nitrogen. To understand the difficulty in removing all impurities from water, many chemical processes rely not on ordinary distilled water but on "triply distilled water"!

HARD WATER

Hard water refers to water containing significant amounts of cations which form precipitates with one or more of the common anions such as sulfate, carbonate and phosphate and also the soap anion.
(Nitrate is also a common anion so why is it not included? Recall that almost all nitrates are soluble)

• The major cations involved are ... Ca⁺², Mg⁺², Fe⁺² and Fe⁺³.
  These four ions are singled out because:
  • Being significant components of our bodies, they are nontoxic.
    (At least in the concentrations found in usable water.)
  • They are commonly and universally found dissolved in water.
    Each of the three elements comprises 2-5 percent of the earth's crust.

EFFECTS of HARD WATER

Hard water produces problems due to the production of precipitates. There are two major areas where precipitates are undesireable.

1. In systems which boil water to produce steam.
   • Hospital steam autoclaves and sterilizers
   • Steam generating power plants
   • Household steam irons
2. In systems which use soap
   • baths
   • washing hands
   • laundry

DEFINITION of HARD WATER

Hard water is in the eyes of the beholder.

It is generally agreed that Water containing less than about 40 ppm CaCO₃ is soft and water containing more than about 150 ppm CaCO₃ is hard.

In terms of ppm calcium, this roundly translates to less than 20 ppm Ca is soft and more than 60 ppm Ca is hard.

The numbers for CaCO₃ and Ca should be in the ratio 100.09/40.08. As you can see there is plenty of uncertainty built into the above numbers.

Salesmen selling water conditioners, will probably set the hard water limit much lower.

WATER SOFTENING Water softening, both commercial and personal, is big business. You can see ads for water softening. On the right is an add which appeared in Parade Magazine, the Sunday supplement 2/1/98.