How Does Water Become Hard?

Water becomes hard by being in contact with soluble, divalent, metallic cations (positive ions having a valence of 2). The two main cations that cause water hardness are calcium (Ca) and magnesium (Mg). Calcium is dissolved in water as it passes over and through limestone deposits. Magnesium is dissolved as water passes over and through dolomite and other magnesium bearing formations. Because groundwater is in contact with these geologic formations for a longer period of time than surface water, groundwater is usually harder than surface water.

Although strontium, aluminum, barium, iron, manganese, and zinc also cause hardness in water, they are not usually present in large enough concentrations to contribute significantly to total hardness.



What Are The Objections To Hard Water?

Hardness was originally defined as the capacity of water to precipitate soap. Calcium and magnesium precipitate soap, forming a curd which causes “bathtub ring” and dingy laundry (yellowing, graying, loss of brightness, and reduced life of washable fabrics), and feels unpleasant on the skin (red, itchy, or dry skin). To counteract these problems, synthetic detergents have been developed. These detergents have additives known as sequestering agents that “tie-up” the hardness ions so they cannot form the troublesome precipitates. Although synthetic detergents overcome these problems, both soap and detergent are wasted by hardness.

Hard water forms scale, usually calcium carbonate, which causes a variety of problems. Left to dry on the surface of glassware, silverware, and plumbing fixtures (shower doors, faucets and sink tops), hard water leaves unsightly scale, called water spots. Scale that forms on the inside of water pipes eventually reduces the water pipes’ carrying capacity. Scale that forms within appliances, pumps, valves, and water meters causes wear on moving parts.

When hard water is heated, scale forms much faster. This creates an insulation problem inside boilers, water heaters, and hot-water lines and increases heating costs.



How Is Hardness Expressed?

Water hardness is, unfortunately, expressed in several different units, and thus it is often necessary to convert from one unit to another when making calculations. The most commonly used units include grains per gallon (gpg), parts per million (ppm), and milligrams per liter (mg/L).

The grain per gallon is based on the old English system of weights and measures. It is based on the average weight of a dry kernel of grain (or wheat). The part per million is a weight to weight ratio. For example, one ppm of calcium means 1 pound of calcium in 1 million pounds per liter, the mg/L is the same as the ppm in the dilute solutions present in most raw and treated water.



To Convert To ... Multiply By

Grains per gallon Milligrams per liter 17.12
Milligrams per liter Grains per gallon 0.05841


Since calcium carbonate is one of the more common causes of hardness, total hardness is usually reported in terms of calcium carbonate concentration (mg/L as CaCO3), using either of two methods:

  1. Calcium and magnesium hardness.

    Hardness caused by calcium is called calcium hardness, regardless of the salts associated with it. Likewise, hardness caused by magnesium is called magnesium hardness. Since calcium and magnesium are normally the only significant minerals that cause hardness, it is generally assumed that:

    Total Hardness =      Calcium Hardness      +      Magnesium Hardness
    (mg/L as CaCO3)         (mg/L as CaCO3)                    (mg/L as CaCO3)

    = 2.50 x Calcium concentration + 4.12 x Magnesium concentration
                  (mg/L as Ca2+)                                   (mg/L as Mg2+)


  1. Carbonate and non-carbonate hardness.

    Carbonate hardness is primarily caused by the carbonate and bicarbonate salts of calcium and magnesium. Non-carbonate hardness is a measure of calcium and magnesium salts other than carbonate and bicarbonate salts, such as calcium sulfate, CaSO4, or magnesium chloride, MgCl2. Total hardness is expressed as the sum of the carbonate hardness and non-carbonate hardness.

    Total hardness =       Carbonate hardness      +         Non-carbonate hardness
    (mg/L as CaCO3)            (mg/L as CaCO3)                           (mg/L as CaCO3)

    The amount of carbonate and non-carbonate hardness depends on the alkalinity of the water.
    When a laboratory reports a value for total hardness of, for instance, 150 mg/l as CaCO3, this indicates that the combined effect of the different hardness causing agents is the same as if the water contained exactly 150 mg/l of CaCO3.



What Degree of Hardness is Considered Objectionable?

The degree of hardness consumers consider objectionable will vary, depending both on the water and the degree of hardness to which consumers have become accustomed. The following table shows a classification of water hardness:

Hardness mg/L as CaCO3
Soft 0 to 75
Moderate 75 to 150
Hard 150 to 300
Very Hard Above 300


The degree of hardness acceptable for a finished water varies with the consumer or industry served. In 1968, the American Water Works Association (AWWA) established a water quality goal for total hardness of 80 to 100 mg/l as calcium carbonate.

It has often been proposed that ideally water should have a total hardness of 75 to 85 mg/l (as CaCO3) and a magnesium hardness of not more than 40 mg/L as CaCO3 to minimize magnesium hydroxide scaling at elevated temperatures, although the actual magnesium concentration that can be present before precipitation of magnesium salt is a function of the pH of the finished water.

In recent years, many cities have allowed the hardness in finished water to approach 110 to 150 mg/L to reduce chemical costs and sludge production. The general use of synthetic detergents has reduced the importance of hardness for soap consumption; however, industrial requirements for a higher-quality feed water for high pressure boilers and cooling towers have generally increased. As industrial waste treatment costs increase, the demand for higher quality water has increased dramatically. Industries purchasing water from municipal supplies have generally been faced with additional water treatment costs, dependent upon the quality of the municipal supply and the intended plant or process use. The degree of water treatment provided, is therefore determined to some degree by the user’s requirements.




How Is Hardness Removed?

“Softening” is the term which refers to the process of hardness removal.




Testing Procedures













Warning: Do not use potassium cyanide to eliminate interferences because it will generate deadly hydrogen cyanide gas when the sulfuric acid solution is added.


Metal Interferences


CDTA Removes Interference Below This Level

Aluminum 50 mg/L
Cobalt 200 mg/L
Copper 100 mg/L
Iron 100 mg/L
Manganese 200 mg/L
Nickel 400 mg/L
Zinc 300 mg/L




Sample Collection, Preservation and Storage




Summary of Method

This test procedure is a combination of the calcium and total hardness procedures. Refer to each method for a detailed description of the methods.