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Use of Cation Exchange Resin

Cation resin is a negatively charged matrix with exchangeable positive ions (cations). Negatively charged ions are fixed and permanently attached – part of the resin structure. Because the bead is negatively charged only positive ions are attracted or exchanged. For water treatment purposes the cation resin is sold in the sodium form (Na+) or hydrogen form (H+).

Cation Resin Applications:

Water Softeners - Hardness Reduction
Strong base cation resin in the sodium (Na+) is commonly used for household, commercial and industrial water softening applications. According to the Water Quality Association, the hardness of water supplies in the United States ranges from 1 to 350 grains (gpg) per gallon or 17.1 to 5,985 ppm. However, most waters range between 3 and 50 gpg.

Softening with cation resin is a simple exchange function. The resin bead is negatively charged and attracts positive ions. When supplied in the sodium form (Na+) the resin will exchange sodium ions for hardness ions – calcium and magnesium. The bead is more strongly attracted to these ions and readily exchanges with the sodium. Once the resin is exhausted it is regenerated with a brine solution (NaCl). The brine overwhelms and releases the calcium and magnesium from the bead. The resin is converted back to the sodium form and ready for re-use.

  • Free chlorine levels above 0.5ppm or any oxidant decrosslink and shorten cation resin life.

  • In applications with high chlorine levels and temperature use a 10% crosslinked gel or macroporous resin.

Water Softeners - Iron & Manganese Removal
The hard water problem is often compounded by contaminants like iron (rust stains) and manganese (black stains). Iron water has a distinctive taste.


  • Softening resin removes ferrous, or soluble iron, in water.

The USEPA Secondary Drinking Water Regulations require less than 0.3 ppm for iron.
Ferrous iron is often referred to as "clear water iron" and commonly found in ground or well water.
  • When sizing a softener compensate for ferrous iron –> for every part per million (ppm) of iron add 1 grain of hardness. For example – if the water sample shows 5 grains hardness and 2 ppm of iron, you would size up the softener for 7, not 5 grains.

  • Ferrous Fe++ not Ferric Fe+++

Ferrous iron sticks to the cation resin and isn't easily exchanged. Regular use of resin cleaners or salt with resin cleaner additives is recommended.
When ferrous iron meets oxygen (air) it changes to ferric iron.  Ferric iron is insoluble and leaves the classic rust stains people are familiar with and often mistakenly refer to as hardness. Ferric iron is best removed through mechanical filtration not cation resin.


  • Manganese can be reduced with cation resin.

  • Manganese is often a companion to iron and is similarly removed. The black or dark brown stain occurs when the soluble manganese ions are oxidized and converted to manganese dioxide.


  • Softening resin has a strong affinity for lead – Pb++.


Strong Acid Cation in the hydrogen form (H+) in combination with strong base anion (SBA) in the hydroxide form (OH), is most often used for demineralization processes such as portable exchange tank (PEDI) operations. In this case, the hydrogen is exchanged for calcium, magnesium and sodium.

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