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Hardness in water, or "hard water", refers to specific minerals that consume soap and cause scaling in water heaters and boilers. The more minerals, the harder the water. Soft water refers to the absence of these minerals.
The term hardness comes from an expression of how difficult or "hard" it is to wash clothes with the water. When soap is mixed with hard water, these minerals combine with the soap and form a precipitate, or a solid. This decreases the cleaning efficiency of the soap and forms soap scum. As more soap is added, solids continue to form until the minerals are depleted. When the minerals are no longer available, the soap forms a lather and works as a cleaning agent.
The minerals that precipitate with soap are polyvalent cations such as calcium, magnesium, iron, manganese, and zinc. The concentration of calcium and magnesium in natural waters generally far exceeds that of any other polyvalent cation. Therefore, hardness is generally considered to be the concentration of calcium and magnesium in water.
Hardness is removed from water systems by precipitation or ion exchange. The treatment method varies depending on the relative amounts of carbonate vs. non-carbonate hardness.
Hach offers many solutions for hard water testing including hard water test kits, hard water test strips, analyzers for hard water, as well as ion-selective eletrodes.
More about how hardness is measured and treated
Hardness is most commonly measured by titration with an EDTA solution. A titration involves adding small amounts of a solution to a water sample until the sample changes color. You can titrate a sample for total hardness using a buret or hard water test kit. You can also measure calcium hardness separately from magnesium hardness by adjusting the pH and using different indicators.
Hach Drop Count Test Kits for total hardness use a dropper to add the EDTA solution to the sample. Hard water test kit model HA-71A, which uses ManVer indicator, works best for natural water samples, especially when iron or manganese is present, or when alkalinity is high. Test kit models 5-B, 5-EP, and 5-EP/MG-L, which use UniVer reagent, work best for industrial samples that may have high concentrations of metals such as copper. Other hard water testing kits are available for measuring calcium and magnesium hardness separately.
Kits using the Digital Titrator can measure hardness concentrations more accurately than drop count titration kits. This is because the Digital Titrator dispenses the EDTA solution in very small increments. Hard water test kits using the Digital Titrator use the ManVer indicator.
Test strips are also available for measuring hardness. Using a hard water test strip, a color develops on the strip and the strip is matched to a chart. The chart shows colors for concentrations of 0, 25, 50, 120, 250, and 425 ppm, or 1, 1.5, 3.7, 15, and 25 gpg. Use hard water test strips when a general range for hardness is sufficient. Test strips should not be used when an exact hardness concentration is required.
Calcium can also be measured using an ion-selective electrode, such as the Radiometer ISE25Ca-9 Calcium Ion Selective Electrode (ISE). An electrode is the best method to use when color or turbidity in the sample interferes with titration or colorimetric methods.
Hach Company also makes several Online Analyzers for continuous hardness monitoring, such as the SP 510 Hardness Monitor and APA 6000 Analyzer. These instruments can activate alarms or pumps when the hardness concentration reaches selected concentrations.
Hach has a complete portfolio of instruments and chemistries with support and services to help you get the right results.