Hardness in water is caused by dissolved minerals, primarily divalent cations, including calcium (Ca2+), magnesium (Mg2+), iron (Fe2+), strontium (Sr2+), zinc (Zn2+) and manganese (Mn2+). Calcium and magnesium ions are usually the only ions present in significant concentrations; therefore, hardness is generally considered to be a measure of the calcium and magnesium content of water. Considerations should be given when other cations contributing to hardness are present in significant amounts.

Hardness (Total, Ca & Mg)

Why Measure Hardness?

In general, hard water forms solid deposits comprised of mainly calcium and magnesium salts and can damage equipment, while soft water may be corrosive and therefore, it is important measuring and knowing levels of hardness in your process water to maintain the delicate balance between scaling and corrosivity.

While some hardness may be acceptable in certain water quality applications, others require zero hardness to prevent scaling and damage to equipment. Therefore, water softening by either precipitation or ion exchange is often necessary to remove hardness. To optimize these processes, it is sometimes important to monitor calcium and magnesium levels separately, along with total hardness.

Additionally, magnesium can interfere with other water quality tests such as nitrogen, ammonium-salicylate methods. Visit these related parameter pages to learn more about ammonium and nitrogen.

At Hach® find the testing equipment, resources, training, and software you need to correctly monitor and manage water hardness in your specific application.

Featured Water Hardness Product Categories

Online Water Hardness Analyzers

Portable pH Meter

Continuously monitor water hardness in your systems. Our online water hardness analyzers offer precision, reliability and innovative features that can help save you time and money.

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Portable Water Hardness Testers

HQD

Light, compact and rugged makes our portable instruments the best choice for on-the-go calcium or magnesium ion measurement.

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Water Hardness Test Kits, Reagents and Consumables

Consumables

Hach's quality test kits, reagents and chemistries are quick, accurate and easy to use.

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Laboratory Hardness Meters and Titrators

Lab

Achieve accurate, quick and reliable water hardness measurement. From spectrophotometers with predosed reagents to reliable titrators - Hach has you covered.

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Which Processes Require Hardness Monitoring?

Excessive hardness of finished water released in the distribution system may cause scaling and if the water is too soft, it can cause corrosion of the pipes. Thus, leaching out lead and copper which may lead to violation of the lead and copper rule (LCR).

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A clear glass of water highlights that water quality monitoring is important for human health. Clarity is deceptive and monitoring hardness guards against iron and copper corrosion.

How is Water Hardness Measured? 

Titration

Hardness is commonly measured by colorimetric titration with an EDTA solution. A titration involves adding indicator and then titrant solution in small increments to a water sample until the sample changes color. You can titrate a sample for total hardness using a burette or use a water hardness test kit. You can also measure calcium hardness separately from magnesium hardness by adjusting the pH and using different indicators.

Drop Count Test Kits

This hardness test uses a dropper to add the EDTA solution to the sample and the drop counts are proportional to the water hardness. Total hardness test kit model HA-71A, uses ManVer indicator and 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 test kits are available for measuring calcium and magnesium hardness separately.

Digital Titrator

Kits using the digital titrator can measure hardness more accurately than drop count titration kits. This is because the digital titrator dispenses the EDTA solution in very small increments with higher precision. Hardness test kits with digital titrator use the ManVer indicator.

Test Strips

When the water hardness test strip is dipped in a water sample, 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 the test strips when a general range for hardness is sufficient. Test strips should not be used when an exact hardness value is required.

Colorimetry or Spectrophotometry 

Use a colorimeter or a spectrophotometer when you need to measure hardness of extremely soft water, where the concentration is expected to be less than 4 mg/L as CaCO3 (calmagite method).

Benchtop/Portable:

Use a spectrophotometer when you need to measure ultra-low hardness in water, where the concentration is expected to be less than 1 mg/L as CaCO3 (chlorophosphonazo method).

Benchtop:

Use a spectrophotometer to measure higher ranges of total, Ca and Mg hardness.

Benchtop:

Ion-Selective Electrodes  

Calcium can also be measured using an ion-selective electrode, such as the Radiometer ISE25Ca or the 9660C Calcium Ion Selective Electrode (ISE). An electrode is the best method to use when color or turbidity in the sample interferes with colorimetric titration or spectrophotometric methods.

Online Analysis 

Online analyzers allow for continuous hardness monitoring. These instruments can activate alarms or control chemical feed pumps when the hardness concentration reaches a selected level.

Water Softening

To reduce undesirable hardness, water must be softened. Softening methods typically fall into two main categories:

Ion Exchange Softening Process

This process exchanges cations causing hardness with non-hardness-causing cations, typically sodium, using either natural or synthetic ion-exchanging matrix, e.g. resin. For example, naturally occurring matrices include greensands and zeolites. Different ion-exchange resins (or matrices) possess varying properties. Synthetic materials tend to have a higher exchange capacity, so they are better for removing higher levels of hardness. Resins using hydrogen as the cation are commonly referred to as demineralizers and usually are comprised of both cation and anion-exchange resins to maintain neutral pH. For health reasons, it is important to note that sodium-based resin will increase the sodium levels in the treated water. This may also provide interference to some ULR hardness methods.

Once the exchange capacity of a resin has been exhausted, most can be regenerated so it is important to monitor effluent hardness to determine when the column needs regeneration. In this two-step process, first the unit is flushed to remove sediment, and then a brine solution is circulated through the resin at certain conditions to replace the accumulated calcium and magnesium ions with the cation used for softening originally.

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Precipitation Softening Process

Precipitation is typically accomplished with the lime-soda process. When lime is added to hard water, it causes a reaction with the carbonate hardness present producing solids that then must be removed from the water. Lime and soda ash can be used together to remove both carbonate and non-carbonate hardness. Compared with calcium precipitation, magnesium precipitation requires twice the amount of chemical additive and generates twice the amount of sludge that must be removed. Excess carbon dioxide must be removed prior to softening because it can impede lime precipitation.

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