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.
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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.
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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.
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Use a colorimeter when you need to measure hardness in extremely soft water, where the concentration is expected to be less than 4 mg/L as CaCO3. Use a colorimeter such as the DR 900 Colorimeter, or a spectrophotometer such as the DR 2700, DR 3900, DR 5000 or DR 6000 Spectrophotometer.
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Calcium can also be measured using an ion-selective electrode, such as the model ISE25Ca Calcium Electrode made by Radiometer Analytical. An electrode is the best method to use when color or turbidity in the sample interferes with titration or colorimetric methods.
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Hach Company also makes several online Hardness Analyzers for continuous hardness monitoring such as the model SP 510 and APA6000 Hardness Analyzers. These instruments can activate alarms or pumps when the hardness concentration reaches selected concentrations.
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