What is the difference between hardness and alkalinity?

Document ID

Document ID TE285

Published Date

Published Date 04/30/2019
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Question
What is the difference between hardness and alkalinity?
Summary
Difference between hardness and alkalinity
Answer
Hardness is the sum of the multivalent metal ions in solution, whereas alkalinity is a measure of the solution’s ability to neutralize acids (sum of hydroxide, carbonate, and bicarbonates). In natural water systems calcium carbonate is usually present and responsible for different characteristics of the water. Both hardness and alkalinity are expressed as a concentration of CaCO3 for convenience of reporting a single number to represent multiple chemicals and for ease in calculating a solution’s carbonate and non-carbonate hardness.

Additional information on levels of hardness and alkalinity present in surface waters of the United States can be found at this link: https://water.usgs.gov/owq/hardness-alkalinity.html#map

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